Archives

Gastrointestinal Disorders in Diabetes

ABSTRACT

 

Gastrointestinal manifestations of type 1 and 2 diabetes are common and represent a substantial cause of morbidity and health care costs, as well as a diagnostic and therapeutic challenge. Predominant among them, and most extensively studied, is abnormally delayed gastric emptying or diabetic gastroparesis. Abnormally increased retention of gastric contents may be associated with symptoms, including nausea, vomiting, postprandial fullness, bloating, and early satiety, which may be debilitating. However, the relationship of upper gastrointestinal symptoms with the rate of gastric emptying is relatively weak. Moreover, gastrointestinal symptoms also occur frequently in people without diabetes, which may compromise the capacity to discriminate gastrointestinal dysfunction resulting from diabetes from common gastrointestinal disorders such as functional dyspepsia. A definitive diagnosis of gastroparesis thus necessitates measurement of gastric emptying by a sensitive technique, such as scintigraphy or a stable-isotope breath test. There is an inter-dependent relationship of gastric emptying with postprandial glycemia. Elevated blood glucose (hyperglycemia) slows gastric emptying while, conversely, the rate of emptying is a major determinant of the glycemic response to a meal. The latter recognition has stimulated the development of dietary and pharmacological (e.g. short-acting GLP-1 receptor agonists) approaches to improve postprandial glycemic control in type 2 diabetes by slowing gastric emptying. The outcome of current management of symptomatic diabetic gastroparesis is often sub-optimal - optimizing glycemic control, the correction of nutritional deficiencies, and use of pharmacotherapy, are important. A number of promising and novel pharmacotherapeutic agents are in development. This chapter focusses on gastric motor function, but also provides an overview of the manifestations of esophageal, gall bladder, small and large intestinal function, in diabetes.

 

INTRODUCTION

 

The gastrointestinal tract extends from the mouth to the anus and performs functions vital to sustaining life including ingestion, breakdown and digestion of nutrients, facilitating nutrient absorption and preparation and expulsion of the waste product. Gastrointestinal symptoms occur commonly in people with diabetes, and include gastro-esophageal reflux, bloating, nausea, constipation, diarrhea, and fecal incontinence. It has been suggested that more than 50% of individuals attending outpatient diabetic clinics will at some stage experience a distressing gastrointestinal symptom. Gastrointestinal motor dysfunction is also common in diabetes and may have an impact on glycemic control. Of the motor dysfunctions, gastroparesis, or delayed gastric emptying, is the most important and will be discussed in relatively greater detail. This chapter is limited to the gastrointestinal manifestations of type 1 and 2 diabetes and does not address other causes of diabetes, such as that related to cystic fibrosis. 

 

GASTROINTESTINAL SYMPTOMS

 

Gastrointestinal symptoms are exhibited frequently in type 1 and 2 diabetes and most, but not all, studies suggest that they are significantly more common in diabetes than in controls without diabetes (1); reported inconsistencies likely reflect discrepancies in the methodology used and the patient populations studied. It should be appreciated that gastrointestinal symptoms are often not volunteered, particularly those considered embarrassing (such as fecal incontinence) and it would not be surprising if current estimates are less than is really the case. Symptoms, unfortunately, continue to be evaluated in clinical trials solely using participant ‘self-report’ despite its appreciated unreliability, rather than simple, validated measures, which are used widely in the assessment of ‘functional’ gastrointestinal disorders (e.g.  irritable bowel syndrome and functional depression) (2)  (1)  Symptoms appear to be more common in women with diabetes, as is the case with functional gastrointestinal disorders (3). While it is unclear whether symptom prevalence varies between type 1 and type 2 diabetes there is no doubt that gastrointestinal symptoms have a substantial negative impact on quality of life in people with diabetes (4). There is, however, a poor correlation between gastrointestinal symptoms and measures of function, such as the rate of gastric emptying. The natural history of gastrointestinal symptoms remains poorly defined, although it is known that onset and disappearance of symptoms is common i.e. there is considerable ‘symptom turnover’ - approximately 15-25 % over a 2-year period has been observed in type 2 patients (1). This symptom turnover has been reported to be associated with the onset of depression, but not with autonomic neuropathy or glycemic control (5).  

 

GASTROINTESTINAL MANIFESTATIONS IN DIABETES

 

Esophagus

 

The esophagus, a muscular tube connecting the pharynx to the stomach, enables propulsion of swallowed food, with a sphincter at either end (the upper and lower esophageal sphincters) to prevent esophago-pharyngeal and gastro-esophageal reflux, respectively.

 

Two common esophageal symptoms are heartburn (as part of gastro-esophageal reflux disease) and dysphagia (potentially indicating esophageal motor dysfunction). Techniques to evaluate esophageal motility include conventional and high-resolution manometry (HRM). Scintigraphy can measure esophageal transit but has not been standardized and is not commonly employed in clinical settings.

 

The relationship between esophageal transit and gastric emptying in diabetes is poor (6). Acute hyperglycemia inhibits esophageal motility (7), and reduces the basal lower esophageal sphincter pressure (8). While the esophagus has been less well studied than the stomach, it is clear that disordered esophageal function occurs frequently and that disordered motility in both the esophagus and stomach may share a similar pathogenesis. It has been postulated that the major mechanism underlying esophageal dysmotility is a reduction of cholinergic activity and vagal parasympathetic dysfunction (9). The pathological abnormalities associated with gastroparesis, such as a reduction in interstitial cells of Cajal and inhibitory intrinsic neurons, have also been postulated to be relevant to esophageal dysmotility (10). Diffuse esophageal muscular hypertrophy was reported in two-thirds of people with diabetes in one case series (11).

 

There are limited evidence-based options for the management of esophageal disorders in diabetes. General measures include lifestyle modifications (improved glycemic control, weight loss, dietary modifications, and physical exercise). Prokinetic agents have been used, albeit with limited evidence to support efficacy. The latter include dopaminergic agents (metoclopramide, domperidone), serotonin receptor agonists (cisapride), and motilin agonists (erythromycin). Botulinum toxin was trialed in a pilot study in patients with achalasia (including those with diabetes) and peripheral neuropathy and improvements in effective peristalsis induction and contraction amplitude were reported (12).

 

Gastro-esophageal reflux disease (GERD) is extremely common in the general population and also frequently seen in diabetes. In non-erosive GERD, treatment involves lifestyle measures (bed elevation of 30 degrees at head- end) and use of proton-pump inhibitors. In a community study, a reduced rate of heartburn was found in type 1 patients when compared with a control population (13), although this observation remains to be confirmed and the implications are unclear.

 

Disordered esophageal motility, especially the elderly, increases the risk of ‘pill-induced esophagitis’, with mucosal injury due to prolonged exposure to impacted medications (14). Diabetes is an independent risk factor (14), and the condition usually presents as chest pain with or without odynophagia. Treatment involves withdrawal of the offending agent and use of proton pump inhibitors (15).

 

Stomach - Diabetic Gastroparesis

 

INTRODUCTION

 

Delayed gastric emptying in diabetes was first reported almost a century ago, but it was Kassander who, in 1958, documented asymptomatic increased gastric retention of barium in diabetes and coined the descriptive term ‘gastroparesis diabeticorum’ (16). Interestingly, Kassander also suggested in their paper that gastroparesis could adversely impact glycemic control. Some sixty years on, diabetic gastroparesis, traditionally defined as abnormally delayed gastric emptying of solid food in the absence of mechanical obstruction, remains a diagnostic and management challenge (17). Gastroparesis occurs in both type 1 and 2 diabetes and may not, necessarily, be indicative of a poor prognosis (18,19).

 

The rate of gastric emptying is now appreciated as a major determinant of postprandial glycemia in both health and diabetes (20), and novel anti-diabetic medications, such as short acting GLP-1 receptor agonists, diminish postprandial glycemic excursions predominantly by slowing gastric emptying, are used widely.

 

EPIDEMIOLOGY OF DIABETIC GASTROPARESIS

 

The ‘true’ incidence and prevalence of diabetic gastroparesis globally remain uncertain largely due to inconsistencies in the definition of gastroparesis, study populations, and methodology. It is, however, clear that diabetes is a leading cause of gastroparesis, accounting for about 30% of cases in tertiary referral studies (17). A recent analysis of data from the follow-up arm of the landmark prospective study in type 1 diabetes, called DCCT-EDIC (Diabetes Control and Complications –Epidemiology of Diabetes Interventions and Complications) found that delayed gastric emptying of a solid meal occurred in 47% of this population, consistent with the prevalence reported in other cross-sectional studies (21). Previously believed to be essentially a complication of advanced type 1 diabetes (T1D), it is now apparent that gastroparesis also occurs frequently in type 2 diabetes (T2D) (16,22). Risk factors for gastroparesis include a long duration of diabetes, the presence of other microvascular complications, female gender, obesity. and smoking (17).  In a recent report from the NIH Gastroparesis Consortium, the proportion of T1D and T2D was comparable (although many more people with T2D have gastroparesis as its prevalence is much higher), although a US-based community study based on symptomatic cases, reported an incidence of approximately 5% in T1D and 1% in T2D (compared with 0.01% in controls) (23). Data from the US indicate that hospitalizations due to diabetic gastroparesis rose 158% between 1995-2004, which may reflect a true increase in incidence and / or greater clinical awareness of the condition (24). Not surprisingly, health care costs related to diabetic gastroparesis have also increased substantially in recent years. It should, however, also be noted that the awareness of the central importance of glycemic control to the development and progression of microvascular complications, and the consequent increased priority in management to improve it, may have led to a reduction in the incidence of gastroparesis. Consistent with this, it has recently been shown that in well-controlled T2D, even when longstanding, the prevalence of gastroparesis is low and, not infrequently, gastric emptying is modestly accelerated (19,25).

 

DIAGNOSIS OF DIABETIC GASTROPARESIS

 

As alluded to, the presence of gastrointestinal symptoms is poorly predictive of delayed gastric emptying. It is well established that patients with debilitating upper GI symptoms may have normal, or even rapid emptying, while others with unequivocally markedly delayed emptying may report few, or no symptoms. Measurement of gastric emptying, after exclusion of mechanical obstruction at the gastric outlet or proximal small intestine is, accordingly, mandatory for a formal diagnosis of gastroparesis, for which scintigraphy, developed in the 1970s, remains the ‘gold standard’ technique. An attempt has been made to standardize the methodology, with the American Neurogastroenterology and Motility Society and the Society of Nuclear Medicine defining gastroparesis by the intra-gastric retention of >60% of a standardized meal at 2 hours and/or >10% at 4 hours (26). The test meal advocated in the consensus statement comprises two egg-whites, two slices of bread and jam (30 g) with water (120 ml), providing 255 kcal with little fat (72% carbohydrate, 24% protein, 2% fat and 2% fiber) (26). While a useful exercise, the probability of universal adoption of a specific meal, especially outside Western cultures, is intuitively low. The advantages of scintigraphy are its capacity for precise, concurrent measurement of both solid and liquid meal components (the ‘consensus’ test meal only labels the solid component); however, it involves radiation exposure and requires sophisticated equipment and technical expertise. Acceptable alternatives include 13C based breath tests and ultrasonography, neither of which involve radiation exposure, although the latter is operator-dependent (22). Newer techniques, such as the wireless motility capsule, MRI, and SPECT imaging have emerged, but at present these should be considered less accurate than scintigraphy and / or only relevant to a research setting (17).

 

PATHOGENESIS OF DIABETIC GASTROPARESIS

 

Gastric emptying is a complex, coordinated process by which chyme is delivered to the small intestine at a tightly regulated rate and involves the gastro-intestinal musculature, nervous systems (intrinsic and extrinsic), gastric ‘pacemaker’ (so-called ‘Interstitial cells of Cajal or ICC), immune cells, and fibroblast-like cells that stain positive for platelet derived growth factor receptor alpha.  In the fasting state, a cyclical pattern of contractile activity known as the ‘migrating motor complex’ (MMC) sweeps from the stomach through to the small intestine, which serves a “housekeeping’ role i.e. facilitating the movement of ingestible food particles and bacteria from the stomach through the intestine (27). There are distinct phases of the MMC: phase I consists of motor quiescence lasting approximately 40 min, phase II, approximately 50 min, is comprised of irregular contractions, and phase III is characterized by regular contractions (at approximately 3 per min in the stomach and about 10-12 per small intestine) for 10 min during which the bulk of indigestible solids are emptied (28). Following meal ingestion, the MMC is replaced by a ‘postprandial’ motor pattern. Solids are then mixed with gastric acid and ground into small particles (usually < 1-2 mm) in the distal stomach. Gastric accommodation is mediated by vagal and nitrergic mechanisms, antral contractions by vagal and intrinsic cholinergic mediation, and pyloric relaxation by nitrergic mechanisms (17). The resultant chyme is delivered through the pylorus to the proximal duodenum predominantly in a pulsatile manner (22,27). It is now appreciated that the rate of emptying is regulated primarily by nutrient-induced inhibitory feedback arising from the small intestine, rather than by ‘intragastric’ mechanisms (29). Digestible solids and high nutrient liquids empty from the stomach in an overall linear fashion as a result of this feedback (6); solid emptying is preceded by an initial so-called ‘lag-phase’ of 20-40 min during which solids are ground into small particles. In contrast to solids, low or non-nutrient liquids empty in an overall, volume-dependent, monoexponential pattern because small intestinal feedback is less (27). A number of gut peptides play a key role in providing intestinal feedback, including GLP-1, CCK, and peptide YY. In contrast, ghrelin and motilin, which accelerate gastric emptying, are suppressed following food intake (22,27). Both the length and region of small intestine exposed to nutrients modulate feedback to slow gastric emptying (30).

 

Disordered gastric emptying represents the outcome of impairments of variable combinations of these diverse components. Advances in understanding the underlying pathophysiology have been made over the past decade, particularly through the efforts of the NIH-funded Gastroparesis Clinical Research Consortium. Histological studies from this group and others have shown a reduction in the number of interstitial cells of Cajal in diabetic gastroparesis, which correlates with the magnitude of delay in emptying (31). Interstitial cells of Cajal loss appears to be driven by an immune infiltrate involving a shift from protective M2, to classically activated, M1 macrophages, with defective regulation of heme oxygenase-1 and resultant oxidative stress.  Altered expression of the Ano-1 gene which influences conduction in the Interstitial cells of Cajal has also been reported (32). A reduction in inhibitory neurons expressing nitric oxide synthase also appears to contribute (31).

 

GASTRIC EMPTYING AND GLYCEMIA (FIGURE 1)

 

Figure 1. Bidirectional relationship between gastric emptying and glycemia. Abbreviations: CCK, cholecystokinin; GIP, gastric inhibitory polypeptide; GLP-1, glucagon-like peptide 1; PYY, peptide YY. Reproduced with permission from Philips et al (22)

 

Gastric emptying exhibits a wide inter-individual variation (ranging between 1-4kcal /min in health and even wider in diabetes because of the high prevalence of gastroparesis and, less often, abnormally rapid emptying) (Figure 2).  Gastric emptying is a major determinant of postprandial glycemia across glucose-tolerant states and these relationships are time- dependent. In individuals with normal glucose tolerance, following a 75g oral glucose drink, the early (approximately 30 min) rise in glucose is directly proportional to the rate of emptying, while the 120 min value (the standard endpoint in an OGTT) is inversely related. This relationship shifts to the right as glucose tolerance worsens, such that both 30- and 120-min glucose values are directly proportional to the gastric emptying rate in type 2 diabetes(33-35).Epidemiological studies indicate that about 50% of people with impaired glucose tolerance or IGT will develop frank type 2 diabetes and, hence, factors affecting progression are of considerable interest. We have shown that the disposition index – a predictor of progression to type 2 diabetes – is inversely related to the gastric emptying rate (36), suggesting that the rate of emptying may influence the progression. There is evidence that the 1-hour plasma glucose level in a 75g oral glucose tolerance test is strongly associated with the risk of future type 2 diabetes (37)  and this is known to be dependent on the rate of gastric emptying (34,35).

 

In type 2 diabetes, slowing of gastric emptying (such as by morphine) reduces the postprandial glycemic profile, while accelerating emptying by pro-kinetics (such as erythromycin) increases it (38). Bypassing the stomach and delivering glucose directly into the small intestine at specified rates (within the physiological span of gastric emptying) via naso- duodenal catheters has been used as a model to characterize the impact of gastric emptying on glycemia. These 'surrogate' studies indicate that gastric emptying is a major determinant of postprandial insulin secretion and the magnitude of the so-called 'incretin' effect (the augmented insulin secretory response to oral or enteral, compared with intravenous, glucose). Moreover, the relative contribution of the two 'incretin' hormones (GIP and GLP-1) to the incretin effect in health varies such that GIP is the predominant contributor when glucose enters the small intestine at 2kcal/ min or less, with GLP-1 contributing only at higher rates of duodenal glucose delivery (3 or 4 kcal per min) (39). It is, therefore, likely that the relative contributions of GIP and GLP-1 to the postprandial insulin response and glycemia depend on an individual’s intrinsic rate of emptying. Variations in blood glucose also affect gastric emptying. Through ‘glucose clamp’ studies, we have shown that abrupt elevations in blood glucose slows gastric emptying in a ‘dose- dependent’ manner, i.e. the slowing is dependent on the magnitude of the elevation in blood glucose (7).  Moreover, when blood glucose is ‘clamped’ at about 8 mmol/L or 144 mg/dL (i.e. physiological hyperglycemia), gastric emptying is modestly slower in both health and well-controlled type 1 diabetes (40). This may, however, not apply to spontaneous elevations in blood glucose (41)  and further clarification is required. On the other hand, acute hypoglycemia (blood glucose about 2.6 mmol/L or 46.8 mg/dL) accelerates gastric emptying markedly in both groups (42), and is likely to represent an important counter-regulatory mechanism. It follows that the acute glycemic environment, by altering gastric emptying, is likely to influence intestinal absorption of nutrients, as well as oral medications, which has hitherto been poorly appreciated in clinical practice. The impact of chronic glycemic control on gastric emptying remains uncertain.

 

Figure 2. Gastric emptying of solids (minced beef) (A), shown as the retention at 100 min (percent); and the gastric emptying of liquids (10% dextrose) (B), shown as the 50% emptying time (minutes) in 101 outpatients with diabetes. The normal range is indicated by the shaded area. Reproduced with permission from Jones et al (43).

 

In people with insulin-treated diabetes (type 1 or type 2), it is important to match exogenous insulin delivery with the availability of carbohydrate to minimize the risk of postprandial hypoglycemia. It is, therefore, intuitively likely that delayed gastric emptying predisposes to lower blood glucose concentrations in the early postprandial period (so-called ‘gastric hypoglycemia’) (44) and subsequent hyperglycemia. A study in type 1 patients reported that insulin requirements were lower in those with gastroparesis during the first 120 min post-meal, but greater during 180-240 min, compared to patients with normal gastric emptying (45). It is increasingly appreciated that greater glycemic variability is associated with worse outcomes (46). Knowledge of the rate of emptying may potentially assist the clinician in developing strategies to reduce postprandial glycemic variability in individual patients, although this needs to be evaluated formally.

 

MANAGEMENT OF SYMPTOMATIC GASTROPARESIS (FIGURE 3)

 

Figure 3. Treatment Algorithm for Diabetic Gastroparesis. PRN, as needed. Reproduced with permission from Du et al (1)

 

General Measures

 

Management of gastroparesis should be individualized. In clinical practice, patients are generally advised to consume small, frequent meals that are low in fat and fiber, with more calories as liquids than solids; ingested solids should be those that fragment readily into small particles (47). It should, however, be noted that this advice has not been rigorously evaluated and may be difficult to adhere to, so that the involvement of a dietitian is recommended (48). While optimizing glycemic control is intuitively important, given the inhibitory effect of acute hyperglycemia on gastric emptying, this has not been clearly established to be the case in the chronic setting, although the use of continuous subcutaneous glucose infusion and continuous glucose monitoring has recently been advocated (49).

 

Concurrent medications should be reviewed and, if possible, those which may slow gastric emptying (e.g. opiates, anticholinergics) ceased. In this regard, it should be appreciated that short-acting GLP-1 receptor agonists (e.g. exenatide BD and lixisenatide) and the amylin analogue, pramlintide (50), improve chronic glycemic control primarily by slowing gastric emptying.

 

Medications

 

Although studies involving pro-kinetic medications for treatment of gastroparesis have nearly all been of short duration and involved a modest number of participants, these drugs are used widely and form the mainstay of therapy. Major limitations are their adverse effect profile and tachyphylaxis i.e. diminution in pharmacological effect over time. Tachyphylaxis is thought to particularly affect motilin agonists, although this has not been well studied. Cisapride (a 5HT4 agonist) was used widely for symptomatic management, but shown subsequently to be associated with cardiac adverse effects (prolonged QT interval and ‘torsades de pointes’) and taken off the market. The most commonly used medications are discussed below. Some prokinetic drugs also have antiemetic properties.

 

Metoclopramide, a dopamine D2 receptor antagonist, improves gastric emptying (48), and can be administered via oral, intranasal, and subcutaneous routes, but is associated with central nervous system adverse events (including tardive dyskinesia), which may be irreversible. Accordingly, the US Food and Drug Administration (FDA) recommends short duration (12 weeks) use only. An intranasal formulation of metoclopramide under development was reported to be efficacious in women, but not men, implying the potential importance of gender in selecting the route of delivery (51). Metoclopramide can also be injected subcutaneously in an attempt to abort attacks of vomiting. It is the only medication that is approved currently by the FDA for the management of gastroparesis.

 

Domperidone is another D2 receptor antagonist, but unlike metoclopramide, does not cross the blood-brain barrier and is associated with fewer adverse events, with apparently comparable improvements in gastric emptying and upper gastrointestinal symptoms (48,52). Domperidone may prolong the QT interval and affect metabolism of other medications through the CYP2D6 pathway (48).

 

The antibiotic, erythromycin, is a motilin receptor agonist and is effective acutely, and inexpensive, but needs to be administered frequently and may also prolong the QT interval and interact with other medications, in this case through the CYP3A4 pathway (48). Acute, intravenous, administration of erythromycin markedly accelerates delayed gastric emptying (53) and may assist in the placement of neuroenteric tubes (54). The gastrokinetic effect of erythromycin is, however, subject to tachyphylaxis (55).

 

A number of novel agents are in Phase 2-3 trials, including ghrelin and 5HT4 receptor agonists. Ghrelin (sometimes referred to as the 'hunger' hormone) is secreted from the fundus of the stomach and has important roles in nutrient sensing and appetite regulation. Administration of ghrelin accelerates gastric emptying in both animals and humans (56). The outcome of phase 2 trials of the ghrelin agonist, relamorelin, have been promising, with a reduction in upper gastrointestinal symptoms in type 1- and 2 patients with gastroparesis as well as an acceleration of gastric emptying (57). An international phase 3 trial is in progress. Similarly, the oral highly selective 5 HT4 agonists, velusetrag (which was marketed for constipation) and prucalopride, accelerate gastric emptying (58,59). A recent study reported that 4 weeks’ of treatment with prucalopride in 32 people with gastroparesis (including 6 with diabetes) improved both symptoms and accelerated gastric emptying, although sub-group analysis of the diabetic cohort was not performed due to small numbers (59).

 

Treatment-Refractory Gastroparesis

 

Gastroparesis refractory to dietary and pharmacological intervention is debilitating for the patient and management represents a substantial challenge. Bypassing the stomach using jejunal or parenteral feeding, may be required to sustain nutrition. Gastric electrical stimulation (GES) using the ‘Enterra’ device) appeared to be a promising therapeutic option when initial unblinded studies were indicative of symptom improvement (22,48) and is currently approved by the FDA for ‘humanitarian exemption’; however, a subsequent blinded study failed to show a difference between periods where the stimulator was switched ‘on’ or ‘off’ (22,48,60,61).  A recently reported randomized cross-over trial reported a reduction in frequency of refractory vomiting following GES for a 4-month period in gastroparesis with or without diabetes but improvement in symptom control did not accelerate gastric emptying or benefit quality of life (62). Similarly, pyloric botulinum toxin injections have fared much better in uncontrolled, than in sham-controlled trials (22).  Surgical and endoscopic interventions, such as pyloroplasty and pyloromyotomy, and acupuncture have been described in literature, but lack controlled outcome data (22,48).

 

Gall Bladder

 

Gall stones are encountered more frequently in people with diabetes, which is not surprising given that risk factors for the development of stones, such as intestinal dysmotility, obesity, and hypertriglyceridemia, are more common in this group (particularly type 2 diabetes) (63). In addition, impairment of gall bladder motility and autonomic neuropathy, as well as factors such as cholesterol supersaturation and crystal nucleation promoting factors, are considered important. Common techniques used to measure gall bladder motor function include ultrasound and scintigraphy. Some studies have found increased fasting gall bladder volume, while in others, there was no difference, or even a reduction, in people with diabetes. It is possible that differences in the techniques employed (ultrasound or scintigraphy), and the presence of autonomic neuropathy may account for these discrepancies. Many studies, however, have reported impairment in postprandial gall bladder emptying in diabetes, sometimes termed ‘diabetic cholecystoparesis’ (63). It is also possible that delayed gastric emptying contributes to delayed emptying from the gall bladder. In health, acute hyperglycemia inhibits gall bladder motility in a dose-dependent manner (64). An increased prevalence of gall bladder-related disorders (including cholecystitis and cholelithiasis) is associated with the use of GLP-1 receptor agonists (65) and may potentially relate to a drug-induced prolongation of gall bladder refilling time (66). Similarly, an increase in gall-bladder disease has been reported post-bariatric surgery in obese individuals (including those with diabetes) with the implication that dramatic weight loss may predispose (67).

 

Small Intestine

 

While diabetic enteropathy is common, it has been studied much less comprehensively than diabetic gastroparesis (68). Symptoms of constipation and diarrhea are discussed in the section on large intestinal disorders in diabetes, which follows.

 

Traditionally, vagal dysfunction has been regarded as the major impairment in diabetic enteropathy. However, as is the case with gastroparesis, recent evidence has suggested a critical role for both interstitial cells of Cajal and nNOS (31). Acute hyperglycemia also has a major effect on postprandial small intestinal motility in health (and, presumably, diabetes) by reducing the amplitude of duodenal and jejunal pressure waves, as well as retarding duodenal-cecal transit (69).

 

Small intestinal bacterial overgrowth (SIBO), probably secondary to altered small intestinal motility, is commonly encountered in diabetes; estimates range between 15-40% in type 1 diabetic cohorts. A major limitation of these studies is lack of a ‘gold standard’ method for diagnosis.

 

There is limited information about small intestinal glucose absorptive function in diabetes but, based on animal models, it has been suggested that carbohydrate digestion is disordered. For example, streptozotocin-induced diabetes in rats, is associated with an increase in mucosal absorption of glucose (70). We have demonstrated that small intestinal glucose absorption is comparable in uncomplicated type 1 patients and healthy controls, but probably affected by both duodenal motility and the prevailing glycemic environment (71) - when blood glucose was elevated, intestinal glucose absorption was increased, while absorption was comparable to that in healthy controls during euglycemia. A fundamental limitation in interpreting the outcome of the numerous studies which have reported the potent modulatory effect of the rate of gastric emptying on postprandial glycemia is their failure to discriminate between effects mediated by changes in gastric emptying from those potentially secondary to changes in small intestinal transit  (72).

 

DIAGNOSIS OF ENTEROPATHY

 

Diabetic enteropathy is often a diagnosis of exclusion. It is essential to exclude underlying non-diabetes related etiologies where relevant – for example, testing for celiac disease in type 1 patients is recommended. It should be appreciated that gastrointestinal adverse effects occur frequently with commonly used anti-diabetic medications. Metformin, GLP-1RAs, SGLT2 inhibitors, and particularly alpha-glucosidase inhibitors (e.g. acarbose), which are used widely, are commonly associated with intestinal symptoms.

 

Small intestinal manometry (measurement of contractile activity) may provide mechanistic insights, but its use is limited to specialized centers. Scintigraphy can quantify small intestinal transit, but the diagnostic significance is uncertain. More recently, technologies, including ingestible wireless capsules (such as the SmartPill) and continuous tracking of capsules (3D-Transit system), have been employed; these are promising, but require further validation before clinical exploitation. Small intestinal bacterial overgrowth can be diagnosed by aspiration and culture of intestinal fluid or breath tests, but both have substantial limitations and neither technique can be regarded as a “gold standard”.

 

MANAGEMENT OF ENTEROPATHY

 

Symptom management with medications is common. Prokinetic agents used for gastroparesis are commonly employed for management of disordered intestinal motility, but much less well evaluated. Small intestinal bacterial overgrowth can be treated with antibiotics, such as rifamixin (most common but expensive), amoxicillin-clavulanic acid, or metronidazole. Not surprisingly, small intestinal bacterial overgrowth frequently relapses.

 

Large Intestine

 

The major function of the colon is to re-absorb water and electrolytes from the intraluminal contents, to concentrate and solidify the waste product, and prepare for its elimination. The most common lower gastrointestinal symptoms are constipation, diarrhea, abdominal pain, and distention. It is difficult to estimate a ‘true’ incidence and prevalence. Cohort studies have reported the presence of chronic constipation in up to 25% of people with type 1 and 2 diabetes, while that of chronic diarrhea is up to 5% (73).  Bytzer et al reported a higher prevalence of constipation and diarrhea in people with type 2 diabetes (15.6% compared with 10% in those without diabetes) (3). A recent report analyzing data from the large-scale US public survey, NHANES, found that chronic diarrhea was more common in people with type 1 and 2 diabetes compared with non-diabetic controls (~ 11% vs 6%) (74).

 

CONSTIPATION

 

The etiology of constipation in diabetes is likely to be multifactorial. A study involving only 10 patients found prolonged colonic transit time in those with constipation (13). Autonomic neuropathy is thought to be important; constipation is more common in those with diabetes and autonomic impairment (75). Validated techniques for evaluation include colonic transit scintigraphy and the use of radio-opaque markers and wireless motility capsules (76), but their utility in routine clinical practice has not been fully established.

 

Management of diabetic constipation must include a medication history review and those that may cause constipation should be ceased, if feasible (figure 4). For mild constipation, the American Diabetes Association recommends lifestyle modification such as increased physical exercise and dietary fiber. Over-the-counter laxatives (bulk, osmotic or stimulatory) such as Senna, Bisacodyl and water- soluble fiber supplements are commonly prescribed. Other medications like lactulose, linaclotide, and lubiprostone (the latter two available by prescription in the United States) have been used. There are no head-to-head trials to determine which agent is superior. However, it has been suggested that lactulose may potentiate glucose- lowering (77). Lubiprostone, which acts by direct activation of CIC-2 chloride channels on enterocytes, has been reported to improve both spontaneous bowel movements and accelerate colon transit in a randomized controlled trial in a cohort with diabetes (78). In a randomized trial cholinesterase inhibition with pyridostigmine in 30 people with diabetes (12 T1D, 18 T2D) and chronic constipation, there were superior improvements in both bowel function and colonic transit compared with placebo (79).

 

Figure 4. Algorithm for Management of Chronic Constipation in Patients with Diabetes.

 

CHRONIC DIARRHEA

 

“Diabetic diarrhea” has been traditionally considered a manifestation of autonomic neuropathy (80). The typical symptom is large volume, painless, nocturnal, diarrhea with or without fecal incontinence. Again, the diagnosis essentially represents one of exclusion and it is important to distinguish diarrhea from fecal incontinence. It should be remembered that widely used glucose-lowering therapies, including metformin (malabsorptive), acarbose (osmotic), and GLP-1 receptor agonists, not infrequently cause diarrhea. It is likely that optimizing glycemic control is important in the management of diabetic diarrhea (81), but again, this has not been rigorously evaluated. Dietary strategies include a low FODMAP (Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols) diet under guidance of a qualified dietitian, although this has not been evaluated specifically for the diabetes population in clinical trials. Loperamide, an over-the-counter mu opioid receptor agonist, is used widely. Bile acid sequestrants, such as cholestyramine and colesevelam, are used when bile salt malabsorption is suspected, and have the added advantage of reducing LDL cholesterol and glycated hemoglobin. Other agents include clonidine, diphenoxylate, octreotide, and ondansetron (figure 5).

 

It has been reported that people with diabetes, especially type 1 diabetes, are more likely to have inflammatory bowel disease (IBD) such as ulcerative colitis. Diabetes also appears to be an independent risk factor for Clostridium difficile infection where metformin appears to be protective, probably via its action on the gut microbiota (82). It has also been suggested that there is a link between diabetes and colorectal malignancy (83), and diabetes is associated with worse outcomes and response to colorectal surgery. Interestingly, some observational studies suggest that metformin may have chemo-preventative properties against colorectal malignancy (84).

 

Figure 5. Algorithm for Management of Chronic Diarrhea in Patients with Diabetes.

 

Rectum and Anus

 

Fecal incontinence occurs more frequently in people with diabetes and is associated with the duration of disease, and the presence of microvascular complications, including autonomic and peripheral neuropathy (85). Both internal anal sphincter tone and anal squeeze pressures are reduced in diabetes compared with healthy controls (86,87). A key step in management is to exclude important differential diagnoses, such as colorectal malignancy and irritable bowel disease (88). No single test can be regarded as ‘gold standard’, but anorectal manometry (conventional, 3D or high resolution) is very useful in clinical practice to estimate ano-rectal motor abnormalities, while barium defecography is useful to detect rectal motory, sensory and structural abnormalities (89).

 

Treatment of fecal incontinence is rarely curative, and the focus of management is to improve symptoms and quality of life. Fecal impaction with overflow can be managed by initial manual removal of stool from the rectum and enemas (promoting evacuation) and the subsequent prescription of bulk laxatives, increasing fiber intake, and toilet training. Operant reconditioning of rectosphincteric responses, called ‘biofeedback’ training, was first described by Engel et al in 1974 (90) and can be useful in treating fecal, as well as urinary, incontinence. The technique involves visual demonstration of voluntary contraction of external anal sphincter (EAS) contraction to the patient and training to improve the quality of the response (both strength and duration). Biofeedback training is effective in the longer term in only about 60% of patients in clinical trials; those with a low bowel satisfaction score and having digital evacuations fare better (91).

 

GASTROINTESTINAL EFFECTS OF ANTI-DIABETIC MEDICATIONS AND THEIR IMPLICATIONS FOR CLINICAL PRACTICE

 

Gastrointestinal adverse effects are extremely common in people treated with glucose-lowering medications for type 2 diabetes. In the case of alpha glucosidase inhibitors such as acarbose and miglitol, these effects (e.g., diarrhea and abdominal distention) are predictable sequelae of the malabsorption of carbohydrate (92) . There is new information in relation to two classes of medications (biguanides and GLP-1 receptor agonists).

 

Metformin, a biguanide of herbal origin, remains a first line pharmacological agent of choice for type 2 diabetes. The precise mechanisms of action remain uncertain, although it clearly has multiple effects, including in the liver (block gluconeogenesis), as an insulin sensitizer, and direct actions through the gut, including slowing of gastric emptying (93) . Up to 25% of people using metformin report gastrointestinal adverse events, particularly diarrhea and nausea. Common outpatient clinic strategies to minimize these include initiating treatment at a low dose (i.e., 500mg/day) and gradually up-titrating to usually ~2000 mg/day, use of extended-release formulations and avoiding ingestion on an empty stomach, although evidence to support these approaches is not robust (94).

 

Similarly, GLP-1 receptor agonists (but not DPP-IV inhibitors which lead to only a modest rise in plasma GLP-1 levels), commonly cause gastrointestinal adverse effects. As mentioned, GLP-1 is a gut-based peptide with a profound, but variable, action to slow gastric emptying. This slowing is more marked when baseline gastric emptying is relatively more rapid and is predictive of the reduction in blood glucose following a meal (95) . GLP-1 plays a physiological role to slow gastric emptying - gastric emptying is accelerated by the specific GLP-1 antagonist, exendin 9-39 (96)  and delayed by exogenous administration of GLP-1 in modestly supra-physiological plasma levels (97). Upper gastrointestinal events induced by GLP-1 are, likely to reflect, in part, delayed emptying. As effects are also observed in the fasting state, a direct action on CNS GLP-1 receptors (most notably, area postrema in the brain stem) has also been postulated. A direct effect on the gut is likely to contribute to lower gastrointestinal adverse events such as diarrhea. GLP-1 secreting cells (specialized entero-endocrine ‘L’ cells) are found throughout the gastrointestinal tract, and GLP-1 may exert a local excitatory action in smooth muscle or through the intramural autonomic plexus to increase motility and induce diarrhea (98,99) . A fundamental limitation of the vast majority of clinical trials involving GLP-1 receptor agonists is that gastrointestinal adverse effects have been assessed using participant recall and not validated questionnaires. Nevertheless, results from large cardiovascular outcome trials relating to the use of GLP-1 agonists indicate that the proportion of participants discontinuing GLP-1 receptor agonists due to adverse gastrointestinal events ranges between 4.5 to 13% (100) . Nausea appears to be the most common symptom (up to 25%), with vomiting and diarrhea reported by about 10% (101). A retrospective analysis of 32 phase-3 trials involving ‘long’ and ‘short’ acting GLP-1 receptor agonists reported that gastrointestinal adverse effects are also dose-dependent, and that ‘long’ acting GLP-1 receptor agonists are associated with less nausea and vomiting, but more diarrhea when compared to short-acting GLP-1 receptor agonists (101). Symptoms are reported most frequently at the time of initiation of a GLP-1 receptor agent and may persist for several hours or days probably dependent on the Tmax of the drug (100). Gradual titration of dose is recommended, although evidence to support this approach is uncontrolled.

 

We, and others, have demonstrated employing the gold standard technique of scintigraphy to quantify gastric emptying and both ‘long’ and ‘short’ acting GLP-1 receptor agonists delay gastric emptying, although the magnitude of this deceleration appears to be greater with ‘short’ acting GLP-1 receptor agonists (95,102-104) . Moreover, it is appreciated that GLP-1 receptor agonists may slow gastric emptying profoundly in doses much lower than those used in the management of type 2 diabetes (2). It had been suggested, incorrectly, that long acting GLP-1 receptor agonists, which are now the most widely used form, have no effect on gastric emptying with sustained use (2). A further limitation of clinical trials of GLP-1 receptor agonists is that gastric emptying has either not been measured or a sub-optimal technique used (105).

 

Instances of apparently GLP-1 receptor agonist-induced gastroparesis are increasingly appearing in the medical literature as case reports (106). The prevalence of marked delay in gastric emptying induced by GLP-1 receptor agonists remains uncertain but has stimulated guidelines in relation to their use prior to elective surgery or endoscopy. For example, the American Society of Anesthesiologists (ASA), has recently published consensus guidelines on pre-operative management of people using GLP-1 agonists and have advised withholding a long-acting agent for at least one week prior to the procedure/surgery (107). Such recommendations lack a strong evidence base. It is uncertain whether these recommendations from the ASA will be universally adopted but it appears intuitively unlikely. Recently a UK-based expert group comprising endocrinologists, anesthetists, and pharmacists have recommended against this generic advice  (107) primarily on the lack of robust data demonstrating an increased risk of aspiration under anesthesia, while being on a GLP-1 receptor agonist, that the recommended duration of avoidance may be inadequate (for example, in people taking 1mg semaglutide, avoidance of one week is likely to reduce the plasma drug concentration by about half, which is still likely to slow gastric emptying), at least in some people and reintroduction of GLP-1 receptor agonists once normal food intake has been established has not clearly defined and there is intuitively the high potential for a deterioration in glycemic control, postoperatively including an increase in glycemic variability. They instead recommend that preoperative assessment for risk of aspiration be individualized.

 

In people co-prescribed with insulin and GLP-1 receptor agonists, there is likely to be an increased risk of a mismatch between insulin delivery and availability and intestinal glucose absorption due to prolonged gastric retention to predispose to hypoglycemia. Clinicians should be circumspect in prescribing a GLP-1 agonist and insulin combination in those who have impaired awareness of hypoglycemia or suspicion of delayed gastric emptying.

 

PANCREATIC EXOCRINE SUFFICIENCY IN DIABETES

 

There is an intricate anatomical association of endocrine and exocrine components of the pancreas which appears to translate to a reciprocal relationship between endocrine and exocrine dysfunction (108). However, a wide variation in the prevalence of pancreatic exocrine insufficiency in diabetes has been reported, with evidence that it is greater in type 1 (approx. 25-75%). compared with type 2 (approx. 25-50%) diabetes (109). The majority of these studies are in hospitalized populations; it is likely that prevalence in the community is lower. Our recent study of community type 2 patients reported a lower prevalence of 9% (110). The etiology in type 1 is thought to be a combination of lack of insulin (+/- glucagon and somatostatin), autoimmunity, autonomic neuropathy, and microvascular damage while the latter two contribute to pancreatic exocrine insufficiency in type 2 diabetes – it has been suggested that this may explain why pancreatic exocrine insufficiency is more common in patients with type 1 diabetes (109). Common symptoms are variable and include diarrhea (steatorrhea), abdominal pain, and failure to thrive in children. It is important to discriminate pancreatic and non-pancreatic causes of malabsorption. The relatively deep-seated location of the pancreas hinders easy assessment of its exocrine function. Diagnostic tests can be direct or indirect (111). Direct tests involve stimulation with exogenous hormones or nutrients while simultaneously collecting pancreatic secretions via duodenal intubation. This technique has many logistical issues (high costs, requirement of expertise, invasive nature) which limits its clinical utility despite being the most sensitive and specific. Examples of indirect tests include the 3-day fecal fat, fecal elastase-1 measurement, and breath tests (14C-triolein). Of these, the most common indirect and non-invasive (as well as relatively inexpensive) test in clinical practice is measurement of fecal elastase 1. It has been suggested that a fecal elastase-1 level less than 200 ug/g stool is indicative of mild pancreatic exocrine insufficiency, and a level of 100 ug/g stool of severe pancreatic exocrine insufficiency (108). It should be appreciated that sensitivity (55%) and specificity (60%) of fecal elastase-1 in diagnosing pancreatic exocrine insufficiency are modest. Measurement of fat-soluble vitamins may be indicated.

 

The principles of general management of pancreatic exocrine insufficiency include consumption of smaller, frequent meals, abstinence from alcohol, and involvement of an experienced dietitian. Pancreatic enzyme replacement therapy is regarded as the cornerstone of treatment (108). It is uncertain whether supplementation with pancreatic enzyme replacement therapy in those with type 2 diabetes and pancreatic exocrine insufficiency reduces postprandial glycemic excursions (110). Adjunctive therapies such as acid-suppressing agents are reserved for those with symptoms despite high-dose pancreatic enzyme replacement therapy.

 

CONCLUSIONS

 

Both gastrointestinal symptoms and dysmotility are common in diabetes and represent an important component of management. Gastric emptying is also a major determinant of postprandial glycemic control and may be modulated therapeutically to improve it. Current management of disordered gastrointestinal function, particularly gastroparesis, is primarily empirical, although a number of novel agents are in development; results of these clinical trials are eagerly anticipated.

 

REFERENCES

 

  1. Du YT, Rayner CK, Jones KL, Talley NJ, Horowitz M. Gastrointestinal Symptoms in Diabetes: Prevalence, Assessment, Pathogenesis, and Management. Diabetes Care 2018; 41:627-637
  2. Jalleh RJ, Jones KL, Nauck M, Horowitz M. Accurate Measurements of Gastric Emptying and Gastrointestinal Symptoms in the Evaluation of Glucagon-like Peptide-1 Receptor Agonists. Ann Intern Med 2023; 176:1542-1543
  3. Bytzer P, Talley NJ, Leemon M, Young LJ, Jones MP, Horowitz M. Prevalence of gastrointestinal symptoms associated with diabetes mellitus: a population-based survey of 15,000 adults. Archives of internal medicine 2001; 161:1989-1996
  4. Talley NJ, Young L, Bytzer P, Hammer J, Leemon M, Jones M, Horowitz M. Impact of chronic gastrointestinal symptoms in diabetes mellitus on health-related quality of life. The American journal of gastroenterology 2001; 96:71-76
  5. Quan C, Talley NJ, Jones MP, Spies J, Horowitz M. Gain and loss of gastrointestinal symptoms in diabetes mellitus: associations with psychiatric disease, glycemic control, and autonomic neuropathy over 2 years of follow-up. The American journal of gastroenterology 2008; 103:2023-2030
  6. Horowitz M, Maddox AF, Wishart JM, Harding PE, Chatterton BE, Shearman DJ. Relationships between oesophageal transit and solid and liquid gastric emptying in diabetes mellitus. European journal of nuclear medicine 1991; 18:229-234
  7. Fraser RJ, Horowitz M, Maddox AF, Harding PE, Chatterton BE, Dent J. Hyperglycaemia slows gastric emptying in type 1 (insulin-dependent) diabetes mellitus. Diabetologia 1990; 33:675-680
  8. De Boer SY, Masclee AA, Lam WF, Lamers CB. Effect of acute hyperglycemia on esophageal motility and lower esophageal sphincter pressure in humans. Gastroenterology 1992; 103:775-780
  9. Lam WF, Masclee AA, de Boer SY, Lamers CB. Hyperglycemia reduces gastric secretory and plasma pancreatic polypeptide responses to modified sham feeding in humans. Digestion 1993; 54:48-53
  10. Monreal-Robles R, Remes-Troche JM. Diabetes and the Esophagus. Curr Treat Options Gastroenterol 2017; 15:475-489
  11. Iyer SK, Chandrasekhara KL, Sutton A. Diffuse muscular hypertrophy of esophagus. The American journal of medicine 1986; 80:849-852
  12. Tack J, Zaninotto G. Therapeutic options in oesophageal dysphagia. Nature reviews Gastroenterology & hepatology 2015; 12:332-341
  13. Maleki D, Locke GR, 3rd, Camilleri M, Zinsmeister AR, Yawn BP, Leibson C, Melton LJ, 3rd. Gastrointestinal tract symptoms among persons with diabetes mellitus in the community. Archives of internal medicine 2000; 160:2808-2816
  14. Kikendall JW. Pill-induced esophagitis. Gastroenterol Hepatol (N Y) 2007; 3:275-276
  15. Dumic I, Nordin T, Jecmenica M, Stojkovic Lalosevic M, Milosavljevic T, Milovanovic T. Gastrointestinal Tract Disorders in Older Age. Can J Gastroenterol Hepatol 2019; 2019:6757524
  16. Kassander P. Asymptomatic gastric retention in diabetics (gastroparesis diabeticorum). Annals of internal medicine 1958; 48:797-812
  17. Camilleri M, Chedid V, Ford AC, Haruma K, Horowitz M, Jones KL, Low PA, Park SY, Parkman HP, Stanghellini V. Gastroparesis. Nat Rev Dis Primers 2018; 4:41
  18. Chang J, Russo A, Bound M, Rayner CK, Jones KL, Horowitz M. A 25-year longitudinal evaluation of gastric emptying in diabetes. Diabetes care 2012; 35:2594-2596
  19. Watson LE, Phillips LK, Wu T, Bound MJ, Jones KL, Horowitz M, Rayner CK. Longitudinal evaluation of gastric emptying in type 2 diabetes. Diabetes research and clinical practice 2019; 154:27-34
  20. Marathe CS, Rayner CK, Jones KL, Horowitz M. Relationships between gastric emptying, postprandial glycemia, and incretin hormones. Diabetes care 2013; 36:1396-1405
  21. Bharucha AE, Batey-Schaefer B, Cleary PA, Murray JA, Cowie C, Lorenzi G, Driscoll M, Harth J, Larkin M, Christofi M, Bayless M, Wimmergren N, Herman W, Whitehouse F, Jones K, Kruger D, Martin C, Ziegler G, Zinsmeister AR, Nathan DM, Diabetes C, Complications Trial-Epidemiology of Diabetes I, Complications Research G. Delayed Gastric Emptying Is Associated With Early and Long-term Hyperglycemia in Type 1 Diabetes Mellitus. Gastroenterology 2015; 149:330-339
  22. Phillips LK, Deane AM, Jones KL, Rayner CK, Horowitz M. Gastric emptying and glycaemia in health and diabetes mellitus. Nature reviews Endocrinology 2015; 11:112-128
  23. Jung HK, Choung RS, Locke GR, 3rd, Schleck CD, Zinsmeister AR, Szarka LA, Mullan B, Talley NJ. The incidence, prevalence, and outcomes of patients with gastroparesis in Olmsted County, Minnesota, from 1996 to 2006. Gastroenterology 2009; 136:1225-1233
  24. Wang YR, Fisher RS, Parkman HP. Gastroparesis-related hospitalizations in the United States: trends, characteristics, and outcomes, 1995-2004. The American journal of gastroenterology 2008; 103:313-322
  25. Boronikolos GC, Menge BA, Schenker N, Breuer TG, Otte JM, Heckermann S, Schliess F, Meier JJ. Upper gastrointestinal motility and symptoms in individuals with diabetes, prediabetes and normal glucose tolerance. Diabetologia 2015; 58:1175-1182
  26. Camilleri M, Parkman HP, Shafi MA, Abell TL, Gerson L, American College of G. Clinical guideline: management of gastroparesis. The American journal of gastroenterology 2013; 108:18-37; quiz 38
  27. Marathe CS, Rayner CK, Jones KL, Horowitz M. Effects of GLP-1 and incretin-based therapies on gastrointestinal motor function. Experimental diabetes research 2011; 2011:279530
  28. Sarna SK. Cyclic motor activity; migrating motor complex: 1985. Gastroenterology 1985; 89:894-913
  29. Lin HC, Kim BH, Elashoff JD, Doty JE, Gu YG, Meyer JH. Gastric emptying of solid food is most potently inhibited by carbohydrate in the canine distal ileum. Gastroenterology 1992; 102:793-801
  30. Rigda RS, Trahair LG, Little TJ, Wu T, Standfield S, Feinle-Bisset C, Rayner CK, Horowitz M, Jones KL. Regional specificity of the gut-incretin response to small intestinal glucose infusion in healthy older subjects. Peptides 2016; 86:126-132
  31. Kashyap P, Farrugia G. Diabetic gastroparesis: what we have learned and had to unlearn in the past 5 years. Gut 2010; 59:1716-1726
  32. Mazzone A, Bernard CE, Strege PR, Beyder A, Galietta LJ, Pasricha PJ, Rae JL, Parkman HP, Linden DR, Szurszewski JH, Ordog T, Gibbons SJ, Farrugia G. Altered expression of Ano1 variants in human diabetic gastroparesis. The Journal of biological chemistry 2011; 286:13393-13403
  33. Jalleh RJ, Jones KL, Rayner CK, Marathe CS, Wu T, Horowitz M. Normal and disordered gastric emptying in diabetes: recent insights into (patho)physiology, management and impact on glycaemic control. Diabetologia 2022; 65:1981-1993
  34. Marathe CS, Horowitz M, Trahair LG, Wishart JM, Bound M, Lange K, Rayner CK, Jones KL. Relationships of Early And Late Glycemic Responses With Gastric Emptying During An Oral Glucose Tolerance Test. J Clin Endocrinol Metab 2015; 100:3565-3571
  35. Jalleh RJ, Wu T, Jones KL, Rayner CK, Horowitz M, Marathe CS. Relationships of Glucose, GLP-1, and Insulin Secretion With Gastric Emptying After a 75-g Glucose Load in Type 2 Diabetes. J Clin Endocrinol Metab 2022; 107:e3850-e3856
  36. Marathe CS, Rayner CK, Lange K, Bound M, Wishart J, Jones KL, Kahn SE, Horowitz M. Relationships of the early insulin secretory response and oral disposition index with gastric emptying in subjects with normal glucose tolerance. Physiological reports 2017; 5
  37. Abdul-Ghani MA, DeFronzo RA. Plasma glucose concentration and prediction of future risk of type 2 diabetes. Diabetes Care 2009; 32 Suppl 2:S194-198
  38. Gonlachanvit S, Hsu CW, Boden GH, Knight LC, Maurer AH, Fisher RS, Parkman HP. Effect of altering gastric emptying on postprandial plasma glucose concentrations following a physiologic meal in type-II diabetic patients. Digestive diseases and sciences 2003; 48:488-497
  39. Marathe CS, Rayner CK, Bound M, Checklin H, Standfield S, Wishart J, Lange K, Jones KL, Horowitz M. Small intestinal glucose exposure determines the magnitude of the incretin effect in health and type 2 diabetes. Diabetes 2014; 63:2668-2675
  40. Schvarcz E, Palmer M, Aman J, Horowitz M, Stridsberg M, Berne C. Physiological hyperglycemia slows gastric emptying in normal subjects and patients with insulin-dependent diabetes mellitus. Gastroenterology 1997; 113:60-66
  41. Aigner L, Becker B, Gerken S, Quast DR, Meier JJ, Nauck MA. Day-to-Day Variations in Fasting Plasma Glucose Do Not Influence Gastric Emptying in Subjects With Type 1 Diabetes. Diabetes Care 2021; 44:479-488
  42. Russo A, Stevens JE, Chen R, Gentilcore D, Burnet R, Horowitz M, Jones KL. Insulin-induced hypoglycemia accelerates gastric emptying of solids and liquids in long-standing type 1 diabetes. The Journal of clinical endocrinology and metabolism 2005; 90:4489-4495
  43. Jones KL, Russo A, Stevens JE, Wishart JM, Berry MK, Horowitz M. Predictors of delayed gastric emptying in diabetes. Diabetes care 2001; 24:1264-1269
  44. Horowitz M, Jones KL, Rayner CK, Read NW. 'Gastric' hypoglycaemia--an important concept in diabetes management. Neurogastroenterology and motility : the official journal of the European Gastrointestinal Motility Society 2006; 18:405-407
  45. Ishii M, Nakamura T, Kasai F, Onuma T, Baba T, Takebe K. Altered postprandial insulin requirement in IDDM patients with gastroparesis. Diabetes care 1994; 17:901-903
  46. Ceriello A, Monnier L, Owens D. Glycaemic variability in diabetes: clinical and therapeutic implications. The lancet Diabetes & endocrinology 2019; 7:221-230
  47. Olausson EA, Storsrud S, Grundin H, Isaksson M, Attvall S, Simren M. A small particle size diet reduces upper gastrointestinal symptoms in patients with diabetic gastroparesis: a randomized controlled trial. The American journal of gastroenterology 2014; 109:375-385
  48. Tornblom H. Treatment of gastrointestinal autonomic neuropathy. Diabetologia 2016; 59:409-413
  49. Calles-Escandon J, Koch KL, Hasler WL, Van Natta ML, Pasricha PJ, Tonascia J, Parkman HP, Hamilton F, Herman WH, Basina M, Buckingham B, Earle K, Kirkeby K, Hairston K, Bright T, Rothberg AE, Kraftson AT, Siraj ES, Subauste A, Lee LA, Abell TL, McCallum RW, Sarosiek I, Nguyen L, Fass R, Snape WJ, Vaughn IA, Miriel LA, Farrugia G, Consortium NGCR. Glucose sensor-augmented continuous subcutaneous insulin infusion in patients with diabetic gastroparesis: An open-label pilot prospective study. PLoS One 2018; 13:e0194759
  50. Samsom M, Szarka LA, Camilleri M, Vella A, Zinsmeister AR, Rizza RA. Pramlintide, an amylin analog, selectively delays gastric emptying: potential role of vagal inhibition. American journal of physiology Gastrointestinal and liver physiology 2000; 278:G946-951
  51. Parkman HP, Carlson MR, Gonyer D. Metoclopramide nasal spray is effective in symptoms of gastroparesis in diabetics compared to conventional oral tablet. Neurogastroenterology and motility : the official journal of the European Gastrointestinal Motility Society 2014; 26:521-528
  52. Patterson D, Abell T, Rothstein R, Koch K, Barnett J. A double-blind multicenter comparison of domperidone and metoclopramide in the treatment of diabetic patients with symptoms of gastroparesis. The American journal of gastroenterology 1999; 94:1230-1234
  53. Urbain JL, Vantrappen G, Janssens J, Van Cutsem E, Peeters T, De Roo M. Intravenous erythromycin dramatically accelerates gastric emptying in gastroparesis diabeticorum and normals and abolishes the emptying discrimination between solids and liquids. Journal of nuclear medicine : official publication, Society of Nuclear Medicine 1990; 31:1490-1493
  54. Griffith DP, McNally AT, Battey CH, Forte SS, Cacciatore AM, Szeszycki EE, Bergman GF, Furr CE, Murphy FB, Galloway JR, Ziegler TR. Intravenous erythromycin facilitates bedside placement of postpyloric feeding tubes in critically ill adults: a double-blind, randomized, placebo-controlled study. Critical care medicine 2003; 31:39-44
  55. Jones KL, Berry M, Kong MF, Kwiatek MA, Samsom M, Horowitz M. Hyperglycemia attenuates the gastrokinetic effect of erythromycin and affects the perception of postprandial hunger in normal subjects. Diabetes care 1999; 22:339-344
  56. Camilleri M, Acosta A. Emerging treatments in Neurogastroenterology: relamorelin: a novel gastrocolokinetic synthetic ghrelin agonist. Neurogastroenterology and motility : the official journal of the European Gastrointestinal Motility Society 2015; 27:324-332
  57. Lembo A, Camilleri M, McCallum R, Sastre R, Breton C, Spence S, White J, Currie M, Gottesdiener K, Stoner E, Group RMT. Relamorelin Reduces Vomiting Frequency and Severity and Accelerates Gastric Emptying in Adults With Diabetic Gastroparesis. Gastroenterology 2016; 151:87-96 e86
  58. Manini ML, Camilleri M, Goldberg M, Sweetser S, McKinzie S, Burton D, Wong S, Kitt MM, Li YP, Zinsmeister AR. Effects of Velusetrag (TD-5108) on gastrointestinal transit and bowel function in health and pharmacokinetics in health and constipation. Neurogastroenterology and motility : the official journal of the European Gastrointestinal Motility Society 2010; 22:42-49, e47-48
  59. Carbone F, Van den Houte K, Clevers E, Andrews CN, Papathanasopoulos A, Holvoet L, Van Oudenhove L, Caenepeel P, Arts J, Vanuytsel T, Tack J. Prucalopride in Gastroparesis: A Randomized Placebo-Controlled Crossover Study. The American journal of gastroenterology 2019; 114:1265-1274
  60. Abell T, McCallum R, Hocking M, Koch K, Abrahamsson H, Leblanc I, Lindberg G, Konturek J, Nowak T, Quigley EM, Tougas G, Starkebaum W. Gastric electrical stimulation for medically refractory gastroparesis. Gastroenterology 2003; 125:421-428
  61. McCallum RW, Snape W, Brody F, Wo J, Parkman HP, Nowak T. Gastric electrical stimulation with Enterra therapy improves symptoms from diabetic gastroparesis in a prospective study. Clinical gastroenterology and hepatology : the official clinical practice journal of the American Gastroenterological Association 2010; 8:947-954; quiz e116
  62. Ducrotte P, Coffin B, Bonaz B, Fontaine P, Des Varannes SB, Zerbib F, Caiazzo R, Charles Grimaud J, Mion F, Hadjadj S, Elie Valensi P, Vuitton L, Charpentier G, Ropert A, Altwegg R, Pouderoux P, Dorval E, Dapoigny M, Duboc H, Yves Benhamou P, Schmidt A, Donnadieu N, Gourcerol G, Guerci B, Group Er. Gastric Electrical Stimulation Reduces Refractory Vomiting in a Randomized Cross-Over Trial. Gastroenterology 2019;
  63. Pazzi P, Scagliarini R, Gamberini S, Pezzoli A. Review article: gall-bladder motor function in diabetes mellitus. Alimentary pharmacology & therapeutics 2000; 14 Suppl 2:62-65
  64. Gielkens HA, van Oostayen JA, Frolich M, Biemond I, Lamers CB, Masclee AA. Dose-dependent inhibition of postprandial gallbladder motility and plasma hormone secretion during acute hyperglycemia. Scandinavian journal of gastroenterology 1998; 33:1074-1079
  65. Marso SP, Daniels GH, Brown-Frandsen K, Kristensen P, Mann JF, Nauck MA, Nissen SE, Pocock S, Poulter NR, Ravn LS, Steinberg WM, Stockner M, Zinman B, Bergenstal RM, Buse JB, Committee LS, Investigators LT. Liraglutide and Cardiovascular Outcomes in Type 2 Diabetes. The New England journal of medicine 2016; 375:311-322
  66. Gether IM, Nexoe-Larsen C, Knop FK. New Avenues in the Regulation of Gallbladder Motility-Implications for the Use of Glucagon-Like Peptide-Derived Drugs. The Journal of clinical endocrinology and metabolism 2019; 104:2463-2472
  67. Pineda O, Maydon HG, Amado M, Sepulveda EM, Guilbert L, Espinosa O, Zerrweck C. A Prospective Study of the Conservative Management of Asymptomatic Preoperative and Postoperative Gallbladder Disease in Bariatric Surgery. Obes Surg 2017; 27:148-153
  68. Cogliandro RF, Rizzoli G, Bellacosa L, De Giorgio R, Cremon C, Barbara G, Stanghellini V. Is gastroparesis a gastric disease? Neurogastroenterology and motility : the official journal of the European Gastrointestinal Motility Society 2019; 31:e13562
  69. Russo A, Fraser R, Horowitz M. The effect of acute hyperglycaemia on small intestinal motility in normal subjects. Diabetologia 1996; 39:984-989
  70. Adachi T, Mori C, Sakurai K, Shihara N, Tsuda K, Yasuda K. Morphological changes and increased sucrase and isomaltase activity in small intestines of insulin-deficient and type 2 diabetic rats. Endocrine journal 2003; 50:271-279
  71. Rayner CK, Schwartz MP, van Dam PS, Renooij W, de Smet M, Horowitz M, Smout AJ, Samsom M. Small intestinal glucose absorption and duodenal motility in type 1 diabetes mellitus. The American journal of gastroenterology 2002; 97:3123-3130
  72. Chaikomin R, Wu KL, Doran S, Jones KL, Smout AJ, Renooij W, Holloway RH, Meyer JH, Horowitz M, Rayner CK. Concurrent duodenal manometric and impedance recording to evaluate the effects of hyoscine on motility and flow events, glucose absorption, and incretin release. Am J Physiol Gastrointest Liver Physiol 2007; 292:G1099-1104
  73. Lysy J, Israeli E, Goldin E. The prevalence of chronic diarrhea among diabetic patients. The American journal of gastroenterology 1999; 94:2165-2170
  74. Sommers T, Mitsuhashi S, Singh P, Hirsch W, Katon J, Ballou S, Rangan V, Cheng V, Friedlander D, Iturrino J, Lembo A, Nee J. Prevalence of Chronic Constipation and Chronic Diarrhea in Diabetic Individuals in the United States. The American journal of gastroenterology 2019; 114:135-142
  75. Prasad VG, Abraham P. Management of chronic constipation in patients with diabetes mellitus. Indian J Gastroenterol 2017; 36:11-22
  76. Rao SS, Camilleri M, Hasler WL, Maurer AH, Parkman HP, Saad R, Scott MS, Simren M, Soffer E, Szarka L. Evaluation of gastrointestinal transit in clinical practice: position paper of the American and European Neurogastroenterology and Motility Societies. Neurogastroenterology and motility : the official journal of the European Gastrointestinal Motility Society 2011; 23:8-23
  77. Panesar PS, Kumari S. Lactulose: production, purification and potential applications. Biotechnol Adv 2011; 29:940-948
  78. Christie J, Shroff S, Shahnavaz N, Carter LA, Harrison MS, Dietz-Lindo KA, Hanfelt J, Srinivasan S. A Randomized, Double-Blind, Placebo-Controlled Trial to Examine the Effectiveness of Lubiprostone on Constipation Symptoms and Colon Transit Time in Diabetic Patients. The American journal of gastroenterology 2017; 112:356-364
  79. Bharucha AE, Low P, Camilleri M, Veil E, Burton D, Kudva Y, Shah P, Gehrking T, Zinsmeister AR. A randomised controlled study of the effect of cholinesterase inhibition on colon function in patients with diabetes mellitus and constipation. Gut 2013; 62:708-715
  80. Celik AF, Osar Z, Damci T, Pamuk ON, Pamuk GE, Ilkova H. How important are the disturbances of lower gastrointestinal bowel habits in diabetic outpatients? The American journal of gastroenterology 2001; 96:1314-1316
  81. Vinik AI, Erbas T. Diabetic autonomic neuropathy. Handb Clin Neurol 2013; 117:279-294
  82. Eliakim-Raz N, Fishman G, Yahav D, Goldberg E, Stein GY, Zvi HB, Barsheshet A, Bishara J. Predicting Clostridium difficile infection in diabetic patients and the effect of metformin therapy: a retrospective, case-control study. Eur J Clin Microbiol Infect Dis 2015; 34:1201-1205
  83. Yuhara H, Steinmaus C, Cohen SE, Corley DA, Tei Y, Buffler PA. Is diabetes mellitus an independent risk factor for colon cancer and rectal cancer? The American journal of gastroenterology 2011; 106:1911-1921; quiz 1922
  84. Franciosi M, Lucisano G, Lapice E, Strippoli GF, Pellegrini F, Nicolucci A. Metformin therapy and risk of cancer in patients with type 2 diabetes: systematic review. PLoS One 2013; 8:e71583
  85. Epanomeritakis E, Koutsoumbi P, Tsiaoussis I, Ganotakis E, Vlata M, Vassilakis JS, Xynos E. Impairment of anorectal function in diabetes mellitus parallels duration of disease. Dis Colon Rectum 1999; 42:1394-1400
  86. Sun WM, Katsinelos P, Horowitz M, Read NW. Disturbances in anorectal function in patients with diabetes mellitus and faecal incontinence. Eur J Gastroenterol Hepatol 1996; 8:1007-1012
  87. Schiller LR, Santa Ana CA, Schmulen AC, Hendler RS, Harford WV, Fordtran JS. Pathogenesis of fecal incontinence in diabetes mellitus: evidence for internal-anal-sphincter dysfunction. The New England journal of medicine 1982; 307:1666-1671
  88. Norton C, Thomas L, Hill J, Guideline Development G. Management of faecal incontinence in adults: summary of NICE guidance. BMJ 2007; 334:1370-1371
  89. Carrington EV, Scott SM, Bharucha A, Mion F, Remes-Troche JM, Malcolm A, Heinrich H, Fox M, Rao SS, International Anorectal Physiology Working G, the International Working Group for Disorders of Gastrointestinal M, Function. Expert consensus document: Advances in the evaluation of anorectal function. Nature reviews Gastroenterology & hepatology 2018; 15:309-323
  90. Engel BT, Nikoomanesh P, Schuster MM. Operant conditioning of rectosphincteric responses in the treatment of fecal incontinence. The New England journal of medicine 1974; 290:646-649
  91. Narayanan SP, Bharucha AE. A Practical Guide to Biofeedback Therapy for Pelvic Floor Disorders. Curr Gastroenterol Rep 2019; 21:21
  92. Hanefeld M. The role of acarbose in the treatment of non-insulin-dependent diabetes mellitus. J Diabetes Complications 1998; 12:228-237
  93. Sansome DJ, Xie C, Veedfald S, Horowitz M, Rayner CK, Wu T. Mechanism of glucose-lowering by metformin in type 2 diabetes: Role of bile acids. Diabetes Obes Metab 2020; 22:141-148
  94. Bonnet F, Scheen A. Understanding and overcoming metformin gastrointestinal intolerance. Diabetes Obes Metab 2017; 19:473-481
  95. Linnebjerg H, Park S, Kothare PA, Trautmann ME, Mace K, Fineman M, Wilding I, Nauck M, Horowitz M. Effect of exenatide on gastric emptying and relationship to postprandial glycemia in type 2 diabetes. Regul Pept 2008; 151:123-129
  96. Deane AM, Nguyen NQ, Stevens JE, Fraser RJ, Holloway RH, Besanko LK, Burgstad C, Jones KL, Chapman MJ, Rayner CK, Horowitz M. Endogenous glucagon-like peptide-1 slows gastric emptying in healthy subjects, attenuating postprandial glycemia. J Clin Endocrinol Metab 2010; 95:215-221
  97. Little TJ, Pilichiewicz AN, Russo A, Phillips L, Jones KL, Nauck MA, Wishart J, Horowitz M, Feinle-Bisset C. Effects of intravenous glucagon-like peptide-1 on gastric emptying and intragastric distribution in healthy subjects: relationships with postprandial glycemic and insulinemic responses. J Clin Endocrinol Metab 2006; 91:1916-1923
  98. Hellström PM, Näslund E, Edholm T, Schmidt PT, Kristensen J, Theodorsson E, Holst JJ, Efendic S. GLP-1 suppresses gastrointestinal motility and inhibits the migrating motor complex in healthy subjects and patients with irritable bowel syndrome. Neurogastroenterol Motil 2008; 20:649-659
  99. Nakamori H, Iida K, Hashitani H. Mechanisms underlying the prokinetic effects of endogenous glucagon-like peptide-1 in the rat proximal colon. Am J Physiol Gastrointest Liver Physiol 2021; 321:G617-g627
  100. Nauck MA, Quast DR, Wefers J, Meier JJ. GLP-1 receptor agonists in the treatment of type 2 diabetes - state-of-the-art. Mol Metab 2021; 46:101102
  101. Bettge K, Kahle M, Abd El Aziz MS, Meier JJ, Nauck MA. Occurrence of nausea, vomiting and diarrhoea reported as adverse events in clinical trials studying glucagon-like peptide-1 receptor agonists: A systematic analysis of published clinical trials. Diabetes Obes Metab 2017; 19:336-347
  102. Maselli D, Atieh J, Clark MM, Eckert D, Taylor A, Carlson P, Burton DD, Busciglio I, Harmsen WS, Vella A, Acosta A, Camilleri M. Effects of liraglutide on gastrointestinal functions and weight in obesity: A randomized clinical and pharmacogenomic trial. Obesity (Silver Spring) 2022; 30:1608-1620
  103. Jones KL, Huynh LQ, Hatzinikolas S, Rigda RS, Phillips LK, Pham HT, Marathe CS, Wu T, Malbert CH, Stevens JE, Lange K, Rayner CK, Horowitz M. Exenatide once weekly slows gastric emptying of solids and liquids in healthy, overweight people at steady-state concentrations. Diabetes Obes Metab 2020; 22:788-797
  104. Jensterle M, Ferjan S, Ležaič L, Sočan A, Goričar K, Zaletel K, Janez A. Semaglutide delays 4-hour gastric emptying in women with polycystic ovary syndrome and obesity. Diabetes Obes Metab 2023; 25:975-984
  105. Horowitz M, Rayner CK, Marathe CS, Wu T, Jones KL. Glucagon-like peptide-1 receptor agonists and the appropriate measurement of gastric emptying. Diabetes Obes Metab 2020; 22:2504-2506
  106. Kalas MA, Galura GM, McCallum RW. Medication-Induced Gastroparesis: A Case Report. J Investig Med High Impact Case Rep 2021; 9:23247096211051919
  107. Dhatariya K, Levy N, Russon K, Patel A, Frank C, Mustafa O, Newland-Jones P, Rayman G, Tinsley S, Dhesi J. Perioperative use of glucagon-like peptide-1 receptor agonists and sodium-glucose cotransporter 2 inhibitors for diabetes mellitus. Br J Anaesth 2024;
  108. Toouli J, Biankin AV, Oliver MR, Pearce CB, Wilson JS, Wray NH, Australasian Pancreatic C. Management of pancreatic exocrine insufficiency: Australasian Pancreatic Club recommendations. The Medical journal of Australia 2010; 193:461-467
  109. Piciucchi M, Capurso G, Archibugi L, Delle Fave MM, Capasso M, Delle Fave G. Exocrine pancreatic insufficiency in diabetic patients: prevalence, mechanisms, and treatment. Int J Endocrinol 2015; 2015:595649
  110. Riceman MD, Bound M, Grivell J, Hatzinikolas S, Piotto S, Nguyen NQ, Jones KL, Horowitz M, Rayner CK, Phillips LK. The prevalence and impact of low faecal elastase-1 in community-based patients with type 2 diabetes. Diabetes research and clinical practice 2019; 156:107822
  111. Altay M. Which factors determine exocrine pancreatic dysfunction in diabetes mellitus? World J Gastroenterol 2019; 25:2699-2705

Understanding Ethical Dilemmas in Pediatric Lipidology- Genetic Testing in Youth

ABSTRACT

 

Over the past 25 years there has been an increasing focus on early identification of individuals at-risk of premature cardiovascular disease (CVD), with the goal of improving outcomes and reducing premature CVD-related events such as myocardial infarction and stroke. In 2011, a National Heart, Lung and Blood Institute (NHLBI) Expert Panel recommended universal cholesterol screening of all children, irrespective of health status and family history, beginning at 10 years-of-age (range 9-11) and, if normal, repeated once between 17 and 20 years-of-age (1). Children found to have significant hypercholesterolemia are encouraged to adopt a heart-healthy lifestyle and, when appropriate, offered treatment with lipid-lowering medication, starting at 8 years-of-age and older. Research studies have convincingly demonstrated the safety and effectiveness of lipid-lowering medications in reducing risk and improving outcomes in adults, providing indirect support for universally cholesterol screening of children. Data from individuals with familial hypercholesterolemia (FH), treated for 20 years with pravastatin starting at a young age, have shown no adverse effects of growth, development, or reproductive function during adulthood. Shared decision-making in this population, however, is complex. Unlike most adults who are capable of making informed healthcare decisions, children have a wide range of developmentally-related intellectual and cognitive function, creating unique challenges in their ability to 1) understand long-term risk and benefit; and 2) make informed decisions regarding testing and medical management. In addition, some children have mental health and developmental disabilities that limit their cognitive abilities and judgement. Furthermore, legal guardians have the moral responsibility and legal right to make decisions on behalf of a minor.  In this article, we will discuss 1) privacy, discrimination, and the legal rights of children; 2) ethical considerations and concerns and 3) recommendations for clinicians when providing medical care of children with disorders of lipid and lipoprotein metabolism.

 

OVERVIEW OF LIPID AND LIPOPROTEIN DISORDERS IN YOUTH

 

Children with abnormal levels of lipids and lipoproteins are generally identified as result of targeted, universal or occasionally, coincidental testing.  Current recommendations for lipid screening of children are listed below.

 

  1. Targeted screening in all children ≥2 years of age in whom:
    1. One or both biologic parents are known to have hypercholesterolemia or are receiving lipid-lowering medications
    2. Who have a family history of premature cardiovascular disease (men <55 years of age and women <65 years of age)
    3. Whose family history is unknown (e.g., children who were adopted)
  2. Universal screening of all children 10 years of age (range 9-11), regardless of general health or the presence/absence of CVD risk factors. If normal, repeat screening is recommended at 17-20 years-of-age.

 

Since hypercholesterolemia is often caused by an underlying genetic mutation, such as in FH, cascade screening of biologic relatives is also recommended.  Cascade screening involves systematic testing of all first-degree relatives (parents and siblings) of a child with FH, followed by testing of second- and third-degree relatives if any of the first-degree relatives are affected. The most practical approach to cascade screening is biochemical testing of cholesterol, which is inexpensive, readily available and can be performed without the need for fasting. However, up to 25% of family members may be misdiagnosed as being either affected or unaffected when screening is based on cholesterol levels alone. Testing for a known genetic mutation in the family combined with fasting or non-fasting LDL-C levels will yield the most definitive information. While helpful if known, the child’s family history is often unknown, incomplete, or inaccurate. Reliance upon family history alone fails to identify as many as 30-60% of children with significant hypercholesterolemia. For additional information see the Endotext chapters entitled “Guidelines for Screening, Prevention, Diagnosis, and Treatment of Dyslipidemia in Children and Adolescents” and “Principles of Genetic Testing for Dyslipidemia in Children”.

 

Abnormalities of lipids and lipoproteins in youth may be caused by genetic mutations, acquired conditions, or both.  Those with acquired conditions, such as obesity and insulin resistance, are encouraged to adopt a heart-healthy lifestyle, which includes a low-fat,   calorically appropriate carbohydrate diet, weight loss if overweight or obese, participation in 30-60 minutes of moderate-to-vigorous physical activity per day and smoking avoidance or cessation.  Those suspected of having a genetic mutation are generally diagnosed based upon clinical criteria with or without genetic testing.

 

Genetic mutations that cause lipid and lipoprotein abnormalities vary depending upon the mode of inheritance (autosomal co-dominant vs autosomal recessive), the type of mutation present (slice vs missense), the number of genes involved (monogenic vs polygenic) and their phenotypic expression. When a genetic mutation is present, its expression may potentially be modified by other gene abnormalities (often small effect mutations) and environmental factors (e.g., obesity, insulin resistance, medications). For additional information see the Endotext chapter entitled “Genetics and Dyslipidemia”.

 

Early identification and treatment of children with clinically suspected or genetically confirmed FH has become increasingly common.  However long-term outcome studies demonstrating the safety and efficacy of this approach are lacking. Since lifestyles and therapeutic options are likely to change over the extended period of time that would be necessary to reach “hard” end points in children with FH, such as myocardial infarction and stroke, outcome studies are unlikely to be forthcoming.  Given the significant benefit statins have shown in reducing CVD-related mortality in adults, it has been suggested that withholding effective treatment in moderate-to-high risk children would be unethical (2). For additional information see the Endotext chapter entitled “Familial Hypercholesterolemia”.

 

A novel approach has been suggested to potentially lower costs and avoid prolonged exposure of at-risk children to lipid-lowering medication, while offering timely and presumably effective intervention.  Rather than continuous treatment implemented at an early age, Robinson and Gidding proposed intermittent lipid-lowering medication guided by noninvasive measures of atherosclerosis, such as carotid intima-media thickness (3). As with conventional approaches, the goal of such therapy would be regression of atherosclerotic lesions, with retreatment periodically throughout adulthood as needed.  While intriguing, the benefits of this recommendation have not been proven.

 

To date recommendations for early identification and treatment of children with hypercholesterolemia have focused primary on the potential benefits.  Fortunately, no significant physical or psychological harms have been shown in children who have undergone early screening and treatment. However, healthcare providers who advocate screening, genetic testing and treatment of children should carefully consider potential ethical issues, including the rights of the child to participate in clinical decision-making, the presumed benefits to the child and the family, as well as potential harms.

 

 

Over the last 50 years, in the U.S. Congress has passed a variety of laws to assure the privacy of an individual’s health information and eliminate discrimination based upon an individual’s health status. While most clinicians have an awareness of these laws, it is unclear how clinicians use this information in clinical decision-making, particularly as it relates to the current or future interests of the child.

 

Privacy

 

In 1996, Congress passed the Health Insurance Portability and Accountability Act or HIPAA. This law mandates the protection and confidential handling of protected health information, including genetic information. Furthermore, HIPAA states that genetic information in the absence of a diagnosis (e.g., predictive genetic test results) cannot be considered a pre-existing condition. Since children with heterozygous FH are rarely affected by their hypercholesterolemia during childhood, genetic testing would be considered “predictive” of adult-onset disease.  Children found to have a pathogenic or presumed pathogenic mutation, therefore, are afforded privacy under HIPPA and are not consider to have a pre-existing condition.

 

The Genetic Information Nondiscrimination Act (GINA), passed in 2008, adds to HIPPA by prohibiting health insurers and employers from asking or requiring a person to take a genetic test and using genetic information in 1) setting insurance rates and 2) making employment decisions.

 

Discrimination and Pre-existing Medical Conditions

 

Prior to 2014, insurance companies based eligibility for and the cost of health insurance on the presence or absence of pre-existing medical conditions.  A pre-existing condition is typically one for which an individual has received treatment or a diagnosis before being enrolled in a health plan.  Because they were determined by insurance providers, criteria defining pre-existing conditions varied widely. This meant that when applying for health insurance individuals, including children, previously diagnosed with and/or treated for hypercholesterolemia were considered to have a pre-existing condition.

 

Since 2014, with the passage of the Affordable Care Act, insurance companies can no longer deny coverage or discriminate against individuals due to a pre-existing condition. Nor can individuals be charged significantly higher premiums, subjected to an extended waiting period, or have their benefits curtailed or coverage withdrawn because of a pre-existing condition.  However, this protection does not extend to an individual’s ability to obtain nor the rates charged for life, disability, and long-term care insurance.

 

Despite these reassurances, in some cases exemptions may apply, particularly for members of the United States military, veterans obtaining healthcare through the Veterans Administration (VA), and individuals who receive services through the Indian Health Service.

 

Children’s Rights

 

A child’s rights can be considered in two parts 1) nurturance rights, i.e., the right to care and protection and 2) self-determination rights, i.e., the right to have some measure of control over their own lives.  Historically, society has focused on the former. Increasingly there is a growing emphasis on shared decision-making in medicine that recognizes children have the right to take an active part in many of the decisions regarding their own lives. While such efforts are commendable, the ability of children to become actively and willfully involved in the decision process is complicated by normal, and sometimes abnormal, growth and development. This raises an important question about a child’s ability to understand their rights in a reasonable and meaningful way (4). It also assumes that healthcare providers are trained, capable of and willing to provide developmentally-appropriate information to children in a comprehendible and non-threatening way.

 

In the 1980s, Melton (5, 6) suggested that children progress through three distinct stage-like levels of reasoning about rights: Level 1, children exhibit an egocentric orientation where they perceive rights in terms of privileges that are bestowed or withdrawn on the whims of an authority figure. Level 2 children see rights as being based on fairness, maintaining social order and obeying rules. Finally, in Level 3 rights are seen in terms of abstract universal principles. Subsequent models favored the gradual acquisition of context specific knowledge (7-9).  When and how well a child progresses from limited to abstract reasoning presents challenges for physicians who strive to involve children in decisions regarding early screening and intervention for CVD risk prevention.

 

MIGHT EARLY DIAGNOSIS AND TREATMENT OF HYPERCHOLESTEROLEMIA COMPROMISE A CHILD’S FUTURE RIGHTS?

 

Laws such as HIPPA, the Affordable Care Act, and GINA protect privacy and prohibit health insurance companies from denying coverage or discriminating against individuals due to a pre-existing condition, including hypercholesterolemia. Nonetheless, current laws do not preclude an individual being denied other forms of coverage, such as life, disability, or long-term care insurance. Furthermore, laws governing privacy, healthcare, and insurance coverage are subject to change over the course of the child's lifetime. This potential vulnerability needs to be considered by clinicians who provide care to children and fully disclosed to the family prior to diagnostic evaluation and treatment of children with hypercholesterolemia. To the extent that they can participate in such conversations, children should be included in the clinical decision-making. The accelerated risk of atherosclerosis beginning in young adults notwithstanding, the urgency of screening and early treatment of children needs to be considered in the context of the child’s overall best interest and, ideally, with their approval.  

 

ETHICAL CONSIDERATIONS AND CONCERNS

 

Since 1953, there has been an impressive increase in new technology and expanded uses of genetic testing and screening. Application of these diagnostic tools in minors has increasingly become commonplace, raising concerns about ethical issues. While pediatric screening and genetic testing are much less common outside of newborn screening, universal screening and increased use of genetic testing has been advocated by many national professional organizations and societies. Justification for such recommendations cite early identification of a child with an underlying genetic abnormality as an opportunity to initiate treatment that may prevent or reduce morbidity or mortality.

 

Over the past 50 years, genetic testing has increasingly played an important role in helping to understand the basis of many disorders of lipid and lipoprotein metabolism, identifying those who are affected and aiding our understanding of an individual’s risk. While only a minority of individuals with hypercholesterolemia who undergo genetic testing are found to have a pathogenic mutation, epidemiologic and Medallion randomization studies suggest these individuals are at significantly higher risk of premature ASCVD-related morbidity and mortality than the general population. 

 

Genetic testing of an asymptomatic child based upon an abnormal blood test and/or positive family history for a specific genetic condition, such as FH, has also been proposed, particularly if early treatment may affect future morbidity or mortality.  Some genetic tests can reasonability predict disease which only manifest in adults. 

 

Ultimately, decisions about whether to offer genetic testing and screening should be driven by the best interest of the child. This, perhaps, is best determined by a thoughtful discussion between the child’s healthcare provider, the parents, and, when appropriate, the child.  Current recommendations and guidelines suggest early intervention to achieve the best outcomes. Yet, there is no clear definition as to the optimum age at which intervention should be recommended, nor clear understanding about a child’s ability to understand and make a rational decision regarding testing and/or treatment.

 

The genetic testing of children raises specific considerations. Because of the need to respect a children's rights, caution has been advised in performing genetic tests during childhood. Newborn genetic testing is now ubiquitous, yet it is not always seen as routine for older children despite specific indications. Testing for drug responsiveness or disease susceptibility is supported by the ethical principle of beneficence when the benefit/risk ratio is in favor of discovering these results during childhood. Possible harms are seen when such knowledge may impact a child negatively, or foreclose future autonomy about the decision to accept the consequences of such testing. Therefore, there is a difference between genetic confirmation in symptomatic children, and that of pre-symptomatic children in which the benefit may accrue later, but the risks may occur in childhood. Such immediate risks potentially include stigmatization by the disease, depression, or decreased self-esteem. Conversely, altered family dynamics may result in parental favoritism, and survivor's guilt in siblings who test negative. This limitation on future autonomy is not confined to just refusing or allowing an adult decision for testing, but also dealing with the impact on future employment, education, and social relationships when the diagnosis is made at an early age.

 

Tests which help diagnose an ongoing, treatable condition that could affect current and future manifestations and complications clearly can be in the child's best interest. However, when a child is asymptomatic and the disorder is late-onset, it is no longer obvious that such a diagnosis during childhood is in the child's best interest. Therefore, it is advised the children only undergo genetic testing when there is immediate medical benefit in childhood, either through diagnosis and treatment of a disease manifesting in the pediatric age range, or a disease whose prevention is possible and should not be delayed. Under these circumstances, informed decision-making is essential, with parental permission being linked to the child's assent whenever possible.

 

CHOLESTEROL SCREENING AND TREATMENT

 

Currently, universally cholesterol testing is recommended for all children in the U.S., starting at 10 years-of-age (range 9-11). The primary purpose of cholesterol screening is to identify individuals with familial hypercholesterolemia.  For those found to have a significant elevation of cholesterol a low-fat diet is recommended. Lipid-lowering medications, such as a statin, are recommended for children with a persistently elevated LDL-C, starting at approximately 8-10 years-of-age. 

 

Genetic Testing

 

Genetic testing of all children suspected of having FH has been recommended (10). The purported benefits of genetic testing are 1) to assist in clinical decision-making regarding the need for lipid-lowering medication, 2) to help determine the appropriate on-treatment goal of LCL cholesterol; and 3) facilitate cascade screening of biologic relatives.

 

To help better understand the complexities of genetic testing and provide guidance, in 2013 both the American Academy Pediatrics (AAP) and the American College of Medical Genetics (ACMG) published recommendations for genetic testing of children. These guidelines are particularly relevant for those providing care for children with lipid and lipoprotein disorders since, with the exception of homozygous disease, children with heterozygous FH are asymptomatic. Hence, genetic testing in this unique population would be considered “predictive” of adult disease.

 

However, although there is much emphasis on early screening and genetic testing of children for FH, children have a variety of genetic conditions that affect other lipids and lipoproteins as well, such as triglycerides. The infantile form of lysosomal acid lipase deficiency, for example, is generally fatal in the absence of early diagnosis and enzyme replacement therapy. Thus, biochemical screening and genetic testing in this condition becomes imperative in order to reduce early morbidity and prevent premature mortality. Examples of other conditions in which there is a sense of urgency include familial chylomicronemia syndrome (FCS), cerebrotendinous xanthomatosis (CTX), and homozygous mutations of MTTP (abetalipoproteinemia), APOB (familial hypobetalipoproteinemia), and SAR1 (chylomicron retention disease).  When considering screening and genetic testing of children with lipid and lipoprotein disorders, therefore, “one size” clearly does not fit all circumstances. Clinicians must consider each child and condition as unique, carefully weighing the presumed benefits and potential harms individually, before making diagnostic and therapeutic recommendations.  

 

In deciding whether a child should undergo predictive genetic testing, the AAP and ACMG emphasize that the focus must be on the child’s medical best interest. Both organizations concluded that unless ameliorative interventions are available during childhood, children should not undergo testing for predispositions to adult-onset conditions and clinicians should generally decline to order testing. With the exception of those with homozygous FH, this suggests that children with heterozygous disease could defer treatment until adulthood. There is convincing evidence using noninvasive techniques, however, that early initiation of lipid-lowering medication can significantly reduce subclinical atherosclerosis. It is presumed that as a consequence of early and persistent LDL-cholesterol lowering that ASCVD-related events will be prevented or delayed. Yet proof of improved outcomes is currently limited and generally inferred from adult data.  

 

The AAP and ACMG did recognize that the potential psychosocial benefits and harms to the child and the extended family also need to be carefully considered. Extending consideration beyond the child’s medical best interest not only acknowledges the traditional deference given to parents about how they raise their children, but also recognizes that the interest of a child is embedded in and dependent on the interests of the family unit.

 

Predictive genetic testing for adult-onset conditions generally should be deferred unless an intervention initiated in childhood may reduce morbidity or mortality. In some families, the psychosocial burden of ambiguity may be so great as to justify testing during childhood, particularly when parents and mature adolescents jointly express interest in doing so.

 

AAP AND ACMG RECOMMENDATIONS

 

Genetic testing performed in children can be considered either as diagnostic or predictive (11).

 

  1. Diagnostic Genetic Testing - Is performed on a child with physical, developmental, or behavioral features consistent with a potential genetic syndrome or for pharmacogenetic drug selection and dosing decisions. Medical benefits include the possibility of preventive or therapeutic interventions, decisions about surveillance, the clarification of diagnosis and prognosis, and recurrence risks. If the medical benefits of a test are uncertain, will not be realized until a later time, or do not clearly outweigh the medical risks, the justification for testing is less compelling.

 

  1. Predictive Genetic Testing - Is performed on an asymptomatic child with a positive family history for a specific genetic condition, particularly if early surveillance or treatment may affect morbidity or mortality. When there is uncertainty that the presence of a genetic mutation will give rise to clinical manifestations, testing is referred to as “pre-dispositional” testing. Most predictive genetic testing for adult-onset conditions is pre-dispositional.

 

Recommendations for Genetic Testing of Children

 

  1. General
    1. Decisions about whether to offer genetic testing and screening should be driven by the best interest of the child.
    2. Genetic testing is best offered in the context of genetic counseling.
  2. Diagnostic Testing
    1. In a child with symptoms of a genetic condition:
      1. Parents or guardians should be informed about the risks and benefits of testing, and their permission should be obtained.
      2. Ideally and when appropriate, the assent of the child should be obtained.
    2. When performed for therapeutic purposes:
      1. Pharmacogenetic testing of children is acceptable, with permission of parents or guardians and, when appropriate, the child’s assent.
      2. If a pharmacogenetic test result carries implications beyond drug targeting or dose-responsiveness, the broader implications should be discussed before testing.
    3. Newborn Screening
      1. The AAP and ACMG support the mandatory offering of newborn screening for all children. Parents should have the option of refusing the procedure, and an informed refusal should be respected.
    4. Carrier Testing
      1. The AAP and ACMG do not support routine carrier testing in minors when such testing does not provide health benefits in childhood. This recommendation accords with previous statements supporting the future decisional autonomy of the minor, who will be able to make an informed choice about testing once he or she reaches the age of majority.
      2. For pregnant adolescents or for adolescents considering reproduction, genetic testing and screening should be offered as clinically indicated, and the risks and benefits should be clearly explained.
    5. Predictive Genetic Testing
      1. Parents or guardians may authorize predictive genetic testing for asymptomatic children at risk of childhood onset conditions.
      2. Ideally, the assent of the child should be obtained.
      3. Predictive genetic testing for adult-onset conditions generally should be deferred unless an intervention initiated in childhood may reduce morbidity or mortality.
      4. An exception might be made for families in whom diagnostic uncertainty poses a significant psychosocial burden, particularly when an adolescent and his or her parents concur in their interest in predictive testing.
      5. For ethical and legal reasons, health care providers should be cautious about providing predictive genetic testing to minors without the involvement of their parents or guardians, even if a minor is mature. Results of such tests may have significant medical, psychological, and social implications, not only for the minor, but also for other family members.

 

Potential Benefits and Harms of Predictive Genetic Testing of Children. Adapted from (11)

Medical

 

Benefits

Possibility of evolving therapeutic interventions, targeted surveillance, refinement of prognosis and clarification of diagnosis

Harms

Misdiagnosis to the extent that genotype does not correlate with phenotype, ambiguous results in which a specific phenotype cannot be predicted and use of ineffective or harmful preventive or therapeutic interventions.

Psychosocial

 

Benefits

Reduction of uncertainty and anxiety, the opportunity for psychological adjustment, the ability to make realistic life plans and sharing the information with family members.

Harms

Alteration of self-image, distortion of parental perception of the child, increased anxiety and guilt, altered expectation by self and others, familial stress related to identification of other at-risk family members, difficulty obtaining life and/or disability insurance, and the detection of misattributed parentage.

Reproductive

 

Benefits

Avoiding the birth of a child with genetic disease or having time to prepare for the birth of a child with genetic disease.

Harms

Changing family-planning decisions on the basis of social pressures.

 

It is essential that parents, guardians and maturing minors receive genetic counseling before undergoing predictive testing, which includes a discussion of the limits of genetic knowledge and treatment capabilities as well as the potential for psychological harm, stigmatization, and discrimination (12).

 

If an adolescent declines genetic testing, and the benefits of knowing will not be relevant for years to decades, the adolescent’s decision should be final. If a minor is young or immature, genetic testing should be delayed until the minor can actively participate. 

 

If predictive testing of conditions for which childhood interventions will ameliorate future harm, this may favor early testing. In such cases, parental authority to determine medical treatment supersedes the minor’s preferences with regard to liberty and privacy.

 

CONCLUSION

 

Although recommended for all individuals, including children, with clinically suspected familial hypercholesterolemia, genetic testing should be approached with caution. Parents and, when appropriate, children should be provided with a comprehensive discussion of the pros and cons of genetic testing, and informed about out-of-pocket costs prior to testing.

 

REFERENCES

 

  1. Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents, National Heart, Lung, and Blood Institute. Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents: summary report. Pediatrics. 2011;128(Suppl 5):S213-S256. doi:10.1542/peds.2009-2107C
  2. Wilson DP, Gidding SS. Atherosclerosis: Is a cure in sight? J Clin Lipidol. 2015;9(5 Suppl):1. doi:S1933-2874(15)00266-4
  3. Robinson JG, Gidding SS. Curing atherosclerosis should be the next major cardiovascular prevention goal. J Am Coll Cardiol. 2014;63(25 Pt A):2779-2785. doi:S0735-1097(14)02138-X
  4. Ruck MD, Abramovitch R, Keating DP. Children's and adolescents' understanding of rights: balancing nurturance and self-determination. Child Dev. 1998;69(2):404-417.
  5. Melton GB. Children's concepts of their rights. J Clin Child Psychol. 1980;9(3):186-190. doi:10.1080/15374418009532985
  6. Melton GB. Child advocacy: Psychological issues and interventions. Plenum; 1983.
  7. Peterson-Badali M, Abramovitch R. Grade related changes in young people's reasoning about plea decisions. Law Hum Behav. 1993;17(5):537-552.
  8. Saywitz KJ. Children’s conceptions of the legal system: “Court is a place to play basketball”. Ceci J, Ross DF, Toglia MP, eds. Perspectives on children’s testimony. Springer-Verlag; 1989:131-157.
  9. Scott ES, Reppucci D, Woolard JL. Evaluating adolescent decision making in legal contexts. Law Hum Behav. 1995;19(3):221-244.
  10. Sturm AC, Knowles JW, Gidding SS, Ahmad ZS, Ahmed CD, Ballantyne CM, Baum SJ, Bourbon M, Carrié A, Cuchel M, de Ferranti SD, Defesche JC, Freiberger T, Hershberger RE, Hovingh GK, Karayan L, Kastelein JJP, Kindt I, Lane SR, Leigh SE, Linton MF, Mata P, Neal WA, Nordestgaard BG, Santos RD, Harada-Shiba M, Sijbrands EJ, Stitziel NO, Yamashita S, Wilemon KA, Ledbetter DH, Rader DJ; the Familial Hypercholesterolemia Foundation. Clinical genetic testing for familial hypercholesterolemia: JACC Scientific Expert Panel. J Am Coll Cardiol. 2018;72(6):662-680. doi:S0735-1097(18)35065-4
  11. American Academy of Pediatrics Committee on Bioethics, Committee on Genetics, and American College of Medical Genetics and Genomics Social, Ethical, Legal Issues Committee. Ethical and policy issues in genetic testing and screening of children. Pediatrics. 2013;131(3):620-622. doi:10.1542/peds.2012-3680
  12. American Academy of Pediatrics Committee on Genetics. Molecular genetic testing in pediatric practice: A subject review. Pediatrics. 2000;106(6):1494-1497. doi:10.1542/peds.106.6.1494

Cholesterol Lowering Drugs

ABSTRACT

 

There are currently several different classes of drugs available for lowering cholesterol levels. There are currently seven HMG-CoA reductase inhibitors (statins) approved for lowering cholesterol levels and they are the first line drugs for treating cholesterol disorders and can lower LDL-C levels by as much as 60%. Statins also are effective in reducing triglyceride levels in patients with hypertriglyceridemia. Statins lower LDL levels by inhibiting HMG-CoA reductase activity leading to decreases in hepatic cholesterol content resulting in an up-regulation of hepatic LDL receptors, which increases the clearance of LDL. The major side effects are muscle complications and an increased risk of diabetes. The different statins have varying drug interactions. Ezetimibe lowers LDL-C levels by approximately 20% by inhibiting cholesterol absorption by the intestines leading to the decreased delivery of cholesterol to the liver, a decrease in hepatic cholesterol content, and an up-regulation of hepatic LDL receptors. Ezetimibe is very useful as add on therapy when statin therapy is not sufficient or in statin intolerant patients. Ezetimibe has few side effects. Bile acid sequestrants lower LDL-C by10-30% by decreasing the absorption of bile acids in the intestine which decreases the bile acid pool consequently stimulating the synthesis of bile acids from cholesterol leading to a decrease in hepatic cholesterol content and an up-regulation of hepatic LDL receptors. Bile acid sequestrants can be difficult to use as they decrease the absorption of multiple drugs, may increase triglyceride levels, and cause constipation and other GI side effects. They do improve glycemic control in patients with diabetes, which is an additional benefit. PCSK9 inhibitors, either monoclonal antibodies or small interfering RNA, lower LDL-C by 50-60% by decreasing PCSK9, which decreases the degradation of LDL receptors. PCSK9 inhibitors also decrease Lp(a) levels. PCSK9 inhibitors are very useful when maximally tolerated statin therapy do not reduce LDL sufficiently and in statin intolerant patients. PCSK9 inhibitors have very few side effects. Bempedoic acid lowers LDL-C by 15-25% by inhibiting hepatic ATP citrate lyase activity resulting in a decrease in cholesterol synthesis in the liver, a decrease in hepatic cholesterol content, and an up-regulation of LDL receptors. Bempedoic acid is employed in patients who do not reach their LDL-C goals on maximally tolerated statin therapy or in patients who do not tolerate statins. Bempedoic acid is associated with elevations in uric acid levels and gouty attacks. Lomitapide and evinacumab are approved for lowering LDL levels in patients with homozygous familiar hypercholesterolemia, as they are not dependent on LDL receptors for decreasing LDL levels. Lomitapide inhibits microsomal triglyceride transfer protein decreasing the formation of chylomicrons in the intestine and VLDL in the liver. Lomitapide has the potential to cause liver toxicity and therefore they were approved with a risk evaluation and mitigation strategy (REMS) to reduce risk. Evinacumab is a monoclonal antibody that inhibits the activity of angiopoietin-like protein 3 resulting in the increased activity of lipoprotein lipase and endothelial cell lipase resulting in a decrease in LDL-C, HDL-C, and triglyceride levels. Mipomersen, which is no longer available, is a second-generation apolipoprotein anti-sense oligonucleotide that decreases apolipoprotein B synthesis resulting in a reduction in the formation and synthesis of VLDL and was approved for the treatment of homozygous familial hypercholesterolemia.

 

INTRODUCTION

 

This chapter will discuss the currently available drugs for lowering total cholesterol levels, especially LDL-C: statins, ezetimibe, bile acid sequestrants, PCSK9 inhibitors, bempedoic acid, lomitapide, mipomersen, and evinacumab. We will not discuss the effect of lifestyle changes or food additives, such as phytosterols, on LDL-C as this is addressed in the chapter entitled “The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels” (1). Additionally, we will not discuss guidelines for determining who to treat, how aggressively to treat, or targets of treatment as these issues are discussed in detail in the chapters entitled “Guidelines for the Management of High Blood Cholesterol” and “Approach to the Patient with Dyslipidemia” (2,3).

 

STATINS

 

Introduction

 

In the 1970s Dr. Akira Endo, working at Sankyo, discovered that compounds isolated from fungi inhibited the activity of HMG-CoA reductase, a key enzyme in the synthesis of cholesterol (4). Further studies at Merck led to the development of the first HMG-CoA reductase inhibitor, lovastatin, approved in 1987 for the treatment of hypercholesterolemia (5). There are currently seven HMG-CoA reductase inhibitors (statins) approved in the United States for lowering cholesterol levels. Three statins are derived from fungi (lovastatin, simvastatin, and pravastatin) and four statins are synthesized (atorvastatin, rosuvastatin, fluvastatin, and pitavastatin). Most of these statins are now generic drugs and therefore they are relatively inexpensive. Which particular statin one elects to use may depend on the degree of cholesterol lowering needed and the potential of drug-drug interactions. Statins are the first line drugs for treating elevated cholesterol levels and therefore one of the most widely utilized class of drugs. Statins have revolutionized the field of preventive cardiology and made an important contribution to the reduction in atherosclerotic cardiovascular events.

 

Effect on Statins on Lipid and Lipoprotein Levels

 

The major effect of statins is lowering LDL-C levels. The effect of the various statins at different doses on LDL-C levels is shown in Table 1. As can be seen in Table 1 different statins have varying abilities to lower LDL-C with maximal reductions of approximately 60% seen with rosuvastatin 40mg. Doubling the dose of a statin results in an approximate 6% further decrease in LDL-C levels. The percent reduction in LDL-C levels is similar in patients with high and low starting LDL-C levels but the absolute decrease is greater if the starting LDL-C is high. Because of this profound ability of statins to lower LDL-C levels, treatment with these drugs as monotherapy is often sufficient to lower LDL-C below target levels.

 

Table 1. Approximate Effect of Different Doses of Statins on LDL-C Levels

% LDL Reduction

Simvastatin (Zocor)

Atorvastatin (Lipitor)

Lovastatin (Mevacor)

Pravastatin (Pravachol)

Fluvastatin (Lescol)

Rosuvastatin (Crestor)

Pitavastatin (Livalo)

27

10mg

-

20mg

20mg

40mg

-

-

34

20mg

10mg

40mg

40mg

80mg

-

1mg

41

40mg

20mg

80mg

80mg

-

-

2mg

48

80mg

40mg

-

-

-

10mg

4mg

54

-

80mg

-

-

-

20mg

-

60

-

-

-

-

-

40mg

-

Data modified from package inserts

 

As would be predicted from the effect of statins on LDL-C levels, statins are also very effective in lowering non-HDL-C levels (LDL-C is the major contributor to non-HDL-C levels) (6,7). In addition, statins also lower plasma triglyceride levels (8,9). The ability of statins to lower triglyceride levels correlates with the reduction in LDL-C (9). Statins that are most efficacious in lowering LDL-C are also most efficacious in lowering plasma triglyceride and VLDL-C levels. Notably the percent reduction in plasma triglyceride levels is dependent on the baseline triglyceride levels (9). For example, in patients with normal triglyceride levels (<150mg/dL), simvastatin 80mg per day lowered plasma triglyceride levels by 11%. In contrast, if plasma triglyceride levels were elevated (> 250mg/dL), simvastatin 80mg per day lowered triglyceride levels by 40% (9). In patients with both elevated LDL-C and triglyceride levels statin therapy can be very effective in improving the lipid profile and are therefore the first line class of drugs to treat patients with mixed hyperlipidemia unless the triglyceride levels are markedly elevated (>500-1000mg/dL). As expected, given the ability of statins to lower LDL-C and triglyceride/VLDL levels, statin therapy is very effective in lowering apolipoprotein B levels (6,7).

 

Of note despite the ability of statins to lower LDL-C, non-HDL-C, and apolipoprotein B levels, statins do not lower Lp(a) levels and may even increase levels (10,11). Finally, statins modestly increase HDL-C levels (8,12,13). In most studies HDL-C levels increase between 5-10% with statin therapy. Interestingly, while low dose atorvastatin increases HDL levels similar to other statins at high doses the effect of atorvastatin is blunted with either very modest increases or no change observed (12).

 

Table 2. Effect of Statins on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

Variable. If TG levels increased will decrease

HDL-C

Small Increase

Lp(a)

No change or small increase

 

Non-Lipid Effects of Statins

 

In addition to effects on lipid metabolism statins also have pleiotropic effects that may not be directly related to alterations in lipid metabolism (14). For example, statins are anti-inflammatory and consistently decrease CRP levels (15). Other pleiotropic effects of statins include anti-proliferative effects, antioxidant properties, anti-thrombosis, improving endothelial dysfunction, and attenuating vascular remodeling (14). Whether these pleiotropic effects contribute to the beneficial effects of statins in preventing cardiovascular disease is uncertain and much of the beneficial effect of statins on cardiovascular disease can be attributed to reductions in lipid levels.

 

Mechanism Accounting for the Statin Induced Lipid Effects

 

Statins are competitive inhibitors of HMG-CoA reductase, which leads to a decrease in cholesterol synthesis in the liver. This inhibition of hepatic cholesterol synthesis results in a decrease in cholesterol in the endoplasmic reticulum resulting in the movement of sterol regulatory element binding proteins (SREBPs) from the endoplasmic reticulum to the golgi where they are cleaved by proteases into active transcription factors (16). The SREBPs then translocate to the nucleus where they increase the expression of a number of genes including HMG-CoA reductase and, most importantly, the LDL receptor (16). The increased expression of HMG-CoA reductase restores hepatic cholesterol synthesis towards normal while the increased expression of the LDL receptor results in an increase in the number of LDL receptors on the plasma membrane of hepatocytes leading to the accelerated clearance of LDL (Figure 1) (16). The increased clearance of LDL accounts for the reduction in plasma LDL-C levels. In patients with a total absence of LDL receptors (Homozygous Familiar Hypercholesterolemia) statin therapy is not very effective in lowering LDL-C levels.

 

Figure 1. Mechanism for the Decrease in LDL Levels

 

In addition to lowering LDL and VLDL levels by accelerating the clearance of lipoproteins some studies have also shown that statins reduce the production and secretion of VLDL particles by the liver (17). This could contribute to the decrease in triglyceride levels. The mechanism by which statins increase HDL-C levels is not clear. The small increase in Lp(a) may be due to increased production as studies have shown that incubating HepG2 hepatocytes with a statin increased the expression of LPA mRNA and apolipoprotein(a) protein (18).

 

Pharmacokinetics and Drug Interactions

 

Statins have different pharmacokinetic properties which can explain clinically important differences in safety and drug interactions (19-22). Most statins are lipophilic except for pravastatin and rosuvastatin, which are hydrophilic. Lipophilic statins can enter cells more easily but the clinical significance of this difference is not clear. Most of the clearance of statins is via the liver and GI tract (19-21). Renal clearance of statins in general is low with atorvastatin having a very low renal clearance making this particular drug the statin of choice in patients with significant renal disease. The half-life of statins varies greatly with lovastatin, pravastatin, simvastatin, and fluvastatin having a short half-life (1-3 hours) while atorvastatin, rosuvastatin, and pitavastatin having a long half-life (19-22). In patient’s intolerant of statins, the use of a long-acting statin every other day or 2 times per week has been employed. Short acting statins are most effective when administered in the evening when HMG-CoA reductase activity is maximal while the efficacy of long-acting statins is equivalent whether given in the AM or PM (23). In patients who prefer to take their statin in the morning one should use a long-acting statin.

 

A key difference between statins is their pathway of metabolism. Simvastatin, lovastatin, and atorvastatin are metabolized by the CYP3A4 enzymes and drugs that affect the CYP3A4 pathway can alter the metabolism of these statins (19-22,24). Fluvastatin is metabolized mainly by CYP2C9 with a small contribution by CYP2C8 (19-21,24). Pitavastatin and rosuvastatin are minimally metabolized by the CYP2C9 pathway (19-21,24). Pravastatin is not metabolized at all via the CYP enzyme system (19-21).

 

Drugs that inhibit CYP3A4 can impede the metabolism of simvastatin, lovastatin, and to a smaller extent atorvastatin resulting in high serum levels of these drugs (19-22,24). These higher levels are associated with adverse effects particularly muscle toxicity. Drugs that inhibit CYP3A4 include intraconazole, ketoconazole, erythromycin, clarithromycin, HIV protease inhibitors (amprenavir, darunavir, fosamprenavir, indinavir, nelfinavir, ritonavir, and saquinavir), amiodarone, diltiazem, verapamil, and cyclosporine (19-22,24). It should be noted that grapefruit juice contains compounds that inhibit CYP3A4 and the consumption of grapefruit juice can significantly increase statin blood levels (25). The inhibition of CYP3A4 by grapefruit juice is dose dependent and increases with the concentration and volume of grapefruit juice ingested. One glass of grapefruit juice everyday can influence the metabolism of statins that are metabolized by the CYP3A4 pathway (25). If a patient requires treatment with a drug that inhibits CYP3A4 the clinician has a number of options to avoid potential drug interactions. One could use a statin that is not metabolized via the CYP3A4 system such as pravastatin or rosuvastatin, one could use an alternative drug to the CYP3A4 inhibitor (for example instead of using erythromycin use azithromycin), or one could temporarily suspend for a short period of time the use of the statin that is metabolized by the CYP3A4 pathway (this is particularly useful when a short course of treatment with an antifungal, antiviral, or antibiotic is required). Drugs that inhibit CYP2C9 do not seem to increase the toxicity of fluvastatin, pitavastatin, or rosuvastatin probably because metabolism via the CYP2C9 pathway is not a dominant pathway.

 

Most statins are transported into the liver and other tissues by organic anion transporting polypeptides, particularly OATP1B1 (19-21,24). Drugs, such as clarithromycin, ritonavir, indinavir, saquinavir, and cyclosporine that inhibit OATP1B1 can increase serum statin levels thereby increasing the risk of statin muscle toxicity (19-21,24). Fluvastatin is the statin that is least affected by OATP1B1 inhibitors. In fact, fluvastatin 40mg per day has been studied in patients receiving renal transplants concomitantly treated with cyclosporine and over a five year study period the risk of myopathy or rhabdomyolysis was not increased in the fluvastatin treated patients compared to those treated with placebo (26).

 

Gemfibrozil inhibits the glucuronidation of statins, which accounts for a significant portion of the metabolism of most statins (24). This can lead to the reduced clearance of statins and elevated blood levels increasing the risk of muscle toxicity (24). The only statin whose metabolism is not altered by gemfibrozil is fluvastatin (24). Notably, fenofibrate, another fibrate that has very similar effects on lipid and lipoprotein levels as gemfibrozil, does not inhibit statin glucuronidation (24). Therefore, in patients on statin therapy who also need treatment with a fibrate one should use fenofibrate and not gemfibrozil in combination with statin therapy. Studies have shown that fenofibrate combined with statins does not significantly increase toxicity (27).

 

There are other drug interactions with statins whose mechanisms are unknown. For example, the lopinavir/ritonavir combination used to treat HIV increases rosuvastatin levels by 2-5-fold and atazanavir/ritonavir increases rosuvastatin levels by 2-6-fold (28-32). Similarly, the tipranavir/ritonavir combination increases rosuvastatin levels 2-fold and atorvastatin levels 8-fold (31). When HIV patients are on these drugs other statins should be used to lower LDL-C levels. The use of statins in patients with HIV is discussed in detail in the Endotext chapter entitled “Lipid Disorders in People with HIV” (33).

 

Thus, despite the excellent safety record of statins, careful attention must be paid to the potential drug-drug interactions. For additional information see Kellick et al (22,24).

 

Effect of Statin Therapy on Clinical Outcomes

 

A large number of studies using a variety of statins in diverse patient populations have shown that statin therapy reduces atherosclerotic cardiovascular disease. The Cholesterol Treatment Trialists have published meta-analyses derived from individual subject data. Their first publication included data from 14 trials with over 90,000 subjects (34). There was a 12% reduction in all-cause mortality in the statin treated subjects, which was mainly due to a 19% reduction in coronary heart disease deaths. Non-vascular causes of death were similar in the statin and placebo groups indicating that statin therapy and lowering LDL-C did not increase the risk of death from other causes such as cancer, respiratory disease, etc. Of particular note there was a 23% decrease in major coronary events per 1 mmol/L (39mg/dL) reduction in LDL-C. Decreases in other vascular outcomes including non-fatal MI, coronary heart disease death, vascular surgery, and stroke were also reduced by 20-25% per 1 mmol/L (39mg/dL) reduction in LDL-C. Additionally, analysis of these studies demonstrated that the greater the reduction in absolute LDL-C levels the greater the decrease in cardiovascular events.  For example, while a 40mg/dL decrease in LDL-C will reduce coronary events by approximately 20%, an 80mg/dL decrease in LDL-C will reduce events by approximately 40%. These results support aggressive lipid lowering with statin therapy.

 

Of note the decrease in the number of events begins to be seen in the first year of therapy indicating that the ability of statins to reduce events occurs relatively quickly and increases over time. The ability of statins to reduce cardiovascular events was seen in a wide diversity of patients including those with and without a history of prior cardiovascular disease, patients over age 65 and younger than age 65, males and females, and patients with and without a history of diabetes or hypertension. Additionally, the beneficial effects of statins were seen regardless of the baseline lipid levels. Subjects with elevated or low LDL-C, HDL-C, or triglyceride levels all had similar decreases in the relative risk of cardiovascular events.

 

A subsequent publication by the Cholesterol Treatment Trialists has focused on five studies with over 39,000 subjects that have compared usual vs. intensive statin therapy (35). It was noted that there was a 0.51mmol/L (20mg/dL) further reduction in LDL-C in the intensively treated subjects. This further decrease in LDL-C resulted in a15% reduction in cardiovascular events. The strong numerical relationship between decreases in LDL-C levels and the reduction in cardiovascular events provides evidence indicating that much of the beneficial effect of statins is accounted for by lipid lowering.

 

In addition, the authors added 7 additional trials to their original comparison of statin treatment vs. placebo for a total of 21 trials with over 129,000 subjects. In this larger cohort a 1mmol/L (39mg/dL) decrease in LDL was associated with a 21% reduction in major cardiovascular events. As seen previously the benefits of statin therapy were seen in a wide variety of subjects including patients older than age 75, obese patients, cigarette smokers, patients with decreased renal function, and patients with low and high HDL-C levels. Additionally, a reduction of cardiovascular events with statin therapy was seen regardless of baseline LDL-C levels.

 

A more recent meta-analysis by the Cholesterol Treatment Trialists examined the effect of statins in 27 trials that included 46,675 women and 127,474 men (36). They found that statin therapy was similarly effective in reducing cardiovascular events in both men and women. Thus, there is an overwhelming database of randomized clinical outcome trials showing the benefits of statin therapy in reducing cardiovascular disease, which, coupled with their excellent safety profile, has resulted in statins becoming a very widely used class of drugs and first line therapy for the prevention of cardiovascular disease.  

 

Effect of Statins Therapy on Clinical Outcomes in Specific Patient Groups

 

PRIMARY PREVENTION

 

While there is no doubt that individuals with pre-existing cardiovascular disease require statin therapy, the use of statins for primary prevention was initially debated. There are now a large number of statin primary prevention studies. The Cholesterol Treatment Trialists reported that statin therapy was very effective in reducing cardiovascular events in subjects without a history of vascular disease and the relative risk reduction was similar to subjects with a history of cardiovascular events (35). Additionally, vascular deaths were reduced by statin treatment even in subjects without a history of vascular disease. As expected, non-vascular deaths were not altered in these subjects without a history of pre-existing vascular disease. Additionally, the Cholesterol Treatment Trialists compared the benefits of statin therapy based on baseline risk of developing cardiovascular disease (<5%, ≥5% to <10%, ≥10% to <20%, ≥20% to <30%, ≥30%) (37). The proportional reduction in major vascular events was at least as big in the two lowest risk categories as in the higher risk categories indicating that subjects at low-risk benefit from statin therapy. Similar to the Cholesterol Treatment Trialists analysis, a Cochrane review published in 2013 on the effect of statins in primary prevention patients reached the following conclusion: “Reductions in all-cause mortality, major vascular events, and revascularizations were found with no excess adverse events among people without evidence of CVD treated with statins” (38). An additional study (HOPE-3 trial), not included in the above analyses, has been completed that focused on intermediate risk patients without cardiovascular disease. In this trial 12,705 men and women who had at least one risk factor for cardiovascular disease were randomized to 10mg rosuvastatin vs. placebo (39). Rosuvastatin treatment resulted in a 27% reduction in LDL-C levels and a 24% decrease in cardiovascular events providing additional evidence that statin treatment can reduce events in primary prevention patients. It is therefore clear that statins are effective in safely reducing events in primary prevention patients.

 

The key issue is “which primary prevention patients should be treated” and this is still controversial. It should be noted that the higher the baseline risk the greater the absolute reduction in events with statin therapy. For example, in a high-risk patient with a 20% risk of developing a vascular event, a 25% risk reduction will result in a 15% risk of an event (absolute decrease of 5%). In contrast in a low-risk patient with a 4% risk of developing a vascular event, a 25% risk reduction will result in a 3% risk (absolute decrease of only 1%). Thus, the absolute benefit of statin therapy over the short term will depend on the risk of developing cardiovascular disease.

 

Additionally, based on the Cholesterol Treatment Trialists results the reduction in cardiovascular events is dependent on the absolute decrease in LDL-C levels. Thus, the effect of statin treatment will be influenced by baseline LDL-C levels. For example, a 50% decrease in LDL-C is 80mg/dL if the starting LDL is 160mg/dL and only 40mg/dL if the starting LDL-C is 80mg/dL. Based on studies showing that a decrease in LDL-C of 1 mmol/L (40mg/dL) reduces cardiovascular events by ~20% the relative benefit of statin therapy will be greater in the patient with the starting LDL-C of 160mg/dL (40% decrease in events) than in the patient with the starting LDL-C of 80mg/dL (20% decrease in events). Thus, decisions on treatment need to factor in both relative risk and baseline LDL levels.

 

Finally, it should be recognized that clinical trials represent short term reductions in LDL-C levels (typically 2-5 years) in a disorder that begins early in life and progresses over decades. Life-long decreases in LDL-C levels due to genetic polymorphisms are associated with a much greater reduction in cardiovascular events than would be expected based on the clinical trial results (40). These results suggest that earlier and longer lasting therapy that decreases LDL-C levels will result in a greater reduction in cardiovascular events (41). An in depth discussion of the benefits of early therapy is discussed in the following reference (42).

 

ELDERLY

 

Few studies have focused on lowering LDL-C in elderly patients, which we define as individuals greater than 75 years of age (this is based on the ACC/AHA guidelines using age 75 in their decision algorithms) (3). The Prosper Trial determined the effect of pravastatin 40mg/day (n= 2891) vs. placebo (n= 2913) on cardiovascular events in older subjects (70-82) with pre-existing vascular disease or who were at high risk for vascular disease (43). The average age in this trial was 75 years of age and approximately 45% had cardiovascular disease. As expected, pravastatin treatment lowered LDL-C by 34% compared to the placebo group. The primary end point was coronary death, non-fatal myocardial infarction, and fatal or non-fatal stroke which was reduced by 15% (HR 0.85, 95% CI 0.74-0.97, p=0.014). However, in the individuals without pre-existing cardiovascular disease pravastatin did not significantly reduce cardiovascular events (HR- 0.94; CI- 0.77–1.15). In contrast, in individuals with cardiovascular disease pravastatin therapy reduced cardiovascular events (HR- 0.78, CI- 0.66–0.93). Thus, this study demonstrated benefits of statin therapy in the elderly with cardiovascular disease but failed to demonstrate benefit in the elderly without cardiovascular disease.

 

A meta-analysis by the Cholesterol Treatment Trialists of 28 trials with 14,483 of 186,854 participants older than 75 years of age found a decrease in cardiovascular events in all age groups including participants older than 75 years of age (Figure 2) (44). Similar to the Prosper Trial a decrease in cardiovascular events was clearly demonstrated in individuals with pre-existing cardiovascular disease (secondary prevention) but in individuals without pre-existing cardiovascular disease (primary prevention) the decrease in cardiovascular events was not statistically significant (Figure 3). Thus, in older patients with cardiovascular disease lowering LDL-C levels with statins clearly reduces cardiovascular events but in older patients without cardiovascular disease the data demonstrating that statins reduce cardiovascular events is less robust but suggests a reduction in cardiovascular events.

 

Figure 2. Effect of Statin Treatment on Major Vascular Events. Modified from (44).

Figure 3. Effect of Statin Treatment on Major Vascular Events in Individuals With and Without Pre-Existing Cardiovascular Disease. Modified from (44).

 

Studies are currently underway to provide definitive information on whether statin therapy is beneficial as primary prevention in the elderly. STAREE (NCT02099123) is a multicenter randomized trial in Australia of atorvastatin 40mg vs. placebo in adults ≥ 70 years of age without cardiovascular disease and PREVENTABLE (NCT04262206) is a multicenter randomized trial in the USA of atorvastatin vs. placebo in adults ≥ 75 years of age without cardiovascular disease (45,46).

 

WOMEN

 

As noted above a meta-analysis by the Cholesterol Treatment Trialists examined the effect of statins in 27 trials that included 46,675 women and 127,474 men (36). They found that statin therapy was similarly effective in reducing cardiovascular events in both men and women.

 

ASIANS

 

Pharmacokinetic data have shown that the serum levels of statins are higher in Asians than in Caucasians (47). Moreover, Asians achieve similar LDL lowering at lower statin doses than Caucasians (47). Therefore, the statin dose used should be lower in Asians. For example, the starting dose of rosuvastatin is 5mg in Asians as compared to 10mg in Caucasians. Additionally, the maximum recommended dose of statin is lower in Japan vs. the United States (Table 3). In contrast, studies suggest that South Asian patients may be treated with atorvastatin and simvastatin at doses typically applied to white patients (48). Studies have demonstrated that statins reduce cardiovascular events in Asians (49,50)

 

Table 3. Maximum Statin Dose in Japan and United States

Statin

Japan

United States

Atorvastatin

40

80

Fluvastatin

60

80

Pravastatin

20

80

Rosuvastatin

20

40

Simvastatin

20

40

 

DIABETES

 

Statin trials, including both primary and secondary prevention trials, have consistently shown the beneficial effect of statins on cardiovascular disease in patients with diabetes (51). The Cholesterol Treatment Trialists analyzed data from 18,686 subjects with diabetes (mostly type 2 diabetes) from 14 randomized trials (52). In the statin treated group there was a 9% decrease in all-cause mortality, a 13% decrease in vascular mortality, and a 21% decrease in major vascular events per 1mmol/L (39mg/dL) reduction in LDL-C. The beneficial effect of statin therapy was seen in both primary and secondary prevention patients. The effect of statin treatment on cardiovascular events in patients with diabetes was similar to that seen in non-diabetic subjects. It should be noted that while the data for patients with type 2 diabetes is robust, the number of patients with type 1 diabetes in these trials is relatively small and the results less definitive. Also, of note is that information on young patients with diabetes (< age 40) is very limited. Thus, these studies indicate that statins are beneficial in reducing cardiovascular disease in patients with diabetes. For addition details on the treatment of dyslipidemia in patients with diabetes see the chapter entitled “Dyslipidemia in Patients with Diabetes” (51).

 

RENAL DISEASE

 

The Cholesterol Treatment Trialists examined the effect of renal function on statin effectiveness. They reported that the relative risk reduction for cardiovascular events was similar if the eGFR was < 60ml/min as compared to > 90 or 60-90 (35). In a follow-up analysis it was reported that the relative risk reduction per 1mMol/l (~39mg/dL) decrease in LDL-C levels with statin therapy was 0·78 for an eGFR ≥60 mL/min, 0·76 for an eGFR 45 to <60 mL/min, 0·85 for an eGFR 30 to <45 mL/min, and 0·85 for an eGFR <30 mL/min in patients not on dialysis (53). In patients on dialysis the relative risk reduction was 0·94 (99% CI 0·79-1·11). Similarly, a meta-analysis of 57 studies with >143,000 participants with renal disease not on dialysis reported a 31% reduction in major cardiovascular events in statin treated subjects compared to placebo groups (54). Thus, in patients with renal disease not on dialysis, treatment with statins is beneficial and should be utilized in this population at high risk for vascular disease.

 

In contrast to the above results, studies examining the role of statins in dialysis patients have not found a benefit from statin therapy. The Deutsche Diabetes Dialyse Studie (4D) randomized 1,255 type 2 diabetic subjects on hemodialysis to either 20 mg atorvastatin or placebo (55). The LDL-cholesterol reduction was similar to that seen in non-dialysis patients but there was no significant reduction in cardiovascular death, nonfatal myocardial infarction, or stroke in the atorvastatin treated compared to the placebo group. Similarly, A Study to Evaluate the Use of Rosuvastatin in Subjects on Regular Hemodialysis (AURORA) randomized 2,776 subjects on hemodialysis to rosuvastatin 10 mg or placebo (56). Again, the LDL-cholesterol lowering in dialysis patients was similar to that seen in other studies but there was no significant effect on the primary endpoint of cardiovascular death, nonfatal myocardial infarction, or stroke. A meta-analysis of 25 studies involving 8,289 dialysis patients found no benefit of statin therapy on major cardiovascular events, cardiovascular mortality, all-cause mortality, or myocardial infarction, despite efficacious lipid lowering. The reason for the failure of statins in patients on maintenance dialysis is unclear but could be due to a number of factors including the possibility that the marked severity of atherosclerosis in end stage renal disease may limit reversal, that different mechanisms of atherosclerosis progression occur in dialysis patients (for example an increased role for inflammation, oxidation, or thrombosis), or that cardiovascular events in this patient population may not be due to atherosclerosis. We would recommend continuing statin therapy in patients on dialysis who have been previously treated with statins but not initiating therapy in the rare statin naïve patient beginning dialysis.

 

Statins are primarily metabolized in the liver and therefore the need to adjust the statin dose is not usually needed in patients with renal disease until the eGFR is < 30ml/min. The effect of renal dysfunction on statin clearance varies from statin to statin (57). For some statins such as atorvastatin, there is no need to adjust the dose in renal disease because there is limited renal clearance (57). However, for other statins it is recommended to adjust the dose in patients when the eGFR is < 30ml/min. In patients with an eGFR < 30ml/min the maximum dose of rosuvastatin is 10mg, simvastatin 40mg, pitavastatin 2mg, pravastatin 20mg, lovastatin 20mg, and fluvastatin 40mg per day (57).

 

For additional information on the treatment of dyslipidemia in patients with renal disease see the chapter entitled “Dyslipidemia in Chronic Kidney Disease” (57).

 

CONGESTIVE HEART FAILURE

 

In the Corona study 5,011 patients with New York Heart Association class II, III, or IV ischemic, systolic heart failure (most were class III) were randomly assigned to receive 10 mg of rosuvastatin or placebo per day (58). While rosuvastatin treatment reduced LDL-C levels by 45% compared to placebo, rosuvastatin did not decrease death from cardiovascular causes, nonfatal myocardial infarction, or nonfatal stroke. Similarly, the GISSI-HF trial randomized 4,574 patients with class II, III, of IV congestive heart failure (most were class II) to 10mg of rosuvastatin or placebo (59). The primary endpoints were time to death, and time to death or admission to hospital for cardiovascular reasons and these were similar in the statin and placebo groups. Why statin treatment was not beneficial in patients with congestive heart failure is unknown.

 

LIVER DISEASE

 

Many patients with liver disease, particularly those with nonalcoholic fatty liver disease (NAFLD), are at high risk for cardiovascular disease and therefore require statin therapy (60). There have been concerns that these patients would not tolerate statin therapy and that statin therapy would worsen their underlying liver disease. Fortunately, there are now studies of statin therapy in patients with abnormal liver function tests and underlying liver disease at baseline (60-62). With a variety of statins, studies have demonstrated no significant worsening of liver disease and in fact several studies have suggested improvement in liver function tests with statin therapy (62). This is true for patients with hepatitis C, NAFLD/NASH, and primary biliary cirrhosis. Additionally, in the GREACE trial, statin treatment reduced cardiovascular events in patients with moderately abnormal liver function tests (transaminases < 3x the upper limit of normal) (63). Thus, in patients with mild liver disease without elevations in bilirubin or abnormalities in synthetic function, statins are safe and reduce the risk of cardiovascular disease. 

 

For additional information on the treatment of dyslipidemia in patients with liver disease see the chapter entitled “Lipid and Lipoprotein Metabolism in Liver Disease” (64).

 

HIV

 

Patients living with HIV have an increased risk of cardiovascular disease (33). A trial randomized 7,769 participants with HIV infection with a low-to-moderate risk of cardiovascular disease to either pitavastatin 4 mg or placebo (65). The primary outcome was the occurrence of cardiovascular death, myocardial infarction, hospitalization for unstable angina, stroke, transient ischemic attack, peripheral arterial ischemia, revascularization, or death from an undetermined cause. In the pitavastatin group cardiovascular events were decreased by 35% (HR, 0.65; 95% CI, 0.48 to 0.90; P=0.002). For additional information on the use of statins in HIV patients see the Endotext chapter “Lipid Disorders in People with HIV” (33).  

 

Statin Side Effects

 

An umbrella review of meta-analyses of observational studies and randomized controlled trials examined 278 unique non-CVD outcomes from 112 meta-analyses of observational studies and 144 meta-analyses of RCTs and found that the only adverse effects associated with statin therapy were the development of diabetes and muscle disorders (66). For a detailed discussion of the side effects of statin therapy a scientific statement from the American Heart Association provides a comprehensive review (67).

 

DIABETES

 

After many years of statin use it was recognized that statins increase the risk of developing diabetes. In a meta-analysis of 13 trials with over 90,000 subjects, there was a 9% increase in the incidence of diabetes during follow-up among subjects receiving statin therapy (68). All statins appear to increase the risk of developing diabetes. In comparisons of intensive vs. moderate statin therapy, Preiss et al observed that patients treated with intensive statin therapy had a 12% greater risk of developing diabetes compared to subjects treated with moderate dose statin therapy (69). Older subjects, obese subjects, and subjects with high glucose levels were at a higher risk of developing diabetes while on statin therapy (70). Thus, statins may be unmasking and accelerating the development of diabetes that would have occurred naturally in these subjects at some point in time. In patients without risk factors for developing diabetes, treatment with statins does not appear to increase the risk of developing diabetes.

 

In patients with diabetes, an analysis of 9 studies with over 9,000 patients with diabetes reported that the patients randomized to statin therapy had a 0.12% higher A1c than the placebo group indicating that statin therapy is associated with only a very small increase in A1c levels in patients with diabetes that is unlikely to be clinically significant (71). Individual studies, such as CARDS and the Heart Protection Study, have also shown only a very modest effect of statins on A1c levels in patients with diabetes (72,73).

 

The mechanism by which statins increase the risk of developing diabetes is unknown (74). A study has demonstrated that a polymorphism in the gene for HMG-CoA reductase that results in a decrease in HMG-CoA reductase activity and a small decrease in LDL levels is also associated with an increase in body weight and plasma glucose and insulin levels (75). Additionally, a cross sectional study that compared the change in BMI in individuals on statins to individuals not on statins observed an increased BMI in the subjects taking statins (+1.3 in stain users vs. + 0.4 in non-users over a 10 year period; p=0.02) (76). These observations suggest that the inhibition of HMG-CoA reductase per se may be contributing to the statin induced increased risk of diabetes via weight gain. However, studies have now shown that polymorphisms in different genes (NPC1L1 and PCSK9) that lead to a decrease in LDL-C levels are also associated with an increase in diabetes suggesting that decreases in LDL-C levels per se alter glucose metabolism and increase the risk of diabetes (74,77). How a decrease in LDL-C levels might affect glucose metabolism is unknown. Clearly further studies are required to understand the mechanisms by which statins increase the risk of developing diabetes.

 

In balancing the benefits and risks of statin therapy it is important to recognize that an increase in plasma glucose levels is a surrogate marker for an increased risk of developing micro and macrovascular disease (i.e., an increase in plasma glucose per se is not an event but rather increases the risk of future events). In contrast, statin therapy is preventing actual clinical events that cause morbidity and mortality. Furthermore, it may take many years for an elevated blood glucose to induce diabetic complications while the reduction in cardiovascular events with statin therapy occurs relatively quickly. Finally, the number of patients needed to treat with statins to avoid one cardiovascular event is much lower (10-20 depending on the type of patient) than the number of patients needed to treat to cause one patient to develop diabetes (100–200 for one extra case of diabetes) (74). Patients on statin therapy, particularly those with risk factors for the development of diabetes, should be periodically screened for the development of diabetes with measurement of fasting glucose or A1c levels.

 

CANCER

 

Analysis of 14 trials with over 90,000 subjects by the Cholesterol Treatment Trialists did not demonstrate an increased risk of cancer or any specific cancer with statin therapy (34). An update with an analysis of 27 trials with over 174,000 participants also did not observe an increase in cancer incidence or death (36). Additionally, no differences in cancer rates were observed with any particular statin.

 

COGNITIVE DYSFUNCTION

 

Several randomized clinical trials have examined the effect of statin therapy on cognitive function and have not indicated any increased risk (78-80). The Prosper Trial was designed to determine whether statin therapy will reduce cardiovascular disease in older subjects (age 70-82) (43). In this trial cognitive function was assessed repeatedly and no difference in cognitive decline was found in subjects treated with pravastatin compared to placebo (43,81). In the Heart Protection Study over 20,000 patients were randomized to simvastatin 40mg or placebo and again no significant differences in cognitive function was observed between the statin vs. placebo group (82). Additionally, a Cochrane review examined the effect of statin therapy in patients with established dementia and identified 4 studies with 1154 participants (83). In this analysis no benefit or harm of statin therapy on cognitive function could be demonstrated in this high-risk group of patients. Thus, randomized clinical trials do not indicate a significant association.

 

HEMORRAGIC STROKE

 

In a scientific statement from the American Heart Association on statin safety reached the following conclusions; “The available data in aggregate show no increased risk of brain hemorrhage with statin use in primary stroke prevention populations. An increased risk in secondary stroke prevention populations is possible, but the absolute risk is very small, and the benefit in reducing overall stroke and other vascular events generally outweighs that risk” (67).

 

LIVER DISEASE

 

It was in initially thought that statins induced liver dysfunction and it was recommended that liver function tests be routinely obtained while patients were taking statins. However, studies have now shown that the risk of liver function test abnormalities in patients taking statins is very small (61). For example, in a survey of 35 randomized studies involving > 74,000 subjects, elevations in transaminases were seen in 1.4% of statin treated subjects and 1.1% of controls (84). Similarly, in a meta-analysis of > 49,000 patients from 13 placebo controlled studies, the incidence of transaminase elevations greater than three times the upper limit of normal was 1.14% in the statin group and 1.05% in the placebo group (85). Moreover, even when the transaminase levels are elevated, repeat testing often demonstrates a return towards normal levels (86). The increases in transaminase levels with statin therapy are dose related with high doses of statins leading to more frequent elevations (87). At this time, routine monitoring of liver function tests in patients taking statins is no longer recommended. However, obtaining baseline liver function tests prior to starting statin therapy is indicated (61). If liver function tests are obtained during statin treatment, one should not be overly concerned with modestly elevated transaminase levels (less than 3x the upper limit of normal) (61). If the transaminase is greater than 3x the upper limit of normal the test should be repeated and if it remains > 3x the upper limit of normal, statin therapy should be stopped and the patient evaluated (61).

 

A more clinically important issue is whether statins lead to an increased risk of liver failure. Studies have suggested that the incidence of liver failure in patients taking statins is very similar to the rate observed in the general population (approx. 1 case per 1 million patient years) (88,89). Thus, statin therapy causing serious liver injury is a very rare event.

 

Non-alcoholic fatty liver disease (NAFLD) is very common and is associates with obesity, metabolic syndrome, diabetes, and cardiovascular disease. In patients with NAFLD studies have shown that statins decrease liver enzymes and reduce steatosis (90).

 

MUSCLE

 

The most common side effect of statin therapy is muscle symptoms. These can range from life threatening rhabdomyolysis to myalgias (Table 4) (91).

 

Table 4. Spectrum of Statin Induced Muscle Disorders (Adapted from J. Clinical Lipidology 8: S58-71, 2014)

Myalgia- aches, soreness, stiffness, tenderness, cramps with normal CK levels

Myopathy- muscle weakness with or without increased CK

Myositis- muscle inflammation

Myonecrosis- mild (CK >3x ULN); moderate (CK> 10x ULN); severe (CK> 50x ULN)

Rhabdomyolysis- myonecrosis with myoglobinuria or acute renal failure

 

Many patients will discontinue the use of statins due to muscle symptoms. Risk factors associated with an increased incidence of statin associated muscle symptoms are listed in Table 5 (92,93).

 

Table 5. Risk Factors for Statin Myopathy

Medications that alter statin metabolism

Older age

Female

Hypothyroidism

Excess alcohol

Vitamin D deficiency

History of muscle disorders

Renal disease

Liver disease

Personal or family history of statin intolerance

Low BMI

Polymorphism in SLCO1B1 gene

High dose statin

Drug-drug interactions

 

 The Cholesterol Treatment Trialists analyzed individual participant data on the development of muscle symptoms from 19 double-blind trials of statin versus placebo with 123,940 participants and four double-blind trials of a more intensive vs. a less intensive statin regimen with 30,724 participants (94). After a median follow-up of 4.3 years 27.1% of the individuals taking a statin vs. 26.6% on placebo reported muscle pain or weakness representing a 3% increase greater than placebo (risk ratio- 1.03; 95% CI 1.01-1.06) (Table 6). The specific muscle symptoms caused by statin therapy, myalgia, muscle cramps or spasm, limb pain, other musculoskeletal pain, or muscle fatigue or weakness were similar to those caused by placebo. The increase in muscle symptoms in the statin treated individuals was manifest in the first year of therapy but in the later years muscle symptoms were similar in the statin treated and placebo groups. The relative risk of statin induced muscle symptoms was greater in women than men. Intensive statin treatment with 40-80 mg atorvastatin or 20-40 mg rosuvastatin resulted in a higher risk of muscle symptoms than less intensive or moderate-intensity regimens but different statins at equivalent LDL-C lowering doses had similar effects on muscle symptoms. This study demonstrates that there is a small increase in muscle symptoms that primarily manifests in the first year of therapy. Statin therapy caused approximately 11 additional complaints of muscle pain or weakness per 1000 patients during the first year, but little excess in later years. Of particularly note is that 26.6% of patients taking a placebo had muscle symptoms demonstrating a very high frequency of this clinical complaint. Given the high prevalence of muscle complaints and the small increase attributed to statins it is very difficult to determine if a muscle complaint is actually due to the statin, which presents great clinical difficulties in patient management.                                                       

 

Table 6. Effect of Statin vs. Placebo on Muscle Symptoms

Symptom

Statin Events

Placebo Events

RR (95% CI)

Myalgia

12.0%

11.7%

1·03 (0·99–1·08)

Other musculoskeletal pain

13.3

13.0

1·03 (0·99–1·08)

Any muscle pain

26.9%

26.3%

1·03 (1·01–1·06)

Any muscle pain or weakness

27.1%

26.6%

1·03 (1·01–1·06)

Modified from (94).

 

While the results of the randomized trials suggest that muscle symptoms are not frequently induced by statin therapy, in typical clinical settings a significant percentage of patients are unable to tolerate statins due to muscle symptoms (in many studies as high as 5-25% of patients) (95-97). Recently there was a randomized trial that explored the issue of myopathy with statin therapy in great detail (98). In this trial the effect of atorvastatin 80mg a day vs. placebo for 6 months on creatine kinase (CK), exercise capacity, and muscle strength was studied in 420 healthy, statin-naive subjects. Atorvastatin treatment led to a modest increase in CK levels (20.8U/L) with no change observed in the placebo group. None of the subjects had an elevation of CK > 10x the upper limits of normal. There were no changes in muscle strength or exercise capacity with atorvastatin treatment. However, myalgia was reported in 19 subjects (9.4%) in the atorvastatin group compared to 10 subjects (4.6%) in the placebo group (p=0.05).  In this study “myalgia” was considered to be present if all of the following occurred: (1) subjects reported new or increased muscle pain, cramps, or aching not associated with ex­ercise; (2) symptoms persisted for at least 2 weeks; (3) symptoms resolved within 2 weeks of stopping the study drug; and (4) symp­toms reoccurred within 4 weeks of restarting the study medication. Notably these myalgias were not associated with elevated CK levels. In the atorvastatin group the myalgias tended to occur soon after therapy (average 35 days) whereas in the placebo group myalgias occur later (average 61 days). In the atorvastatin group the symptoms were predominantly localized to the legs and included aches, cramps, and fatigue, whereas in the placebo group they were more diverse including whole body fatigue, foot cramps, worsening of pain in previous injuries, and groin pain. A number of conclusions can be reached from this study. First, statin treatment does in fact increase the incidence of myalgias. Second, a substantial number of patients treated with placebo will also develop myalgias. Third, clinically differentiating statin induced myalgias from placebo induced myalgias is difficult, as there are no specific symptoms, signs, or biomarkers that clearly distinguish between the two. It should be recognized that the patient population typically treated with statins (patients 50-80 years of age) often have muscle symptoms in the absence of statin therapy and it is therefore difficult to be certain that the muscle symptoms described by the patient are actually due to statin therapy.

 

Additionally, when patients know that they are taking a statin they are more likely to have muscle symptoms (i.e. the nocebo effect). This was nicely demonstrated in the ASCOT-LLA extension trial (99). In the initial phase of the study the patients were randomly assigned to atorvastatin 10 mg (n= 5101) or matching placebo (n= 5079) in a double-blind fashion. During the 3.3 years of the double blinded phase adverse muscle symptoms were very similar in the atorvastatin and placebo groups (HR 1.03; p=0.72). This double-blind phase was followed by a non-blinded non-randomized extension where 6409 patients were treated with atorvastatin 10mg and 3490 were untreated. During the 2.3 years of this extension study muscle symptoms were significantly increased in the atorvastatin group (HR 1·41; p=0.006).    

 

In a very small study in the Annals of Internal Medicine eight patients with “statin related myalgia” were re-challenged with statin or placebo and there were no statistically significant differences in the recurrence of myalgias on the statin or placebo (100). This approach has been expanded upon in other studies. In 120 patients with “statin induced myalgia” patients were randomized in a double blinded crossover trial to either simvastatin 20mg per day or placebo and the occurrence of muscle symptoms was determined (101). Only 36% of these patients were confirmed to actually have statin induced myalgia (presence of symptoms on simvastatin without symptoms on placebo). In a similar study, Nissen and colleagues studied 491 patients with “statin induced myalgia” treating with either atorvastatin 20mg per day or placebo in a double-blind crossover trial (102). In this trial 42.6% of patients were confirmed to have statin induced muscle symptoms. In a trial of 156 patients with prior statin induced muscle symptoms patients were treated with alternating periods of atorvastatin 20mg or placebo (103). In this trial no difference in muscle symptoms was found between the statin and placebo treatment periods. A smaller crossover trial in 49 patients who had stopped statin therapy also found no difference in muscle symptoms when patients were taking atorvastatin 20mg or placebo (104)

 

Thus, while statin induced myalgias are a real entity careful studies have shown that in the majority of patients with “statin induced muscle symptoms” the symptoms are not actually due to statin therapy. In the clinic it is difficult to be certain whether the muscle symptoms are actually due to true statin intolerance or to other factors. The approach to treating these patients will be discussed later in this chapter (Treatment of Stain Intolerant Patients). While some patients will not tolerate statin therapy due to myalgias, this side effect does not appear to result in serious morbidity or long-term consequences. In contrast, studies have found that discontinuing statins increases the risk of myocardial infarctions and death from cardiovascular disease (105,106).

 

Fortunately, the more serious muscle related side effects of statin therapy are rare. In a meta-analysis of 21 statin vs. placebo trials there was an excess risk of rhabdomyolysis of 1.6 patients per 100,000 patient years or a standardized rate of 0.016/patient years (86). Other studies report a rate of rhabdomyolysis between 0.03- 0.16 per 1,000 patient years (107). Similarly, the risk of statin induced myositis (muscle symptoms with an increase in CK 10 times the upper limits of normal) is also very low. In an analysis of 21 randomized trials myositis occurred in only 5 patients per 100,000 person years or 0.05/1000 patient years (86). The higher the dose of statin used the greater the risk of myositis and rhabdomyolysis. In a comparison of five trials that compared high dose statin vs. low dose statin there was an excess risk of rhabdomyolysis of 4 per 10,000 people treated (35). The likely basis for an increased risk of myositis or rhabdomyolysis is elevated statin blood levels, which are more likely to occur with high doses of statins. In the development of statins, manufacturers have studied higher doses that are not approved for clinical use. For example, simvastatin and pravastatin at 160mg per day were studied but discontinued due to an increased incidence of muscle side effects (108,109). The use of simvastatin 80mg per day, a previously approved dose, was discontinued due to an increased risk of muscle side effects. Similarly, pitavastatin at doses greater than 4mg per day was investigated, but development was abandoned when an increased risk of rhabdomyolysis was observed. Along similar lines, in many of the patients that develop rhabdomyolysis, the etiology can be linked to the use of other drugs that alter statin metabolism thereby increasing statin blood levels (93). For example, prior to drug interactions being recognized the use of cyclosporine, gemfibrozil, HIV protease inhibitors, and erythromycin in conjunction with certain statins was linked with the development of rhabdomyolysis (93). Finally, common variants in SLCO1B1, which encodes the organic anion-transporting polypeptide OATP1B1, are strongly associated with an increased risk of statin-induced myopathy (110). OATP1B1 facilitates the transport of statins into the liver and certain polymorphisms are associated with an increased risk of developing statin induced muscle disorders, due to the decreased transport of statins into the liver resulting in increased blood levels (111). The exact mechanism by which elevated blood levels induce muscle toxicity remains to be elucidated.

 

Recently it has been recognized that a very small number of patients taking statins develop a progressive autoimmune necrotizing myopathy, which is characterized by progressive symmetric proximal muscle weakness, elevated CK levels (typically >10x the ULN), and antibodies against HMG-CoA reductase (112). It is estimated that this occurs in 2 or 3 per 100,000 patients treated with a statin (112). This myopathy may begin soon after initiating statin therapy or develop after a patient has been on statins for many years (112). Muscle biopsy reveals necrotizing myopathy without severe inflammation (112). In contrast to the typical muscle disorders induced by statin therapy, the autoimmune myopathy progresses despite discontinuing therapy. Spontaneous improvement is not typical and most patients will need to be treated with immunosuppressive therapy (glucocorticoids plus methotrexate, azathioprine, or mycophenolate mofetil) (112). It should be recognized that this disorder can occur in individuals that have not been exposed to statin therapy (113). Statins likely potentiate the development of this disorder in susceptible individuals, perhaps by increasing HMG-CoA reductase levels.

 

From the above certain conclusions can be reached. First, the risk of serious muscle disorders due to statin therapy is very small, particularly if one is aware of the potential drug interactions that increase the risk. Second, the muscle toxicity is usually linked to elevated statin blood levels and the higher the dose of the statin the more likely the chance of developing toxicity. Third, myalgias in patients taking statins are very common and can be due to statin treatment. However, in the individual patient, it is very difficult to know if the myalgia is actually secondary to statin therapy and in many, if not most patients, the myalgias are not due to statin therapy. Fourth, the muscle symptoms that occur in association with statin treatment are a major reason why patients discontinue statin use and therefore better diagnostic algorithms and treatments are required to allow patients to better comply with these highly effective treatments to reduce cardiovascular disease. 

 

Contraindications

 

Previously statins were contraindicated in pregnant women or lactating women. However, in July 2021 the FDA requested the removal of the strongest recommendation against using statins during pregnancy. They continue to advise against the use of statins in pregnancy given the limited data and quality of information available. The decision of whether to continue a statin during pregnancy requires shared decision-making between the patient and clinician, and healthcare professionals need to discuss the risks versus the benefits in high-risk women, such as those with homozygous FH or prior ASCVD events, that may benefit from statin therapy. For a detailed discussion of the use of statins during pregnancy see the Endotext chapter entitled “Effect of Pregnancy on Lipid Metabolism and Lipoprotein Levels” (114).

 

In addition, liver function tests should be obtained prior to initiating statin treatment and moderate to severe liver disease is a contraindication to statin therapy (61). 

 

Summary

 

An enormous data base has accumulated which demonstrates that statins are very effective at reducing the risk of cardiovascular disease and that statins have an excellent safety profile. The risk benefit ratio of treating patients with statins is very favorable and has resulted in this class of drugs being widely utilized to lower serum lipid levels and to reduce the risk of cardiovascular disease and death.

 

EZETIMBE (ZETIA)

 

Introduction

 

Ezetimibe (Zetia) inhibits the absorption of cholesterol by the intestine thereby resulting in modest decreases in LDL-C levels (115). Ezetimibe is primarily used in combination with statin therapy when statin treatment alone does not lower LDL-C levels sufficiently or when patients only tolerate a low statin dose. It may also be used as monotherapy or in combination with other lipid lowering drugs to lower LDL-C levels in patients with statin intolerance. Finally, it is the drug of choice in patients with the rare genetic disorder sitosterolemia, which is discussed in detail in the chapter “Sitosterolemia” (116). Ezetimibe is relatively inexpensive as it is now a generic drug.

 

Effect of Ezetimibe on Lipid and Lipoprotein Levels

 

Pandor and colleagues have published a meta-analysis of ezetimibe monotherapy that included 8 studies with 2,722 patients (117). They reported that ezetimibe decreased LDL-C levels by 18.6%, decreased triglyceride levels by 8.1%, and increased HDL-C levels by 3% compared to placebo. In a pooled analysis by Morrone and colleagues of 27 studies with 11, 714 subjects treated with ezetimibe in combination with statin therapy similar results were observed (118). Specifically, LDL-C levels were decreased by 15.1%, non-HDL-C levels by 13.5%, triglycerides by 4.7%, apolipoprotein B levels by 10.8%, and HDL-C levels were increased by 1.6%. The combination of a high dose potent statin plus ezetimibe can lower LDL-C levels by 70% (119). A meta-analysis of the effect of ezetimibe on Lp(a) revealed that with either monotherapy or combination with statin there was no change in Lp(a) levels (120). The effect of ezetimibe on lipid parameters occurs quickly and can be seen after 2 weeks of treatment. In patients with Heterozygous Familial Hypercholesterolemia who have marked elevations in LDL-C levels, the addition of ezetimibe to statin therapy resulted in a further 16.5% decrease in LDL-C levels (121). Thus, in comparison with statins, ezetimibe treatment produces modest decreases in LDL-C levels (15-20%). In addition to these changes in lipid parameters, ezetimibe in combination with a statin decreased hs-CRP by 10-19% compared to statin monotherapy (122,123). However, ezetimibe alone does not decrease hs-CRP levels (123).

 

Table 7. Effect of Ezetimibe on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

Small decrease

HDL-C

Small increase

Lp(a)

No change

 

Mechanisms Accounting for the Ezetimibe Induced Lipid Effects

 

NPC1L1 (Niemann-Pick C1-like 1 protein) is highly expressed in the intestine with the greatest expression in the proximal jejunum, which is the major site of intestinal cholesterol absorption (124,125). Knock out animals deficient in NPC1L1 have been shown to have a decrease in intestinal cholesterol absorption (124). Ezetimibe binds to NPC1L1 and inhibits cholesterol absorption (115,124,125). In animals lacking NPC1L1, ezetimibe has no effect on intestinal cholesterol absorption, demonstrating that ezetimibe’s effect on cholesterol absorption is mediated via NPC1L1 (115,125). Thus, a major site of action of ezetimibe is to block the absorption of cholesterol by the intestine (115,125). Cholesterol in the intestinal lumen is derived from both dietary cholesterol (approximately 25%) and biliary cholesterol (approximately 75%); thus the majority is derived from the bile (125). As a consequence, even in patients that have very little cholesterol in their diet, ezetimibe will decrease cholesterol absorption. While ezetimibe is very effective in blocking intestinal cholesterol absorption it does not interfere with the absorption of triglycerides, fatty acids, bile acids, or fat-soluble vitamins including vitamin D and K.

 

When intestinal cholesterol absorption is decreased the chylomicrons formed by the intestine contain less cholesterol and thus the delivery of cholesterol from the intestine to the liver is diminished (126). This results in a decrease in the cholesterol content of the liver, leading to the activation of SREBPs, which enhance the expression of LDL receptors resulting in an increase in LDL receptors on the plasma membrane of hepatocytes (Figure 1) (126). Thus, similar to statins the major mechanism of action of ezetimibe is to decrease the levels of cholesterol in the liver resulting in an increase in the number of LDL receptors leading to the increased clearance of circulating LDL (126). In addition, the decreased cholesterol delivery to the liver may also decrease the formation and secretion of VLDL (126).

 

In addition to NPC1L1 expression in the intestine this protein is also expressed in the liver where it mediates the transport of cholesterol from the bile back into the liver (127). The inhibition of NPC1L1 in the liver will result in the increased secretion of cholesterol in bile and thereby could also contribute to a decrease in the cholesterol content of the liver and an increase in LDL receptor expression and a decrease in VLDL production.

 

Pharmacokinetics and Drug Interactions

 

Following absorption by intestinal cells ezetimibe is rapidly glucuronidated. The glucuronidated ezetimibe is then secreted into the portal circulation and rapidly taken up by the liver where it is secreted into the bile and transported back to the intestine (115). This enterohepatic circulation repeatedly returns ezetimibe to its site of action (note glucuronidated ezetimibe is a very effective inhibitor of NPC1L1) (115). Additionally, this enterohepatic circulation accounts for the long duration of action of ezetimibe and limits peripheral tissue exposure (115). Ezetimibe is not significantly excreted by the kidneys and thus the dose does not need to be adjusted in patients with renal disease.

 

Ezetimibe is not metabolized by the P450 system and does not have many drug interactions (115). It should be noted that cyclosporine does increase ezetimibe levels.

 

Effect of Ezetimibe Therapy on Clinical Outcomes

 

There have been a limited number of ezetimibe clinical outcome trials. Two have studied the effect of ezetimibe in combination with a statin vs. placebo making it virtually impossible to determine if ezetimibe per se has beneficial effects. However, one study has compared ezetimibe plus a statin vs. a statin alone and one study compared ezetimibe vs. placebo. Finally, a study compared moderate-intensity statin with ezetimibe vs. high-intensity statin monotherapy.

 

SEAS TRIAL

 

The SEAS Trial was a randomized trial of 1,873 patients with mild-to-moderate, asymptomatic aortic stenosis (128). The patients received either simvastatin 40mg per day in combination with ezetimibe 10mg per day vs. placebo daily. The primary outcome was a composite of major cardiovascular events, including death from cardiovascular causes, aortic-valve replacement, non-fatal myocardial infarction, hospitalization for unstable angina pectoris, heart failure, coronary-artery bypass grafting, percutaneous coronary intervention, and non-hemorrhagic stroke. Secondary outcomes were events related to aortic-valve stenosis and ischemic cardiovascular events. Simvastatin plus ezetimibe lowered LDL-C levels by 61% compared to placebo. There were no significant differences in the primary outcome between the treated vs. placebo groups. Similarly, the need for aortic valve replacement was also not different between the treated and placebo groups. However, fewer patients had ischemic cardiovascular events in the simvastatin plus ezetimibe treated group than in the placebo group (hazard ratio, 0.78; 95% CI, 0.63 to 0.97; P=0.02), which was primarily accounted for by a decrease in the number of patients who underwent coronary-artery bypass grafting. The design of this study does not allow for one to determine if the beneficial effect on ischemic cardiovascular events typically produced by statin therapy was enhanced by the addition of ezetimibe.

 

SHARP TRIAL

 

The SHARP Trial was a randomized trial of 9,270 patients with chronic kidney disease (3,023 on dialysis and 6,247 not on dialysis) with no known history of myocardial infarction or coronary revascularization (129). Patients were randomly assigned to simvastatin 20 mg plus ezetimibe 10 mg daily vs. placebo. The primary outcome was first major atherosclerotic event (non-fatal myocardial infarction or coronary death, non-hemorrhagic stroke, or any arterial revascularization procedure). Treatment with simvastatin plus ezetimibe resulted in a decrease in LDL-C of 0.85 mmol/L (~34mg/dL). This decrease in LDL-C was associated with a 17% reduction in major atherosclerotic events. In patients on hemodialysis there was a 5% decrease in cardiovascular events that was not statistically significant. Unfortunately, similar to the SEAS Trial, it is impossible to determine whether the addition of ezetimibe improved outcomes above and beyond what would have occurred with statin treatment alone.

 

IMPROVE-IT TRIAL

 

The IMPROVE-IT Trial tested whether the addition of ezetimibe to statin therapy would provide an additional beneficial effect in patients with the acute coronary syndrome (130). The IMPROVE-IT Trial was a large trial with over 18,000 patients randomized to simvastatin 40mg vs. simvastatin 40mg + ezetimibe 10mg per day. On treatment LDL-C levels were 70mg/dL in the statin alone group vs. 54mg/dL in the statin + ezetimibe group. There was a small but significant 6.4% decrease in major cardiovascular events (cardiovascular death, MI, documented unstable angina requiring rehospitalization, coronary revascularization, or stroke) in the statin + ezetimibe group (HR 0.936 CI (0.887, 0.988) p=0.016). Cardiovascular death, non-fatal MI, or non-fatal stroke were reduced by 10% (HR 0.90 CI (0.84, 0.97) p=0.003). There was a significant 21% reduction in ischemic stroke (HR, 0.79; 95% CI, 0.67-0.94; P=0.008) and a nonsignificant increase in hemorrhagic stroke (HR, 1.38; 95% CI, 0.93-2.04; P=0.11) (131). Patients with a prior stroke were at a higher risk of stroke recurrence and the risk of a subsequent stroke was reduced by 40% (HR, 0.60; 95% CI, 0.38-0.95; P=0.030) with ezetimibe added to simvastatin therapy (131). In patients with diabetes or other high risk factors the benefits of adding ezetimibe to statin therapy was enhanced (132). In fact, patients without DM and at low or moderate risk demonstrated no benefit with the addition of ezetimibe to simvastatin (132). Similarly, patients who also had peripheral arterial disease or a history of cerebral vascular disease also had the greatest absolute benefits from the addition of ezetimibe (133). Thus, the addition of ezetimibe to statin therapy is of greatest benefit in patients at high risk (for example patients with diabetes, peripheral vascular disease, cerebrovascular disease, etc.).

 

The results of this study have a number of important implications. First, it demonstrates that combination therapy has benefits above and beyond statin therapy alone. Second, it provides further support for the hypothesis that lowering LDL per se will reduce cardiovascular events. The reduction in cardiovascular events was similar to what one would predict based on the Cholesterol Treatment Trialists results. Third, it suggests that lowering LDL levels into the 50s will have benefits above and beyond lowering LDL levels to the 70mg/dL range in patients with diabetes or other factors that result in a high risk for cardiovascular events. These results have implications for determining goals of therapy and provide support for combination therapy.

 

EWTOPIA 75

 

This was a multicenter, randomized trial in Japan that examined the preventive efficacy of ezetimibe for patients aged ≥75 years (mean age 80.6 years), with elevated LDL-C (≥140 mg/dL) without a history of coronary artery disease who were not taking lipid lowering drugs (134). Patients were randomized to ezetimibe 10mg (n=1,716) or usual care (n=1,695) and followed for 4.1 years. The primary outcome was a composite of sudden cardiac death, myocardial infarction, coronary revascularization, or stroke. In the ezetimibe group LDL-C was decreased by 25.9% and non-HDL-C by 23.1% while in the usual care group LDL-C was decreased by 18.5% and non-HDL-C by 16.5% (p<0.001 for both lipid parameters). By the end of the trial 9.6% of the patients in the usual care group and 2.1% of the ezetimibe group were taking statins. Ezetimibe reduced the incidence of the primary outcome by 34% (HR 0.66; P=0.002). Additionally, composite cardiac events were reduced by 60% (HR 0.60; P=0.039) and coronary revascularization by 62% (HR 0.38; P=0.007) in the ezetimibe group vs. the control group. There was no difference in the incidence of stroke or all-cause mortality between the groups. It should be noted that the reduction in cardiovascular events was much greater than one would expect based on the absolute difference in LDL-C levels (121mg/dL in ezetimibe group vs. 132mg/dL). As stated by the authors “Given the open-label nature of the trial, its premature termination, and issues with follow-up, the magnitude of benefit observed should be interpreted with caution.” Nevertheless, this study provides additional support that ezetimibe can reduce cardiovascular events.

 

RACING TRIAL

 

The RACING trial was a randomized, open-label trial in patients with atherosclerotic cardiovascular disease carried out in South Korea (135). Patients were randomly assigned to either rosuvastatin 10 mg with ezetimibe 10 mg (n= 1894) or rosuvastatin 20 mg (n= 1886). The primary endpoint was cardiovascular death, major cardiovascular events, or non-fatal stroke. The median LDL-C level during the study was 58mg/dL in the combination therapy group and 66mg/dL in the statin monotherapy group (p<0·0001). The primary endpoint occurred in 9.1% of the patients in the combination therapy group and 9·9% of the patients in the high-intensity statin monotherapy group (non-inferior). Non-inferiority was observed in patients with LDL-C levels < 100mg/dL and >100mg/dL and in patients greater than 75 years of age (136,137).

 

This study demonstrates that moderate intensity statin with ezetimibe was non-inferior to high-intensity statin therapy with regards to cardiovascular death, major cardiovascular events, or non-fatal stroke. Interestingly a lower prevalence of discontinuation or dose reduction caused by intolerance to the study drug was seen with combination therapy. This indicates that using a moderate intensity dose of a statin with ezetimibe is a useful strategy in patients that do not tolerate high intensity statin therapy.

 

Side Effects

 

Ezetimibe has not demonstrated significant side effects. In monotherapy trials, the effect on liver function tests was similar to placebo. In a meta-analysis by Toth et al. of 27 randomized trials in > 20,000 participants evaluating statin plus ezetimibe vs. statin alone the incidence of liver function test abnormalities was slightly greater in the combination therapy group (statin alone- 0.35% vs. statin plus ezetimibe 0.56%) (138). In contrast, Luo and colleagues in a meta-analysis of 20 randomized with > 14,000 subjects did not observe a difference in liver function tests in the ezetimibe plus statin vs. statin alone group (139). With regards to muscle side effects, a meta-analysis of seven randomized trials by Kashani and colleagues found that monotherapy with ezetimibe or ezetimibe in combination with a statin did not increase the risk of myositis compared to placebo or monotherapy with a statin (140). Similarly, Luo et al also did not observe that combination therapy with ezetimibe and a statin increased the risk of myositis (139). In a meta-analysis by Savarese et al. of 7 randomized long-term studies including SEAS, SHARP, and IMPROVE-IT, the incidence of cancer was similar in patients treated with ezetimibe vs. patients not treated with ezetimibe (141). This confirms a previous study that also did not demonstrate an increased cancer risk in the three largest ezetimibe trials (142). Ezetimibe does not appear to have adverse effects on fasting glucose levels or A1c levels (143).

 

Thus, over many years of use ezetimibe has been shown to be a very safe drug without major side effects.

 

Contraindications

 

Ezetimibe is contraindicated in patients with active liver disease. The use of ezetimibe during pregnancy and lactation has not been studied.  

 

Summary

 

Ezetimibe has a modest ability to lower LDL-C levels and can be a very useful adjunct to statin therapy. When added to statin therapy it will lower the LDL-C by an additional 15-20% which is equivalent to three titrations of the statin dose (for example adding ezetimibe is equivalent to increasing atorvastatin from 10mg to 80mg per day). Additionally, the combination of a high dose of a potent statin (rosuvastatin 40mg per day) with ezetimibe was able to lower the LDL by approximately 70%, which will allow many patients to reach their LDL goal (123). In patient’s intolerant of statins who either cannot take a statin or can only take low doses of a statin, ezetimibe is extremely useful in further lowering LDL-C. The ease of taking ezetimibe, the lack of serious side effects, and that it is inexpensive as it is now a generic drug make it an obvious second choice drug after statins to lower LDL-C levels.   

 

BILE ACID SEQUESTRANTS 

 

Introduction

 

There are three bile acid sequestrants approved for use in the United States. The first bile acid sequestrant, cholestyramine (Questran), was developed in the 1950s and was the second drug available to lower cholesterol levels (niacin was the first drug). Colestipol (Colestid) was developed in the 1970s and is very similar to cholestyramine. In 2000, Colesevelam (Welchol) was approved. Colesevelam has enhanced binding and affinity for bile acids compared to cholestyramine and colestipol and therefore can be given in much lower doses reducing some side effects (144).

 

​Cholestyramine is available as a powder and the dose ranges from 8-24 grams per day given with meals. Colestipol is available as a tablet and the dose ranges from 2-16 grams per day given with meals or granules and the dose ranges from 5-30 grams per day given with meals. The dose of colesevelam is 3.75 grams per day and can be given as tablets (​take 6 tablets once daily or 3 tablets twice daily), oral suspension (​take one packet once daily), or chewable bars (take one bar once daily). Because bile acid sequestrants mechanism of action starts with the binding of bile acids in the intestine (see below) these drugs are most effective when administered with meals.

 

Effect of Bile Acid Sequestrants on Lipid and Lipoprotein Levels

 

The major effect of bile acid sequestrants is to lower LDL-C levels in a dose dependent fashion. Depending upon the specific drug and dose the decrease in LDL-C ranges from approximately 5 to 30% (144-146). The effect of monotherapy with bile acid sequestrants on LDL-C levels observed in various studies is shown in table 8.

 

Table 8. Effect of Bile Acid Sequestrants on LDL-C

Drug

LDL lowering

Cholestyramine 4g/day

7% decrease

Cholestyramine 24g/day

28% decrease

Colestipol 4g/day

12% decrease

Colestipol 16g/day

24% decrease

Colesevelam 3.8g/day

15% decrease

Colesevelam 4.3g/day

18% decrease

 

Bile acid sequestrants are typically used in combination with statins and the addition of bile acid sequestrants to statin therapy will result in a further 10% to 25% decrease in LDL-C levels (144-146). Combination therapy can result in a 60% reduction in LDL-C levels when high doses of potent statins are combined with high doses of bile acid sequestrants. Bile acid sequestrants will also further lower LDL-C levels by as much as 18% when added to statins and ezetimibe (147). This is particularly useful in patients with Heterozygous Familial Hypercholesterolemia who can have very high LDL-C levels at baseline. Additionally, in patients who are statin intolerant, the combination of a bile acid sequestrant and ezetimibe resulted in an additional 10-20% decrease in LDL-C compared to either drug alone  (148,149). Thus, both in monotherapy and in combination with other drugs that lower LDL-C levels, bile acid sequestrants are effective in lowering LDL-C levels

 

Bile acid sequestrants have a very modest effect on HDL-C levels, typically resulting in a 3-9% increase (144-146). The effect of bile acid sequestrants on triglyceride levels varies (144-146). In patients with normal triglyceride levels, bile acid sequestrants increase triglyceride levels by a small amount. However, as baseline triglyceride levels increase, the effect of bile acid sequestrants on plasma triglyceride levels becomes greater, and can result in substantial increases in triglyceride levels. In patients with triglycerides > 400mg/dL the use of bile acid sequestrants is contraindicated.

 

Table 9. Effect of Bile Acid Sequestrants on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

Variable. If TG levels elevated will increase significantly

HDL-C

Small Increase

Lp(a)

No change

 

Non-Lipid Effects of Bile Acid Sequestrants

 

Bile acid sequestrants have been shown to reduce fasting glucose and hemoglobin A1c levels (150). Colesevelam has been most intensively studied and in a number of different studies colesevelam has decreased A1c levels by approximately 0.5-1.0% in patients also treated with a variety of glucose lowering drugs including metformin, sulfonylureas, and insulin. The Food and Drug Administration (FDA) has approved colesevelam for improving glycemic control in patients with type 2 diabetes.

 

Bile acid sequestrants decrease CRP. For example, Devaraj et al have shown that colesevelam decreases hs-CRP by 18% compared to placebo (151). In combination with a statin, colesevelam reduced hs-CRP levels by 23% compared to statin alone (152). 

 

Mechanisms Accounting for Bile Acid Sequestrants Induced Lipid Effects

 

Bile acid sequestrants bind bile acids in the intestine, preventing their reabsorption in the terminal ileum leading to the increased fecal excretion of bile acids (153). This decrease in bile acid reabsorption reduces the size of the bile acid pool, which stimulates the conversion of cholesterol into bile acids in the liver (153). This increase in bile acid synthesis decreases hepatic cholesterol levels leading to the activation of SREBPs that up-regulate the expression of the enzymes required for the synthesis of cholesterol and the expression of LDL receptors (153). The increase in hepatic LDL receptors results in the increased clearance of LDL from the circulation leading to a decrease in serum LDL-C levels (Figure 1). Thus, similar to statins and ezetimibe, bile acids lower plasma LDL-C levels by decreasing hepatic cholesterol levels, which stimulates LDL receptor production and thereby accelerates the clearance of LDL from the blood.

 

The key regulator of bile acid synthesis is FXR (farnesoid X receptor), a nuclear hormone receptor that forms a heterodimer with RXR to regulate gene transcription (154,155). Bile acids down-regulate cholesterol 7α hydroxylase, the first enzyme in the bile acid synthetic pathway by several FXR mediated mechanisms. In the ileum, bile acids via FXR stimulate the production of FGF19, which is secreted into the portal vein and inhibits cholesterol 7α hydroxylase expression in the liver (154). Additionally, in the liver, bile acids activate FXR leading to the increased expression of SHP (small heterodimer partner), which inhibits the transcription of cholesterol 7α hydroxylase (155). Thus, a decrease in bile acids will lead to the decreased activation of FXR in the liver and intestines and thereby result in an increase in cholesterol 7α hydroxylase expression and the increased conversion of cholesterol to bile acids resulting in a decrease in hepatic cholesterol content.

 

Decreased activation of FXR can also explain the adverse effects of bile acid sequestrants on triglyceride levels (156,157). Activation of FXR increases the expression of apolipoprotein C-II, apolipoprotein A-V, and the VLDL receptor, proteins that decrease plasma triglyceride levels while decreasing the expression of apolipoprotein C-III, a protein that is associated with increases in plasma triglycerides (156,157). Thus, activation of FXR would be expected to decrease triglyceride levels as increases in apolipoprotein C-II, apolipoprotein A-V, and the VLDL receptor and decreases in apolipoprotein C-III would reduce plasma triglyceride levels. With bile acid sequestrants the activation of FXR would be reduced and decreases in the expression of apolipoprotein C-II, apolipoprotein A-V, and the VLDL receptor and increased expression of apolipoprotein C-III would increase plasma triglyceride levels.

 

The mechanism by which treatment with bile acid sequestrants improves glycemic control is unclear (158). 

 

Pharmacokinetics and Drug Interactions

 

Bile acid sequestrants are not absorbed and not altered by digestive enzymes and thus their primary effects are localized to the intestine (144-146). It should be noted that bile acid sequestrants can indirectly have systemic effects by decreasing the reabsorption of bile acids and thereby reducing the exposure of cells to bile acids, which are biologically active compounds.

 

Unfortunately, in the intestine bile acid sequestrants can impede the absorption of many other drugs (144-146). This is particularly true for cholestyramine and colestipol which are used in large quantities (maximum doses- cholestyramine 24 grams per day; colestipol 30 grams per day). In contrast, colesevelam, which requires a much lower quantity of drug because of its high affinity and binding capacity for bile salts, has less of an effect on the absorption of other drugs (recommended dose of colesevelam 3.75 grams/day). Of particular note colesevelam does not interfere with absorption of statins, fenofibrate, or ezetimibe. A list of some of the drugs whose absorption is affected by cholestyramine or colestipol is shown in table 10 and a list of drugs whose absorption is affected by colesevelam is shown in table 11.

 

Table 10.  Some of the Drugs Affected by Cholestyramine/Colestipol

Statins

Ezetimibe

Gemfibrozil

Fenofibrate

Thiazides

Furosemide

Spironolactone

Digoxin

Warfarin

L-thyroxine

Corticosteroids

Vitamin K

Cyclosporine

Raloxifine

NSAIDs

Sulfonylureas

Aspirin

Beta blockers

Tricyclic

 

 

Table 11. Some of the Drugs Affected by Colesevelam

L-thyroxine

Cyclosporine

Glimepiride

Glipizide

Glyburide

Phenytoin

Olmesartan

Warfarin

Oral contraceptives

 

 

 

 

It is currently recommended that medications should be taken either 4 hours before or 4 hours after taking bile acid sequestrants. This is particularly important with drugs that have a narrow toxic/therapeutic window, such as thyroid hormone, digoxin, or warfarin. It can be very difficult for many patients, particularly those on multiple medications, to take bile acid sequestrants given the need to separate pill ingestion.

 

Cholestyramine and colestipol may also interfere with the absorption of fat-soluble vitamins. Taking a multivitamin 4 hours before or after these drugs can reduce the likelihood of a vitamin deficiency.

 

Effect of Bile Acid Sequestrants on Clinical Outcomes

 

The Lipid Research Clinics Coronary Primary Prevention Trial (LRC-CPPT) of cholestyramine vs. placebo was the first large drug study to explore the effect of specifically lowering LDL-C on cardiovascular outcomes (159). LRC-CPPT was a multicenter, randomized, double-blind study in 3,806 asymptomatic middle-aged men with primary hypercholesterolemia. The treatment group received cholestyramine 24 grams per day and the control group received a placebo for an average of 7.4 years. In the cholestyramine group total and LDL-C was decreased by 8.5% and 12.6% as compared to the placebo group. In the cholestyramine group there was a 19% reduction in risk (p < 0.05) of the primary end point accounted for by a 24% reduction in definite CHD death and a 19% reduction in nonfatal myocardial infarction. In addition, the incidence rates for new positive exercise tests, angina, and coronary bypass surgery were reduced by 25%, 20%, and 21%, respectively, in the cholestyramine group. The reduction in events correlated with the decrease in LDL-C levels (160). Of note, compliance with cholestyramine 24 grams per day was limited with many patients taking much less than the prescribed doses. These results indicate that lowering LDL-C with bile acid sequestrant monotherapy reduces cardiovascular disease.

 

In addition to the LRC-CPPT clinical outcome study, two studies have examined the effect of cholestyramine monotherapy on angiographic changes in the coronary arteries. The National Heart, Lung, and Blood Institute Type II Coronary Intervention Study and the St Thomas Atherosclerosis Regression Study reported that cholestyramine decreased the progression of atherosclerosis (161,162). There are a number of studies that have employed bile acid sequestrants in combination with other drugs and have shown a reduction in the progression of atherosclerosis or an increase in the regression of atherosclerosis but given the use of multiple drugs it is difficult to attribute the beneficial effects to the bile acid sequestrants (163-165). Unfortunately, there are no clinical outcome studies comparing statins alone vs. statins plus bile acid sequestrants.

 

Side Effects

 

Bile acid sequestrants do not have major systemic side effects as they are not absorbed and remain in the intestinal tract. However, they do cause gastrointestinal (GI) side effects (144-146). Constipation is a very common side effect and can be severe. In addition, patients will often complain of bloating, abdominal discomfort, and aggravation of hemorrhoids. Because of GI distress, a significant number of patients will discontinue therapy with bile acid sequestrants. These GI side effects are much more common with cholestyramine and colestipol compared to colesevelam, which is much better tolerated. One can reduce or ameliorate these GI side effects by increasing hydration, adding fiber to the diet (psyllium), and using stool softeners. Notably, bile acid sequestrants do not cause liver or muscle problems.

 

One should also be aware that bile acid sequestrants can be difficult for many patients to take. Colestipol and colesevelam pills are large and can be difficult for some patients to swallow. Additionally, patients need to take a large number of these pills (colesevelam- 6 pills per day; colestipol- as many as 16 pills per day). The granular forms of cholestyramine and colestipol do not dissolve and are ingested as a suspension in liquid. Many patients find mixing with water leads to an unpalatable mixture that is difficult to take. Sometimes mixing with fruit juice, apple sauce, mash potatoes, etc. make the mixture more palatable. The suspension form of colesevelam with either 1.875 or 3.75 grams is preferred by many patients.

 

As noted, earlier bile acid sequestrants can increase triglyceride levels, particularly in patients with elevated baseline triglyceride levels.

 

Contraindications

 

Bile acid sequestrants usually should be avoided in patients with pre-existing GI disorders. Bile acid sequestrants are contraindicated in patients with recent or repeated intestinal obstruction and patients with plasma triglyceride levels > 400mg/dL. In contradistinction from other lipid lowering drugs, bile acid sequestrants are not contraindicated during pregnancy or lactation (category B) (166). In women of child bearing age who are planning to become pregnant bile acid sequestrants can be a good choice to lower LDL levels.

 

Summary

 

Bile acid sequestrants are useful secondary drugs for the treatment of elevated LDL-C levels. They are typically used in combination with statin therapy as a second line drug or as an addition to statin plus ezetimibe therapy as a third line drug. In statin intolerant patients the combination of ezetimibe and a bile acid sequestrant is frequently employed. Bile acid sequestrants can be difficult drugs for patients to take due to GI side effects, difficulty taking the medication, and the need to avoid taking these drugs with other medications. To improve compliance with these drugs the clinician needs to spend time educating the patient on how to take these drugs and how to avoid side effects. Because of these difficulties other cholesterol lowering drugs are used more commonly than bile acid sequestrants. In patients with type 2 diabetes who need an improvement in glycemic control and LDL-C lowering colesevelam can be used to target both abnormalities.

 

PCSK9 MONOCLONAL ANTIBODIES

 

Introduction

 

In 2015 two monoclonal antibodies that inhibit PCSK9 (proprotein convertase subtilisin kexin type 9) were approved for the lowering of LDL-C levels. Alirocumab (Praluent) is produced by Regeneron/Sanofi and evolocumab (Repatha) is produced by Amgen (167,168). Alirocumab is administered as either 75mg or 150mg subcutaneously every 2 weeks or 300mg once a month while evolocumab is administered as either 70mg subcutaneously every 2 weeks or 420mg subcutaneously once a month.

 

Effect of PCSK inhibitors on Lipid and Lipoprotein Levels

 

There are a large number of studies that have examined the effect of PCSK9 inhibitors on lipid and lipoprotein levels. A meta-analysis of 24 studies comprising 10,159 patients reported a reduction in LDL-C levels of approximately 50% and in an increase in HDL of 5-8% (169). Notably, in 12 RCTs with 6,566 patients, Lp(a) levels were reduced by 25-30% (169). The higher the baseline Lp(a) the greater the reduction with treatment (170). It should be recognized that most LDL-C lowering drugs (statins, ezetimibe, bempedoic acid, and bile acid sequestrants) do not lower Lp(a) levels. PCSK9 inhibitors have not been shown to decrease hs-CRP levels (171).

 

MONOTHERAPY

 

Both alirocumab and evolocumab have been studied as monotherapy vs. ezetimibe. In the Mendel-2 study patients were randomly assigned to evolocumab, placebo, or ezetimibe (172). In the evolocumab group, LDL-C levels decreased by 57% while in the ezetimibe group LDL-C levels decreased by 18% compared to placebo. Additionally, non-HDL-C was decreased by 49%, apolipoprotein B by 47%, triglycerides by 5.3% (NS), and Lp(a) by 18.5% while HDL levels increased by 5.5% in the evolocumab treated subjects. In a study of alirocumab vs. ezetimibe, LDL-C levels were reduced by 47% in the alirocumab group and 16% in the ezetimibe group (173). In addition, alirocumab decreased non-HDL-C by 41%, apolipoprotein B by 37%, triglycerides by 12%, and Lp(a) by 17% and increased HDL by 6%. Thus, PCSK9 monoclonal antibodies are very effective in lowering pro-atherogenic lipoproteins when used in monotherapy and have a more robust effect than ezetimibe.

 

IN COMBINATION WITH STATINS

 

In the Odyssey Combo I study, patients on maximally tolerated statin therapy were randomized to alirocumab or placebo (174). Similar to monotherapy results, when alirocumab was added to statin therapy there was a further decrease in LDL-C levels by 46%, non-HDL-C by 38%, apolipoprotein B by 36%, and Lp(a) by 15% with an increase in HDL of 7% and no change in triglyceride levels. In the Odyssey Combo II study, patients on maximally tolerated statin therapy were randomized to alirocumab vs. ezetimibe (175). Alirocumab reduced LDL levels by 51% while ezetimibe reduced LDL by 21%, demonstrating that even when added to statin therapy, alirocumab has a significantly greater ability to reduce LDL-C levels than ezetimibe. In Odyssey Combo II, non-HDL-C levels were decreased by 42%, apolipoprotein B by 41%, triglycerides by 13%, and Lp(a) by 28% while HDL increased by 9% in the alirocumab treated group. In the Laplace-2 study, evolocumab was added to various statins used at different doses (176). It didn’t make any difference which statin was being used (atorvastatin, rosuvastatin, or simvastatin) or what dose (atorvastatin 10mg or 80mg; rosuvastatin 5mg or 40mg); the addition of evolocumab resulted in an approximately 60% further decrease in LDL-C levels beyond statin alone. Additionally, the Laplace-2 trial also showed that evolocumab was much more potent than ezetimibe when added to statin therapy (evolocumab resulted in an approximately 60% decrease in LDL vs. while ezetimibe resulted in an approximately 20-25% reduction).

 

IN COMBINATION WITH STATINS AND EZETIMIBE  

 

When evolocumab was added to patients receiving atorvastatin 80mg and ezetimibe 10mg there was 48% further reduction in LDL-C levels indicating that even in patients on very aggressive lipid lowering therapy the addition of a PCSK9 inhibitor can still result in a marked reduction in LDL-C (177). In addition to decreasing LDL-C there was also a 41% decrease in non-HDL-C, a 38% decrease in apolipoprotein B, and a 19% decrease in Lp(a) when evolocumab was added to statin plus ezetimibe therapy.

 

PATIENTS WITH HETEROZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA  

 

Both alirocumab and evolocumab have been tested in patients with Heterozygous Familial Hypercholesterolemia (178,179). In the Rutherford-2 trial, evolocumab lowered LDL-C by 60%, non-HDL-C by 56%, apolipoprotein B by 49%, Lp(a) by 31%, and triglycerides by 22% while increasing HDL by 8% (178). In the Odyssey FH I and FH II studies, alirocumab lowered LDL-C by approximately 55%, non-HDL-C by ~50%, apolipoprotein B by ~43%, Lp(a) by ~19% and triglycerides by ~14% while increasing HDL by ~7% (179). Thus, in these difficult to treat patients PCSK9 monoclonal antibodies were still very effective at lowering pro-atherogenic lipoproteins.

 

PATIENTS WITH HOMOZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA  

 

Evolocumab resulted in a 21-31% decrease in LDL-C levels compared to placebo in patients with Homozygous Familial Hypercholesterolemia (180,181). The response to therapy appears to be dependent on the underlying genetic cause. Patients with mutations in the LDL receptor leading to the expression of defective receptors respond to therapy whereas patients with mutations leading to negative receptors (null variants) have a poor response (180-182). Given the mechanism by which PCSK9 inhibitors lower LDL-C levels it is not surprising that patients that do not have any functional LDL receptors will not respond to therapy (see section on Mechanism of Lipid Lowering). Alirocumab decreased LDL-C by 35.6%, non-HDL-C by 32.9%, apolipoprotein B by 29.8%, and lipoprotein (a) by 28.4% (183). Given that PCSK9 monoclonal antibodies decrease LDL-C levels in some patients with Familial Hypercholesterolemia these drugs can be useful in this very difficult to treat patient population.

 

STATIN INTOLERANT PATIENTS  

 

A number of studies have examined the effect of PCSK9 monoclonal antibodies in statin intolerant patients (myalgias) and compared the response to ezetimibe treatment (102,184,185). As expected, treatment with a PCSK9 inhibitor was more effective in lowering LDL-C levels than ezetimibe. Importantly, muscle symptoms were less frequent in the PCSK9 treated patients than those treated with ezetimibe, indicating that PCSK9 monoclonal antibodies will be an effective treatment choice in statin intolerant patients with myalgias.

 

PATIENTS WITH DIABETES  

 

A meta-analysis of three trials with 413 patients with type 2 diabetes found that in patients with type 2 diabetes evolocumab caused a 60% decrease in LDL-C compared to placebo and a 39% decrease in LDL-C compared to ezetimibe treatment (186). In addition, in patients with type 2 diabetes, evolocumab decreased non-HDL-C 55% vs. placebo and 34% vs. ezetimibe) and Lp(a) (31% vs. placebo and 26% vs. ezetimibe). These beneficial effects were not affected by glycemic control, insulin use, renal function, and cardiovascular disease status. Thus, PCSK9 inhibitors are effective therapy in patients with type 2 diabetes and the beneficial effects on pro-atherogenic lipoproteins is similar to what is observed in non-diabetic patients.

 

PATIENTS WITH HYPERTRIGLYCERIDEMIA  

 

There are no studies that have examined the effect of PCSK9 monoclonal antibodies in patients with marked elevations in triglyceride levels (>400mg/dL).

 

Table 12. Effect of PCSK9 Inhibitors on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

No change or small decrease

HDL-C

Small Increase

Lp(a)

Decrease

 

Mechanism Accounting for the PCSK9 Inhibitor Induced Lipid Effects

 

The linkage of PCSK9 with lipoprotein metabolism was first identified by Abifadel and colleagues in 2003, when they demonstrated that certain mutations in PCSK9 could result in the phenotypic appearance of Familiar Hypercholesterolemia (187). Subsequent studies demonstrated that gain of function mutations in PCSK9 are an uncommon cause of Familiar Hypercholesterolemia (167,168,188). In 2005 it was shown that loss of function mutations in PCSK9 resulted in lower LDL-C levels and this decrease in LDL-C levels was associated with a reduction in the risk of cardiovascular events (189,190).

 

The main route of clearance of clearance of plasma LDL is via LDL receptors in the liver (191). When the LDL particle binds to the LDL receptor the LDL particle- LDL receptor complex is taken into the liver by endocytosis (191). The LDL particle and the LDL receptor then disassociate and the LDL lipoprotein particle is delivered to lysosomes where it is degraded and the LDL receptor returns to the plasma membrane (Figure 2) (191). After endocytosis LDL receptors recirculate back to the plasma membrane over 100 times.

 

PCSK9 is predominantly expressed in the liver and secreted into the circulation. Once extracellular, PCSK9 can bind to the LDL receptor and alter the metabolism of the LDL receptor (192,193). Instead of the LDL receptor recycling to the plasma membrane the LDL receptor bound to PCSK9 remains associated with the LDL particle and is delivered to the lysosomes where it is also degraded (Figure 4) (192,193). This results in a decrease in the number of plasma membrane LDL receptors resulting in the decreased clearance of circulating LDL leading to elevations in plasma LDL-C levels.

 

The PCSK9 monoclonal antibodies bind PCSK9 preventing the PCSK9 from interacting with LDL receptors and thereby preventing PCSK9 from inducing LDL receptor degradation (192,193). The decreased LDL receptor degradation results in an increase in hepatic LDL receptors on the plasma membrane leading to the increased clearance of LDL and decreases in plasma LDL-C levels (194,195). Thus, similar to statins, ezetimibe, bempedoic acid, and bile acid sequestrants, PCSK9 inhibitors are reducing plasma LDL-C levels by up-regulating hepatic LDL receptors. The difference is that PCSK9 inhibitors are decreasing the degradation of LDL receptors while statins, ezetimibe, bempedoic acid, and bile acid sequestrants stimulate the production of LDL receptors.

 

Figure 4. PCSK9 Directs LDL Receptor to Degradation in Lysosome.

 

The expression of PCSK9 is stimulated by SREBP-2 (192,193). Statins and other drugs that lower hepatic cholesterol levels lead to the activation of SREBP-2 and thereby increase plasma PCSK9 levels (192,193). Inhibition of PCSK9 with monoclonal antibodies is more effective in lowering plasma LDL-C levels in patients on statin therapy due to the higher levels of plasma PCSK9 in these individuals.

 

The mechanism by which PCSK9 inhibitors reduce Lp(a) levels is unclear. Studies have shown that PCSK9 inhibitors increase the catabolism of lipoprotein(a) particles (196,197). In some circumstances PCSK9 inhibitors may also decrease the production rate (197). It has been postulated that increasing hepatic LDL receptor levels in the setting of marked reductions in circulating LDL levels will result in the clearance of Lp(a) by liver LDL receptors (198).

 

Pharmacokinetics and Drug Interactions

 

PCSK9 monoclonal antibodies are eliminated primarily by cellular endocytosis, phagocytosis, and target-mediated clearance. They are not metabolized or cleared by the liver or kidneys and therefore there is no need to adjust the dose in patients with either liver or kidney disease. There are no interactions with the cytochrome P450 system or transport proteins and thus the risk of drug-drug interactions is minimal. Currently there are no reported drug-drug interactions with PCSK9 monoclonal antibodies.

 

Effect of PCSK9 Inhibitors on Clinical Outcomes

 

FOURIER TRIAL

 

The FOURIER trial was a randomized, double-blind, placebo-controlled trial of evolocumab vs. placebo in 27,564 patients with atherosclerotic cardiovascular disease and an LDL-C level of 70 mg/dL or higher who were on statin therapy (199). The primary end point was cardiovascular death, myocardial infarction, stroke, hospitalization for unstable angina, or coronary revascularization and the key secondary end point was cardiovascular death, myocardial infarction, or stroke. The median duration of follow-up was 2.2 years. Baseline LDL-C levels were 92mg/dL and evolocumab resulted in a 59% decrease in LDL levels (LDL-C level on treatment approximately 30mg/dL). Evolocumab treatment significantly reduced the risk of the primary end point (hazard ratio, 0.85; 95% confidence interval (CI), 0.79 to 0.92; P<0.001) and the key secondary end point (hazard ratio, 0.80; 95% CI, 0.73 to 0.88; P<0.001). The results were consistent across key subgroups, including the subgroup of patients in the lowest quartile for baseline LDL-C levels (median, 74 mg/dL). Of note, a similar decrease in cardiovascular events occurred in patients with diabetes treated with evolocumab and glycemic control was not altered (200). Additionally, in patients with peripheral arterial disease evolocumab also reduced cardiovascular events (201). Further analysis has shown that in the small number of patients with a baseline LDL-C level less than 70mg/dL, evolocumab reduced cardiovascular events to a similar degree as in the patients with an LDL-C greater than 70mg/dL (202). The lower the on-treatment LDL-C levels (down to levels below 20mg/dL), the lower the cardiovascular event rate, suggesting that greater reductions in LDL-C levels will result in greater reductions in cardiovascular disease (203). Finally, the relative risk reductions with evolocumab for the cardiovascular events tended to be greater in high-risk subgroups (20% for those with a more recent MI, 18% with multiple prior MI, and 21% with residual multivessel coronary artery disease), whereas the relative risk reduction was 5% to 8% in patients without these risk factors (204). This observation suggests that certain groups of patients will derive greater benefit from the addition of a PCSK9 inhibitor.

 

It should be noted that that the duration of the FOURIER trial was very short and it is well recognized from previous statin trials that the beneficial effects of lowering LDL-C levels take time with only modest effects observed during the first year of treatment. In the FOURIER trial the reduction of cardiovascular death, myocardial infarction, or stroke was 16% during the first year but was 25% beyond 12 months.

 

ODYSSEY TRIAL

 

The ODYSSEY trial was a multicenter, randomized, double-blind, placebo-controlled trial involving 18,924 patients who had an acute coronary syndrome 1 to 12 months earlier, an LDL-C level of at least 70 mg/dL, a non-HDL-C level of at least 100 mg/dL, or an apolipoprotein B level of at least 80 mg/dL while on high intensity statin therapy or the maximum tolerated statin dose (205). Patients were randomly assigned to receive alirocumab 75 mg every 2 weeks or matching placebo. The dose of alirocumab was adjusted to target an LDL-C level of 25 to 50 mg/dL. The primary end point was a composite of death from coronary heart disease, nonfatal myocardial infarction, fatal or nonfatal ischemic stroke, or unstable angina requiring hospitalization. During the trial LDL-C levels in the placebo group was 93-103mg/dL while in the alirocumab group LDL-C levels were 40mg/dL at 4 months, 48mg/dL at 12 months, and 66mg/dL at 48 months (the increase with time was due to discontinuation of alirocumab or a decrease in dose). The primary endpoint was reduced by 15% in the alirocumab group (HR 0.85; 95% CI 0.78 to 0.93; P<0.001). In addition, total mortality was reduced by 15% in the alirocumab group (HR 0.85; 95% CI 0.73 to 0.98). The absolute benefit of alirocumab was greatest in patients with a baseline LDL-C level greater than 100mg/dL. In patients with an LDL-C level > than 100mg/dL the number needed to treat with alirocumab to prevent an event was only 16. It should be noted that the duration of this trial was very short (median follow-up 2.8 years) which may have minimized the beneficial effects. Additionally, because alirocumab 75mg every 2 weeks was stopped if the LDL-C level was < 15mg/dL on two consecutive measurements the beneficial effects may have been blunted (7.7% of patients randomized to alirocumab were switched to placebo).

 

SUMMARY OF OUTCOME TRIALS

 

It should be noted that that the duration of the PCSK9 outcome trials were relatively short and it is well recognized from previous statin trials that the beneficial effects of lowering LDL-C levels take time with only modest effects observed during the first year of treatment. In the FOURIER trial the reduction of cardiovascular death, myocardial infarction, or stroke was 16% during the first year but was 25% beyond 12 months. In the ODYSSEY trial the occurrence of cardiovascular events was similar in the alirocumab and placebo group during the first year of the study with benefits of alirocumab appearing after year one. Thus, the long-term benefits of treatment with a PCSK9 inhibitor may be greater than that observed during these relatively short-term studies.

 

GLAGOV TRIAL

 

While not an outcome trial the GLAGOV trial provides further support for the benefits of further lowering of LDL-C levels with a PCSK9 inhibitor added to statin therapy (206). This trial was a double-blind, placebo-controlled, randomized trial of evolocumab vs. placebo in 968 patients presenting for coronary angiography. The primary efficacy measure was the change in percent atheroma volume (PAV) from baseline to week 78, measured by serial intravascular ultrasonography (IVUS) imaging. Secondary efficacy measures included change in normalized total atheroma volume (TAV) and percentage of patients demonstrating plaque regression. As expected, there was a marked decrease in LDL-C levels in the evolocumab group (Placebo 93mg/dL vs. evolocumab 37mg/dL; p<0.001). PAV increased 0.05% with placebo and decreased 0.95% with evolocumab (P < .001) while TAV decreased 0.9 mm3 with placebo and 5.8 mm3 with evolocumab (P < .001). There was a linear relationship between achieved LDL-C and change in PAV (i.e., the lower the LDL-C the greater the regression in atheroma volume down to an LDL-C of 20mg/dL). Additionally, evolocumab induced plaque regression in a greater percentage of patients than placebo (64.3% vs 47.3%; P < .001 for PAV and 61.5% vs 48.9%; P < .001 for TAV). These results demonstrate the anti-atherogenic effects of PCSK9 inhibitors. Other trials in different patient populations have also shown that treatment with PCSK9 inhibitors are anti-atherogenic (207,208). 

 

VENOUS THROMBOEMBOLISM

 

In the FOURIER trial treatment with evolocumab resulted in a reduction in venous thromboembolism (VTE) (HR 0.71; 95% CI, 0.50-1.00; P=0.05) (209). Interestingly no effect was observed in the 1st year (HR, 0.96; 95% CI, 0.57-1.62) but a 46% reduction in VTE (HR, 0.54; 95% CI, 0.33-0.88; P=0.014) beyond 1 year occurred. In patients with low baseline Lp(a) levels, evolocumab reduced Lp(a) by only 7 nmol/L and had no effect on VTE risk but in patients with high baseline Lp(a) levels, evolocumab reduced Lp(a) by 33 nmol/L and risk of VTE by 48% (HR, 0.52; 95% CI, 0.30-0.89; P=0.017). In the ODYSSEY OUTCOMES trial, the risk of VTE was reduced but just missed being statistically significant (HR, 0.67; 95% CI, 0.44-1.01; P=0.06) (210). A meta-analysis of FOURIER and ODYSSEY OUTCOMES demonstrated a 31% relative risk reduction in VTE with PCSK9 inhibition (HR, 0.69; 95% CI, 0.53-0.90; P=0.007) (209).

 

Side Effects

 

The major side effect of PCSK9 monoclonal antibodies has been injection site reactions including erythema, itching, swelling, pain, and tenderness. Allergic reactions have been reported and as with any protein there is potential immunogenicity. In general side effects have been minimal, which is not surprising, as monoclonal antibodies do not typically have off target side effects. Since PCSK9 does not appear to have important functions other than regulating LDL receptor degradation, it is not surprising that inhibiting PCSK9 function has not resulted in major side effects.

 

A meta-analysis of 20 randomized controlled trials with 68,123 subjects found a very modest effect on fasting glucose (mean difference 1.88 mg/dL) and A1c levels (mean difference 0.032%) and did not observe an increased risk of developing diabetes (211). It should be recognized that the duration of these trials was relatively short (median follow-up 78 weeks) and therefore further long-term studies are required.

 

In the large outcome trials (ODYSSEY and FOURIER) there was no significant difference between the PCSK9 treated group vs. the placebo group with regard to adverse events (including new-onset diabetes and neurocognitive events). The only exception was the expected increase in injection-site reactions in the patients treated with a PCSK9 inhibitor. Additionally, in a subgroup of patients from the FOURIER trial a prospective study of cognitive function (EBBINGHAUS Study) was carried out and no significant differences in cognitive function was observed over a median of 19 months in the PCSK9 treated vs. placebo group (212). It should be recognized that while short-term treatment with PCSK9 inhibitors have not demonstrated any significant side effects it is possible that long-term use could lead to unexpected side-effects.

 

An issue of concern is whether lowering LDL-C to very low levels has the potential to cause toxicity. In a number of the PCSK9 studies a significant number of patients had LDL-C levels < 25mg/dL. For example, in the Odyssey long term study 37% of patients on alirocumab had two consecutive LDL-C levels below 25mg/dL and in the Osler long term study in patients treated with evolocumab 13% had values below 25mg/dL (213,214). In these short term PCSK9 studies, toxicity from very low LDL-C levels has not been observed. Additionally, in patients with Familial Hypobetalipoproteinemia LDL levels can be very low and these patients do not have any major disorders other than hepatic steatosis, which is not mechanistically due to low LDL-C levels (215). Similarly, there are rare individuals who are homozygous for loss of function mutations in the PCSK9 gene and they also do not appear to have major medical issues (168). Finally, in a number of statin trials there have been patients with very low LDL-C levels and an increased risk of side effects has not been consistently observed in those patients (216-218). Thus, with the limited data available there does not appear to be a major risk of markedly lowering LDL-C levels.   

 

Contraindications

 

Other than a history of a hypersensitivity to these drugs there are currently no contraindications. There are no studies during pregnancy or lactation.

 

Summary

 

PCSK9 monoclonal antibodies robustly reduce LDL-C levels when used as monotherapy, in combination with statins, or when added to the combination of statins + ezetimibe. In distinction to most other cholesterol lowering drugs the PCSK9 inhibitors also decrease Lp(a) levels. Outcome studies have clearly demonstrated that decreasing LDL-C levels with PCSK9 inhibitors reduces cardiovascular events. The side effect profile appears to be very favorable and there are no drug-drug interactions. The major limitation is the high expense of these drugs, which has limited their widespread use.

 

INCLISIRAN (LEQVIO)

 

Introduction

 

Inclisiran (Leqvio) is a double-stranded, siRNA (small interfering RNA) conjugated on the sense strand with triantennary N-acetylgalactosamine (GalNAc) to facilitate uptake into hepatocytes (219). In hepatocytes, inclisiran stimulates the catalytic breakdown of PCSK9 mRNA thereby reducing the hepatic synthesis of PCSK9 and markedly decreasing plasma PCSK9 levels (219,220). The recommended dose of inclisiran is 284 mg by subcutaneous injection, followed with a repeat injection at 3 months, and then every 6 months (package insert). If a dose is missed by more than 3 months it is recommended to repeat the dosage schedule described above (package insert). It is recommended that inclisiran be administered by a healthcare professional.

 

Effect on Inclisiran on Lipid and Lipoprotein Levels

 

There have been several large trials examining the efficacy of inclisiran. The ORION-10 trial was conducted in the United States and included adults with atherosclerotic cardiovascular disease on a maximally tolerated statin with an LDL-C > 70mg/dL (220). Patients were randomized to inclisiran 284mg (n=781) at initial visit, 3 months, 9 months, and 15 months or placebo (n=780) and followed for 540 days. After 3 months the LDL-C was reduced by approximately 50% and this reduction was sustained throughout the duration of the trial (at 540 days the LDL-c was reduced by 52.3% (P<0.001)). As expected, total cholesterol (-33%), non-HDL-C (-47%), and apolipoprotein B (-43%) were also decreased. Additionally, triglyceride (-13%) and Lp(a) (-26%) levels were decreased while HDL-C levels (+5.1%) and hsCRP (+8.8%) were slightly increased. ORION-11 was a very similar trial with an identical protocol conducted in Europe and South Africa and included adults with ASCVD or an ASCVD risk equivalent on maximally tolerated statin therapy (inclisiran n=810 and placebo n=807) (220). At 540 days LDL-C was reduced by 49.9% (P<0.001). Changes in other lipid parameters were similar to those observed in ORION 10. Subgroup analysis revealed that in both the ORION 10 and 11 trials that all subgroups had a similar reduction in LDL-C levels with inclisiran therapy including subjects with diabetes, moderate renal impairment, and greater than 75 years of age (220). Statin therapy and whether statin therapy was moderate intensity or high intensity also did not affect the reduction in LDL-C (220). Additionally, in patients with renal disease, including individuals with an estimated creatinine clearance between 15-29 mL/min, the reduction in LDL-C levels with inclisiran administration were similar to individuals with normal renal function (221). The decrease in LDL-C with inclisiran treatment has been shown to persist for 4 years (222).

 

HETEROZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA

 

The effect of inclisiran on LDL-C levels was determined in patients with heterozygous familial hypercholesterolemia who were randomized to receive subcutaneous injections of inclisiran 284mg (n= 242) or placebo (n=240) on days 1, 90, 270, and 450 (223). The mean baseline LDL-C level was 153±54mg/dL and 90% of the patients were receiving statins with most on high intensity statins (75%). At day 510 LDL-C levels were reduced by 47.9% compared to placebo (P<0.001). The reduction in LDL-C was similar in all genotypes of familial hypercholesterolemia. Total cholesterol was reduced by 33%, non-HDL-C by 44%, Lp(a) by 17.2%, and triglycerides by 12%. HDL-C and hsCRP were not markedly altered.

 

HOMOZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA

 

A small study reported that inclisiran treatment lowered LDL-C levels in 3 of 4 patients with homozygous familiar hypercholesterolemia  (17.5% to 37% decrease) but less than that seen in individuals with fully functioning LDL receptors (224). A larger more recent trial failed to demonstrate a decrease in LDL-C levels with inclisiran treatment (225). Of note there was considerable variation in the LDL-C response, which could be due to differences in genetic variants. Individuals with null-null LDL receptor variants (i.e. no functioning LDL receptors) are unlikely to respond to inclisiran due to the absence of LDL receptors and the group treated with inclisiran in this study was enriched in patients with this genotype, which could explain the absence of a significant reduction in LDL-C.     

 

Mechanisms Accounting for Inclisiran Induced Lipid Effects

 

The mechanism of action of inclisiran is the same as for PCSK9 monoclonal antibodies (219). Briefly, decreasing the production of PCSK9 in the liver, the primary source of circulating PCSK9, leads to a decrease in plasma PCSK9 levels resulting in a decrease in LDL receptor degradation (219). An increase in the number of hepatic LDL receptors increases the clearance of LDL leading to a decrease in LDL-C levels (219).  

 

Pharmacokinetics and Drug Interactions

 

There are no drug interactions. The reduction in LDL-C occurs within 14 days after drug administration and persists for an extended period of time allowing for administration every 6 months.

 

Effect of Inclisiran on Clinical Outcomes

 

No outcome studies are currently available. A cardiovascular outcome study (ORION-4) is ongoing and includes 15,000 patients with established ASCVD. The trial duration is five years and completion is expected in 2024 (NCT03705234) (ClinicalTrials.gov, 2020a).

 

Side Effects

 

The only adverse reactions associated with inclisiran were injection site reactions including rash, pain, and erythema (220). In an analysis of 7 studies with 3,576 patients treated with inclisiran for up to 6 years and 1,968 patients treated with placebo for up to 1.5 years, hepatic, muscle, and kidney events; incident diabetes; and elevations of creatine kinase or creatinine were not increased in patients treated with inclisiran (226).

 

Contraindications

 

In patients with severe hepatic or renal impairment inclisiran should be used with caution as there is limited data and experience in these patients. There are no studies during pregnancy or lactation.

 

Summary

 

Inclisiran very effectively lowers LDL-C levels. The major advantage of this drug compared to PCSK9 monoclonal antibodies is the ability to administer inclisiran every 6 months, which may improve compliance.

 

BEMPEDOIC ACID (NEXLETOL)

 

Introduction

 

Bempedoic acid was approved in the US in February 2020 and is an adenosine triphosphate-citrate lyase (ACL) inhibitor. It is administered orally once daily with or without food at a dose of 180mg (Nexletol). It is also available as a combination tablet containing 180 mg of bempedoic acid and 10 mg of ezetimibe (Nexlizet).

 

Effect on Bempedoic on Lipid and Lipoprotein Levels

 

EFFECT WITHOUT STATINS

 

In a study that randomized 345 patients with hypercholesterolemia (LDL-C 158mg/dL) and a history of intolerance to statin to either bempedoic acid or placebo (2:1), bempedoic acid decreased LDL-C by 21.4%, non-HDL-C by 17.9%, and apolipoprotein B by 15% (227). One third of patients were on background non-statin therapy most commonly ezetimibe and fish oil. Triglyceride levels were not altered but there was a small decrease in HDL-C levels that was statistically significant (-4.5%).

 

IN COMBINATION WITH STATINS

 

There have been two large trials that determined the effect of adding bempedoic acid to statin therapy. In a study that randomized 779 patients on maximally tolerated statin therapy +/- ezetimibe (only a small number on ezetimibe) with an LDL-C level greater than 70mg/dL (baseline LDL-C 120mg/dL) to either bempedoic acid or placebo it was observed that bempedoic acid decreased LDL-C levels by 17.4% compared to placebo (p<0.001) (228). In addition, non-HDL-C and apolipoprotein B levels were decreased by 13% compared to placebo while there was no significant change in triglyceride levels. Bempedoic acid decreased HDL-C levels by approximately 6%. In a similar study, patients with atherosclerotic cardiovascular disease, heterozygous familial hypercholesterolemia, or both with an LDL-C level greater than 70 mg/dL (baseline LDL-C 103mg/dL) while on maximally tolerated statin therapy with or without additional lipid-lowering therapy (only a small number on ezetimibe) were randomized to bempedoic acid (n= 1,488) or placebo (n= 742) (229). Compared to placebo, treatment with bempedoic acid decreased LDL-C by 18.1%, non-HDL-C by 13.5%, and apolipoprotein B by 11.9%. Triglyceride levels were unchanged but HDL-C decreased by 5.92%. Of note in both of the above studies the decrease in LDL-C was maintained over 52 weeks.

 

Notably, the addition of bempedoic acid to atorvastatin 80mg per day was still capable of significantly decreasing LDL-C (22%), non-HDL-C (13%), and apolipoprotein B (-15%) compared to placebo (230). The addition of bempedoic acid to high dose atorvastatin therapy did not cause meaningful changes in atorvastatin pharmacokinetics.   

 

IN COMBINATION WITH EZETIMIBE

 

Patients on maximally tolerated statin therapy with LDL-C levels greater 100 mg/dL if they had cardiovascular disease and/or Familiar Hypercholesterolemia or greater than 130 mg/dL if they had multiple CVD risk factors were randomized to bempedoic acid + ezetimibe, bempedoic acid alone, ezetimibe alone, or placebo (231). The key results of this study are shown in Table 14. Changes from baseline in HDL-C and triglyceride level were modest (<10%) in all treatment groups. In another study patients with a history of statin intolerance on ezetimibe therapy were randomized to bempedoic acid (n=181) or placebo (n= 88) (232). Compared to placebo, bempedoic acid decreased LDL-C by 28.5%, non-HDL-C by -23.6%, and apolipoprotein B by -19.3%. As seen in other studies bempedoic acid did not alter triglyceride levels but slightly decreased HDL-C levels (approximately 6% decrease compared to placebo).

 

Table 14. Effect of Bempedoic Acid and Ezetimibe on Lipid Parameters (231)

 

LDL-C

Non-HDL-C

Apo B

hsCRP

Bempedoic acid + ezetimibe

-38%

-33.7%

-30.1

-35.1

Bempedoic acid

-19%

-15.9%

-17.3

-31.9

Ezetimibe

-25%

-21.7

-20.8

-8.2

 Results are percent decrease compared to the placebo group.

 

Summary

 

Bempedoic acid typically lowers LDL-C by 15-25%, non-HDL-C by 10-20%, and apolipoprotein B levels by 10-20% with no significant effects on triglyceride levels. HDL-C levels decrease by 5-8% and Lp(a) are unchanged (233).

 

Table 15. Effect of Bempedoic Acid on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

No change

HDL-C

Small decrease

Lp(a)

No change

 

Non-Lipid Effects of Bempedoic Acid

 

Bempedoic acid decreases hsCRP levels (see table 14, 16).

 

Table 16. Effect of Bempedoic Acid on hsCRP Levels

Reference

Percent decrease in hsCRP

(227)

-24.3

(228)

-8.7

(229)

-21.5

(230)

-44

(232)

-31

 

In the CLEAR Outcome study with a median follow-up of 3.4 years there was no difference in the development of new onset diabetes in the bempedoic acid and placebo groups (429 of 3848, -11·1% with bempedoic acid vs 433 of 3749, 11·5% with placebo; HR 0.95; 95% CI 0.83-1.09) (234). Additionally, during the study HbA1c concentrations and fasting glucose levels were similar between the bempedoic acid and placebo groups in patients who had either prediabetes or normoglycemia. In the CLEAR Outcome study in patients with diabetes the prevalence of worsening diabetes was similar in the bempedoic acid and placebo group (235,236)   

 

Mechanisms Accounting for Bempedoic Acid Induced Lipid Effects

 

Bempedoic acid is a potent inhibitor of ATP-citrate lyase, which catalyzes the formation of acetyl-CoA in the cytoplasm (237). Acetyl-CoA is a precursor for the synthesis of cholesterol (figure 5). The inhibition of ATP-citrate lyase by bempedoic acid decreases cholesterol synthesis in liver reducing hepatic intracellular cholesterol levels (237). Of note, bempedoic acid is a pro-drug and conversion to its CoA-derivative by very-long-chain acyl-CoA synthetase-1 is required for inhibition of cholesterol synthesis (237). Very-long-chain acyl-CoA synthetase-1 is highly expressed in the liver but is not expressed in adipose tissue, kidney, intestine or skeletal muscle (237). The inability of bempedoic acid to be activated in muscle and inhibit cholesterol synthesis suggests that bempedoic acid is unlikely result in muscle toxicity.

 

Figure 5. Inhibition of Cholesterol Synthesis by Bempedoic Acid.

 

The decrease in plasma LDL-C levels in patients treated with bempedoic acid is primarily due to an increase in hepatic LDL receptors secondary to the inhibition of cholesterol synthesis resulting in a reduction in hepatic cholesterol levels (237). It should be noted that bempedoic acid also decreases circulating LDL-C levels in LDL receptor deficient mice and LDL receptor deficient miniature pigs indicating that mechanisms in addition to up-regulation of hepatic LDL receptors may contribute to the decrease in LDL-C levels (237). The inhibition of hepatic cholesterol synthesis may decrease the production and secretion of VLDL, which could contribute to a decrease in LDL-C.

 

Pharmacokinetics and Drug Interactions

 

No dose adjustments are required in patients with mild or moderate renal or hepatic impairment or in the elderly (package insert). Concomitant use of bempedoic acid with simvastatin or pravastatin causes an increase in the concentrations of these drugs and therefore may increase the risk of myopathy (package insert). This drug interaction may be secondary to bempedoic acid inhibiting organic anion-transporting polypeptide OATP1B1. It is recommended to avoid concomitant use of bempedoic acid with simvastatin greater than 20 mg/day or pravastatin 40mg/day. While concomitant administration of bempedoic acid with atorvastatin or rosuvastatin elevated the area under the curve by 1.7-fold these elevations were generally within the individual statin exposures and do not impact dosing recommendations (package insert).

 

Effect of Bempedoic Acid on Clinical Outcomes

 

In animal models of atherosclerosis, treatment with bempedoic acid had favorable effects on atherosclerosis (237). Moreover, genetic variants of ATP citrate lyase that lower LDL-C levels are associated with a decrease in cardiovascular disease suggesting that bempedoic acid will have favorable effects on reducing the risk of cardiovascular disease (238).

 

The CLEAR Outcome trial was a double-blind, randomized, placebo-controlled trial involving patients with cardiovascular disease or at high risk of cardiovascular disease who were unable or unwilling to take statins ("statin-intolerant" patients) (239). The patients were randomized to bempedoic acid 180 mg (n= 6992) or placebo (n= 6978) and the median duration of follow-up was 40.6 months. As expected, LDL-C levels were decreased by 21% in the bempedoic group compared to placebo (29mg/dL difference). The primary endpoint, death from cardiovascular causes, nonfatal myocardial infarction, nonfatal stroke, or coronary revascularization, was reduced by 13% in the bempedoic acid group (HR 0.87; 95% CI 0.79 to 0.96; P = 0.004). Bempedoic acid also decreased fatal and non-fatal myocardial infarctions and coronary revascularization but had no significant effects on fatal or nonfatal stroke, death from cardiovascular causes, and death from any cause. In the patients who were at high risk for cardiovascular disease (primary prevention), 66% had diabetes, and the primary endpoint was reduced by 30% in the bempedoic acid group (HR 0.70; 95% CI, 0.55-0.89; P = .002) (235). In patients with diabetes with or without cardiovascular disease the primary endpoint was reduced by 17% in the bempedoic acid group (HR 0.83; 95% CI 0.72-0.95) (234). This study clearly demonstrates that treatment with bempedoic acid reduces the risk cardiovascular events.     

 

Side Effects

 

HYPERURICEMIA

 

In clinical trials, 26% of bempedoic acid-treated patients with normal baseline uric acid values experienced hyperuricemia one or more times versus 9.5% in the placebo group (package insert). In the CLEAR Outcomes trial elevated uric acid levels occurred in 10.9% of the patients on bempedoic acid compared to 5.6% taking the placebo (239). The increase in uric acid is due to bempedoic acid inhibiting renal tubular OAT2. The Increase in uric acid levels typically occurred within the first 4 weeks of treatment and persisted throughout treatment. After 12 weeks of treatment, the mean placebo-adjusted increase in uric acid compared to baseline was 0.8 mg/dL for patients treated with bempedoic acid (package insert). Elevations in blood uric acid levels may lead to the development of gout. Gout was reported in 1.5% of patients treated with bempedoic acid vs. 0.4% of patients treated with placebo. The risk for gout attacks were higher in patients with a prior history of gout (11.2% for bempedoic acid treatment vs. 1.7% in the placebo group) (package insert). In patients with no prior history of gout only 1% of patients treated with bempedoic acid and 0.3% of the placebo group had a gouty attack (package insert). In the CLEAR Outcomes trial gout was increased in the bempedoic acid group (3.1% vs. 2.1%) (239).

 

TENDON RUPTURE

 

In clinical trials tendon rupture occurred in 0.5% of patients treated with bempedoic acid vs. 0% of placebo treated patients and involved the rotator cuff (the shoulder), biceps tendon, or Achilles tendon (package insert). Tendon rupture occurred within weeks to months of starting bempedoic acid and occurred more frequently in patients over 60 years of age, in those taking corticosteroid or fluoroquinolone drugs, in patients with renal failure, and in patients with previous tendon disorders. In the CLEAR Outcomes trial tendon rupture was similar in the bempedoic acid and placebo group (bempedoic acid 1.2% and placebo 0.9%) (239).

 

RENAL FUNCTION

 

Bempedoic acid treatment resulted in a mean increase in serum creatinine of 0.05 mg/dL compared to baseline. Approximately 3.8% of patients treated with bempedoic acid had BUN levels that doubled vs. 1.5% in the placebo group and about 2.2% of patients treated with bempedoic acid had creatinine values that increased by 0.5 mg/dL vs. 1.1% in the placebo group (package insert). Renal function returned to baseline when bempedoic acid was discontinued. In the CLEAR Outcomes trial renal impairment was increased in the bempedoic acid group (11.5% vs.8.6%) as was the change from baseline creatinine (0.05±0.2 mg/dL vs. 0.01±0.2 mg/dL)  (239).

 

CHOLELITHIASIS

 

In the CLEAR Outcomes trial cholelithiasis was increased in the bempedoic acid group (2.2 vs 1.2) (239).

 

BENIGN PROSTATIC HYPERPLASIA

 

Bempedoic acid was associated with an increased risk of benign prostatic hyperplasia (BPH) in men with no reported history of BPH, occurring in 1.3% of NEXLETOL-treated patients versus 0.1% of placebo-treated patients (package insert).

 

MISCELLANEOUS LABORATORY ABNORMALITIES

 

Approximately 5.1% of patients on bempedoic acid vs. 2.3% on placebo had decreases in hemoglobin levels of 2 or more g/dL and below the lower limit of normal on one or more occasion. Anemia was reported in 2.8% of patients treated with bempedoic acid and 1.9% of patients treated with placebo. Hemoglobin decrease was generally asymptomatic and did not require medical intervention (package insert).

 

Approximately 9.0% of bempedoic acid treated patients with a normal baseline leukocyte count decreased leukocyte count to less than the lower limit of normal on one or more occasions vs. 6.7% in the placebo group. The leukocyte decrease was generally asymptomatic and did not require medical intervention (package insert).

 

Approximately 10.1% of bempedoic acid treated patients vs. 4.7% in the placebo group had

increases in platelet counts of 100× 109/L or more on one or more occasion. The platelet count increase was asymptomatic, did not result in an increased risk for thromboembolic events, and did not require medical intervention (package insert).

 

Increases to more than 3× the upper limit of normal (ULN) in AST occurred in 1.4% of patients treated with bempedoic acid vs. 0.4% of placebo patients, and increases to more than 5× ULN occurred in 0.4% of bempedoic acid treated patients vs. 0.2% of placebo-treated patients. Increases in ALT were similar in bempedoic acid treated patients and placebo-treated patients. Elevations in transaminases were generally asymptomatic and not associated with elevations ≥2× ULN in bilirubin or with cholestasis. In most cases, the elevations were transient and resolved or improved with continued therapy or after discontinuation of therapy (package insert).

 

Contraindications

 

The use of bempedoic acid during pregnancy and lactation has not been studied (package insert).

 

Summary

 

In patients on statins and ezetimibe with an LDL-C that is not at goal the addition of bempedoic acid is a reasonable third drug. In addition, in patients that cannot tolerate statin therapy the combination of ezetimibe and bempedoic acid may allow for the lowering of LDL-C to goal. One can expect a reduction in LDL-C of approximately 15-25% with bempedoic acid monotherapy therapy or when used in combination with other LDL-C lowering drugs.

 

LOMITAPID (JUXTAPID)

 

Introduction

 

Lomitapide (Juxtapid), a selective microsomal triglyceride transfer protein inhibitor, was approved in December 2012 for lowering LDL-C levels in adults with Homozygous Familial Hypercholesterolemia (240-242). As will be discussed below it lowers LDL-C levels by an LDL receptor independent mechanism.

 

Effect on Lomitapide on Lipid and Lipoprotein Levels

 

The effect of lomitapide on lipid and lipoprotein levels has been studied in patients with Homozygous Familial Hypercholesterolemia. The pivotal study was a 78-week single arm open label study in 29 patients receiving treatment for Homozygous Familial Hypercholesterolemia (243). Lomitapide was initiated at 5mg per day and was up-titrated to 60mg per day based on tolerability and liver function tests. On an intention to treat basis, LDL-C was decreased by 40% and apolipoprotein B by 39%. In patients who were actually taking lomitapide, LDL-C levels were reduced by 50%. In addition to decreasing LDL-C levels, non-HDL-C levels were decreased by 50%, Lp(a) by 15%, and triglycerides by 45%. Interestingly HDL and apolipoprotein A-I levels were decreased by 12% and 14% respectively in this study. Follow-up revealed that the decrease in LDL-C could be sustained for a prolonged period of time (294 weeks) (244).

 

The effect of lomitapide has also been studied in patients without Homozygous Familial Hypercholesterolemia. A study by Samaha and colleagues compared the effect of ezetimibe and lomitapide in patients with elevated cholesterol levels(245). Patients were treated with ezetimibe alone, lomitapide alone, or the combination of ezetimibe and lomitapide. Ezetimibe monotherapy led to a 20–22% decrease in LDL-C levels, lomitapide monotherapy led to a dose dependent decrease in LDL-cholesterol levels (19% at 5.0 mg, 26% at 7.5 mg and 30% at 10 mg). Combined therapy produced a larger dose-dependent decrease in LDL-C levels (35%, 38% and 46%, respectively).  Additionally, lomitapide decreased triglycerides by 10%, non-HDL-C by 27%, apolipoprotein B by 24%, and Lp(a) by 17%.

 

The above studies demonstrate that lomitapide decreases LDL-C, non-HDL-C, triglycerides, and Lp(a) levels.

 

Mechanism Accounting for the Lomitapide Induced Lipid Effects

 

Lomitapide is a selective inhibitor of microsomal triglyceride transfer protein (MTTP) (240-242). MTTP is located in the endoplasmic reticulum of hepatocytes and enterocytes where it plays a key role in transferring triglycerides onto newly synthesized apolipoprotein B leading to the formation of VLDL and chylomicrons (246). Loss of function mutations in both alleles of MTTP results in abetalipoproteinemia, which is characterized by the virtual absence of apolipoprotein B, VLDL, chylomicrons, and LDL in the plasma due to the failure of the liver and intestine to produce VLDL and chylomicrons (215). Lomitapide by inhibiting MTTP activity reduces the secretion of chylomicrons by the intestine and VLDL by the liver leading to a decrease in LDL, apolipoprotein B, triglycerides, non-HDL-C, and Lp(a) (240-242). 

 

Pharmacokinetics and Drug Interactions

 

Lomitapide is extensively metabolized in the liver by the CYP3A4 pathway (240,241). Therefore, lomitapide is contraindicated in patients on strong CYP3A4 inhibitors and lower doses should be used in patients on weak inhibitors. Of particular note, in patients on atorvastatin the maximal dose of lomitapide is 30mg per day and lomitapide should not be used in patients taking more than 20mg of simvastatin (240,241). Lomitapide can increase warfarin levels and therefore close monitoring is required. Finally, given the risk of liver abnormalities (see side effect section) the avoidance of alcohol or a reduction in alcohol intake is prudent.

 

Effect of Lomitapide on Clinical Outcomes

 

There are no clinical outcome trials but it is presumed that lowering LDL-C levels in patients with Homozygous Familial Hypercholesterolemia will reduce cardiovascular events. After initiating lomitapide therapy 1.7 cardiovascular events per 1000 patient months on treatment was observed vs. 26.1 cardiovascular events per 1000 patient months in a comparison cohort (247).

 

Side Effects

 

As expected from its mechanism of action lomitapide causes side effects in the GI tract and liver. In the GI tract diarrhea, nausea, vomiting, and dyspepsia occur very commonly (240-242). In the pivotal study in patients with Homozygous Familial Hypercholesterolemia, 90% of the patients developed GI symptoms during drug titration (243). GI side effects are potentiated by high fat meals and it is therefore recommended that dietary fat be limited. Approximately 10% of patients will discontinue lomitapide, mostly from diarrhea. Lomitapide also reduces the absorption of fat soluble vitamins and therefore patients need to take vitamin supplements (240,241). Additionally, it may also block the absorption of essential fatty acids and it is therefore recommended that supplements of essential fatty acids also be provided (at least 200 mg linoleic acid, 210 mg alpha-linolenic acid (ALA), 110 mg eicosapentaenoic acid (EPA), and 80 mg docosahexaenoic acid (DHA) (240,241).

 

Blocking the formation of VLDL in the liver can lead to fatty liver with elevated liver enzymes (240-242). Approximately 30% of patients will develop increased transaminase levels but in the small number of patients studied this has not resulted in liver failure. After stopping the drug, the transaminases have returned to normal. Whether long term treatment with lomitapide will lead to an increase in liver disease is unknown. There is a single case of a patient with lipoprotein lipase deficiency who was treated for 13 years with lomitapide who developed steatohepatitis and fibrosis (248). In an observational study of a small number of patients on lomitapide for > 5 years liver failure or cirrhosis was not noted (249). In another study in Italy, 34 patients were treated with lomitapide for more than 9 years and elevations in hepatic fat were mild-to-moderate, hepatic stiffness remained normal, and the mean FIB-4 score remained below the fibrosis threshold (250). The studies suggest that in most patients’ severe liver disease will not develop. To reduce the risk of liver dysfunction it is important that patients avoid or limit alcohol intake and avoid drugs that inhibit Cyp3A4 activity.

 

Because of the high potential risk of serious complications the FDA has mandated several measures to ensure that patients are closely followed and monitored for liver toxicity ((Risk Evaluation and Mitigation Strategy (REMS) Program) (240,241). ALT, AST, alkaline phosphatase, and total bilirubin should be measured before initiating treatment. During the first year, liver function tests should be measured prior to each increase in dose or monthly, whichever occurs first. After the first year, liver function tests should be measured at least every 3 months and before any increase in dose.

 

Contraindications

 

Lomitapide should not be used during pregnancy and in patients with moderate or severe liver disease. In addition, it should not be used in patients on strong CYP3A4 inhibitors.

 

Summary

 

Lomitapide is approved only for the treatment of lipid disorders in patients with Homozygous Familiar Hypercholesterolemia. The frequent GI side effects and the potential risk of serious liver disease greatly limit the use of this drug and it should be reserved for the patients in which more benign therapies are not sufficient in lowering LDL-C into a reasonable range. It is used as an adjunct to other lipid lowering therapies and lipoprotein apheresis in patients with Homozygous Familiar Hypercholesterolemia.

 

MIPOMERSEN (KYNAMRO)

 

Introduction

 

Mipomersen (Kynamro) is a second generation apolipoprotein B antisense oligonucleotide that was approved in January 2013 for the treatment of patients with Homozygous Familiar Hypercholesterolemia (241,242,251). It is administered as a 200mg subcutaneous injection once a week (241,242,251). As will be discussed below, it lowers LDL-C levels by an LDL receptor independent mechanism. In May 2018 sales were discontinued due to safety concerns related to increased liver transaminases and fatty liver.

 

Effect on Mipomersen on Lipid and Lipoprotein Levels

 

In the pivotal trial, 51 patients with Homozygote Familial Hypercholesterolemia on treatment were randomized to additional treatment with mipomersen (n= 34) or placebo (n=17) and followed for 26 weeks (252). Mipomersen lowered LDL-C levels by 21% and apolipoprotein B levels by 24% compared to placebo. In addition, non-HDL-C was decreased by 21.6%, triglycerides by 17%, and Lp(a) by 23% while HDL and apolipoprotein A-I were increased by 11.2% and 3.9% respectively.

 

Mipomersen has also been studied in patients with Heterozygous Familial Hypercholesterolemia. In a double-blind, placebo-controlled, randomized trial, patients on maximally tolerated statin therapy were treated weekly with subcutaneous mipomersen 200 mg or placebo for 26 weeks (253). LDL-C levels decreased by 33% in the mipomersen group compared to placebo. Additionally, mipomersen significantly reduced apolipoprotein B by 26%, triglycerides by 14%, and Lp(a) by 21% compared to placebo with no significant changes in HDL-C levels. In an extension follow-up study the beneficial effects of mipomersen were maintained for at least 2 years (254). 

 

In a meta-analysis of 8 randomized studies with 462 subjects with either non-specified hypercholesterolemia or Heterozygous Familial Hypercholesterolemia, Panta and colleagues reported that mipomersen decreased LDL-C levels by 32% compared to placebo (255). Additionally, non-HDL-C was decreased by 31%, apolipoprotein B by 33%, triglycerides by 36%, and Lp(a) by 26% with no effect on HDL-C levels.

 

Mechanism Accounting for the Mipomersen Induced Lipid Effects

 

Apolipoprotein B 100 is the main structural protein of VLDL and LDL and is required for the formation of VLDL and LDL (191). Familiar Hypobetalipoproteinemia is a genetic disorder due to a mutation of one apolipoprotein B allele that is characterized by very low concentrations of LDL and apolipoprotein B due to the decreased production of lipoproteins by the liver (215). Mipomersen, an apolipoprotein B antisense oligonucleotide, mimics Familiar Hypobetalipoproteinemia by inhibiting apolipoprotein B 100 production in the liver by pairing with apolipoprotein B mRNA preventing its translation (241,242,251). This decrease in apolipoprotein B synthesis results in a decrease in hepatic VLDL production leading to a decrease in LDL levels.

 

Pharmacokinetics and Drug Interactions

 

No significant drug interactions have been reported. Given the risk of liver abnormalities (see side effect section) the avoidance of alcohol or a reduction in alcohol intake would be prudent.

 

Effect of Mipomersen on Clinical Outcomes

 

There are no clinical outcome trials but it is presumed that lowering LDL-C levels in patients with Homozygous Familial Hypercholesterolemia will reduce cardiovascular events. In a study comparing cardiovascular events in patients with Homozygous Familial Hypercholesterolemia in the 24 months prior to initiating mipomersen therapy and after initiating mipomersen revealed a decrease in events (prior to treatment 61.5% of patients had an event vs. 9.6% after initiating mipomersen; P < .0001) (256). In this trial mipomersen resulted in a mean absolute reduction in LDL-C of 70 mg/dL (-28%), non-HDL cholesterol of 74 mg/dL (-26%), and Lp(a) of 11 mg/dL (-17%).

 

Side Effects

 

The most common side effect is injection site reactions, which occur in 75-98% of patients and typically consist of one or more of the following: erythema, pain, tenderness, pruritus, and local swelling (241,242,251).  Additional, influenza like symptoms, which typically occur within 2 days after an injection, occur in 30-50% of patients and include one or more of the following: influenza-like illness, pyrexia, chills, myalgia, arthralgia, malaise or fatigue which result in a substantial percentage of patients discontinuing therapy (241,242,251).

 

A major safety concern is liver toxicity (241,242,251). By inhibiting VLDL formation and secretion the risk of fatty liver is increased. Fatty liver has been observed in 5-20% of patients treated with mipomersen (241,242,251). In 10-15% of patients treated with mipomersen increases in transaminases occur (241,242,251). Additionally, liver biopsies from 7 patients after a minimum of 6 months of mipomersen therapy have demonstrated the presence of fatty liver although there was no inflammation despite elevations in liver enzymes (257). Liver function should be measured prior to initiating therapy and monthly during the first year and every 3 months after the first year. Fortunately, when treatment is discontinued liver function tests and fatty liver return to normal.

 

Because of the potential for liver toxicity this drug is no longer available.

 

Contraindications

 

Mipomersen is contraindicated in patients in patients with liver disease or severe renal disease. Mipomersen is not recommended for use during pregnancy or lactation. In animal studies mipomersen has not resulted in fetal abnormalities.

 

Summary

 

Mipomersen was approved only for the treatment of lipid disorders in patients with Homozygous Familiar Hypercholesterolemia. The potential risk of serious liver disease greatly limits the use of this drug and therefore it was reserved for patients in which more benign therapies were not sufficient in lowering LDL-C into a reasonable range. It was used as an adjunct to other lipid lowering therapies in patients with Homozygous Familiar Hypercholesterolemia but because of safety concerns is no longer available.

 

EVINACUMAB (EVKEEZA)

 

Introduction

 

Evinacumab is a human monoclonal antibody against angiopoietin-like protein 3 (ANGPTL3). It is approved for the treatment of Homozygous Familial Hypercholesterolemia. Evinacumab decreases LDL-C levels by mechanisms independent of LDL receptor activity. The recommended dose of evinacumab is 15 mg/kg administered by intravenous infusion over 60 minutes every 4 weeks.

 

Effect on Evinacumab on Lipid and Lipoprotein Levels

 

HOMOZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA

 

A double-blind, placebo-controlled trial randomly treated patients with Homozygous Familial Hypercholesterolemia with an intravenous infusion of evinacumab 15 mg/Kg every 4 weeks (n= 43) or placebo (n= 22) (258). The individuals in this trial were on lipid lowering therapy (94% were on a statin with 77% on a high-intensity statin, 77% on a PCSK9 inhibitor, 75% on ezetimibe, 25% on lomitapide, and 34% undergoing apheresis) and the mean baseline LDL-C level was approximately 250-260mg/dL. After 24 weeks of treatment patients in the evinacumab group had a 47% reduction in LDL-C levels vs. a 1.9% increase in the placebo group (table 17). This decrease in LDL-C levels was observed after 2 weeks of therapy and was observed regardless of concomitant use of other lipid lowering drugs or apheresis. Notably, in individuals with null-null LDL receptor variants evinacumab resulted in a 43% decrease in LDL-C levels indicating that evinacumab therapy was effective in the absence of functional LDL receptors. As expected, total cholesterol, non-HDL-C cholesterol, and apo B levels were also decreased. Moreover, triglyceride levels decreased 55% and HDL-C levels decreased 30% with evinacumab administration while Lp(a) levels were unchanged.

 

Table 17. Effect of Evinacumab on Lipid Levels in Homozygous Familial Hypercholesterolemia

 

LDL-C

Apo B

Non-HDL-C

TG

HDL-C

Baseline mg/dL

255

171

278

124

44

Evinacumab % Change

−47%

−41%

−50%

−55%

−30%

Placebo  % Change

+2%

−5%

+2%

−5%

+1%

 

REFRACTORY HYPERCHOLESTEROLEMIA

 

In a double-blind, placebo-controlled trial, patients with refractory hypercholesterolemia with a screening LDL-C level > 70 mg/dL with atherosclerosis or LDL-C > 100 mg/dL without atherosclerosis were randomized to receive subcutaneous or intravenous evinacumab or placebo (259). The hypercholesterolemia was refractory to treatment with a PCSK9 inhibitor and a statin at a maximum tolerated dose, with or without ezetimibe. In this trial a number of different treatment regimens of evinacumab were employed (intravenous or subcutaneous; different doses) and in this summary only the results of intravenous evinacumab 15 mg/kg every 4 weeks (39 patients) vs. placebo (34 patients) will be presented. Baseline LDL-C levels were approximately 145mg/dL. After 16 weeks of treatment the LDL-C level was decreased by 50% with evinacumab administration vs. a 0.6% decrease with placebo. An extension of this trial for 72 weeks found that the reduction in LDL-C were sustained (260). The decrease in LDL-C was observed after 2 weeks of treatment. As expected, total cholesterol, non-HDL-C, and apo B levels also decreased in the evinacumab group. Evinacumab administration decreased triglyceride levels by 53% and HDL-C levels by 31%. In contrast to the results in the homozygous familiar hypercholesterolemia study described above in this study evinacumab decreased Lp(a) levels by 16%. The effect of the subcutaneous administration of evinacumab on lipid levels was similar to that observed with intravenous administration.

 

The effect of evinacumab on triglyceride levels in patients with marked hypertriglyceridemia is described in the Endotext chapter “Triglyceride Lowering Drugs” (261).

 

Mechanism Accounting for the Evinacumab Induced Lipid Effects

 

ANGPTL3 inhibits lipoprotein lipase (LPL) activity thereby slowing the clearance of VLDL and chylomicrons resulting in an increase in plasma triglyceride levels (262,263). Mice deficient in ANGPTL3 have lower plasma triglyceride levels while mice overexpressing ANGPTL3 have elevated plasma triglyceride levels (263). Evinacumab by inhibiting the ability of ANGPTL3 to inhibit LPL activity will accelerate the clearance of TG rich lipoproteins decreasing plasma triglyceride levels (263). Furthermore, ANGPTL3 has also been shown to reduce endothelial lipase activity (263). Endothelial lipase is a phospholipase that catabolizes phospholipids on HDL and accelerates HDL clearance (264,265). Evinacumab by inhibiting the ability of ANGPTL3 to inhibit endothelial lipase activity will lead to a decrease in HDL levels (266).

 

The mechanism(s) that explain the decrease in LDL-C levels with evinacumab administration is not completely understood. A study has demonstrated that the increase in endothelial lipase activity induced by evinacumab leads to VLDL remodeling and lipid depletion that increases VLDL clearance when the LDL receptor is absent (267). This decrease in VLDL, the precursor of LDL, limits LDL particle production resulting in a reduction in plasma LDL-C levels (267). Kinetic studies in four patients with homozygous familial hypercholesterolemia observed that evinacumab markedly increased the fractional catabolic rate of IDL (intermediate-density lipoprotein) and LDL apoB (268). Whether decreases in VLDL production also plays a role in the decrease in LDL-C levels with evinacumab treatment requires additional studies. It should be noted that inhibition of ANGPTL3 decreases LDL-C levels independent of LDL receptor activity (269).

 

Pharmacokinetics and Drug Interactions

 

There are no significant drug interactions.

 

Effect of Evinacumab on Clinical Outcomes

 

There are no cardiovascular outcome studies. In two patients with homozygous Familial Hypercholesterolemia evinacumab therapy markedly reduced LDL-C levels with a concomitant decrease in plaque volume determined by coronary computed tomography angiography (268).  

 

Homozygosity for loss-of-function mutations in ANGPTL3 is associated with significantly lower plasma levels of LDL-C, HDL-C, and triglycerides (familial combined hypolipidemia) (215,263,270). Heterozygous carriers of loss-of-function mutations in ANGPTL3, which occur at a frequency of about 1:300, have significantly lower total cholesterol, LDL-C, and triglyceride levels than noncarriers (263). Moreover, patients carrying loss-of-function variants in ANGPTL3 have a significantly lower risk of coronary artery disease (271,272). Additionally, in an animal model of atherosclerosis treatment with evinacumab decreased atherosclerotic lesion area and necrotic content (271). Taken together these observations suggest that inhibiting ANGPTL3 with evinacumab will reduce cardiovascular disease.

 

Side Effects

 

Serious hypersensitivity reactions have occurred with evinacumab. In clinical trials, 1 (1%) of evinacumab treated patients experienced anaphylaxis vs. 0% of patients who received placebo (package insert).

 

Contraindications

 

Based on animal studies, evinacumab may cause fetal harm when administered to pregnant patients (package insert). Patients should be advised of the potential risks to the fetus of pregnancy. Patients who may become pregnant should be advised to use effective contraception during treatment with evinacumab and for at least 5 months following the last dose.

 

Summary

 

In patients with Homozygous Familiar Hypercholesterolemia the ability of evinacumab to lower LDL-C levels independent of LDL receptor activity makes this agent very useful in these patients. Most patients with Homozygous Familial Hypercholesterolemia do not achieve goal LDL-C levels with triple drug therapy with maximally tolerated statin therapy, ezetimibe, and a PCSK9 inhibitor and therefore the addition of evinacumab will be needed in many of these patients. Evinacumab is also effective in patients with refractory hypercholesterolemia but the drug is not yet FDA approved in this situation. Nevertheless, one can foresee in patients with refractory hypercholesterolemia at high risk for cardiovascular events the use of evinacumab. In addition to lowering LDL-C levels evinacumab also lowers triglyceride levels and could be useful in selected patients with very severe hypertriglyceridemia (261,273).  

 

APPROACH TO TREATING PATIENTS WITH HYPERCHOLESTEROLEMIA

 

Introduction

 

The issues of deciding who to treat, how aggressive to treat, and the goals of therapy are discussed in detail in the chapter “Guidelines for the Management of High Blood Cholesterol” and therefore will not be addressed in this chapter (3). Additionally, the role of life style changes to lower LDL-C is discussed in great depth in chapter “The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels” and therefore will also not be addressed here (1). Rather we will focus on how to use the drugs discussed in this chapter to treat various categories of patients. The factors to consider when deciding which drugs are appropriate to use for lowering plasma LDL-C levels are; the efficacy in lowering LDL-C levels, the effect on other lipid and lipoprotein levels, the ability to reduce cardiovascular events, the side effects of drug therapy, the ease of complying with the drug regimen, and the cost of the drugs. Many statins and ezetimibe are generic drugs and therefore they are relatively inexpensive.

 

Isolated Hypercholesterolemia with Cardiovascular Disease

 

In patients with isolated hypercholesterolemia and cardiovascular disease, initial drug therapy should be high intensity statin therapy (atorvastatin 40-80mg or rosuvastatin 20-40mg). In patients with cardiovascular disease, one should aim to lower the LDL-C to below 70mg/dL. Many experts, based on studies comparing statin alone vs. statin + ezetimibe or statin + a PCSK9 inhibitor, would recommend a more aggressive LDL-C goal in high-risk patients (LDL-C <55mg/dL). If statin therapy alone is not sufficient adding ezetimibe, is a reasonable next step. Because a considerable amount of data indicates that the lower the LDL-C the greater the reduction in cardiovascular events many experts would use a combination of high intensity statin therapy plus ezetimibe in all high-risk patients to maximize LDL-C reduction. Ezetimibe is inexpensive, easy to take, has few side effects, will modestly lower LDL-C, and has been shown in combination with statins to further reduce cardiovascular events. High dose statin and ezetimibe will lower LDL-C by as much as 70%, which will lower LDL-C to goal in a large number of patients who do not have a genetic basis for their elevated LDL-C levels. If the combination of statin plus ezetimibe does not lower the LDL to goal one can add a third drug. If the LDL is close to goal, one could add a bile acid sequestrant such as colesevelam or bempedoic acid. If the LDL is not very close to goal one could instead use a statin +/- ezetimibe plus a PCSK9 inhibitor, which will result in marked reductions in LDL-C levels. If the patient has diabetes with a moderately elevated A1c level using a bile acid sequestrant such as colesevelam instead of ezetimibe or in combination with ezetimibe could improve both glycemic control and further lower LDL levels. If the cost of PCSK9 inhibitors decrease the earlier use of these drugs will become feasible.

 

Isolated Hypercholesterolemia in Primary Prevention

 

In patients with isolated hypercholesterolemia (LDL-C < 190mg/dL) without cardiovascular disease initial drug therapy is with a statin. The statin dose should be chosen based on the percent reduction in LDL-C required to lower the LDL-C level to below the target goal (typically < 100mg/dL but if multiple risk factors with a high risk for cardiovascular events is present many experts would aim for <70mg/dL). As discussed earlier, the side effects of statin therapy increase with higher doses so one should not automatically start with high doses, but instead should choose a dose balancing the benefits and risks. Generic statins are inexpensive drugs and are very effective in both lowering LDL-C levels and reducing cardiovascular events. Additionally, they have an excellent safety profile. If the initial statin dose does not lower LCL-C sufficiently, one can then increase the dose or add ezetimibe. If the maximal statin dose does not lower LDL-C sufficiently adding ezetimibe is a reasonable next step if the LDL-C level is in a reasonable range and an additional 20-25% reduction in LDL will be sufficient. High dose statin and ezetimibe will lower LDL-C by as much as 70%, which will lower LDL-C to goal in the majority of patients who do not have a genetic basis for their elevated LDL-C levels. If the combination of statin plus ezetimibe does not lower the LDL-C to goal one can add a third drug, such as bempedoic acid or colesevelam. If the patient has diabetes with a moderately elevated A1c level using colesevelam instead of ezetimibe or in combination with ezetimibe could improve both glycemic control and further lower LDL-C levels.

 

Mixed Hyperlipidemia

 

In patients with mixed hyperlipidemia (elevated LDL-C and triglyceride levels) Initial drug therapy should also be a generic statin unless triglyceride levels are greater than 500-1000mg/dL. If triglycerides are > 500-1000mg/dL initial therapy is directed at lowering triglyceride levels (261). In addition to lowering LDL-C levels, statins are also effective in lowering triglyceride levels particularly when the triglycerides are elevated. If LDL-C is not lowered sufficiently ezetimibe is a reasonable next step. Bile acid sequestrants are not appropriate drugs in patients with hypertriglyceridemia. The approach to the patient whose LDL-C levels are at goal but the triglycerides and non-HDL-C are still elevated is discussed in the chapter on triglyceride lowering drugs (261).

 

Heterozygous Familial Hypercholesterolemia

 

In patients with Heterozygous Familial Hypercholesterolemia or other disorders with very elevated LDL-C levels (>190mg/dL), high doses of a potent statin such as atorvastatin 40-80mg or rosuvastatin 20-40mg are the first step to lower LDL-C levels. In many patients this will not be sufficient. If the LDL-C levels are above goal then adding ezetimibe is a reasonable next step. If after ezetimibe the LDL-C is still slightly above goal triple drug therapy with bempedoic acid or a bile acid sequestrant can be employed. If on statin alone or with the combination of statin and ezetimibe the LDL-C still needs to be markedly reduced a PCSK9 inhibitor may be a better choice as these drugs can markedly lower LDL-C levels.

 

Homozygous Familiar Hypercholesterolemia

 

In patients with Homozygous Familiar Hypercholesterolemia initial therapy with a maximally tolerated statin and ezetimibe can be instituted. This will likely not result in an acceptable LDL-C level and then one can add a PCSK9 inhibitor. Because these therapies depend on LDL receptor activity to lower LDL-C a high percentage of patients will not reach goal and then one can add lomitapide and/or evinacumab, drugs that lower LDL-C levels independent of LDL receptor activity. Because side effects are fewer with evinacumab this is the preferred initial drug in most patients. Studies have shown that with the addition of evinacumab many patients will reach acceptable LDL-C levels. If LDL-C levels are still not acceptable one could then initiate lipoprotein apheresis (274).  

 

Statin Intolerance

 

Statin intolerance is frequently due to myalgias but on occasion can be due other issues, such as increased liver or muscle enzymes, cognitive dysfunction, or other neurological disorders. The percentage of patients who are “statin intolerant” varies greatly but in clinical practice a significant number of patients have difficulty taking statins.

 

As discussed earlier it can be difficult to determine if the muscle symptoms that occur when a patient is taking a statin are actually due to the statin or are unrelated to statin use. The first step in a “statin intolerant patient” is to take a careful history of the nature and location of the muscle symptoms and the timing of onset in relation to statin use to determine whether the presentation fits the typical picture for statin induced myalgias. The characteristic findings with a statin induced myalgia are shown in table 18 and findings that are not typical for statin induced myalgia are shown in table 19. The disappearance of symptoms within a few weeks of stopping statins and the reappearance after restarting statins is very suggestive of the symptoms being due to true statin intolerance. An on-line tool (htpp://tools.acc.org/statinintolerance/#!/) and an app produced by the ACC/AHA are available. This tool characterizes patients based on 8 criteria into possible vs. unlikely to have statin induced muscle symptoms (table 20)

 

Table 18. Characteristic Findings with Statin Induced Myalgia

Symmetric

Proximal muscles

Muscle pain, tenderness, weakness, cramps

Symptom onset < 4 weeks after starting statin or dose increase

Improves within 2-4 weeks of stopping statin

Cramping is unilateral and involves small muscles of hands and feet

Same symptoms occur with re-challenge within 4 weeks

 

Table 19. Symptoms Atypical in Statin Induced Myalgia

Unilateral

Asymmetric

Small muscles

Joint or tendon pain

Shooting pain, muscle twitching or tingling

Symptom onset > 12 weeks

No improvement after discontinuing statin

 

Table 20. Diagnosis of Statin Associated Muscle Symptoms

Symptom timing

Symptom type

Symptom location

Sex

Age

Race/ethnicity

CK elevation > 5 times the upper limit of normal

Known risk factors for statin induced muscle symptoms and non-statin causes of muscle symptoms

 

One should also check a CK level but this is almost always in the normal range. If the CK is not elevated and the symptoms do not suggest a statin induced myalgia one can often reassure the patient and continue statin therapy. This is often successful and studies have shown that many patients that stop taking statins due to “statin induced myalgia” can be successfully treated with a statin. If the CK is elevated it should be repeated after instructing the patient to avoid exercise for 48 hours. Also, the CK levels should be compared to CK levels prior to starting therapy. If the CK remains elevated (3x upper limit of normal) the statin should be discontinued. Similarly, if the CK is normal but the symptoms are suggestive of a statin induced myalgia the statin should also be discontinued. The next step is to determine if one can identify reversible factors that could be increasing statin toxicity (hypothyroidism, drug interactions).  If none are identified the next step after the myalgias have resolved is to try a low dose of a different statin that is metabolized by a different pathway (for example instead of atorvastatin, which is metabolized by the CYP3A4 pathway, rosuvastatin, which has a different pathway of metabolism). Because statin side effects are dose related, a low dose of a statin may often be tolerated. One can also try several different statins as sometimes a patient may tolerate one statin and not others. A meta-analysis has shown that every other day administration of statins is as effective as daily administration in lowering lipid levels and therefore is a very reasonable strategy (275). In some instances, using a long-acting statin (rosuvastatin or atorvastatin) 1-3 times per week can work (we usually start with once per week and then slowly increase frequency as tolerated) (276). In these circumstances (low doses or 1-3 times per week) the reduction in LDL-C may not be sufficient but one can use combination therapy with other drugs such as ezetimibe, bempedoic acid, bile acid sequestrants, or PCSK9 inhibitors to achieve LDL target goals.

 

Many providers have combined Coenzyme Q10 with statins to prevent statin induced myalgias. However, randomized trials with Coenzyme Q10 supplementation have not consistently shown benefit (277-282). A trial, which carefully screened patients to make sure they actually had statin induced myalgias, failed to show a benefit from Coenzyme Q10 supplementation (101). It has also been recommended that vitamin D supplementation be used to prevent statin induced myalgias but a large randomized trial failed to show a reduction in muscle symptoms with vitamin D therapy (283).

 

If after trying various approaches a patient still has myalgias and is unable to tolerate statin therapy one needs to utilize other approaches to lower LDL levels. Similarly, if there are other reasons why a patient cannot take a statin, such as developing muscle pathology, one will also need to utilize other approaches to lower LDL levels. These patients can be treated with ezetimibe, bempedoic acid, bile acid sequestrants, or PCSK 9 inhibitors either as monotherapy or in combination to achieve LDL goals.

 

There are patients who will refuse statins and other drug therapy because they do not believe in taking pharmaceuticals but will take natural products. In these patients we have employed red yeast rice, which decreases LDL-C because it contains a form of lovastatin (284,285). It is effective but one should recognize that the quality control is not similar to the standards of pharmaceutical products and that there can be batch to batch variations. Furthermore, there is a risk of drug-drug interactions if used with inhibitors of CYP3A4. However, in this particular patient population, who refuses to take statins or other drugs, this can be a reasonable alternative. If a patient just refuses statins (usually based on a belief that statins are toxic) we will employ other cholesterol lowering drugs.

 

CONCLUSIONS

 

With currently available drugs to lower LDL-C levels we are now able to markedly reduce LDL-C levels and achieve our LDL-C goals in the vast majority of patients and thereby reduce the risk of cardiovascular disease. Patients with Homozygous Familial Hypercholesterolemia and some patients with Heterozygous Familial Hypercholesterolemia still present major clinical challenges and it can be very difficult in these patients to achieve LDL-C goals.

 

ACKNOWLEDGEMENTS

 

This work was supported by grants from the Northern California Institute for Research and Education.

 

REFERENCES

 

  1. Feingold KR. The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2021.
  2. Feingold KR. Approach to the Patient with Dyslipidemia. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  3. Grundy SM, Feingold KR. Guidelines for the Management of High Blood Cholesterol. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2022.
  4. Endo A. A gift from nature: the birth of the statins. Nat Med 2008; 14:1050-1052
  5. Alberts AW. Discovery, biochemistry and biology of lovastatin. Am J Cardiol 1988; 62:10J-15J
  6. Ballantyne CM, Andrews TC, Hsia JA, Kramer JH, Shear C, Efficacy ASGACC, Safety S. Correlation of non-high-density lipoprotein cholesterol with apolipoprotein B: effect of 5 hydroxymethylglutaryl coenzyme A reductase inhibitors on non-high-density lipoprotein cholesterol levels. Am J Cardiol 2001; 88:265-269
  7. Jones PH, Hunninghake DB, Ferdinand KC, Stein EA, Gold A, Caplan RJ, Blasetto JW. Effects of rosuvastatin versus atorvastatin, simvastatin, and pravastatin on non-high-density lipoprotein cholesterol, apolipoproteins, and lipid ratios in patients with hypercholesterolemia: additional results from the STELLAR trial. Clin Ther 2004; 26:1388-1399
  8. Jones PH, Davidson MH, Stein EA, Bays HE, McKenney JM, Miller E, Cain VA, Blasetto JW. Comparison of the efficacy and safety of rosuvastatin versus atorvastatin, simvastatin, and pravastatin across doses (STELLAR* Trial). Am J Cardiol 2003; 92:152-160
  9. Stein EA, Lane M, Laskarzewski P. Comparison of statins in hypertriglyceridemia. Am J Cardiol 1998; 81:66B-69B
  10. Bos S, Yayha R, van Lennep JE. Latest developments in the treatment of lipoprotein (a). Curr Opin Lipidol 2014; 25:452-460
  11. van Capelleveen JC, van der Valk FM, Stroes ES. Current therapies for lowering lipoprotein (a). J Lipid Res 2016; 57:1612-1618
  12. Adams SP, Tsang M, Wright JM. Lipid-lowering efficacy of atorvastatin. Cochrane Database Syst Rev 2015; 3:CD008226
  13. Adams SP, Sekhon SS, Wright JM. Lipid-lowering efficacy of rosuvastatin. Cochrane Database Syst Rev 2014; 11:CD010254
  14. Liao JK. Clinical implications for statin pleiotropy. Curr Opin Lipidol 2005; 16:624-629
  15. Joshi PH, Jacobson TA. Therapeutic options to further lower C-reactive protein for patients on statin treatment. Curr Atheroscler Rep 2010; 12:34-42
  16. Goldstein JL, Brown MS. A century of cholesterol and coronaries: from plaques to genes to statins. Cell 2015; 161:161-172
  17. Huff MW, Burnett JR. 3-Hydroxy-3-methylglutaryl coenzyme A reductase inhibitors and hepatic apolipoprotein B secretion. Curr Opin Lipidol 1997; 8:138-145
  18. Tsimikas S, Gordts P, Nora C, Yeang C, Witztum JL. Statin therapy increases lipoprotein(a) levels. Eur Heart J 2020; 41:2275-2284
  19. Causevic-Ramosevac A, Semiz S. Drug interactions with statins. Acta Pharm 2013; 63:277-293
  20. Hu M, Tomlinson B. Evaluation of the pharmacokinetics and drug interactions of the two recently developed statins, rosuvastatin and pitavastatin. Expert Opin Drug Metab Toxicol 2014; 10:51-65
  21. Sirtori CR. The pharmacology of statins. Pharmacol Res 2014; 88:3-11
  22. Bellosta S, Corsini A. Statin drug interactions and related adverse reactions: an update. Expert Opin Drug Saf 2018; 17:25-37
  23. Awad K, Serban MC, Penson P, Mikhailidis DP, Toth PP, Jones SR, Rizzo M, Howard G, Lip GYH, Banach M. Effects of morning vs evening statin administration on lipid profile: A systematic review and meta-analysis. J Clin Lipidol 2017; 11:972-985 e979
  24. Kellick KA, Bottorff M, Toth PP, The National Lipid Association's Safety Task Force. A clinician's guide to statin drug-drug interactions. J Clin Lipidol 2014; 8:S30-46
  25. Lee JW, Morris JK, Wald NJ. Grapefruit Juice and Statins. Am J Med 2016; 129:26-29
  26. Holdaas H, Fellstrom B, Jardine AG, Holme I, Nyberg G, Fauchald P, Gronhagen-Riska C, Madsen S, Neumayer HH, Cole E, Maes B, Ambuhl P, Olsson AG, Hartmann A, Solbu DO, Pedersen TR. Effect of fluvastatin on cardiac outcomes in renal transplant recipients: a multicentre, randomised, placebo-controlled trial. Lancet 2003; 361:2024-2031
  27. ACCORD Study Group, Ginsberg HN, Elam MB, Lovato LC, Crouse JR, 3rd, Leiter LA, Linz P, Friedewald WT, Buse JB, Gerstein HC, Probstfield J, Grimm RH, Ismail-Beigi F, Bigger JT, Goff DC, Jr., Cushman WC, Simons-Morton DG, Byington RP. Effects of combination lipid therapy in type 2 diabetes mellitus. N Engl J Med 2010; 362:1563-1574
  28. Busti AJ, Bain AM, Hall RG, 2nd, Bedimo RG, Leff RD, Meek C, Mehvar R. Effects of atazanavir/ritonavir or fosamprenavir/ritonavir on the pharmacokinetics of rosuvastatin. J Cardiovasc Pharmacol 2008; 51:605-610
  29. Gervasoni C, Riva A, Rizzardini G, Clementi E, Galli M, Cattaneo D. Potential association between rosuvastatin use and high atazanavir trough concentrations in ritonavir-treated HIV-infected patients. Antivir Ther 2015; 20:449-451
  30. Kiser JJ, Gerber JG, Predhomme JA, Wolfe P, Flynn DM, Hoody DW. Drug/Drug interaction between lopinavir/ritonavir and rosuvastatin in healthy volunteers. J Acquir Immune Defic Syndr 2008; 47:570-578
  31. Pham PA, la Porte CJ, Lee LS, van Heeswijk R, Sabo JP, Elgadi MM, Piliero PJ, Barditch-Crovo P, Fuchs E, Flexner C, Cameron DW. Differential effects of tipranavir plus ritonavir on atorvastatin or rosuvastatin pharmacokinetics in healthy volunteers. Antimicrob Agents Chemother 2009; 53:4385-4392
  32. van der Lee M, Sankatsing R, Schippers E, Vogel M, Fatkenheuer G, van der Ven A, Kroon F, Rockstroh J, Wyen C, Baumer A, de Groot E, Koopmans P, Stroes E, Reiss P, Burger D. Pharmacokinetics and pharmacodynamics of combined use of lopinavir/ritonavir and rosuvastatin in HIV-infected patients. Antivir Ther 2007; 12:1127-1132
  33. Sarkar S, Brown TT. Lipid Disorders in People with HIV. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  34. Baigent C, Keech A, Kearney PM, Blackwell L, Buck G, Pollicino C, Kirby A, Sourjina T, Peto R, Collins R, Simes R, Cholesterol Treatment Trialists Collaborators. Efficacy and safety of cholesterol-lowering treatment: prospective meta-analysis of data from 90,056 participants in 14 randomised trials of statins. Lancet 2005; 366:1267-1278
  35. Cholesterol Treatment Trialists Collaboration, Baigent C, Blackwell L, Emberson J, Holland LE, Reith C, Bhala N, Peto R, Barnes EH, Keech A, Simes J, Collins R. Efficacy and safety of more intensive lowering of LDL cholesterol: a meta-analysis of data from 170,000 participants in 26 randomised trials. Lancet 2010; 376:1670-1681
  36. Cholesterol Treatment Trialists Collaboration, Fulcher J, O'Connell R, Voysey M, Emberson J, Blackwell L, Mihaylova B, Simes J, Collins R, Kirby A, Colhoun H, Braunwald E, La Rosa J, Pedersen TR, Tonkin A, Davis B, Sleight P, Franzosi MG, Baigent C, Keech A. Efficacy and safety of LDL-lowering therapy among men and women: meta-analysis of individual data from 174,000 participants in 27 randomised trials. Lancet 2015; 385:1397-1405
  37. Cholesterol Treatment Trialists Collaboration, Mihaylova B, Emberson J, Blackwell L, Keech A, Simes J, Barnes EH, Voysey M, Gray A, Collins R, Baigent C. The effects of lowering LDL cholesterol with statin therapy in people at low risk of vascular disease: meta-analysis of individual data from 27 randomised trials. Lancet 2012; 380:581-590
  38. Taylor F, Huffman MD, Macedo AF, Moore TH, Burke M, Davey Smith G, Ward K, Ebrahim S. Statins for the primary prevention of cardiovascular disease. Cochrane Database Syst Rev 2013; 1:CD004816
  39. Yusuf S, Bosch J, Dagenais G, Zhu J, Xavier D, Liu L, Pais P, Lopez-Jaramillo P, Leiter LA, Dans A, Avezum A, Piegas LS, Parkhomenko A, Keltai K, Keltai M, Sliwa K, Peters RJ, Held C, Chazova I, Yusoff K, Lewis BS, Jansky P, Khunti K, Toff WD, Reid CM, Varigos J, Sanchez-Vallejo G, McKelvie R, Pogue J, Jung H, Gao P, Diaz R, Lonn E. Cholesterol Lowering in Intermediate-Risk Persons without Cardiovascular Disease. N Engl J Med 2016;
  40. Ference BA. Mendelian randomization studies: using naturally randomized genetic data to fill evidence gaps. Curr Opin Lipidol 2015; 26:566-571
  41. Brown MS, Goldstein JL. Biomedicine. Lowering LDL--not only how low, but how long? Science 2006; 311:1721-1723
  42. Feingold KR. Maximizing the benefits of cholesterol-lowering drugs. Curr Opin Lipidol 2019; 30:388-394
  43. Shepherd J, Blauw GJ, Murphy MB, Bollen EL, Buckley BM, Cobbe SM, Ford I, Gaw A, Hyland M, Jukema JW, Kamper AM, Macfarlane PW, Meinders AE, Norrie J, Packard CJ, Perry IJ, Stott DJ, Sweeney BJ, Twomey C, Westendorp RG. Pravastatin in elderly individuals at risk of vascular disease (PROSPER): a randomised controlled trial. Lancet 2002; 360:1623-1630
  44. Cholesterol Treatment Trialists Collaboration. Efficacy and safety of statin therapy in older people: a meta-analysis of individual participant data from 28 randomised controlled trials. Lancet 2019; 393:407-415
  45. Joseph J, Pajewski NM, Dolor RJ, Sellers MA, Perdue LH, Peeples SR, Henrie AM, Woolard N, Jones WS, Benziger CP, Orkaby AR, Mixon AS, VanWormer JJ, Shapiro MD, Kistler CE, Polonsky TS, Chatterjee R, Chamberlain AM, Forman DE, Knowlton KU, Gill TM, Newby LK, Hammill BG, Cicek MS, Williams NA, Decker JE, Ou J, Rubinstein J, Choudhary G, Gazmuri RJ, Schmader KE, Roumie CL, Vaughan CP, Effron MB, Cooper-DeHoff RM, Supiano MA, Shah RC, Whittle JC, Hernandez AF, Ambrosius WT, Williamson JD, Alexander KP. Pragmatic evaluation of events and benefits of lipid lowering in older adults (PREVENTABLE): Trial design and rationale. J Am Geriatr Soc 2023; 71:1701-1713
  46. Zoungas S, Curtis A, Spark S, Wolfe R, McNeil JJ, Beilin L, Chong TT, Cloud G, Hopper I, Kost A, Nelson M, Nicholls SJ, Reid CM, Ryan J, Tonkin A, Ward SA, Wierzbicki A. Statins for extension of disability-free survival and primary prevention of cardiovascular events among older people: protocol for a randomised controlled trial in primary care (STAREE trial). BMJ Open 2023; 13:e069915
  47. Liao JK. Safety and efficacy of statins in Asians. Am J Cardiol 2007; 99:410-414
  48. Gupta M, Braga MF, Teoh H, Tsigoulis M, Verma S. Statin effects on LDL and HDL cholesterol in South Asian and white populations. J Clin Pharmacol 2009; 49:831-837
  49. Sakamoto T, Kojima S, Ogawa H, Shimomura H, Kimura K, Ogata Y, Sakaino N, Kitagawa A. Effects of early statin treatment on symptomatic heart failure and ischemic events after acute myocardial infarction in Japanese. Am J Cardiol 2006; 97:1165-1171
  50. Pais P, Jung H, Dans A, Zhu J, Liu L, Kamath D, Bosch J, Lonn E, Yusuf S. Impact of blood pressure lowering, cholesterol lowering and their combination in Asians and non-Asians in those without cardiovascular disease: an analysis of the HOPE 3 study. Eur J Prev Cardiol 2019; 26:681-697
  51. Feingold KR. Dyslipidemia in Patients with Diabetes. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  52. Cholesterol Treatment Trialists Collaboration, Kearney PM, Blackwell L, Collins R, Keech A, Simes J, Peto R, Armitage J, Baigent C. Efficacy of cholesterol-lowering therapy in 18,686 people with diabetes in 14 randomised trials of statins: a meta-analysis. Lancet 2008; 371:117-125
  53. Cholesterol Treatment Trialists Collaboration , Herrington WG, Emberson J, Mihaylova B, Blackwell L, Reith C, Solbu MD, Mark PB, Fellstrom B, Jardine AG, Wanner C, Holdaas H, Fulcher J, Haynes R, Landray MJ, Keech A, Simes J, Collins R, Baigent C. Impact of renal function on the effects of LDL cholesterol lowering with statin-based regimens: a meta-analysis of individual participant data from 28 randomised trials. Lancet Diabetes Endocrinol 2016; 4:829-839
  54. Su X, Zhang L, Lv J, Wang J, Hou W, Xie X, Zhang H. Effect of Statins on Kidney Disease Outcomes: A Systematic Review and Meta-analysis. Am J Kidney Dis 2016;
  55. Wanner C, Krane V, Marz W, Olschewski M, Mann JF, Ruf G, Ritz E. Atorvastatin in patients with type 2 diabetes mellitus undergoing hemodialysis. N Engl J Med 2005; 353:238-248
  56. Fellstrom BC, Jardine AG, Schmieder RE, Holdaas H, Bannister K, Beutler J, Chae DW, Chevaile A, Cobbe SM, Gronhagen-Riska C, De Lima JJ, Lins R, Mayer G, McMahon AW, Parving HH, Remuzzi G, Samuelsson O, Sonkodi S, Sci D, Suleymanlar G, Tsakiris D, Tesar V, Todorov V, Wiecek A, Wuthrich RP, Gottlow M, Johnsson E, Zannad F. Rosuvastatin and cardiovascular events in patients undergoing hemodialysis. N Engl J Med 2009; 360:1395-1407
  57. Rosenstein K, Tannock LR. Dyslipidemia in Chronic Kidney Disease. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2022.
  58. Kjekshus J, Apetrei E, Barrios V, Bohm M, Cleland JG, Cornel JH, Dunselman P, Fonseca C, Goudev A, Grande P, Gullestad L, Hjalmarson A, Hradec J, Janosi A, Kamensky G, Komajda M, Korewicki J, Kuusi T, Mach F, Mareev V, McMurray JJ, Ranjith N, Schaufelberger M, Vanhaecke J, van Veldhuisen DJ, Waagstein F, Wedel H, Wikstrand J. Rosuvastatin in older patients with systolic heart failure. N Engl J Med 2007; 357:2248-2261
  59. Tavazzi L, Maggioni AP, Marchioli R, Barlera S, Franzosi MG, Latini R, Lucci D, Nicolosi GL, Porcu M, Tognoni G. Effect of rosuvastatin in patients with chronic heart failure (the GISSI-HF trial): a randomised, double-blind, placebo-controlled trial. Lancet 2008; 372:1231-1239
  60. Arvind A, Osganian SA, Cohen DE, Corey KE. Lipid and Lipoprotein Metabolism in Liver Disease. In: Feingold KR, Anawalt B, Boyce A, Chrousos G, Dungan K, Grossman A, Hershman JM, Kaltsas G, Koch C, Kopp P, Korbonits M, McLachlan R, Morley JE, New M, Perreault L, Purnell J, Rebar R, Singer F, Trence DL, Vinik A, Wilson DP, eds. Endotext. South Dartmouth (MA) 2019.
  61. Bays H, Cohen DE, Chalasani N, Harrison SA, The National Lipid Association's Statin Safety Task Force. An assessment by the Statin Liver Safety Task Force: 2014 update. J Clin Lipidol 2014; 8:S47-57
  62. Herrick C, Bahrainy S, Gill EA. Statins and the Liver. Endocrinol Metab Clin North Am 2016; 45:117-128
  63. Athyros VG, Tziomalos K, Gossios TD, Griva T, Anagnostis P, Kargiotis K, Pagourelias ED, Theocharidou E, Karagiannis A, Mikhailidis DP. Safety and efficacy of long-term statin treatment for cardiovascular events in patients with coronary heart disease and abnormal liver tests in the Greek Atorvastatin and Coronary Heart Disease Evaluation (GREACE) Study: a post-hoc analysis. Lancet 2010; 376:1916-1922
  64. Cohen DE, Corey KE. Lipid and Lipoprotein Metabolism in Liver Disease. In: De Groot LJ, Beck-Peccoz P, Chrousos G, Dungan K, Grossman A, Hershman JM, Koch C, McLachlan R, New M, Rebar R, Singer F, Vinik A, Weickert MO, eds. Endotext. South Dartmouth (MA) 2019.
  65. Grinspoon SK, Fitch KV, Zanni MV, Fichtenbaum CJ, Umbleja T, Aberg JA, Overton ET, Malvestutto CD, Bloomfield GS, Currier JS, Martinez E, Roa JC, Diggs MR, Fulda ES, Paradis K, Wiviott SD, Foldyna B, Looby SE, Desvigne-Nickens P, Alston-Smith B, Leon-Cruz J, McCallum S, Hoffmann U, Lu MT, Ribaudo HJ, Douglas PS. Pitavastatin to Prevent Cardiovascular Disease in HIV Infection. N Engl J Med 2023; 389:687-699
  66. He Y, Li X, Gasevic D, Brunt E, McLachlan F, Millenson M, Timofeeva M, Ioannidis JPA, Campbell H, Theodoratou E. Statins and Multiple Noncardiovascular Outcomes: Umbrella Review of Meta-analyses of Observational Studies and Randomized Controlled Trials. Ann Intern Med 2018; 169:543-553
  67. Newman CB, Preiss D, Tobert JA, Jacobson TA, Page RL, 2nd, Goldstein LB, Chin C, Tannock LR, Miller M, Raghuveer G, Duell PB, Brinton EA, Pollak A, Braun LT, Welty FK. Statin Safety and Associated Adverse Events: A Scientific Statement From the American Heart Association. Arterioscler Thromb Vasc Biol 2019; 39:e38-e81
  68. Sattar N, Preiss D, Murray HM, Welsh P, Buckley BM, de Craen AJ, Seshasai SR, McMurray JJ, Freeman DJ, Jukema JW, Macfarlane PW, Packard CJ, Stott DJ, Westendorp RG, Shepherd J, Davis BR, Pressel SL, Marchioli R, Marfisi RM, Maggioni AP, Tavazzi L, Tognoni G, Kjekshus J, Pedersen TR, Cook TJ, Gotto AM, Clearfield MB, Downs JR, Nakamura H, Ohashi Y, Mizuno K, Ray KK, Ford I. Statins and risk of incident diabetes: a collaborative meta-analysis of randomised statin trials. Lancet 2010; 375:735-742
  69. Preiss D, Seshasai SR, Welsh P, Murphy SA, Ho JE, Waters DD, DeMicco DA, Barter P, Cannon CP, Sabatine MS, Braunwald E, Kastelein JJ, de Lemos JA, Blazing MA, Pedersen TR, Tikkanen MJ, Sattar N, Ray KK. Risk of incident diabetes with intensive-dose compared with moderate-dose statin therapy: a meta-analysis. JAMA 2011; 305:2556-2564
  70. Feingold KR. Dyslipidemia in Diabetes. In: Feingold KR, Anawalt B, Boyce A, Chrousos G, Dungan K, Grossman A, Hershman JM, Kaltsas G, Koch C, Kopp P, Korbonits M, McLachlan R, Morley JE, New M, Perreault L, Purnell J, Rebar R, Singer F, Trence DL, Vinik A, Wilson DP, eds. Endotext. South Dartmouth (MA) 2020.
  71. Erqou S, Lee CC, Adler AI. Statins and glycaemic control in individuals with diabetes: a systematic review and meta-analysis. Diabetologia 2014; 57:2444-2452
  72. Colhoun HM, Betteridge DJ, Durrington PN, Hitman GA, Neil HA, Livingstone SJ, Thomason MJ, Mackness MI, Charlton-Menys V, Fuller JH. Primary prevention of cardiovascular disease with atorvastatin in type 2 diabetes in the Collaborative Atorvastatin Diabetes Study (CARDS): multicentre randomised placebo-controlled trial. Lancet 2004; 364:685-696
  73. Collins R, Armitage J, Parish S, Sleigh P, Peto R. MRC/BHF Heart Protection Study of cholesterol-lowering with simvastatin in 5963 people with diabetes: a randomised placebo-controlled trial. Lancet 2003; 361:2005-2016
  74. Sattar N. Statins and diabetes: What are the connections? Best Pract Res Clin Endocrinol Metab 2023; 37:101749
  75. Swerdlow DI, Preiss D, Kuchenbaecker KB, Holmes MV, Engmann JE, Shah T, Sofat R, Stender S, Johnson PC, Scott RA, Leusink M, Verweij N, Sharp SJ, Guo Y, Giambartolomei C, Chung C, Peasey A, Amuzu A, Li K, Palmen J, Howard P, Cooper JA, Drenos F, Li YR, Lowe G, Gallacher J, Stewart MC, Tzoulaki I, Buxbaum SG, van der AD, Forouhi NG, Onland-Moret NC, van der Schouw YT, Schnabel RB, Hubacek JA, Kubinova R, Baceviciene M, Tamosiunas A, Pajak A, Topor-Madry R, Stepaniak U, Malyutina S, Baldassarre D, Sennblad B, Tremoli E, de Faire U, Veglia F, Ford I, Jukema JW, Westendorp RG, de Borst GJ, de Jong PA, Algra A, Spiering W, Maitland-van der Zee AH, Klungel OH, de Boer A, Doevendans PA, Eaton CB, Robinson JG, Duggan D, Consortium D, Consortium M, InterAct C, Kjekshus J, Downs JR, Gotto AM, Keech AC, Marchioli R, Tognoni G, Sever PS, Poulter NR, Waters DD, Pedersen TR, Amarenco P, Nakamura H, McMurray JJ, Lewsey JD, Chasman DI, Ridker PM, Maggioni AP, Tavazzi L, Ray KK, Seshasai SR, Manson JE, Price JF, Whincup PH, Morris RW, Lawlor DA, Smith GD, Ben-Shlomo Y, Schreiner PJ, Fornage M, Siscovick DS, Cushman M, Kumari M, Wareham NJ, Verschuren WM, Redline S, Patel SR, Whittaker JC, Hamsten A, Delaney JA, Dale C, Gaunt TR, Wong A, Kuh D, Hardy R, Kathiresan S, Castillo BA, van der Harst P, Brunner EJ, Tybjaerg-Hansen A, Marmot MG, Krauss RM, Tsai M, Coresh J, Hoogeveen RC, Psaty BM, Lange LA, Hakonarson H, Dudbridge F, Humphries SE, Talmud PJ, Kivimaki M, Timpson NJ, Langenberg C, Asselbergs FW, Voevoda M, Bobak M, Pikhart H, Wilson JG, Reiner AP, Keating BJ, Hingorani AD, Sattar N. HMG-coenzyme A reductase inhibition, type 2 diabetes, and bodyweight: evidence from genetic analysis and randomised trials. Lancet 2015; 385:351-361
  76. Sugiyama T, Tsugawa Y, Tseng CH, Kobayashi Y, Shapiro MF. Different time trends of caloric and fat intake between statin users and nonusers among US adults: gluttony in the time of statins? JAMA Intern Med 2014; 174:1038-1045
  77. Higuchi S, Izquierdo MC, Haeusler RA. Unexplained reciprocal regulation of diabetes and lipoproteins. Curr Opin Lipidol 2018; 29:186-193
  78. Rojas-Fernandez C, Hudani Z, Bittner V. Statins and cognitive side effects: what cardiologists need to know. Cardiol Clin 2015; 33:245-256
  79. Rojas-Fernandez CH, Goldstein LB, Levey AI, Taylor BA, Bittner V, The National Lipid Association's Safety Task Force. An assessment by the Statin Cognitive Safety Task Force: 2014 update. J Clin Lipidol 2014; 8:S5-16
  80. Richardson K, Schoen M, French B, Umscheid CA, Mitchell MD, Arnold SE, Heidenreich PA, Rader DJ, deGoma EM. Statins and cognitive function: a systematic review. Ann Intern Med 2013; 159:688-697
  81. Trompet S, van Vliet P, de Craen AJ, Jolles J, Buckley BM, Murphy MB, Ford I, Macfarlane PW, Sattar N, Packard CJ, Stott DJ, Shepherd J, Bollen EL, Blauw GJ, Jukema JW, Westendorp RG. Pravastatin and cognitive function in the elderly. Results of the PROSPER study. J Neurol 2010; 257:85-90
  82. Collins R, Armitage J, Parish S, Sleight P, Peto R. Effects of cholesterol-lowering with simvastatin on stroke and other major vascular events in 20536 people with cerebrovascular disease or other high-risk conditions. Lancet 2004; 363:757-767
  83. McGuinness B, Craig D, Bullock R, Malouf R, Passmore P. Statins for the treatment of dementia. Cochrane Database Syst Rev 2014; 7:CD007514
  84. Kashani A, Phillips CO, Foody JM, Wang Y, Mangalmurti S, Ko DT, Krumholz HM. Risks associated with statin therapy: a systematic overview of randomized clinical trials. Circulation 2006; 114:2788-2797
  85. de Denus S, Spinler SA, Miller K, Peterson AM. Statins and liver toxicity: a meta-analysis. Pharmacotherapy 2004; 24:584-591
  86. Law M, Rudnicka AR. Statin safety: a systematic review. Am J Cardiol 2006; 97:52C-60C
  87. Alsheikh-Ali AA, Maddukuri PV, Han H, Karas RH. Effect of the magnitude of lipid lowering on risk of elevated liver enzymes, rhabdomyolysis, and cancer: insights from large randomized statin trials. J Am Coll Cardiol 2007; 50:409-418
  88. Russo MW, Scobey M, Bonkovsky HL. Drug-induced liver injury associated with statins. Semin Liver Dis 2009; 29:412-422
  89. Tolman KG. Defining patient risks from expanded preventive therapies. Am J Cardiol 2000; 85:15E-19E
  90. Boutari C, Pappas PD, Anastasilakis D, Mantzoros CS. Statins' efficacy in non-alcoholic fatty liver disease: A systematic review and meta-analysis. Clin Nutr 2022; 41:2195-2206
  91. Rosenson RS, Baker SK, Jacobson TA, Kopecky SL, Parker BA, The National Lipid Association's Muscle Safety Expert Panel. An assessment by the Statin Muscle Safety Task Force: 2014 update. J Clin Lipidol 2014; 8:S58-71
  92. Stroes ES, Thompson PD, Corsini A, Vladutiu GD, Raal FJ, Ray KK, Roden M, Stein E, Tokgozoglu L, Nordestgaard BG, Bruckert E, De Backer G, Krauss RM, Laufs U, Santos RD, Hegele RA, Hovingh GK, Leiter LA, Mach F, Marz W, Newman CB, Wiklund O, Jacobson TA, Catapano AL, Chapman MJ, Ginsberg HN, European Atherosclerosis Society Consensus Panel. Statin-associated muscle symptoms: impact on statin therapy-European Atherosclerosis Society Consensus Panel Statement on Assessment, Aetiology and Management. Eur Heart J 2015; 36:1012-1022
  93. Thompson PD, Clarkson P, Karas RH. Statin-associated myopathy. JAMA 2003; 289:1681-1690
  94. Cholesterol Treatment Trialists Collaboration. Effect of statin therapy on muscle symptoms: an individual participant data meta-analysis of large-scale, randomised, double-blind trials. Lancet 2022; 400:832-845
  95. Bruckert E, Hayem G, Dejager S, Yau C, Begaud B. Mild to moderate muscular symptoms with high-dosage statin therapy in hyperlipidemic patients--the PRIMO study. Cardiovasc Drugs Ther 2005; 19:403-414
  96. Buettner C, Davis RB, Leveille SG, Mittleman MA, Mukamal KJ. Prevalence of musculoskeletal pain and statin use. J Gen Intern Med 2008; 23:1182-1186
  97. Cohen JD, Brinton EA, Ito MK, Jacobson TA. Understanding Statin Use in America and Gaps in Patient Education (USAGE): an internet-based survey of 10,138 current and former statin users. J Clin Lipidol 2012; 6:208-215
  98. Parker BA, Capizzi JA, Grimaldi AS, Clarkson PM, Cole SM, Keadle J, Chipkin S, Pescatello LS, Simpson K, White CM, Thompson PD. Effect of statins on skeletal muscle function. Circulation 2013; 127:96-103
  99. Gupta A, Thompson D, Whitehouse A, Collier T, Dahlof B, Poulter N, Collins R, Sever P. Adverse events associated with unblinded, but not with blinded, statin therapy in the Anglo-Scandinavian Cardiac Outcomes Trial-Lipid-Lowering Arm (ASCOT-LLA): a randomised double-blind placebo-controlled trial and its non-randomised non-blind extension phase. Lancet 2017; 389:2473-2481
  100. Joy TR, Monjed A, Zou GY, Hegele RA, McDonald CG, Mahon JL. N-of-1 (single-patient) trials for statin-related myalgia. Ann Intern Med 2014; 160:301-310
  101. Taylor BA, Lorson L, White CM, Thompson PD. A randomized trial of coenzyme Q10 in patients with confirmed statin myopathy. Atherosclerosis 2015; 238:329-335
  102. Nissen SE, Stroes E, Dent-Acosta RE, Rosenson RS, Lehman SJ, Sattar N, Preiss D, Bruckert E, Ceska R, Lepor N, Ballantyne CM, Gouni-Berthold I, Elliott M, Brennan DM, Wasserman SM, Somaratne R, Scott R, Stein EA. Efficacy and Tolerability of Evolocumab vs Ezetimibe in Patients With Muscle-Related Statin Intolerance: The GAUSS-3 Randomized Clinical Trial. JAMA 2016; 315:1580-1590
  103. Herrett E, Williamson E, Brack K, Beaumont D, Perkins A, Thayne A, Shakur-Still H, Roberts I, Prowse D, Goldacre B, van Staa T, MacDonald TM, Armitage J, Wimborne J, Melrose P, Singh J, Brooks L, Moore M, Hoffman M, Smeeth L. Statin treatment and muscle symptoms: series of randomised, placebo controlled n-of-1 trials. BMJ 2021; 372:n135
  104. Howard JP, Wood FA, Finegold JA, Nowbar AN, Thompson DM, Arnold AD, Rajkumar CA, Connolly S, Cegla J, Stride C, Sever P, Norton C, Thom SAM, Shun-Shin MJ, Francis DP. Side Effect Patterns in a Crossover Trial of Statin, Placebo, and No Treatment. J Am Coll Cardiol 2021; 78:1210-1222
  105. Nielsen SF, Nordestgaard BG. Negative statin-related news stories decrease statin persistence and increase myocardial infarction and cardiovascular mortality: a nationwide prospective cohort study. Eur Heart J 2016; 37:908-916
  106. De Vera MA, Bhole V, Burns LC, Lacaille D. Impact of statin adherence on cardiovascular disease and mortality outcomes: a systematic review. Br J Clin Pharmacol 2014; 78:684-698
  107. Cziraky MJ, Willey VJ, McKenney JM, Kamat SA, Fisher MD, Guyton JR, Jacobson TA, Davidson MH. Risk of hospitalized rhabdomyolysis associated with lipid-lowering drugs in a real-world clinical setting. J Clin Lipidol 2013; 7:102-108
  108. Davidson MH, Stein EA, Dujovne CA, Hunninghake DB, Weiss SR, Knopp RH, Illingworth DR, Mitchel YB, Melino MR, Zupkis RV, Dobrinska MR, Amin RD, Tobert JA. The efficacy and six-week tolerability of simvastatin 80 and 160 mg/day. Am J Cardiol 1997; 79:38-42
  109. Rosenson RS, Bays HE. Results of two clinical trials on the safety and efficacy of pravastatin 80 and 160 mg per day. Am J Cardiol 2003; 91:878-881
  110. Search Collaborative Group, Link E, Parish S, Armitage J, Bowman L, Heath S, Matsuda F, Gut I, Lathrop M, Collins R. SLCO1B1 variants and statin-induced myopathy--a genomewide study. N Engl J Med 2008; 359:789-799
  111. Hopewell JC, Reith C, Armitage J. Pharmacogenomics of statin therapy: any new insights in efficacy or safety? Curr Opin Lipidol 2014; 25:438-445
  112. Mammen AL. Statin-Associated Autoimmune Myopathy. N Engl J Med 2016; 374:664-669
  113. Mohassel P, Mammen AL. Anti-HMGCR Myopathy. J Neuromuscul Dis 2018; 5:11-20
  114. Wild R, Feingold KR. Effect of Pregnancy on Lipid Metabolism and Lipoprotein Levels. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  115. Bruckert E, Giral P, Tellier P. Perspectives in cholesterol-lowering therapy: the role of ezetimibe, a new selective inhibitor of intestinal cholesterol absorption. Circulation 2003; 107:3124-3128
  116. Liebeskind A, Peterson AL, Wilson D. Sitosterolemia. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  117. Pandor A, Ara RM, Tumur I, Wilkinson AJ, Paisley S, Duenas A, Durrington PN, Chilcott J. Ezetimibe monotherapy for cholesterol lowering in 2,722 people: systematic review and meta-analysis of randomized controlled trials. J Intern Med 2009; 265:568-580
  118. Morrone D, Weintraub WS, Toth PP, Hanson ME, Lowe RS, Lin J, Shah AK, Tershakovec AM. Lipid-altering efficacy of ezetimibe plus statin and statin monotherapy and identification of factors associated with treatment response: a pooled analysis of over 21,000 subjects from 27 clinical trials. Atherosclerosis 2012; 223:251-261
  119. Ballantyne CM, Weiss R, Moccetti T, Vogt A, Eber B, Sosef F, Duffield E. Efficacy and safety of rosuvastatin 40 mg alone or in combination with ezetimibe in patients at high risk of cardiovascular disease (results from the EXPLORER study). Am J Cardiol 2007; 99:673-680
  120. Sahebkar A, Simental-Mendia LE, Pirro M, Banach M, Watts GF, Sirtori C, Al-Rasadi K, Atkin SL. Impact of ezetimibe on plasma lipoprotein(a) concentrations as monotherapy or in combination with statins: a systematic review and meta-analysis of randomized controlled trials. Sci Rep 2018; 8:17887
  121. Kastelein JJ, Akdim F, Stroes ES, Zwinderman AH, Bots ML, Stalenhoef AF, Visseren FL, Sijbrands EJ, Trip MD, Stein EA, Gaudet D, Duivenvoorden R, Veltri EP, Marais AD, de Groot E. Simvastatin with or without ezetimibe in familial hypercholesterolemia. N Engl J Med 2008; 358:1431-1443
  122. Sager PT, Capece R, Lipka L, Strony J, Yang B, Suresh R, Mitchel Y, Veltri E. Effects of ezetimibe coadministered with simvastatin on C-reactive protein in a large cohort of hypercholesterolemic patients. Atherosclerosis 2005; 179:361-367
  123. Ballantyne CM, Houri J, Notarbartolo A, Melani L, Lipka LJ, Suresh R, Sun S, LeBeaut AP, Sager PT, Veltri EP. Effect of ezetimibe coadministered with atorvastatin in 628 patients with primary hypercholesterolemia: a prospective, randomized, double-blind trial. Circulation 2003; 107:2409-2415
  124. Yu L. The structure and function of Niemann-Pick C1-like 1 protein. Curr Opin Lipidol 2008; 19:263-269
  125. Turley SD, Dietschy JM. Sterol absorption by the small intestine. Curr Opin Lipidol 2003; 14:233-240
  126. Telford DE, Sutherland BG, Edwards JY, Andrews JD, Barrett PH, Huff MW. The molecular mechanisms underlying the reduction of LDL apoB-100 by ezetimibe plus simvastatin. J Lipid Res 2007; 48:699-708
  127. Pramfalk C, Jiang ZY, Parini P. Hepatic Niemann-Pick C1-like 1. Curr Opin Lipidol 2011; 22:225-230
  128. Rossebo AB, Pedersen TR, Boman K, Brudi P, Chambers JB, Egstrup K, Gerdts E, Gohlke-Barwolf C, Holme I, Kesaniemi YA, Malbecq W, Nienaber CA, Ray S, Skjaerpe T, Wachtell K, Willenheimer R. Intensive lipid lowering with simvastatin and ezetimibe in aortic stenosis. N Engl J Med 2008; 359:1343-1356
  129. Baigent C, Landray MJ, Reith C, Emberson J, Wheeler DC, Tomson C, Wanner C, Krane V, Cass A, Craig J, Neal B, Jiang L, Hooi LS, Levin A, Agodoa L, Gaziano M, Kasiske B, Walker R, Massy ZA, Feldt-Rasmussen B, Krairittichai U, Ophascharoensuk V, Fellstrom B, Holdaas H, Tesar V, Wiecek A, Grobbee D, de Zeeuw D, Gronhagen-Riska C, Dasgupta T, Lewis D, Herrington W, Mafham M, Majoni W, Wallendszus K, Grimm R, Pedersen T, Tobert J, Armitage J, Baxter A, Bray C, Chen Y, Chen Z, Hill M, Knott C, Parish S, Simpson D, Sleight P, Young A, Collins R. The effects of lowering LDL cholesterol with simvastatin plus ezetimibe in patients with chronic kidney disease (Study of Heart and Renal Protection): a randomised placebo-controlled trial. Lancet 2011; 377:2181-2192
  130. Cannon CP, Blazing MA, Giugliano RP, McCagg A, White JA, Theroux P, Darius H, Lewis BS, Ophuis TO, Jukema JW, De Ferrari GM, Ruzyllo W, De Lucca P, Im K, Bohula EA, Reist C, Wiviott SD, Tershakovec AM, Musliner TA, Braunwald E, Califf RM. Ezetimibe Added to Statin Therapy after Acute Coronary Syndromes. N Engl J Med 2015; 372:2387-2397
  131. Bohula EA, Wiviott SD, Giugliano RP, Blazing MA, Park JG, Murphy SA, White JA, Mach F, Van de Werf F, Dalby AJ, White HD, Tershakovec AM, Cannon CP, Braunwald E. Prevention of Stroke with the Addition of Ezetimibe to Statin Therapy in Patients With Acute Coronary Syndrome in IMPROVE-IT (Improved Reduction of Outcomes: Vytorin Efficacy International Trial). Circulation 2017; 136:2440-2450
  132. Giugliano RP, Cannon CP, Blazing MA, Nicolau JC, Corbalan R, Spinar J, Park JG, White JA, Bohula EA, Braunwald E. Benefit of Adding Ezetimibe to Statin Therapy on Cardiovascular Outcomes and Safety in Patients With Versus Without Diabetes Mellitus: Results From IMPROVE-IT (Improved Reduction of Outcomes: Vytorin Efficacy International Trial). Circulation 2018; 137:1571-1582
  133. Bonaca MP, Gutierrez JA, Cannon C, Giugliano R, Blazing M, Park JG, White J, Tershakovec A, Braunwald E. Polyvascular disease, type 2 diabetes, and long-term vascular risk: a secondary analysis of the IMPROVE-IT trial. Lancet Diabetes Endocrinol 2018; 6:934-943
  134. Ouchi Y, Sasaki J, Arai H, Yokote K, Harada K, Katayama Y, Urabe T, Uchida Y, Hayashi M, Yokota N, Nishida H, Otonari T, Arai T, Sakuma I, Sakabe K, Yamamoto M, Kobayashi T, Oikawa S, Yamashita S, Rakugi H, Imai T, Tanaka S, Ohashi Y, Kuwabara M, Ito H. Ezetimibe Lipid-Lowering Trial on Prevention of Atherosclerotic Cardiovascular Disease in 75 or Older (EWTOPIA 75): A Randomized, Controlled Trial. Circulation 2019; 140:992-1003
  135. Kim BK, Hong SJ, Lee YJ, Hong SJ, Yun KH, Hong BK, Heo JH, Rha SW, Cho YH, Lee SJ, Ahn CM, Kim JS, Ko YG, Choi D, Jang Y, Hong MK. Long-term efficacy and safety of moderate-intensity statin with ezetimibe combination therapy versus high-intensity statin monotherapy in patients with atherosclerotic cardiovascular disease (RACING): a randomised, open-label, non-inferiority trial. Lancet 2022; 400:380-390
  136. Lee B, Hong SJ, Rha SW, Heo JH, Hur SH, Choi HH, Kim KJ, Kim JH, Kim HK, Kim U, Choi YJ, Lee YJ, Lee SJ, Ahn CM, Ko YG, Kim BK, Choi D, Hong MK, Jang Y, Kim JS. Moderate-intensity statin plus ezetimibe vs high-intensity statin according to baseline LDL-C in the treatment of atherosclerotic cardiovascular disease: A post-hoc analysis of the RACING randomized trial. Atherosclerosis 2023; 386:117373
  137. Lee SH, Lee YJ, Heo JH, Hur SH, Choi HH, Kim KJ, Kim JH, Park KH, Lee JH, Choi YJ, Lee SJ, Hong SJ, Ahn CM, Kim BK, Ko YG, Choi D, Hong MK, Jang Y, Kim JS. Combination Moderate-Intensity Statin and Ezetimibe Therapy for Elderly Patients With Atherosclerosis. J Am Coll Cardiol 2023; 81:1339-1349
  138. Toth PP, Morrone D, Weintraub WS, Hanson ME, Lowe RS, Lin J, Shah AK, Tershakovec AM. Safety profile of statins alone or combined with ezetimibe: a pooled analysis of 27 studies including over 22,000 patients treated for 6-24 weeks. Int J Clin Pract 2012; 66:800-812
  139. Luo L, Yuan X, Huang W, Ren F, Zhu H, Zheng Y, Tang L. Safety of coadministration of ezetimibe and statins in patients with hypercholesterolaemia: a meta-analysis. Intern Med J 2015; 45:546-557
  140. Kashani A, Sallam T, Bheemreddy S, Mann DL, Wang Y, Foody JM. Review of side-effect profile of combination ezetimibe and statin therapy in randomized clinical trials. Am J Cardiol 2008; 101:1606-1613
  141. Savarese G, De Ferrari GM, Rosano GM, Perrone-Filardi P. Safety and efficacy of ezetimibe: A meta-analysis. Int J Cardiol 2015; 201:247-252
  142. Peto R, Emberson J, Landray M, Baigent C, Collins R, Clare R, Califf R. Analyses of cancer data from three ezetimibe trials. N Engl J Med 2008; 359:1357-1366
  143. Wu H, Shang H, Wu J. Effect of ezetimibe on glycemic control: a systematic review and meta-analysis of randomized controlled trials. Endocrine 2018; 60:229-239
  144. Aldridge MA, Ito MK. Colesevelam hydrochloride: a novel bile acid-binding resin. Ann Pharmacother 2001; 35:898-907
  145. Heel RC, Brogden RN, Pakes GE, Speight TM, Avery GS. Colestipol: a review of its pharmacological properties and therapeutic efficacy in patients with hypercholesterolaemia. Drugs 1980; 19:161-180
  146. Insull W, Jr. Clinical utility of bile acid sequestrants in the treatment of dyslipidemia: a scientific review. South Med J 2006; 99:257-273
  147. Huijgen R, Abbink EJ, Bruckert E, Stalenhoef AF, Imholz BP, Durrington PN, Trip MD, Eriksson M, Visseren FL, Schaefer JR, Kastelein JJ. Colesevelam added to combination therapy with a statin and ezetimibe in patients with familial hypercholesterolemia: a 12-week, multicenter, randomized, double-blind, controlled trial. Clin Ther 2010; 32:615-625
  148. Zema MJ. Colesevelam HCl and ezetimibe combination therapy provides effective lipid-lowering in difficult-to-treat patients with hypercholesterolemia. Am J Ther 2005; 12:306-310
  149. Bays H, Rhyne J, Abby S, Lai YL, Jones M. Lipid-lowering effects of colesevelam HCl in combination with ezetimibe. Curr Med Res Opin 2006; 22:2191-2200
  150. Fonseca VA, Handelsman Y, Staels B. Colesevelam lowers glucose and lipid levels in type 2 diabetes: the clinical evidence. Diabetes Obes Metab 2010; 12:384-392
  151. Devaraj S, Autret B, Jialal I. Effects of colesevelam hydrochloride (WelChol) on biomarkers of inflammation in patients with mild hypercholesterolemia. Am J Cardiol 2006; 98:641-643
  152. Bays HE, Davidson M, Jones MR, Abby SL. Effects of colesevelam hydrochloride on low-density lipoprotein cholesterol and high-sensitivity C-reactive protein when added to statins in patients with hypercholesterolemia. Am J Cardiol 2006; 97:1198-1205
  153. Einarsson K, Ericsson S, Ewerth S, Reihner E, Rudling M, Stahlberg D, Angelin B. Bile acid sequestrants: mechanisms of action on bile acid and cholesterol metabolism. Eur J Clin Pharmacol 1991; 40 Suppl 1:S53-58
  154. Kliewer SA, Mangelsdorf DJ. Bile Acids as Hormones: The FXR-FGF15/19 Pathway. Dig Dis 2015; 33:327-331
  155. Chiang JY. Bile acids: regulation of synthesis. J Lipid Res 2009; 50:1955-1966
  156. Porez G, Prawitt J, Gross B, Staels B. Bile acid receptors as targets for the treatment of dyslipidemia and cardiovascular disease. J Lipid Res 2012; 53:1723-1737
  157. Edwards PA, Kast HR, Anisfeld AM. BAREing it all: the adoption of LXR and FXR and their roles in lipid homeostasis. J Lipid Res 2002; 43:2-12
  158. Bronden A, Knop FK. Gluco-Metabolic Effects of Pharmacotherapy-Induced Modulation of Bile Acid Physiology. J Clin Endocrinol Metab 2020; 105
  159. The Lipid Research Clinics Coronary Primary Prevention Trial results. I. Reduction in incidence of coronary heart disease. JAMA 1984; 251:351-364
  160. The Lipid Research Clinics Coronary Primary Prevention Trial results. II. The relationship of reduction in incidence of coronary heart disease to cholesterol lowering. JAMA 1984; 251:365-374
  161. Levy RI, Brensike JF, Epstein SE, Kelsey SF, Passamani ER, Richardson JM, Loh IK, Stone NJ, Aldrich RF, Battaglini JW, et al. The influence of changes in lipid values induced by cholestyramine and diet on progression of coronary artery disease: results of NHLBI Type II Coronary Intervention Study. Circulation 1984; 69:325-337
  162. Watts GF, Lewis B, Brunt JN, Lewis ES, Coltart DJ, Smith LD, Mann JI, Swan AV. Effects on coronary artery disease of lipid-lowering diet, or diet plus cholestyramine, in the St Thomas' Atherosclerosis Regression Study (STARS). Lancet 1992; 339:563-569
  163. Blankenhorn DH, Nessim SA, Johnson RL, Sanmarco ME, Azen SP, Cashin-Hemphill L. Beneficial effects of combined colestipol-niacin therapy on coronary atherosclerosis and coronary venous bypass grafts. JAMA 1987; 257:3233-3240
  164. Brown G, Albers JJ, Fisher LD, Schaefer SM, Lin JT, Kaplan C, Zhao XQ, Bisson BD, Fitzpatrick VF, Dodge HT. Regression of coronary artery disease as a result of intensive lipid-lowering therapy in men with high levels of apolipoprotein B. N Engl J Med 1990; 323:1289-1298
  165. Kane JP, Malloy MJ, Ports TA, Phillips NR, Diehl JC, Havel RJ. Regression of coronary atherosclerosis during treatment of familial hypercholesterolemia with combined drug regimens. JAMA 1990; 264:3007-3012
  166. Ito MK, McGowan MP, Moriarty PM, National Lipid Association Expert Panel on Familial Hypercholesterolemia. Management of familial hypercholesterolemias in adult patients: recommendations from the National Lipid Association Expert Panel on Familial Hypercholesterolemia. J Clin Lipidol 2011; 5:S38-45
  167. Giugliano RP, Sabatine MS. Are PCSK9 Inhibitors the Next Breakthrough in the Cardiovascular Field? J Am Coll Cardiol 2015; 65:2638-2651
  168. McKenney JM. Understanding PCSK9 and anti-PCSK9 therapies. J Clin Lipidol 2015; 9:170-186
  169. Navarese EP, Kolodziejczak M, Schulze V, Gurbel PA, Tantry U, Lin Y, Brockmeyer M, Kandzari DE, Kubica JM, D'Agostino RB, Sr., Kubica J, Volpe M, Agewall S, Kereiakes DJ, Kelm M. Effects of Proprotein Convertase Subtilisin/Kexin Type 9 Antibodies in Adults With Hypercholesterolemia: A Systematic Review and Meta-analysis. Ann Intern Med 2015; 163:40-51
  170. O'Donoghue ML, Fazio S, Giugliano RP, Stroes ESG, Kanevsky E, Gouni-Berthold I, Im K, Lira Pineda A, Wasserman SM, Ceska R, Ezhov MV, Jukema JW, Jensen HK, Tokgozoglu SL, Mach F, Huber K, Sever PS, Keech AC, Pedersen TR, Sabatine MS. Lipoprotein(a), PCSK9 Inhibition, and Cardiovascular Risk. Circulation 2019; 139:1483-1492
  171. Sahebkar A, Di Giosia P, Stamerra CA, Grassi D, Pedone C, Ferretti G, Bacchetti T, Ferri C, Giorgini P. Effect of monoclonal antibodies to PCSK9 on high-sensitivity C-reactive protein levels: a meta-analysis of 16 randomized controlled treatment arms. Br J Clin Pharmacol 2016; 81:1175-1190
  172. Koren MJ, Lundqvist P, Bolognese M, Neutel JM, Monsalvo ML, Yang J, Kim JB, Scott R, Wasserman SM, Bays H. Anti-PCSK9 monotherapy for hypercholesterolemia: the MENDEL-2 randomized, controlled phase III clinical trial of evolocumab. J Am Coll Cardiol 2014; 63:2531-2540
  173. Roth EM, Taskinen MR, Ginsberg HN, Kastelein JJ, Colhoun HM, Robinson JG, Merlet L, Pordy R, Baccara-Dinet MT. Monotherapy with the PCSK9 inhibitor alirocumab versus ezetimibe in patients with hypercholesterolemia: results of a 24 week, double-blind, randomized Phase 3 trial. Int J Cardiol 2014; 176:55-61
  174. Kereiakes DJ, Robinson JG, Cannon CP, Lorenzato C, Pordy R, Chaudhari U, Colhoun HM. Efficacy and safety of the proprotein convertase subtilisin/kexin type 9 inhibitor alirocumab among high cardiovascular risk patients on maximally tolerated statin therapy: The ODYSSEY COMBO I study. Am Heart J 2015; 169:906-915 e913
  175. Cannon CP, Cariou B, Blom D, McKenney JM, Lorenzato C, Pordy R, Chaudhari U, Colhoun HM. Efficacy and safety of alirocumab in high cardiovascular risk patients with inadequately controlled hypercholesterolaemia on maximally tolerated doses of statins: the ODYSSEY COMBO II randomized controlled trial. Eur Heart J 2015; 36:1186-1194
  176. Robinson JG, Nedergaard BS, Rogers WJ, Fialkow J, Neutel JM, Ramstad D, Somaratne R, Legg JC, Nelson P, Scott R, Wasserman SM, Weiss R. Effect of evolocumab or ezetimibe added to moderate- or high-intensity statin therapy on LDL-C lowering in patients with hypercholesterolemia: the LAPLACE-2 randomized clinical trial. JAMA 2014; 311:1870-1882
  177. Blom DJ, Hala T, Bolognese M, Lillestol MJ, Toth PD, Burgess L, Ceska R, Roth E, Koren MJ, Ballantyne CM, Monsalvo ML, Tsirtsonis K, Kim JB, Scott R, Wasserman SM, Stein EA. A 52-week placebo-controlled trial of evolocumab in hyperlipidemia. N Engl J Med 2014; 370:1809-1819
  178. Raal FJ, Stein EA, Dufour R, Turner T, Civeira F, Burgess L, Langslet G, Scott R, Olsson AG, Sullivan D, Hovingh GK, Cariou B, Gouni-Berthold I, Somaratne R, Bridges I, Scott R, Wasserman SM, Gaudet D. PCSK9 inhibition with evolocumab (AMG 145) in heterozygous familial hypercholesterolaemia (RUTHERFORD-2): a randomised, double-blind, placebo-controlled trial. Lancet 2015; 385:331-340
  179. Kastelein JJ, Ginsberg HN, Langslet G, Hovingh GK, Ceska R, Dufour R, Blom D, Civeira F, Krempf M, Lorenzato C, Zhao J, Pordy R, Baccara-Dinet MT, Gipe DA, Geiger MJ, Farnier M. ODYSSEY FH I and FH II: 78 week results with alirocumab treatment in 735 patients with heterozygous familial hypercholesterolaemia. Eur Heart J 2015; 36:2996-3003
  180. Raal FJ, Honarpour N, Blom DJ, Hovingh GK, Xu F, Scott R, Wasserman SM, Stein EA. Inhibition of PCSK9 with evolocumab in homozygous familial hypercholesterolaemia (TESLA Part B): a randomised, double-blind, placebo-controlled trial. Lancet 2015; 385:341-350
  181. Raal FJ, Hovingh GK, Blom D, Santos RD, Harada-Shiba M, Bruckert E, Couture P, Soran H, Watts GF, Kurtz C, Honarpour N, Tang L, Kasichayanula S, Wasserman SM, Stein EA. Long-term treatment with evolocumab added to conventional drug therapy, with or without apheresis, in patients with homozygous familial hypercholesterolaemia: an interim subset analysis of the open-label TAUSSIG study. Lancet Diabetes Endocrinol 2017; 5:280-290
  182. Stein EA, Honarpour N, Wasserman SM, Xu F, Scott R, Raal FJ. Effect of the proprotein convertase subtilisin/kexin 9 monoclonal antibody, AMG 145, in homozygous familial hypercholesterolemia. Circulation 2013; 128:2113-2120
  183. Blom DJ, Harada-Shiba M, Rubba P, Gaudet D, Kastelein JJP, Charng MJ, Pordy R, Donahue S, Ali S, Dong Y, Khilla N, Banerjee P, Baccara-Dinet M, Rosenson RS. Efficacy and Safety of Alirocumab in Adults With Homozygous Familial Hypercholesterolemia: The ODYSSEY HoFH Trial. J Am Coll Cardiol 2020; 76:131-142
  184. Stroes E, Colquhoun D, Sullivan D, Civeira F, Rosenson RS, Watts GF, Bruckert E, Cho L, Dent R, Knusel B, Xue A, Scott R, Wasserman SM, Rocco M. Anti-PCSK9 antibody effectively lowers cholesterol in patients with statin intolerance: the GAUSS-2 randomized, placebo-controlled phase 3 clinical trial of evolocumab. J Am Coll Cardiol 2014; 63:2541-2548
  185. Moriarty PM, Thompson PD, Cannon CP, Guyton JR, Bergeron J, Zieve FJ, Bruckert E, Jacobson TA, Kopecky SL, Baccara-Dinet MT, Du Y, Pordy R, Gipe DA. Efficacy and safety of alirocumab vs ezetimibe in statin-intolerant patients, with a statin rechallenge arm: The ODYSSEY ALTERNATIVE randomized trial. J Clin Lipidol 2015; 9:758-769
  186. Sattar N, Preiss D, Robinson JG, Djedjos CS, Elliott M, Somaratne R, Wasserman SM, Raal FJ. Lipid-lowering efficacy of the PCSK9 inhibitor evolocumab (AMG 145) in patients with type 2 diabetes: a meta-analysis of individual patient data. Lancet Diabetes Endocrinol 2016; 4:403-410
  187. Abifadel M, Varret M, Rabes JP, Allard D, Ouguerram K, Devillers M, Cruaud C, Benjannet S, Wickham L, Erlich D, Derre A, Villeger L, Farnier M, Beucler I, Bruckert E, Chambaz J, Chanu B, Lecerf JM, Luc G, Moulin P, Weissenbach J, Prat A, Krempf M, Junien C, Seidah NG, Boileau C. Mutations in PCSK9 cause autosomal dominant hypercholesterolemia. Nat Genet 2003; 34:154-156
  188. Warden BA, Fazio S, Shapiro MD. Familial Hypercholesterolemia: Genes and Beyond. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2021.
  189. Cohen J, Pertsemlidis A, Kotowski IK, Graham R, Garcia CK, Hobbs HH. Low LDL cholesterol in individuals of African descent resulting from frequent nonsense mutations in PCSK9. Nat Genet 2005; 37:161-165
  190. Cohen JC, Boerwinkle E, Mosley TH, Jr., Hobbs HH. Sequence variations in PCSK9, low LDL, and protection against coronary heart disease. N Engl J Med 2006; 354:1264-1272
  191. Feingold KR. Introduction to Lipids and Lipoproteins. In: Feingold KR, Anawalt B, Boyce A, Chrousos G, Dungan K, Grossman A, Hershman JM, Kaltsas G, Koch C, Kopp P, Korbonits M, McLachlan R, Morley JE, New M, Perreault L, Purnell J, Rebar R, Singer F, Trence DL, Vinik A, Wilson DP, eds. Endotext. South Dartmouth (MA) 2021.
  192. Horton JD, Cohen JC, Hobbs HH. PCSK9: a convertase that coordinates LDL catabolism. J Lipid Res 2009; 50 Suppl:S172-177
  193. Lambert G, Sjouke B, Choque B, Kastelein JJ, Hovingh GK. The PCSK9 decade. J Lipid Res 2012; 53:2515-2524
  194. Reyes-Soffer G, Pavlyha M, Ngai C, Thomas T, Holleran S, Ramakrishnan R, Karmally W, Nandakumar R, Fontanez N, Obunike J, Marcovina SM, Lichtenstein AH, Matthan NR, Matta J, Maroccia M, Becue F, Poitiers F, Swanson B, Cowan L, Sasiela WJ, Surks HK, Ginsberg HN. Effects of PCSK9 Inhibition With Alirocumab on Lipoprotein Metabolism in Healthy Humans. Circulation 2017; 135:352-362
  195. Watts GF, Chan DC, Dent R, Somaratne R, Wasserman SM, Scott R, Burrows S, Barrett PHR. Factorial Effects of Evolocumab and Atorvastatin on Lipoprotein Metabolism. Circulation 2017; 135:338-351
  196. Watts GF, Chan DC, Pang J, Ma L, Ying Q, Aggarwal S, Marcovina SM, Barrett PHR. PCSK9 Inhibition with alirocumab increases the catabolism of lipoprotein(a) particles in statin-treated patients with elevated lipoprotein(a). Metabolism 2020; 107:154221
  197. Ying Q, Chan DC, Pang J, Marcovina SM, Barrett PHR, Watts GF. PCSK9 inhibition with alirocumab decreases plasma lipoprotein(a) concentration by a dual mechanism of action in statin-treated patients with very high apolipoprotein(a) concentration. J Intern Med 2022; 291:870-876
  198. Raal FJ, Giugliano RP, Sabatine MS, Koren MJ, Blom D, Seidah NG, Honarpour N, Lira A, Xue A, Chirovolu P, Jackson S, Di M, Peach M, Somaratne R, Wasserman SM, Scott R, Stein EA. PCSK9 inhibition-mediated reduction in Lp(a) with evolocumab: an analysis of 10 clinical trials and the role of the LDL receptor. J Lipid Res 2016;
  199. Sabatine MS, Giugliano RP, Wiviott SD, Raal FJ, Blom DJ, Robinson J, Ballantyne CM, Somaratne R, Legg J, Wasserman SM, Scott R, Koren MJ, Stein EA, Open-Label Study of Long-Term Evaluation against LDLCI. Efficacy and safety of evolocumab in reducing lipids and cardiovascular events. N Engl J Med 2015; 372:1500-1509
  200. Sabatine MS, Leiter LA, Wiviott SD, Giugliano RP, Deedwania P, De Ferrari GM, Murphy SA, Kuder JF, Gouni-Berthold I, Lewis BS, Handelsman Y, Pineda AL, Honarpour N, Keech AC, Sever PS, Pedersen TR. Cardiovascular safety and efficacy of the PCSK9 inhibitor evolocumab in patients with and without diabetes and the effect of evolocumab on glycaemia and risk of new-onset diabetes: a prespecified analysis of the FOURIER randomised controlled trial. Lancet Diabetes Endocrinol 2017; 5:941-950
  201. Bonaca MP, Nault P, Giugliano RP, Keech AC, Pineda AL, Kanevsky E, Kuder J, Murphy SA, Jukema JW, Lewis BS, Tokgozoglu L, Somaratne R, Sever PS, Pedersen TR, Sabatine MS. Low-Density Lipoprotein Cholesterol Lowering With Evolocumab and Outcomes in Patients With Peripheral Artery Disease: Insights From the FOURIER Trial (Further Cardiovascular Outcomes Research With PCSK9 Inhibition in Subjects With Elevated Risk). Circulation 2018; 137:338-350
  202. Giugliano RP, Keech A, Murphy SA, Huber K, Tokgozoglu SL, Lewis BS, Ferreira J, Pineda AL, Somaratne R, Sever PS, Pedersen TR, Sabatine MS. Clinical Efficacy and Safety of Evolocumab in High-Risk Patients Receiving a Statin: Secondary Analysis of Patients With Low LDL Cholesterol Levels and in Those Already Receiving a Maximal-Potency Statin in a Randomized Clinical Trial. JAMA Cardiol 2017; 2:1385-1391
  203. Giugliano RP, Pedersen TR, Park JG, De Ferrari GM, Gaciong ZA, Ceska R, Toth K, Gouni-Berthold I, Lopez-Miranda J, Schiele F, Mach F, Ott BR, Kanevsky E, Pineda AL, Somaratne R, Wasserman SM, Keech AC, Sever PS, Sabatine MS. Clinical efficacy and safety of achieving very low LDL-cholesterol concentrations with the PCSK9 inhibitor evolocumab: a prespecified secondary analysis of the FOURIER trial. Lancet 2017; 390:1962-1971
  204. Sabatine MS, De Ferrari GM, Giugliano RP, Huber K, Lewis BS, Ferreira J, Kuder JF, Murphy SA, Wiviott SD, Kurtz CE, Honarpour N, Keech AC, Sever PS, Pedersen TR. Clinical Benefit of Evolocumab by Severity and Extent of Coronary Artery Disease. Circulation 2018; 138:756-766
  205. Schwartz GG, Steg PG, Szarek M, Bhatt DL, Bittner VA, Diaz R, Edelberg JM, Goodman SG, Hanotin C, Harrington RA, Jukema JW, Lecorps G, Mahaffey KW, Moryusef A, Pordy R, Quintero K, Roe MT, Sasiela WJ, Tamby JF, Tricoci P, White HD, Zeiher AM. Alirocumab and Cardiovascular Outcomes after Acute Coronary Syndrome. N Engl J Med 2018; 379:2097-2107
  206. Nicholls SJ, Puri R, Anderson T, Ballantyne CM, Cho L, Kastelein JJ, Koenig W, Somaratne R, Kassahun H, Yang J, Wasserman SM, Scott R, Ungi I, Podolec J, Ophuis AO, Cornel JH, Borgman M, Brennan DM, Nissen SE. Effect of Evolocumab on Progression of Coronary Disease in Statin-Treated Patients: The GLAGOV Randomized Clinical Trial. JAMA 2016; 316:2373-2384
  207. Raber L, Ueki Y, Otsuka T, Losdat S, Haner JD, Lonborg J, Fahrni G, Iglesias JF, van Geuns RJ, Ondracek AS, Radu Juul Jensen MD, Zanchin C, Stortecky S, Spirk D, Siontis GCM, Saleh L, Matter CM, Daemen J, Mach F, Heg D, Windecker S, Engstrom T, Lang IM, Koskinas KC. Effect of Alirocumab Added to High-Intensity Statin Therapy on Coronary Atherosclerosis in Patients With Acute Myocardial Infarction: The PACMAN-AMI Randomized Clinical Trial. JAMA 2022; 327:1771-1781
  208. Perez de Isla L, Diaz-Diaz JL, Romero MJ, Muniz-Grijalvo O, Mediavilla JD, Argueso R, Sanchez Munoz-Torrero JF, Rubio P, Alvarez-Banos P, Ponte P, Manas D, Suarez Gutierrez L, Cepeda JM, Casanas M, Fuentes F, Guijarro C, Angel Barba M, Saltijeral Cerezo A, Padro T, Mata P. Alirocumab and Coronary Atherosclerosis in Asymptomatic Patients with Familial Hypercholesterolemia: The ARCHITECT Study. Circulation 2023; 147:1436-1443
  209. Marston NA, Gurmu Y, Melloni GEM, Bonaca M, Gencer B, Sever PS, Pedersen TR, Keech AC, Roselli C, Lubitz SA, Ellinor PT, O'Donoghue ML, Giugliano RP, Ruff CT, Sabatine MS. The Effect of PCSK9 (Proprotein Convertase Subtilisin/Kexin Type 9) Inhibition on the Risk of Venous Thromboembolism. Circulation 2020; 141:1600-1607
  210. Schwartz GG, Steg PG, Szarek M, Bittner VA, Diaz R, Goodman SG, Kim YU, Jukema JW, Pordy R, Roe MT, White HD, Bhatt DL. Peripheral Artery Disease and Venous Thromboembolic Events After Acute Coronary Syndrome: Role of Lipoprotein(a) and Modification by Alirocumab: Prespecified Analysis of the ODYSSEY OUTCOMES Randomized Clinical Trial. Circulation 2020; 141:1608-1617
  211. de Carvalho LSF, Campos AM, Sposito AC. Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) Inhibitors and Incident Type 2 Diabetes: A Systematic Review and Meta-analysis With Over 96,000 Patient-Years. Diabetes Care 2018; 41:364-367
  212. Giugliano RP, Mach F, Zavitz K, Kurtz C, Im K, Kanevsky E, Schneider J, Wang H, Keech A, Pedersen TR, Sabatine MS, Sever PS, Robinson JG, Honarpour N, Wasserman SM, Ott BR. Cognitive Function in a Randomized Trial of Evolocumab. N Engl J Med 2017; 377:633-643
  213. Koren MJ, Giugliano RP, Raal FJ, Sullivan D, Bolognese M, Langslet G, Civeira F, Somaratne R, Nelson P, Liu T, Scott R, Wasserman SM, Sabatine MS. Efficacy and safety of longer-term administration of evolocumab (AMG 145) in patients with hypercholesterolemia: 52-week results from the Open-Label Study of Long-Term Evaluation Against LDL-C (OSLER) randomized trial. Circulation 2014; 129:234-243
  214. Robinson JG, Farnier M, Krempf M, Bergeron J, Luc G, Averna M, Stroes ES, Langslet G, Raal FJ, El Shahawy M, Koren MJ, Lepor NE, Lorenzato C, Pordy R, Chaudhari U, Kastelein JJ. Efficacy and safety of alirocumab in reducing lipids and cardiovascular events. N Engl J Med 2015; 372:1489-1499
  215. Shapiro MD, Feingold KR. Monogenic Disorders Causing Hypobetalipoproteinemia. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2024.
  216. LaRosa JC, Grundy SM, Kastelein JJ, Kostis JB, Greten H, Treating to New Targets Steering C, Investigators. Safety and efficacy of Atorvastatin-induced very low-density lipoprotein cholesterol levels in Patients with coronary heart disease (a post hoc analysis of the treating to new targets [TNT] study). Am J Cardiol 2007; 100:747-752
  217. Wiviott SD, Cannon CP, Morrow DA, Ray KK, Pfeffer MA, Braunwald E. Can low-density lipoprotein be too low? The safety and efficacy of achieving very low low-density lipoprotein with intensive statin therapy: a PROVE IT-TIMI 22 substudy. J Am Coll Cardiol 2005; 46:1411-1416
  218. Everett BM, Mora S, Glynn RJ, MacFadyen J, Ridker PM. Safety profile of subjects treated to very low low-density lipoprotein cholesterol levels (<30 mg/dl) with rosuvastatin 20 mg daily (from JUPITER). Am J Cardiol 2014; 114:1682-1689
  219. Sinning D, Landmesser U. Low-density Lipoprotein-Cholesterol Lowering Strategies for Prevention of Atherosclerotic Cardiovascular Disease: Focus on siRNA Treatment Targeting PCSK9 (Inclisiran). Curr Cardiol Rep 2020; 22:176
  220. Ray KK, Wright RS, Kallend D, Koenig W, Leiter LA, Raal FJ, Bisch JA, Richardson T, Jaros M, Wijngaard PLJ, Kastelein JJP. Two Phase 3 Trials of Inclisiran in Patients with Elevated LDL Cholesterol. N Engl J Med 2020; 382:1507-1519
  221. Wright RS, Collins MG, Stoekenbroek RM, Robson R, Wijngaard PLJ, Landmesser U, Leiter LA, Kastelein JJP, Ray KK, Kallend D. Effects of Renal Impairment on the Pharmacokinetics, Efficacy, and Safety of Inclisiran: An Analysis of the ORION-7 and ORION-1 Studies. Mayo Clin Proc 2020; 95:77-89
  222. Ray KK, Troquay RPT, Visseren FLJ, Leiter LA, Scott Wright R, Vikarunnessa S, Talloczy Z, Zang X, Maheux P, Lesogor A, Landmesser U. Long-term efficacy and safety of inclisiran in patients with high cardiovascular risk and elevated LDL cholesterol (ORION-3): results from the 4-year open-label extension of the ORION-1 trial. Lancet Diabetes Endocrinol 2023; 11:109-119
  223. Raal FJ, Kallend D, Ray KK, Turner T, Koenig W, Wright RS, Wijngaard PLJ, Curcio D, Jaros MJ, Leiter LA, Kastelein JJP. Inclisiran for the Treatment of Heterozygous Familial Hypercholesterolemia. N Engl J Med 2020; 382:1520-1530
  224. Hovingh GK, Lepor NE, Kallend D, Stoekenbroek RM, Wijngaard PLJ, Raal FJ. Inclisiran Durably Lowers Low-Density Lipoprotein Cholesterol and Proprotein Convertase Subtilisin/Kexin Type 9 Expression in Homozygous Familial Hypercholesterolemia: The ORION-2 Pilot Study. Circulation 2020; 141:1829-1831
  225. Raal F, Durst R, Bi R, Talloczy Z, Maheux P, Lesogor A, Kastelein JJP. Efficacy, Safety, and Tolerability of Inclisiran in Patients With Homozygous Familial Hypercholesterolemia: Results From the ORION-5 Randomized Clinical Trial. Circulation 2024; 149:354-362
  226. Wright RS, Koenig W, Landmesser U, Leiter LA, Raal FJ, Schwartz GG, Lesogor A, Maheux P, Stratz C, Zang X, Ray KK. Safety and Tolerability of Inclisiran for Treatment of Hypercholesterolemia in 7 Clinical Trials. J Am Coll Cardiol 2023; 82:2251-2261
  227. Laufs U, Banach M, Mancini GBJ, Gaudet D, Bloedon LT, Sterling LR, Kelly S, Stroes ESG. Efficacy and Safety of Bempedoic Acid in Patients With Hypercholesterolemia and Statin Intolerance. J Am Heart Assoc 2019; 8:e011662
  228. Goldberg AC, Leiter LA, Stroes ESG, Baum SJ, Hanselman JC, Bloedon LT, Lalwani ND, Patel PM, Zhao X, Duell PB. Effect of Bempedoic Acid vs Placebo Added to Maximally Tolerated Statins on Low-Density Lipoprotein Cholesterol in Patients at High Risk for Cardiovascular Disease: The CLEAR Wisdom Randomized Clinical Trial. JAMA 2019; 322:1780-1788
  229. Ray KK, Bays HE, Catapano AL, Lalwani ND, Bloedon LT, Sterling LR, Robinson PL, Ballantyne CM. Safety and Efficacy of Bempedoic Acid to Reduce LDL Cholesterol. N Engl J Med 2019; 380:1022-1032
  230. Lalwani ND, Hanselman JC, MacDougall DE, Sterling LR, Cramer CT. Complementary low-density lipoprotein-cholesterol lowering and pharmacokinetics of adding bempedoic acid (ETC-1002) to high-dose atorvastatin background therapy in hypercholesterolemic patients: A randomized placebo-controlled trial. J Clin Lipidol 2019; 13:568-579
  231. Ballantyne CM, Laufs U, Ray KK, Leiter LA, Bays HE, Goldberg AC, Stroes ES, MacDougall D, Zhao X, Catapano AL. Bempedoic acid plus ezetimibe fixed-dose combination in patients with hypercholesterolemia and high CVD risk treated with maximally tolerated statin therapy. Eur J Prev Cardiol 2019:2047487319864671
  232. Ballantyne CM, Banach M, Mancini GBJ, Lepor NE, Hanselman JC, Zhao X, Leiter LA. Efficacy and safety of bempedoic acid added to ezetimibe in statin-intolerant patients with hypercholesterolemia: A randomized, placebo-controlled study. Atherosclerosis 2018; 277:195-203
  233. Ridker PM, Lei L, Ray KK, Ballantyne CM, Bradwin G, Rifai N. Effects of bempedoic acid on CRP, IL-6, fibrinogen and lipoprotein(a) in patients with residual inflammatory risk: A secondary analysis of the CLEAR harmony trial. J Clin Lipidol 2023; 17:297-302
  234. Ray KK, Nicholls SJ, Li N, Louie MJ, Brennan D, Lincoff AM, Nissen SE. Efficacy and safety of bempedoic acid among patients with and without diabetes: prespecified analysis of the CLEAR Outcomes randomised trial. Lancet Diabetes Endocrinol 2024; 12:19-28
  235. Nissen SE, Nicholls SJ, Lincoff AM. Bempedoic Acid for Primary Prevention of Cardiovascular Events-Reply. JAMA 2023; 330:1696-1697
  236. Bays HE, Bloedon LT, Lin G, Powell HA, Louie MJ, Nicholls SJ, Lincoff AM, Nissen S. Safety of bempedoic acid in patients at high cardiovascular risk and with statin intolerance. J Clin Lipidol 2023;
  237. Burke AC, Telford DE, Huff MW. Bempedoic acid: effects on lipoprotein metabolism and atherosclerosis. Curr Opin Lipidol 2019; 30:1-9
  238. Ference BA, Ray KK, Catapano AL, Ference TB, Burgess S, Neff DR, Oliver-Williams C, Wood AM, Butterworth AS, Di Angelantonio E, Danesh J, Kastelein JJP, Nicholls SJ. Mendelian Randomization Study of ACLY and Cardiovascular Disease. N Engl J Med 2019; 380:1033-1042
  239. Nissen SE, Lincoff AM, Brennan D, Ray KK, Mason D, Kastelein JJP, Thompson PD, Libby P, Cho L, Plutzky J, Bays HE, Moriarty PM, Menon V, Grobbee DE, Louie MJ, Chen CF, Li N, Bloedon L, Robinson P, Horner M, Sasiela WJ, McCluskey J, Davey D, Fajardo-Campos P, Petrovic P, Fedacko J, Zmuda W, Lukyanov Y, Nicholls SJ. Bempedoic Acid and Cardiovascular Outcomes in Statin-Intolerant Patients. N Engl J Med 2023; 388:1353-1364
  240. Neef D, Berthold HK, Gouni-Berthold I. Lomitapide for use in patients with homozygous familial hypercholesterolemia: a narrative review. Expert Rev Clin Pharmacol 2016; 9:655-663
  241. Gouni-Berthold I, Berthold HK. Mipomersen and lomitapide: Two new drugs for the treatment of homozygous familial hypercholesterolemia. Atheroscler Suppl 2015; 18:28-34
  242. Rader DJ, Kastelein JJ. Lomitapide and mipomersen: two first-in-class drugs for reducing low-density lipoprotein cholesterol in patients with homozygous familial hypercholesterolemia. Circulation 2014; 129:1022-1032
  243. Cuchel M, Meagher EA, du Toit Theron H, Blom DJ, Marais AD, Hegele RA, Averna MR, Sirtori CR, Shah PK, Gaudet D, Stefanutti C, Vigna GB, Du Plessis AM, Propert KJ, Sasiela WJ, Bloedon LT, Rader DJ, Phase 3 Ho FHLSi. Efficacy and safety of a microsomal triglyceride transfer protein inhibitor in patients with homozygous familial hypercholesterolaemia: a single-arm, open-label, phase 3 study. Lancet 2013; 381:40-46
  244. Blom DJ, Averna MR, Meagher EA, du Toit Theron H, Sirtori CR, Hegele RA, Shah PK, Gaudet D, Stefanutti C, Vigna GB, Larrey D, Bloedon LT, Foulds P, Rader DJ, Cuchel M. Long-Term Efficacy and Safety of the Microsomal Triglyceride Transfer Protein Inhibitor Lomitapide in Patients With Homozygous Familial Hypercholesterolemia. Circulation 2017; 136:332-335
  245. Samaha FF, McKenney J, Bloedon LT, Sasiela WJ, Rader DJ. Inhibition of microsomal triglyceride transfer protein alone or with ezetimibe in patients with moderate hypercholesterolemia. Nat Clin Pract Cardiovasc Med 2008; 5:497-505
  246. Hussain MM, Shi J, Dreizen P. Microsomal triglyceride transfer protein and its role in apoB-lipoprotein assembly. J Lipid Res 2003; 44:22-32
  247. Blom DJ, Cuchel M, Ager M, Phillips H. Target achievement and cardiovascular event rates with Lomitapide in homozygous Familial Hypercholesterolaemia. Orphanet J Rare Dis 2018; 13:96
  248. Sacks FM, Stanesa M, Hegele RA. Severe hypertriglyceridemia with pancreatitis: thirteen years' treatment with lomitapide. JAMA Intern Med 2014; 174:443-447
  249. Underberg JA, Cannon CP, Larrey D, Makris L, Blom D, Phillips H. Long-term safety and efficacy of lomitapide in patients with homozygous familial hypercholesterolemia: Five-year data from the Lomitapide Observational Worldwide Evaluation Registry (LOWER). J Clin Lipidol 2020; 14:807-817
  250. Larrey D, D'Erasmo L, O'Brien S, Arca M. Long-term hepatic safety of lomitapide in homozygous familial hypercholesterolaemia. Liver Int 2023; 43:413-423
  251. Agarwala A, Jones P, Nambi V. The role of antisense oligonucleotide therapy in patients with familial hypercholesterolemia: risks, benefits, and management recommendations. Curr Atheroscler Rep 2015; 17:467
  252. Raal FJ, Santos RD, Blom DJ, Marais AD, Charng MJ, Cromwell WC, Lachmann RH, Gaudet D, Tan JL, Chasan-Taber S, Tribble DL, Flaim JD, Crooke ST. Mipomersen, an apolipoprotein B synthesis inhibitor, for lowering of LDL cholesterol concentrations in patients with homozygous familial hypercholesterolaemia: a randomised, double-blind, placebo-controlled trial. Lancet 2010; 375:998-1006
  253. Stein EA, Dufour R, Gagne C, Gaudet D, East C, Donovan JM, Chin W, Tribble DL, McGowan M. Apolipoprotein B synthesis inhibition with mipomersen in heterozygous familial hypercholesterolemia: results of a randomized, double-blind, placebo-controlled trial to assess efficacy and safety as add-on therapy in patients with coronary artery disease. Circulation 2012; 126:2283-2292
  254. Santos RD, Duell PB, East C, Guyton JR, Moriarty PM, Chin W, Mittleman RS. Long-term efficacy and safety of mipomersen in patients with familial hypercholesterolaemia: 2-year interim results of an open-label extension. Eur Heart J 2015; 36:566-575
  255. Panta R, Dahal K, Kunwar S. Efficacy and safety of mipomersen in treatment of dyslipidemia: a meta-analysis of randomized controlled trials. J Clin Lipidol 2015; 9:217-225
  256. Duell PB, Santos RD, Kirwan BA, Witztum JL, Tsimikas S, Kastelein JJP. Long-term mipomersen treatment is associated with a reduction in cardiovascular events in patients with familial hypercholesterolemia. J Clin Lipidol 2016; 10:1011-1021
  257. Hashemi N, Odze RD, McGowan MP, Santos RD, Stroes ES, Cohen DE. Liver histology during Mipomersen therapy for severe hypercholesterolemia. J Clin Lipidol 2014; 8:606-611
  258. Raal FJ, Rosenson RS, Reeskamp LF, Hovingh GK, Kastelein JJP, Rubba P, Ali S, Banerjee P, Chan KC, Gipe DA, Khilla N, Pordy R, Weinreich DM, Yancopoulos GD, Zhang Y, Gaudet D. Evinacumab for Homozygous Familial Hypercholesterolemia. N Engl J Med 2020; 383:711-720
  259. Rosenson RS, Burgess LJ, Ebenbichler CF, Baum SJ, Stroes ESG, Ali S, Khilla N, Hamlin R, Pordy R, Dong Y, Son V, Gaudet D. Evinacumab in Patients with Refractory Hypercholesterolemia. N Engl J Med 2020; 383:2307-2319
  260. Rosenson RS, Burgess LJ, Ebenbichler CF, Baum SJ, Stroes ESG, Ali S, Khilla N, McGinniss J, Gaudet D, Pordy R. Longer-Term Efficacy and Safety of Evinacumab in Patients With Refractory Hypercholesterolemia. JAMA Cardiol 2023; 8:1070-1076
  261. Feingold KR. Triglyceride Lowering Drugs. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2024.
  262. Mattijssen F, Kersten S. Regulation of triglyceride metabolism by Angiopoietin-like proteins. Biochim Biophys Acta 2012; 1821:782-789
  263. Kersten S. New insights into angiopoietin-like proteins in lipid metabolism and cardiovascular disease risk. Curr Opin Lipidol 2019; 30:205-211
  264. Jaye M, Lynch KJ, Krawiec J, Marchadier D, Maugeais C, Doan K, South V, Amin D, Perrone M, Rader DJ. A novel endothelial-derived lipase that modulates HDL metabolism. Nat Genet 1999; 21:424-428
  265. Ishida T, Choi S, Kundu RK, Hirata K, Rubin EM, Cooper AD, Quertermous T. Endothelial lipase is a major determinant of HDL level. J Clin Invest 2003; 111:347-355
  266. Shimamura M, Matsuda M, Yasumo H, Okazaki M, Fujimoto K, Kono K, Shimizugawa T, Ando Y, Koishi R, Kohama T, Sakai N, Kotani K, Komuro R, Ishida T, Hirata K, Yamashita S, Furukawa H, Shimomura I. Angiopoietin-like protein3 regulates plasma HDL cholesterol through suppression of endothelial lipase. Arterioscler Thromb Vasc Biol 2007; 27:366-372
  267. Adam RC, Mintah IJ, Alexa-Braun CA, Shihanian LM, Lee JS, Banerjee P, Hamon SC, Kim HI, Cohen JC, Hobbs HH, Van Hout C, Gromada J, Murphy AJ, Yancopoulos GD, Sleeman MW, Gusarova V. Angiopoietin-like protein 3 governs LDL-cholesterol levels through endothelial lipase-dependent VLDL clearance. J Lipid Res 2020; 61:1271-1286
  268. Reeskamp LF, Millar JS, Wu L, Jansen H, van Harskamp D, Schierbeek H, Gipe DA, Rader DJ, Dallinga-Thie GM, Hovingh GK, Cuchel M. ANGPTL3 Inhibition With Evinacumab Results in Faster Clearance of IDL and LDL apoB in Patients With Homozygous Familial Hypercholesterolemia. Arterioscler Thromb Vasc Biol 2021:ATVBAHA120315204
  269. Wang Y, Gusarova V, Banfi S, Gromada J, Cohen JC, Hobbs HH. Inactivation of ANGPTL3 reduces hepatic VLDL-triglyceride secretion. J Lipid Res 2015; 56:1296-1307
  270. Musunuru K, Pirruccello JP, Do R, Peloso GM, Guiducci C, Sougnez C, Garimella KV, Fisher S, Abreu J, Barry AJ, Fennell T, Banks E, Ambrogio L, Cibulskis K, Kernytsky A, Gonzalez E, Rudzicz N, Engert JC, DePristo MA, Daly MJ, Cohen JC, Hobbs HH, Altshuler D, Schonfeld G, Gabriel SB, Yue P, Kathiresan S. Exome sequencing, ANGPTL3 mutations, and familial combined hypolipidemia. N Engl J Med 2010; 363:2220-2227
  271. Dewey FE, Gusarova V, Dunbar RL, O'Dushlaine C, Schurmann C, Gottesman O, McCarthy S, Van Hout CV, Bruse S, Dansky HM, Leader JB, Murray MF, Ritchie MD, Kirchner HL, Habegger L, Lopez A, Penn J, Zhao A, Shao W, Stahl N, Murphy AJ, Hamon S, Bouzelmat A, Zhang R, Shumel B, Pordy R, Gipe D, Herman GA, Sheu WHH, Lee IT, Liang KW, Guo X, Rotter JI, Chen YI, Kraus WE, Shah SH, Damrauer S, Small A, Rader DJ, Wulff AB, Nordestgaard BG, Tybjaerg-Hansen A, van den Hoek AM, Princen HMG, Ledbetter DH, Carey DJ, Overton JD, Reid JG, Sasiela WJ, Banerjee P, Shuldiner AR, Borecki IB, Teslovich TM, Yancopoulos GD, Mellis SJ, Gromada J, Baras A. Genetic and Pharmacologic Inactivation of ANGPTL3 and Cardiovascular Disease. N Engl J Med 2017; 377:211-221
  272. Stitziel NO, Khera AV, Wang X, Bierhals AJ, Vourakis AC, Sperry AE, Natarajan P, Klarin D, Emdin CA, Zekavat SM, Nomura A, Erdmann J, Schunkert H, Samani NJ, Kraus WE, Shah SH, Yu B, Boerwinkle E, Rader DJ, Gupta N, Frossard PM, Rasheed A, Danesh J, Lander ES, Gabriel S, Saleheen D, Musunuru K, Kathiresan S. ANGPTL3 Deficiency and Protection Against Coronary Artery Disease. J Am Coll Cardiol 2017; 69:2054-2063
  273. Ahmad Z, Banerjee P, Hamon S, Chan KC, Bouzelmat A, Sasiela WJ, Pordy R, Mellis S, Dansky H, Gipe DA, Dunbar RL. Inhibition of Angiopoietin-Like Protein 3 With a Monoclonal Antibody Reduces Triglycerides in Hypertriglyceridemia. Circulation 2019; 140:470-486
  274. Feingold KR. Lipoprotein Apheresis. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  275. Awad K, Mikhailidis DP, Toth PP, Jones SR, Moriarty P, Lip GYH, Muntner P, Catapano AL, Pencina MJ, Rosenson RS, Rysz J, Banach M. Efficacy and Safety of Alternate-Day Versus Daily Dosing of Statins: a Systematic Review and Meta-Analysis. Cardiovasc Drugs Ther 2017; 31:419-431
  276. Kennedy SP, Barnas GP, Schmidt MJ, Glisczinski MS, Paniagua AC. Efficacy and tolerability of once-weekly rosuvastatin in patients with previous statin intolerance. J Clin Lipidol 2011; 5:308-315
  277. Banach M, Serban C, Sahebkar A, Ursoniu S, Rysz J, Muntner P, Toth PP, Jones SR, Rizzo M, Glasser SP, Lip GY, Dragan S, Mikhailidis DP, Lipid, Blood Pressure Meta-analysis Collaboration G. Effects of coenzyme Q10 on statin-induced myopathy: a meta-analysis of randomized controlled trials. Mayo Clin Proc 2015; 90:24-34
  278. Schaars CF, Stalenhoef AF. Effects of ubiquinone (coenzyme Q10) on myopathy in statin users. Curr Opin Lipidol 2008; 19:553-557
  279. Skarlovnik A, Janic M, Lunder M, Turk M, Sabovic M. Coenzyme Q10 supplementation decreases statin-related mild-to-moderate muscle symptoms: a randomized clinical study. Med Sci Monit 2014; 20:2183-2188
  280. Bogsrud MP, Langslet G, Ose L, Arnesen KE, Sm Stuen MC, Malt UF, Woldseth B, Retterstol K. No effect of combined coenzyme Q10 and selenium supplementation on atorvastatin-induced myopathy. Scand Cardiovasc J 2013; 47:80-87
  281. Caso G, Kelly P, McNurlan MA, Lawson WE. Effect of coenzyme q10 on myopathic symptoms in patients treated with statins. Am J Cardiol 2007; 99:1409-1412
  282. Fedacko J, Pella D, Fedackova P, Hanninen O, Tuomainen P, Jarcuska P, Lopuchovsky T, Jedlickova L, Merkovska L, Littarru GP. Coenzyme Q(10) and selenium in statin-associated myopathy treatment. Can J Physiol Pharmacol 2013; 91:165-170
  283. Hlatky MA, Gonzalez PE, Manson JE, Buring JE, Lee IM, Cook NR, Mora S, Bubes V, Stone NJ. Statin-Associated Muscle Symptoms Among New Statin Users Randomly Assigned to Vitamin D or Placebo. JAMA Cardiol 2023; 8:74-80
  284. Becker DJ, Gordon RY, Halbert SC, French B, Morris PB, Rader DJ. Red yeast rice for dyslipidemia in statin-intolerant patients: a randomized trial. Ann Intern Med 2009; 150:830-839, W147-839
  285. Halbert SC, French B, Gordon RY, Farrar JT, Schmitz K, Morris PB, Thompson PD, Rader DJ, Becker DJ. Tolerability of red yeast rice (2,400 mg twice daily) versus pravastatin (20 mg twice daily) in patients with previous statin intolerance. Am J Cardiol 2010; 105:198-204

Endocrinology of Pregnancy

ABSTRACT

A coordinated sequence of events must occur in order to establish and successfully maintain a healthy pregnancy. Synchrony between the development of the early embryo and establishment of a receptive endometrium is necessary to allow implantation and subsequent progression of pregnancy. The endocrinology of human pregnancy involves endocrine and metabolic changes that result from physiological alterations at the boundary between mother and fetus. Known as the feto-placental unit (FPU), this interface is a major site of protein and steroid hormone production and secretion. Many of the endocrine and metabolic changes that occur during pregnancy can be directly attributed to hormonal signals originating from the FPU. The initiation and maintenance of pregnancy depends primarily on the interactions of neuronal and hormonal factors. Proper timing of these neuro-endocrine events within and between the placental, fetal, and maternal compartments is critical in directing fetal growth and development and in coordinating the timing of parturition. Maternal adaptations to hormonal changes that occur during pregnancy directly affect the development of the fetus and placenta. Gestational adaptations that take place in pregnancy include establishment of a receptive endometrium; implantation and the maintenance of early pregnancy; modification of the maternal system in order to provide adequate nutritional support for the developing fetus; and preparation for parturition and subsequent lactation.

INTRODUCTION

A coordinated sequence of events must occur in order to establish and successfully maintain a healthy pregnancy.  The endocrinology of human pregnancy involves endocrine and metabolic changes that result from physiological alterations at the boundary between mother and fetus. Known as the feto-placental unit (FPU), this interface is a major site of protein and steroid hormone production and secretion (Figure 1). Additionally, it serves as an endocrine, respiratory, alimentary, and excretory organ, facilitating the exchange of nutrients and metabolic products between the mother and fetus. The fetus is dependent on this effective exchange with the mother for its proper intrauterine growth and development. Thus, it is not surprising that the fetus initiates and influences maternal adaptations to optimize this exchange via complex hormonal mechanisms. Many of the endocrine and metabolic changes that occur during pregnancy can be directly attributed to hormonal signals originating from the FPU. The initiation and maintenance of pregnancy depends primarily on the interactions of neuronal and hormonal factors. Proper timing of these neuro-endocrine events within and between the placental, fetal, and maternal compartments is critical in directing fetal growth and development and in coordinating the timing of parturition. Maternal adaptations to hormonal changes that occur during pregnancy directly affect the development of the fetus and placenta. Gestational adaptations that take place in pregnancy include establishment of a receptive endometrium; implantation and the maintenance of early pregnancy; modification of the maternal system in order to provide adequate nutritional support for the developing fetus; and preparation for parturition and subsequent lactation.

 Figure 1. The interface between mother and fetus, known as the feto-placental unit (FPU), is a major site of protein and steroid hormone production and secretion. 

ENDOMETRIAL RECEPTIVITY 

The menstrual cycle, involves a synchronous production of ovarian steroid hormones, estrogen and progesterone, which induces structural and functional changes within the endometrium in anticipation for embryo implantation and the establishment of a pregnancy.  During the luteal phase, under the primary influence of progesterone, the proliferative endometrium changes into secretory endometrium, which is well vascularized and composed of spiral arteries. A favorable environment for implantation is established via chemokines, growth factors, and cell adhesion molecules (CAMs) produced by the glandular secretory endometrium (1). The chemokines and CAMs serve to attract the blastocyst to the specific sites of implantation where the endometrium is strategically prepared for invasion and placentation (1). When implantation does not occur, a timely regression and destruction of the fully developed endometrium leads to menstruation. However, if implantation occurs, the endometrium continues to grow and undergoes further morphological and molecular changes to provide supportive environment for the growing embryo (2).

Endometrial “receptivity” refers to this physiological state when the endometrium allows a blastocyst to attach, firmly adhere, penetrate, and induce localized changes in the endometrial stroma resulting in decidualization (2). The specific period, known as the “implantation window” opens 4-5 days after endogenous or exogenous progesterone stimulation and closes approximately 9-10 days later (3, 4). Implantation has three stages: apposition, adhesion and penetration. Apposition is an initial unstable adhesion of the blastocyst to the endometrial surface.  This stage is characterized histologically by the appearance of microprotrusions from the apical surface of the epithelium, termed pinopodes, occurring six days after ovulation and retained for 24 hours during the implantation window. The pinopods express chemokines and CAMs, which attract the blastocyst floating within the endometrial cavity to appose.  Additionally, the smooth surface of the pinopodes facilitates the apposition of the blastocyst to the endometrium.  Further encouraging the blastocyst to appose to the pinopods is the removal of adhesion inhibiting mucin, while the areas between pinopods have been shown to express MUC-1, which prevents embryo adhesion (5).  Once the blastocyst is apposed, a stronger attachment is achieved through local paracrine signaling between the embryo and the endometrium. At this stage, the blastocyst is sufficiently adherent to the endometrium as to resist dislocation of the blastocyst by flushing the uterine lumen. The first sign of the attachment reaction coincides with a localized increase in stromal vascular permeability which is manifested as stromal edema at the site of blastocyst attachment (6).  Thus, vascular changes also appear to be an important factor in establishing endometrial receptivity. Following adhesion, the embryo invades through the luminal epithelium into the stroma to establish a relationship with the maternal vasculature. In response to this invasion and the presence of progesterone stimulation, the endometrial stromal cells undergo a process termed decidualization by which they differentiate and become specialized decidual stromal cells. Decidualization is essential for the survival and continued development of the pregnancy. In humans, decidual changes occur throughout the entire endometrium during the luteal phase even in the absence of an embryo, but become widespread in early gestation. These decidual stromal cells are very metabolically active and support the implanting embryo by secreting a wide array of hormones and growth factors including prolactin, relaxin, insulin-like growth factors (IGFs) and insulin growth factor binding proteins (IGFBPs). The endometrial stromal cells are the precursors of decidual stromal cells and appear to originate from both resident uterine mesenchymal stem cells as well as adult bone marrow-derived stem cells (7, 8). Interestingly, bone marrow-derived progenitors have been shown to give rise to functional prolactin-producing decidual stromal cells in decidua of pregnant mice, and appear to play an important role in implantation and pregnancy maintenance (9). The bone marrow is also the source of many leukocytes that infiltrate the endometrium during the secretory phase. In humans, a large influx of leukocytes to the uterus occurs in response to ovulation and rising ovarian P4 production, elevating them to 40% of all endometrial cells in the mid-late secretory phase of the menstrual cycle (10). This gain in leukocyte numbers is primarily due to the accumulation of uterine natural killer (uNK) cells. Studies in mice additionally show that the selected entry of uNK cells into early decidua optimizes angiogenesis and promotes decidual spiral artery vascular remodeling. This influences the timing of uterine lumen closure and thereby the appropriate rate of early fetal development including initiation of trophoblast invasion (11). Macrophages are the second most abundant leukocyte population in the luteal phase endometrium. In addition to uterine NK cells and macrophages, the endometrium contains T cells with no apparent cyclic changes, and rare populations of dendritic cells in luteal phase endometrium, both of which become more abundant in the pregnant decidua. The composition and function of these immune cells at the implantation site and the maternal-fetal interface are highly specialized to foster embryo and placental development and to minimize the chance of immune rejection (12).   

Progesterone is essential in mediating the changes that the endometrium undergoes in the luteal phase in preparation for embryo implantation (13). The effects of progesterone on the uterus have been elucidated through elegant experiments in knockout mice as well as studies using progesterone receptor (PR) antagonists. Mice with global PR knockout are infertile due to defects in ovulation and implantation (14). Their endometrium displays hypertrophy and inflammation of the glandular epithelium associated with failure to undergo decidualization. Mice with a specific knockout in PR-B isoform, however, have normal ovarian function, implantation and reproductive capacity (15, 16). In contrast, mice with a specific knockout in PR-A exhibit lack of decidualization in the endometrial stroma along with endometrial epithelial hyperplasia and inflammation (15, 16), indicating that PR-A is critical for embryo implantation and the normal function of the endometrial epithelium and stroma, while PR-B promotes epithelial hyperplasia of the endometrium. Moreover, administration of the progesterone antagonist mifepristone (RU486) in humans during pregnancy induces abortion, fetal loss or parturition, depending on the gestational age (17, 18). If administered at low doses at the mid- or late follicular phase, it prevents pregnancy by delaying endometrial maturation, while at high doses it delays the LH surge and inhibits ovulation (19, 20).   

The key to endometrial receptivity is the dynamic and precisely controlled molecular and cellular events that involve coordinated effects of autocrine, paracrine, and endocrine factors. Analysis of the transcriptosome of the endometrium during the implantation window using microarray technology has revealed numerous genes that are up- and down-regulated during the “window of implantation” when compared with late proliferative phase endometrium (4, 21). In particular, transcription factors such as the homeobox (HOX) genes are essential for endometrial receptivity by mediating some functions of the sex steroids. HOXA10- and HOXA11-deficient mice have uterine factor infertility due to an implantation defect (22, 23). Both HOXA10 and HOXA11 mRNAs are expressed in human endometrial epithelial and stromal cells; their expression is upregulated by estrogen and progesterone, and is significantly higher in the mid- and late-secretory phases, coinciding with time of embryo implantation (24, 25). As transcription factors, HOX genes regulate other downstream target genes specific to the implantation window, including pinopodes, β3 integrin and insulin-like growth factor-binding protein-1 (IGFBP-1), leading to the proper development of the endometrium and receptivity to implantation (26). Other growth factors, cytokines, and transcription factors produced by the endometrium also assist in the establishment of endometrial receptivity (26, 27).  Impaired endometrial receptivity is considered to be a major limiting factor for the establishment of a pregnancy. Implantation during this time of uterine receptivity is associated with high (85%) success rate for continuing a pregnancy, whereas implantation after cycle day 25 has a much lower success rate (11%) (28).

IMPLANTATION

Pregnancy-related proteins can be found in maternal circulation shortly after fertilization. For example, platelet activating factor (PAF)-like substance, which is produced by the fertilized ovum, is present almost immediately (29-32). After ovulation and fertilization, the embryo remains in the ampullary portion of the fallopian tube for up to 3 days. The embryo undergoes a sequence of cell divisions and differentiation that is not dependent on the hormonal milieu of the fallopian tube or the uterus, as fertilization and early embryonic development occur successfully in vitro. The developing conceptus travels toward the uterus, through the isthmic portion of the tube, for approximately 10 hours, and then enters the uterus as an embryo at the 2- to 8-cell stage (33, 34). With further development, between 3-6 days after fertilization, the embryo becomes a blastocyst floating unattached in the endometrial cavity (34). A schematic representation of the pre-implantation phase of pregnancy is shown in Figure 2. Before implantation, the blastocyst also secretes specific substances that enhance endometrial receptivity. Successful implantation requires precise synchronization between blastocyst development and endometrial maturation. Indeed, there appears to be a cross-talk between the embryo and the endometrium with the endometrium acting as a biosensor that is able to respond favorably to competent embryos but less favorably to incompetent poorly viable embryos destined to fail (35). Ultimately, implantation failure is the result of impaired embryo developmental competence or impaired endometrial receptivity, both having negative effects on the embryo-endometrium cross-talk. It is estimated that embryos account for one third of implantation failures, while suboptimal endometrial receptivity and aberrant embryo-endometrial cross-talk are responsible for the remaining two-thirds (36).

Figure 2. A diagrammatic summary of the ovarian cycle leading to embryo development as it occurs during the first week after fertilization. (Adapted from (37), with permission)

To date, little information exists regarding regulation of steroid production in the embryo. The early embryo and its surrounding cumulus cells secrete detectable estradiol and progesterone well before the time of implantation (38, 39). Mechanical removal of these cells results in the cessation of steroid secretion, while return of the removed cells through co-culture results in restoration of steroid secretion (38). Given this finding, steroid production by the conceptus is thought to be negligible by the time it has reached the endometrial cavity, since it is gradually denuded of cumulus cells as it travels through the fallopian tube.

Conceptus-secreted progesterone may itself affect tubal motility as the conceptus is carried to the uterus (40). Progesterone, by action mediated through catecholamines and prostaglandins (PG), is believed to relax utero-tubal musculature. Moreover, progesterone is thought to be important in tubal-uterine transport of the embryo to the uterine cavity, since receptors for progesterone are found in highest concentrations in the mucosa of the distal one third of the fallopian tube. Estradiol, also secreted by these structures, may balance the progesterone effect so as to maintain the desired level of tubal motility and tone (40). Progesterone antagonizes estrogen-augmented uterine blood flow through depletion of estrogen receptors in the cytoplasm (41). Likewise, estrogen and progesterone also appear to balance one another in the maintenance of blood flow at the implantation site. Both estrogen and progesterone are known to upregulate the expression of multiple angiogenic factors in the uterus, including VEGF, bFGF, PDGF, and TGF-β (42). It is well known that estrogen stimulates an increase in uterine angiogenesis, blood flow and vasodilation by acting both directly on endothelial cells, and/or indirectly on other endometrial cell types via numerous potential promoters (43). In pregnant baboons and sheep, estrogen stimulated uterine and placental blood flow (44). Estrogen treatment significantly increased the paracellular cleft width between endometrial endothelial cells within 6 h considered to result in the increased vascular permeability associated with estrogen administration (45). Unlike estrogen, the angiogenic effects of progesterone in the uterus are believed to occur without concurrent vasodilation (46), as there was no change in endometrial endothelial paracellular cleft width 6 h after progesterone treatment in baboons (45). However, much is still unknown regarding uterine blood flow regulation in pregnancy and how the implanting embryo may influence this process. Human chorionic gonadotropin (hCG) messenger ribonucleic acid (mRNA) is detectable in the blastomeres of 6- to 8-cell embryos; however, it is not detectable in blastocyst culture media until the 6th day (47-49). After implantation is initiated, the embryo is actively secreting hCG, which can be detected in maternal serum as early as the 8th day after ovulation. However, due to the absence of direct vascular communication, secretion of hCG into the maternal circulation is initially limited (50). The primary role of hCG is to prolong the biosynthetic activity of the corpus luteum, which allows continued progesterone production and maintenance of the gestational endometrium. As implantation progresses, the conceptus continues to secrete hCG and other pregnancy-related proteins, and resumes detectable steroid production (38, 39, 51).

Termed trophectoderm (aka outer cell mass), blastomeres lining the periphery of the blastocyst are destined to form the placenta and can be identified at 5 days post-fertilization. The main structural and functional units of the placenta are the chorionic villi, which increase significantly in number during the first trimester of pregnancy. The structure of the chorionic villi is pictured in Figure 3. The villous structure provides a tremendous absorptive surface area to facilitate exchange between the maternal and fetal circulation. The maternal blood arrives from the spiral arteries and circulates through the intervillous space. Fetal blood moves in the core of the chorionic villi within the villous vessels; thus, fetal and maternal blood is never mixed in this system. The key cells inside the chorionic villi are the cytotrophoblasts. They have the ability to proliferate, invade and migrate or to differentiate, through aggregation and fusion, to form a syncytial layer of multi-nucleate cells lining the placental villi, known as the syncytiotrophoblasts.

By 10 days post-fertilization, 2 distinct layers of invading trophoblasts have formed. The inner layer, the cytotrophoblasts, is composed of individual, well-defined and rapidly dividing cells. The outer layer, the syncytiotrophoblasts, is a thicker layer comprised of a continuous cell mass lacking distinct cell borders. Syncytiotrophoblasts line the fetal side of the intervillous space opposite the decidualized endometrium of the maternal side. Immunohistochemically, cytotrophoblasts stain for hypothalamic-like protein hormones: gonadotropin releasing hormone (GnRH), corticotrophin releasing hormone (CRH), and thyrotropin releasing hormone (TRH) (52-64). Juxtaposed syncytotrophoblasts stain immunohistochemically for the corresponding pituitary-like peptide hormones: human chorionic gonadotropin (hCG; analogous to pituitary luteinizing hormone, LH), adrenocorticotropic hormone (ACTH) and human chorionic thyrotropin (hCT). Anatomically, this arrangement suggests that these 2 layers mirror the paracrine relationship of the hypothalamic-pituitary axis (52-64).

Syncytiotrophoblasts, the principal site of placental steroid and protein hormone biosynthesis, have a large surface area and line the intervillous space which exposes them directly to maternal bloodstream without the vascular endothelium and basement membrane which separates them from the fetal circulation (Figure 3-5). This anatomic arrangement explains why placental proteins are secreted almost exclusively into the maternal circulation in concentrations much higher than those in the fetus (65). The syncytiotrophoblast layer contains the abundant subcellular machinery characteristic of cells primarily responsible for hormone synthesis. Amino acids of maternal origin are assembled into pro-hormones. Pro-hormones are then packaged into early secretory granules and transferred across the trophoblastic cell membranes as mature granules. Mature granules become soluble as circulating hormones in maternal blood as they pass through the intervillous space (65).

Figure 3. A. A depiction of a blastocyst implanting in the uterus. B. A longitudinal section of a chorionic villus at the feto-maternal interface at about 10 weeks' gestation. The villous serves as a bridge between maternal and fetal compartments. C. Human placental ultra-structure seen in cross section. Syncytiotrophoblasts line the fetal surface of the intervillous space and interact with the maternal blood supply to secrete placental hormones directly into the circulation. Decidua lines the maternal surface of the intervillous space and secretes protein hormones. (From (66), with permission)

DECIDUA AND DECIDUAL HORMONES

The decidua is the endometrium of pregnancy. Decidualized endometrium is a site of maternal steroid and protein biosynthesis that relates directly to the maintenance and protection of the pregnancy from immunologic rejection. For instance, decidual tissue secretes cortisol, and in combination with hCG and progesterone secreted by the conceptus, cortisol produced by the decidua acts to suppress the maternal immune response conferring the immunologic privilege required by the implanting conceptus (67, 68).

Decidual Prolactin

Decidual prolactin is a peptide hormone having chemical and biological properties identical to pituitary prolactin (69). Prolactin, derived from decidualized endometrium, is first detectable in the endometrium at a time corresponding to implantation-cycle day 23. Progesterone is known to induce decidual prolactin secretion (70). Scant decidual prolactin enters the fetal or maternal circulation after it is transported across the fetal membranes from the adherent decidua and is released into the amniotic fluid (71). Unaffected by bromocriptine administration, decidual production of prolactin takes place independent of dopaminergic control (69).

Decidual prolactin secretion rises in parallel with the gradual rise in maternal serum prolactin seen until 10 weeks’ gestation, then it rises rapidly until 20 weeks, and falls as term approaches (72). Decidua-derived prolactin serves to regulate fluid and electrolyte flux through fetal membranes by reducing permeability of the amnion in the fetal-to-maternal direction (69-71, 73-77). Circulating prolactin in the fetus is secreted by the fetal pituitary gland, while prolactin found in the maternal circulation is secreted by the maternal pituitary gland under the influence of estrogens.  Unlike decidual prolactin, these circulating levels are both suppressed by maternal ingestion of bromocriptine.

Decidual Insulin-like Growth Factor Binding Protein-1 (IGFBP-1)

IGF binding protein-1 (IGFBP-1) is a peptide hormone that originates from decidual stromal cells. In non-pregnant women, circulating IGFBP-1 does not change during cycling of the endometrium, while IGFBP-3 is the main circulating IGFBP. During pregnancy, however, there is a several-fold increase in serum IGFBP-1 levels that begins during the first trimester, peaks during the second trimester, and falls briefly only to peak a second time before term (78). IGFBP-1 inhibits the binding of insulin-like growth factor (IGF) to receptors in the decidua and inhibits fetal growth. Newborn birth weight correlates directly with maternal IGF-1 levels, and inversely with circulating IGFBP-1 levels (79).  

Progesterone-Associated Endometrial Protein (PAEP)

Previously known as pregnancy protein-14, PAEP is a glycoprotein hormone synthesized by secretory and decidualized endometrium that is detectable around cycle day 24 (80). In serum, it rises sharply around cycle day 22 to 24, reaching its peak value at the onset of menstruation; if pregnancy occurs, levels remain high (81). In pregnancy, PAEP rises in parallel with hCG (78). Like hCG, PAEP is thought to have immunosuppressant properties in pregnancy (80). PAEP levels are often low in those patients with conditions, such as ectopic pregnancy, in which there is little decidual tissue produced (82).

PROLONGATION OF CORPUS LUTEUM FUNCTION

Primary steroid products of the corpus luteum are progesterone, 17β-progesterone, estradiol and androstenedione. Low-density lipoprotein (LDL) cholesterol is the main precursor responsible for corpus luteum progesterone production (83). Between 6- and 7-weeks’ gestation, corpus luteum function naturally begins to decline. During this luteal-placental transition period, production of progesterone shifts to the developing placenta (Figure 4).

Pulsatile pituitary LH secretion in the early luteal phase followed by hCG secreted from the implanting conceptus act to stimulate progesterone production from the corpus luteum. Removal of the corpus luteum before 6 weeks of gestation increases the risk of abortion (67a). Thus, regarding early pregnancy, progesterone is considered the most important steroid product in this group because progesterone alone can maintain a pregnancy that would otherwise abort in a lutectomized woman (84). For example, exogenous progesterone, given to an agonadal woman pregnant through egg-donor in vitro fertilization (IVF), maintains the pregnancy through the first trimester until placental progesterone secretion is established (85). For this reason, in patients with corpus luteum dysfunction or in whom the corpus luteum has been removed surgically, supplementation with exogenous progesterone is frequently initiated and extended beyond approximately 10 weeks of gestation, the critical period of the luteal-placental shift.

Figure 4. A shift in progesterone production from the corpus luteum to the placenta occurs at approximately the 7th to 9th week of gestation. The small, shaded area represents the estimated duration of this functional transition. (From (86), with permission)

In women with first-trimester threatened abortion, progesterone concentrations at the time of initial evaluation are often predictive of ultimate outcome (87). Abortion will occur in approximately 80% of those with progesterone concentrations under 10 ng/mL; viable pregnancies are virtually never observed at concentrations of <5.0 ng/mL (88).

Corpus Luteum Relaxin

Relaxin is a peptide hormone produced by the corpus luteum, and not detected in non-pregnant women or men.  Although it is argued that the corpus luteum is the sole source of relaxin in pregnancy, it has also been identified in the placenta, decidua and chorion (89-91). The maternal serum concentrations of relaxin rise during the first trimester, when the corpus luteum is dominant, and decline in the second trimester. Interestingly, when women with a normal pregnancy were compared with pregnant women using egg donor (therefore, no corpus luteum), relaxin was only identified in the women with a pregnancy derived from her own eggs.  However, the duration of pregnancy and labor outcomes were not different between the two groups (92).  The presence of relaxin suggests that it may play a role in early pregnancy, but its function is still unclear. 

In animals, relaxin softens (ripens) the cervix, inhibits uterine contractions, and relaxes the pubic symphysis (93).  These changes are similar to those seen during human labor.  Additionally, in vitro studies of human cervical stromal cells have shown that relaxin induces changes consistent with cervical ripening (94, 95).  Human relaxin primarily binds to relaxin receptors in the decidua and chorionic cytotrophoblasts (96).  Relaxin, originating in the decidua and binding to its receptors in the fetal membranes, increases cytokine levels that can activate matrix metalloproteinases and lead to rupture of fetal membranes and labor (97). Thus, relaxin may play a facilitatory role in labor, however its role is still not clearly defined.  

PLACENTAL COMPARTMENT

Unique to mammals, the placenta plays a major role in balancing fetal growth and development with maternal homeostasis. The fetus develops in an environment where respiration, alimentation and excretory functions are provided by the placenta. The human placenta is hemochorial, which means the chorion is in direct contact with maternal blood. Cyto- and syncytiotrophoblast cells of the placenta have direct access to the maternal circulation.  In contrast, the trophoblast layer prevents most maternal hormones from entering the fetal compartment, and consequently the fetal/placental endocrine system generally develops and functions independently of that of the mother.  Over time, the placenta has evolved as a system through which viviparity or livebirth could take place with dependable success.

The placenta functions, to some extent, as a hypothalamic-pituitary-end organ-like entity owing to the inherent ability of this type of system, with its stimulatory and inhibitory feedback mechanisms, to dynamically regulate factors that affect fetal growth and development under a variety of conditions. In the fully developed hypothalamic-pituitary-end organ schema of humans, neural inputs to the hypothalamus serve to regulate the secretion of hypothalamic releasing hormone peptides. However, in the placenta there are no such direct neural inputs, and the exact mechanism(s) responsible for regulation of the secretion of hypothalamic-like placental peptides is unknown.

Changes in maternal hormone concentrations play a critical role in modulating the metabolic and immunologic changes required for successful outcome in pregnancy. The fetus and placenta produce and secrete steroids and peptides into the maternal circulation as well as stimulate maternal hormone production. The origins and amounts of the fetal and placental hormones secreted during pregnancy changes dramatically over the course of the gestational period. Some of the pregnancy-related protein hormones previously discussed are, in part, responsible for the altered steroid concentrations typical of pregnancy.

Placental Steroid Hormones

The placenta is a site of active steroidogenesis which depends on highly integrated and active interactions with both mother and fetus. This is consequent to an elegant complementary of enzymatic deficiencies between placental and fetal compartments (Table 1). The placenta is characterized by significant aromatase, sulfatase, and 11b-hydroxysteroid dehydrogenase type 2 activities juxtaposed with a lack of P450C17 (17a-hydroxylase and 17/20 lyase) activity.

Table 1. Enzymatic Limitations by Compartment

Fetal

Placental

3b-hydroxysteroid dehydrogenase

17a-hydroxylase

 

StAR protein

17/20 lyase

16α-hydroxylase

PLACENTAL PROGESTERONE

The placenta is the main source of progesterone during pregnancy. From the luteal phase to term, maternal progesterone levels rise six- to eight-fold. (Figures 5 and 8) Although, progesterone originates almost entirely from the corpus luteum before 6 weeks' gestational age, its production shifts more to the placenta after the 7th week. Beyond 10 weeks, the placenta is the major definitive source of progesterone (51, 98).

While the placenta produces large amounts of progesterone, it has a limited capacity to synthesize cholesterol de novo (Figure 7). Maternal cholesterol enters the trophoblasts in the form of low-density lipoprotein (LDL) cholesterol which serves as the principal precursor for the biosynthesis of progesterone by the placenta (51, 83, 99). The fetal contribution of progesterone is negligible. This is evident as progesterone levels remain high even after fetal demise.  In the non-human primate estrogen regulates placental progesterone production (100). Progesterone concentrations are less than 1 ng/mL during the follicular phase of the normal menstrual cycle (101, 102). However, in the luteal phase of cycles in which fertilization occurs, progesterone concentrations rise from about 1-2 ng/mL on the day of the LH surge to a plateau of approximately 10-35 ng/mL over the subsequent 7 days. Concentrations remain within this luteal-phase range from the 10th week from the last menstrual flow, and then show a sustained rise that continues until term (Figure 5). At term, progesterone concentrations can range from 100-300 ng/mL (51). Most of the progesterone produced in the placenta enters the maternal circulation.

Figure 5. Relative values of circulating concentrations (mean ±SEM) of progesterone and 17α-progesterone during the course of human pregnancy from fertilization to term. The data displayed demonstrates values before and after the luteinizing hormone (LH) surge. Gestational ages are calculated from last menstrual flow. (Adapted from (103), with permission)

The human deciduas and fetal membranes also synthesize and metabolize progesterone (104).  In this case, neither cholesterol nor LDL-cholesterol are significant substrates; pregnenolone sulfate may be the most important precursor.  Progesterone has been shown to exert important functions in implantation and parturition to include promotion of endometrial decidualization; inhibition of smooth muscle contractility; decrease in prostaglandin (PG) formation, which helps maintain myometrial quiescence and prevent the onset of uterine contractions; and inhibition of immune responses like those involved in graft rejection. It is believed to work in concert with hCG and decidual cortisol to inhibit T-lymphocyte-mediated tissue rejection and confer immunologic privilege to the implanted conceptus and developing placenta (105, 106). In animal models, progesterone extends the survival of transplanted human trophoblasts, and high intervillous concentrations of progesterone are of major importance in blocking the cellular immune rejection of the foreign antigens originating from the pregnancy (106).

In addition to its roles in endometrial and myometrial function, progesterone also serves as a substrate for fetal adrenal gland production of glucocorticoids (cortisol) and mineralocorticoids (aldosterone) (107). This important function is consequent to the deficiency of 3b-hydroxysteroid dehydrogenase (3b-HSD) activity in the active fetal zone of the fetal adrenal gland.

PLACENTAL 17α-HYDROXYPROGESTERONE

Like progesterone, during the first several weeks of gestation and through the time of the luteal-placental shift, 17α-hydroxyprogesterone concentrations primarily reflect the steroidogenic status of the corpus luteum (108). However, by the tenth week of gestation, 17 α-hydroxyprogesterone has returned to baseline levels, indicating that the placenta has little 17 α-hydroxylase activity.  During the third trimester the placenta uses fetal D5-sulfoconjugated precursors to secrete increasing amounts of 17α-hydroxyprogesterone, and at this point the placenta becomes the major source of this hormone at term (108).

Concentrations of 17α-hydroxyprogesterone are less than 0.5 ng/mL during the follicular phase of normal menstrual cycles. In cycles leading to pregnancy, 17α-hydroxyprogesterone concentrations rise to about 1 ng/ml on the day of the LH surge, decline slightly for about 1 day, and rise again over the subsequent 4-5 days reaching a level of 1-2 ng/ml. Concentrations then increase slightly to a mean of approximately 2 ng/ml (luteal phase levels) by the end of the 12th week. This level remains stable until a gestational age of about 32 weeks at which time it begins an abrupt, sustained rise at about 37 weeks to approximately 7 ng/ml, a level that persists until term (108) (Figures 5 and 8). The rise in 17α-hydroxyprogesterone that begins at 32 weeks strongly correlates with the fetal maturational processes known to begin at this time. Hence, 17α-hydroxyprogesterone concentration exhibits a bimodal pattern in normal pregnancy.

PLACENTAL 17β-ESTRADIOL

The corpus luteum is the exclusive source of 17β-estradiol during the first 5-6 weeks of gestation. After the first trimester, the placenta is the major source of circulating 17β-estradiol (51). The rate of estrogen production and the level of circulating estrogens increase markedly during pregnancy. Concentrations of 17β-estradiol are less than 0.1 ng/mL during the follicular phase of the cycle and reach about 0.4 ng/mL during the luteal phase of normal menstrual cycles (101). Following fertilization, 17β-estradiol increases gradually to a range of 6-30 ng/mL at term (102) (Figures 6 and 8). Because it is deficient in 17-hydroxylase enzyme activity and 17-20 desmolase (lyase) activity, the placenta is unable to convert progestogens to estrogens. Thus, the placenta relies on 19-carbon androgen precursors produced by the fetal and maternal adrenal glands. Sources of estrogen biosynthesis by the maternal-fetal-placental unit are depicted in Figure 8. The major source of fetal adrenal dihydroepiandrostenediene sulfate (DHEAS) is LDL-cholesterol circulating in the fetal blood. A minor source of fetal adrenal DHEAS is derived from pregnenolone secreted by the placenta. Twenty percent of fetal cholesterol is derived from the maternal compartment. Since amniotic fluid cholesterol levels are negligible, the main source of cholesterol is the fetal liver. As gestation advances, increasing quantities of 17β-estradiol are synthesized from the conversion of circulating maternal and fetal DHEAS by the placenta. At term, approximately equal amounts of estrogens are produced from circulating maternal DHEAS and fetal DHEAS (51, 109). The fetal endocrine system is notable for extensive conjugation of steroids with sulfate. Consequently, the placenta relies on sulfatase activity to cleave sulfate conjugates in the fetal compartment. Naturally occurring placental sulfatase deficiency results in a low estrogen state in pregnancy (110).

The cytochrome P450 aromatase enzyme is responsible for converting 19-carbon precursors to estrogen (111). The efficiency of this enzyme affords the fetus protection from virilization even in the presence of large amounts of aromatizable androgens.

Figure 6. Relative values of circulating concentrations (mean ±SEM) of 17β-estradiol, estriol and estrone during the course of human pregnancy from fertilization to term. Data displayed demonstrate values before and after the luteinizing hormone (LH) surge. Gestational ages are calculated from last menstrual flow. (Adapted from (112), with permission)

The vasodilatory function of estrogens in pregnancy are well described. In animal models, direct estrogen injection into the uterine arteries produces striking increases in blood flow. Without question, 17β-estradiol is the most potent estrogen in this role. Estriol and estrone, though less active, also produce this effect (113). Because the exposure of the utero-placental bed to direct estriol secretion is enormous, estriol may be the principal up-regulator of uterine blood flow. This may be the dominant role of estriol in human pregnancy (113). Estrogen regulated mechanisms may also allow the fetus to govern production and secretion of progesterone during the third trimester. In primates, estrogen regulates the biosynthesis of placental progesterone by regulating the availability of LDL-cholesterol for conversion to pregnenolone and its downstream steroid products (114). Estrogens are also thought to contribute to mammary gland development and fetal adrenal gland function.

PLACENTAL ESTRADIOL

Estriol is first detectable in maternal serum at 9 weeks of gestation (51, 109, 115, 116). This temporal relationship closely corresponds to the early stages of steroidogenic maturation in the fetal adrenal cortex (51). Hence, the continued production of estriol is dependent upon the presence of a living fetus. Concentrations of estriol are less than 0.01 ng/ml in non-pregnant women. First detectable at approximately 0.05 ng/ml by 9 weeks, estriol increases gradually to a range of approximately 10-30 ng/ml at term (51, 98, 115, 117). Between 35- and 40-weeks gestational age, estriol concentrations increase sharply in a pattern that reflects a final surge of intrauterine steroidogenesis just prior to term (Figures 6 and 8).

Figure 7. Synthesis of estrogen and progesterone within and between the maternal, placental and fetal compartments. (Adapted from (118), with permission)

The placenta lacks 16a-hydroxylase activity and consequently, estriol with its 16a-hydroxyl group, must be synthesized from an immediate fetal precursor. The fetal liver provides 16a-hydroxylation of DHEAS for placental estriol synthesis. Interestingly, hepatic 16a-hydroxylation activity disappears postnatally.

Figure 8. Circulating maternal steroid hormone levels throughout early pregnancy. The first-trimester relationship of these steroid hormones to human chorionic gonadotropin (hCG) is shown.

Progestogens
Progesterone o--o--o-
17-a-hydroxyprogesterone -Δ-Δ-Δ-

Estrogens
17-
β-estradiol ---
Estriol -o-o-o-
Estrone -x-x-x-

Human chorionic gonadotropin (hCG)
-Δ-Δ-Δ- 
(From ref. 89, with permission)

PLACENTAL ESTRONE

For the first 4-6 weeks of pregnancy, estrone originates primarily from maternal sources such as the ovaries, adrenals, or peripheral conversion (102). Later, the placenta secretes increasing quantities of estrone from the conversion of circulating maternal and fetal DHEAS. The placenta continues to be the major source of circulating estrone for the remainder of the pregnancy (51). Estrone concentrations are less than 0.1 ng/mL during the follicular phase and may reach a maximum of 0.3 ng/mL during the luteal phase of a normal menstrual cycle. Following fertilization, estrone concentrations remain within the luteal phase range through weeks 6-10 of gestation (98). Subsequently there is a gradual increase to a wide range of 2-30 ng/ml at term (51, 98, 102) (Figures 6 and 8). In the absence of fetal adrenal gland function, as in the case of anencephaly, maternal estrogen levels are extremely low, suggesting that the maternal contribution of DHEAS to total estrogen synthesis is negligible.

Placental Protein Hormones

As detailed previously, the placental cytotrophoblast-syncytiotrophoblast relationship mirrors the hypothalamic-pituitary system. The surface of the syncytiotrophoblast is in direct contact with maternal blood within the intervillous space, and consequently, placental proteins are preferentially secreted into the maternal compartment. Table 2 outlines the various peptides associated with the endocrinology of human pregnancy.

Table 2. Pregnancy Specific Protein Hormones by Compartment

Fetal

Placental

Maternal

Alpha-fetoprotein

Hypothalamic-like (cytotrophoblast)         

- GnRH                                                

- CRH                                     

- TRH                                     

- GHRH                                  

- Somatostatin           

Pituitary-like (syncytiotrophoblast)           

- hCG

- hGH

- ACTH

- hPL

- hCT                                      

- Oxytocin

Growth factors

- Inhibin

- Activin                                              

- IGF-I/IGF-II

Other proteins

- Pregnancy specific β1-glycoprotein

- PAPP-A

Decidual derived

-Prolactin

-IGFBP-1

-PP14

Corpus luteum derived

-Relaxin

PLACENTAL PROTEINS: HYPOTHALAMIC-LIKE PROTEINS

Placental Gonadotropin Releasing Hormone (GnRH)

Gonadotropin releasing hormone derived from the placenta is biologically and immunologically similar to the hypothalamic decapeptide GnRH (54). Gonadotropin releasing hormone activity has been localized to the cytotrophoblast cells along the outer surface of the syncytiocytotrophoblast layer. Human chorionic gonadotropin (hCG) has been localized to the adjacent syncytiocytotrophoblast layer. GnRH production peaks at about 8 weeks’ gestation and then decreases as the pregnancy advances in gestational age (54-57). Furthermore, GnRH levels parallel those of hCG in both the placenta and maternal circulation (57).

Placental GnRH stimulates hCG release through a dose-dependent, paracrine mechanism (119). There is little augmentation of hCG secretion by GnRH in first trimester placental culture, because hCG production is already close to maximum (57). In contrast, at mid-trimester there is a marked dose-dependent GnRH augmentation of hCG release in vitro, with this effect diminishing in the term placenta. Likely due to the low affinity of placental GnRH receptors and dilution effect of the maternal circulation, intravenous administration of GnRH during pregnancy does not increase serum hCG. Thus, it seems most likely that locally produced placental GnRH is responsible for stimulation of placental hCG production via paracrine mechanisms (119). GnRH release is increased by estrogen, activin-A, insulin and prostaglandins, and inhibited by progesterone, inhibin, follistatin and endogenous opiates (120).

Placental Corticotrophin Releasing Hormone (CRH)


Placental CRH is structurally similar to the hypothalamic peptide, CRH (121, 122). Due to this similarity, it is easily measured in amniotic fluid as well as fetal and maternal plasma. Pro-CRH mRNA is present in cytotrophoblasts (123). CRH is also intensely immunoreactive in the decidua (53). CRH is found in maternal serum at low levels during the first and second trimesters of uncomplicated pregnancies, but rises dramatically in the third trimester of normal gestations or earlier if there are pregnancy complications resulting from such factors as prematurity, diabetes, or hypertension.(124). The highest levels of CRH are found at labor and delivery. Although concentrations of CRH in fetal plasma are lower than those found in maternal plasma, there exists a significant correlation between maternal and fetal plasma CRH (124). There is a 3-fold rise, in amniotic fluid CRH between the second and third trimester (124, 125). Placenta-derived CRH stimulates placental ACTH release in a dose-dependent manner in vitro (126, 127). Corticotrophin releasing hormone and ACTH are both released into fetal and maternal circulation; their activity is moderated by maternal CRH binding proteins (124).

Placental CRH participates in the surge of fetal glucocorticoids associated with late third trimester fetal maturation (124, 126, 128). When uterine blood flow is restricted, secretion of both CRH and ACTH is increased. Corticotrophin releasing hormone is a potent utero-placental vasodilator (129, 130). Corticotrophin releasing hormone is released into the fetal circulation in response to fetal stress and in conditions leading to fetal growth restriction (131-133). High circulating maternal CRH is believed to be responsible for the elevated plasma ACTH and cortisol found in pregnancy, which renders them unresponsive to feedback suppression of plasma cortisol (124-126, 128, 134). Corticotrophin releasing hormone stimulates prostaglandin synthesis in fetal membranes and placenta. In pre-eclampsia, fetal asphyxia, premature labor, and other conditions leading to fetal growth restriction CRH is frequently elevated (131-133).

Placental Thyrotropin Releasing Hormone

Thyrotropin releasing hormone is found in the cytotrophoblast layer; however, this molecule is different from the tripeptide produced by the hypothalamus (135). It is localized primarily in the syncytiotrophoblast but also in the fetal and maternal blood vessels as well as in the extravillous trophoblast. The concentration of TRH is higher in the fetal circulation, which is likely due rapid protease degradation on the maternal side (136). Since hCG is regarded as the principal placenta-derived thyroid stimulator, a significant role for TRH is uncertain, although it may be involved in thyroid function regulation during fetal life (137).

Placental Growth Hormone Releasing Hormone (GHRH)

GHRH has also been identified in the human placenta, but its cellular localization and function are unknown (126). Its structure is identical to that of the hypothalamic GHRH peptide. The levels of placental GHRH do not contribute to maternal circulating levels of the extra villous the presence of GHRH receptor in the placenta GHRH does not regulate placental growth hormone production. 

Somatostatin (SRIF)  

Somatostatin (SRIF) is a peptide that exerts a variety of regulatory actions interacting with G protein-coupled receptors. Placental somatostatin has been found in early pregnancy villi, cytotrophoblast and in the decidua; while its binding sites have been identified in term placental membranes and cytotrophoblast (64, 138, 139). The amount of placental somatostatin decreases with increasing gestation and it does not contribute to maternal circulating levels of the peptide.  The role of placental somatostatin remains unclear.

PLACENTAL PROTEINS: PITUITARY-LIKE HORMONES

Placental Human Chrorionic Gonadotropin (hCG)

Human chorionic gonadotropin is a glycoprotein structurally similar to follicle stimulating hormone (FSH), luteinizing hormone (LH), and thyroid stimulating hormone (TSH). It is similar to luteinizing hormone (LH) in action. As is true of the other glycoprotein hormones, hCG is composed of 2 non-identical subunits that associate non-covalently (52, 140). The α subunit consists of an amino acid sequence essentially identical to and shared with the other pituitary glycoprotein hormones. On the other hand, the β subunit is structurally similar to the α subunit yet it differs enough to confer specific biologic activity on the intact dimeric hormone. The subunits differ primarily at the carboxyl terminus where the β subunit of hCG has a 30-amino-acid tailpiece that is not present in the human LH β subunit. Glycosylation in this region of HCG accounts for the longer half-life (32-37 hours) of hCG relative to LH (24h vs. 2h, respectively). The molecular weight of the hCG dimer is estimated at 36.7 kDa with the α subunit contributing 14.5 kDa and the β subunit 22.2 kDa (140). The hCG α subunit is found in the cytotrophoblast layer only (57, 60).

As mentioned previously, hCG mRNA is detectable in embryos as early as the 6- to 8-cell stage (47). After implantation of the conceptus, hCG is detectable in the syncytiotrophoblast layer (outer trophectoderm layer) (57, 60-62). Human chorionic gonadotropin is secreted by the syncytiotrophoblasts of the placenta into both the fetal and maternal circulation. Plasma levels increase, doubling in concentration every 2-3 days between 60 and 90 days of gestation. At 3-4 weeks' gestation, the mean doubling time of dimeric hCG is 2.0 ±1.0 days and increases to about 3.5 ±1.5 days at 9-10 weeks (57). The average peak hCG level is approximately 110,000 mIU/mL and occurs at 10 weeks’ gestation (57). Between 12 and 16 weeks, average hCG decreases rapidly with the concentration halving every 2.5 ±1.0 days before reaching 25% of first trimester peak values. Levels continue to fall from 16 to 22 weeks at a slower rate (mean halving rate of 4.0 ±2.0 days) to become approximately 10% of peak first trimester values (57). During the third trimester mean hCG levels rise in gradual, yet significant, manner from 22 weeks until term (57). Interestingly, hCG levels are comparatively higher in women bearing female fetuses.

Human chorionic gonadotropin secretion is related directly to the mass of hCG-secreting trophoblastic tissues. In vivo, the release of hCG has been correlated with the widths of trophoblast tissue from 4 to 20 weeks and with placental weight from 20 to 38 weeks, respectively (57). The rapidly rising hCG seen between 3-4 and 9-10 weeks’ gestation coincides with the proliferation of immature trophoblastic villi and the extent of the syncytial layer (57). As expected, declining hCG levels are associated with a relative reduction in the mass of the syncytiotrophoblast and cytotrophoblast tissue. From 20-22 weeks until term a gradual increase in dimeric hCG corresponds with a similar increase in placental weight and villus volume (57).

Thus, in early gestation rising hCG levels reflect the histological finding of a rapidly proliferating and increasingly invasive placenta. Later in pregnancy, declining hCG levels are associated with a relative reduction in the number and mass of trophoblasts; therefore, hCG levels mirror the placenta's morphologic transformation from an organ of invasion to an organ of transfer (57).

Levels of the β subunit of hCG mirror those of dimeric hCG. The α subunit, undetectable until around 6 weeks' gestation, rises in a sigmoid fashion to reach peak levels at 36 weeks. Levels of the individual subunits are very low relative to dimeric hCG; they are approximately 2,000-fold to 150-fold less than dimeric forms at 6 and 35 weeks, respectively) (57).

With respect to the regulation of hCG production and secretion, hCG secretion appears to be related to placental GnRH release (119). In vitro, hCG is released in pulses at a frequency and amplitude that correlate with the release of placental GnRH (119). In addition, hCG production is stimulated by glucocorticoids and suppressed by DHEAS (126). In vitro, cyclic AMP (cAMP) analogues augment hCG secretion. In humans, similar to pituitary secretion of gonadotropins, decidual inhibin and prolactin inhibit hCG production by term trophoblasts whereas decidua-derived activin augments it (140, 141), with stimulation by estrogen and a negative feedback by progesterone.

Human chorionic gonadotropin, the primary luteotropic factor involved in supporting and maintaining the corpus luteum, ensures the continuous secretion of progesterone until the placenta can perform this function (142). It has immunosuppressive properties, likely involving maternal T-lymphocyte function and it possesses thyrotropic activity (143). Human chorionic gonadotropin may stimulate steroidogenesis in the early fetal testes resulting in virilization and sexual differentiation in males (144, 145). The functions of hCG are summarized in Figure 9.

Figure 9. The physiological roles of human chorionic gonadotropin (hCG) during the course of human pregnancy from fertilization to term. (Adapted from (146), with permission)

Placental Growth Hormone (GH)

Growth hormone is a single-chain peptide hormone structurally related to prolactin and human chorionic somatomammotropin (hCS). Up to the first 15-20 weeks of pregnancy, pituitary growth hormone (GH) is the main form present in the maternal circulation. From 15-20 weeks to term, placental GH gradually replaces pituitary GH, which eventually becomes undetectable (147-151). In contrast to the pulsatile output of pituitary GH, the daily profile of placental GH release is non-pulsatile (150). Syncytiotrophoblasts directly bathing in maternal blood are the site of placental GH synthesis. This cell layer is the placental site of the major glucose transporter, Glut1, and responds to rapid variations in maternal blood glucose levels by modifying placental GH secretion (152, 153).

The rate of secretion of pituitary GH is known to change rapidly, depending on the net result of multiple stimulatory and inhibitory input. The regulation of placental GH is quite different. The rate of synthesis of placental GH, and thus the maternal circulating levels, increases with the growth of the placenta (154). Growth hormone releasing hormone (GHRH) does not modulate placental GH expression in vitro, in vivo, or in the presence of glucose (155, 156). Figure 10 shows both the stimulatory and inhibitory mediators of maternal pituitary GH output, including the influence of placental growth hormone.

Production of maternal insulin-like growth factor-1 (IGF-I) is regulated by placental growth hormone. IGF-1 concentrations in the maternal plasma, studied in a large number of pregnancies, correlate with the corresponding placental GH. The IGF-1 levels do not vary significantly during the first weeks of gestation, but then increase gradually from 165 ±44.5 mg/L at about 24-25 weeks' gestation, and reach levels of 330.5 ±63.5 mg/L in a manner similar to the increases seen in placental GH. It should be noted that circulating maternal IGF-I levels also reflect placental IGF-I secretion. This growth factor, however, does not appear to be strongly expressed in human placenta; in particular; it is not expressed in the syncytiotrophoblast cell layer (157).

The biologic activities of GH and related peptide hormones can be classified into two general categories: somatogenic and lactogenic. Somatogenic activities are related to linear bone growth and alterations in carbohydrate metabolism (158, 159). The primary function of GH is to protect nutrient availability for the fetus. Via local and hepatic IGF-1, placental GH stimulates gluconeogenesis and lipolysis in the maternal compartment.

Figure 10. Shown is a representation of the hypothalamic-growth hormone-IGF-I axis, with details of its modification during pregnancy. A. In the non-pregnant state, pituitary GH secretion is regulated through hypothalamic control. Pituitary GH regulates the secretion of IGF-I, which, in turn, exerts negative feedback action on GH at the hypothalamic-pituitary level. B. During the latter half of pregnancy, the GH-IGF axis is inhibited by large amounts of estrogen. The large increase in placental GH exerts an inhibitory effect on GH secretion mediated by placental GH on the hypothalamus and pituitary. (From (160), with permission)

Placental Human Placental Lactogen (hPL), [Human Chorionic Somatomammotropin (hCS)]

Human placental lactogen is a single-chain polypeptide with two intramolecular disulfide bridges. The structures of hPL, prolactin, and growth hormone are very similar. Eighty-five percent of its amino acids are identical to human pituitary growth hormone and human pituitary prolactin (69, 161). Furthermore, hPL shares biologic properties with both growth hormone and prolactin (69, 161). Thus, it has primarily lactogenic activity but also exhibits some growth hormone-like activity; therefore, it is also referred to as chorionic growth hormone (hCGH) or human chorionic somatomammotropin (hCS). Human placental lactogen is secreted from the syncytiotrophoblast cell layer. Unlike hCG concentrations, levels of hPL rise with advancing gestational age and plateau at term. Human placental lactogen is first detectable during the fifth week of gestation, and rises throughout pregnancy maintaining a constant hormone weight to placenta weight relationship (162). Concentrations reach their highest levels during the third trimester, rising from approximately 3.5 µg/mL to 25 µg/mL at term (162). Although the level of hPL in serum at term is the highest of all placenta-derived protein hormones, its clearance from the circulation is so rapid that it cannot be detected after the first post-partum day.

Since hPL is secreted primarily into the maternal circulation, most of its functions occur at sites of action in maternal tissues. Human placental lactogen is thought to be responsible for the marked rise in maternal plasma IGF-1 concentrations as the pregnancy approaches term (162-164). Human placental lactogen exerts metabolic effects during pregnancy via IGF-I. It is associated with insulin resistance, enhances insulin secretion which stimulates lipolysis, increases circulating free fatty acids, and inhibits gluconeogenesis; in effect, it antagonizes insulin action, induces glucose intolerance, as well as lipolysis and proteolysis in the maternal system (69). In response to fasting and glucose loading, hPL levels rise and fall (162). These metabolic effects favor the transport of ketones and glucose to the fetus in the fasting and fed states, respectively.

Circulating levels of glucose and amino acids are reduced, while levels of free fatty acids, ketones, and triglycerides are increased. The secretion of insulin is augmented in response to a glucose load. The fuel requirements of the developing fetus are met primarily by glucose. It provides the energy needed for protein synthesis and serves as a precursor for the fat synthesis and glycogen formation. Fetal blood glucose levels are generally 10-20 mg/100 ml below those of the maternal circulation; thus, diffusion and facilitated transport favor the net movement of glucose from mother to fetus.

Pregnancy is associated with profound alterations in maternal metabolism. The fetal-maternal relationship favors glucose use by the fetus and forces the maternal tissues to increase their use of alternative energy sources. The endocrine hallmark of this hormonal environment is insulin resistance. Several hormones prevalent during pregnancy are believed to responsible for this altered milieu: estrogens, progesterone, glucocorticoids, human placental lactogen (hPL) and placental GH. Additionally, placental cytokines such as tumor necrosis factor-alpha (TNF-α) contribute to this metabolic state (165).

Placental Adrenocorticotropic Hormone (ACTH)

Placental ACTH is structurally similar to pituitary ACTH (166-178). Under the paracrine influence of placental CRH released from proximal cytotrophoblasts, placental ACTH is secreted by syncytiotrophoblasts into the maternal circulation (179-181). Circulating maternal ACTH is increased above non-pregnancy levels, but still remains within the normal range (182, 183).

Placental ACTH stimulates an increase in circulating maternal free cortisol that is resistant to dexamethasone suppression (179, 182). Thus, relative hypercortisolism in pregnancy occurs despite high-normal ACTH concentrations. This situation is possible due to two main differences in endocrine relationships during pregnancy. First, the maternal response to exogenous CRH is blunted (182). Second, a paradoxical relationship exists between placental CRH, ACTH, and their end-organ product, cortisol; glucocorticoids augment placental CRH and ACTH secretion, not suppress it (127, 180). This positive feedback mechanism allows an increase in glucocorticoid secretion in times of stress in excess of the amount necessary if the mother were not pregnant (127).

Placental Human Chorionic Thyrotropin (hCT)

Human chorionic thyrotropin is structurally similar to pituitary TSH, but it does not possess the common α subunit (135). The placental content of hCT is very small (58). Human chorionic gonadotropin possesses 1/4000th of the thyrotropic activity of TSH, and is thought to exert a more significant effect on the maternal thyroid than does hCT (137), particularly in conditions with high hCG levels such as trophoblastic disease.

PLACENTAL PROTEINS: GROWTH FACTORS  

Placental Inhibin/Activin/Follistatin  

Inhibin and activin are heterodimeric glycoproteins with the former comprised of an α and β subunit and the latter composed of two β subunits.  Inhibin is secreted by the corpus luteum and is present in decidualized endometrium (184, 185). Inhibin and activin dimers have been localized to the syncytiotrophoblast layer, but their individual subunits have been localized to both cytotrophoblasts and syncytiotrophoblasts (186).

Inhibin begins to increase in the maternal circulation above non-pregnant levels by 12 days post-fertilization, dramatically increasing at about 5 weeks' gestation to peak at 8-10 weeks. Subsequently, levels decrease at 12-13 weeks and stabilize until around 30 weeks before they rise again as term approaches (185). The early fluctuations in inhibin levels reflect release from the corpus luteum, whereas the increase seen in the third trimester originates from the placenta and decidua. After delivery, inhibin is undetectable. The inhibin A dimer is the principal bioactive inhibin secreted during pregnancy. Quantification of inhibin A is part of the prenatal quad screen that can be administered during pregnancy at a gestational age of 16–18 weeks. An elevated inhibin A (along with an increased beta-hCG, decreased AFP, and a decreased estriol) is suggestive of the presence of a fetus with Down syndrome.

Activin-A is the major trophoblastic activin product, which similarly increases in maternal circulation throughout pregnancy and peaks at term (187). Interestingly, higher levels of activin-A are found in mid-gestation in women with preeclampsia (188, 189).  Similar to their action in the ovarian follicle, inhibin and activin are regulators within the placenta for the production of GnRH, HCG, and steroids; as expected, activin is stimulatory and inhibin is inhibitory. 

Follistatin is the activin-binding protein expressed in placenta, membranes, and decidua (190). Since follistatin binds activin, it antagonizes the stimulatory effects of activin on placental steroid and peptide production. 

Placental Insulin-Like Growth Factors-I and II (IGF I and II)

Without question, the most important site of IGF-I and IGF-II production is the placenta (191). IGF-I and IGF-II are involved in prenatal growth and development. These growth factors do not cross the placenta into the fetal circulation; however, they may be involved in placental growth (192, 193).  An increase in maternal IGF-I levels during pregnancy with a rapid decrease after delivery indicates a significant placental influence.  There is however, no change in IGF-II levels throughout pregnancy.  In animal studies, the IGF-I produced in the placenta regulates the transfer of nutrients across the placenta to the fetus and thus enhances growth.  Interestingly, neonates with intrauterine growth restriction have reduced levels of IGF-I. IGF-II secreted by the placenta is also important in influencing β cell sensitivity to glucose and modulation of maternal insulin and glucose concentrations during pregnancy (194).

Placental Soluble FMS-Like Tyrosine Kinase (SFLT-1) and Souble Endoglin (sENG)

Soluble Flt-1 is a circulating splice variant of Flt-1, the receptor for VEGF and placental growth factor (PLGF), while sENG is the circulating receptor for transforming growth factor-β (TGF-β). VEGF, PLGF, TGF-β as well as other pro-angiogenic proteins are known to be essential for normal placental and fetal vascular development. Soluble Flt-1 and sENG are almost undetectable in the circulation of non-pregnant individuals, and are produced in large quantities by the placenta leading to marked elevation in their circulating levels during pregnancy which steadily rise until term (195, 196). These two soluble receptors are increased in serum and placentas of preeclamptic women compared to normal pregnancies and their abnormal elevation presages the development of preeclampsia. Experimental evidence indicates that sENG cooperates with sFlt-1 to induce endothelial dysfunction in vitro and preeclampsia in vivo (197). It is thought that sFlt-1 and sENG neutralize their ligands, reducing the concentration of VEGF, PLGF, and TGF-b in maternal circulation, which results in a shift in the angiogenic balance towards anti-angiogenesis, which in turn leads to endothelial damage and the clinical onset of the syndrome. However, large prospective studies have failed to show sufficient accuracy of these biomarkers for clinical utility in preeclampsia prediction (198, 199).

PLACENTAL PEPTIDE HORMONES: OTHER PLACENTAL PEPTIDES

In addition to the pregnancy-related proteins produced analogous to hypothalamic and pituitary glycoproteins, the placenta also produces several other proteins that have no known analogues in the non-pregnant state. These proteins have been isolated and identified from serum drawn during pregnancy or purified from placental tissue. Figure 11 shows the changes in concentration of each of these pregnancy-related proteins throughout gestation.

Placental Pregnancy-Specific b1-Glycoprotein (SP1)

Pregnancy-specific b1-glycoprotein is a glycoprotein hormone that can be detected about 18-23 days after ovulation. It is secreted from trophoblast cells (200, 201). Initially, it exhibits a 2- to 3-day doubling time, reaching peak concentrations between 100-200 ng/mL at term. Pregnancy-specific b1-glycoprotein has immunosuppressive effects on T-lymphocyte proliferation, and is thought to be involved in preventing rejection of the implanting conceptus (202).

Placental Pregnancy-Associated Plasma Protein-A (PAPP-A)

Pregnancy-associated plasma protein-A is the largest of the pregnancy-related glycoproteins. It originates, mainly, from placental syncytiotrophoblasts (203, 204). Pregnancy-associated plasma protein-A can first be detected at approximately 32-33 days after ovulation. With a 3-day doubling time, its levels initially rise rapidly, and then continue to rise more slowly until term (203). Like SP-1 and hCG, PAPP-A is believed to play an immunosuppressive role in pregnancy (204). It has recently gained favor as a clinically useful, first-trimester screening marker for Down syndrome (trisomy 21). Authors have confirmed decreased PAPP-A levels in association with early pregnancy failure (205). However, when compared with serum hCG and progesterone measurements to evaluate the clinical usefulness of PAPP-A values in predicting the outcome of early pregnancy, hCG and progesterone remained the best clinical tools (206).

Placental Protein-5 (PP5)

This glycoprotein is produced by the syncytiotrophoblasts. It is detected beginning at 42 days after ovulation, and steadily rises until term (207). Placental protein-5 has anti-thrombin and anti-plasmin activities, and is believed to be a naturally occurring blood coagulation inhibitor active at the implantation site (208).

Figure 11. Maternal serum concentrations of human chorionic gonadotropin (hCG) and some other pregnancy-associated protein hormones (SP-1, PAPP-A, PP-5) throughout pregnancy. The timing of implantation, missed menses and parturition is shown to demonstrate the temporal relationships. (Modified from (209), with permission)

PLACENTAL METABOLIC PROTEINS  

Placental Leptin  

Leptin is a key regulator of satiety and body mass index (BMI), and its levels are thought to reflect the amount of energy stores and nutritional state (210).  The placenta is the principal source of leptin during pregnancy (211). Most of the leptin produced by the placenta is secreted into the maternal circulation, and as a consequence leptin levels are elevated during pregnancy.  In the first trimester, maternal plasma leptin levels are double nonpregnant values and continue to increase during the second and third trimesters (212-214).  In the second and third trimesters leptin is also expressed in the chorion and amnion (215).  The amount of leptin directed to the fetus is uncertain, and its role in fetal development is also unclear.  Leptin levels decline to normal nonpregnant levels within 24 hours after delivery (216). Interestingly, leptin levels during pregnancy do not correlate with BMI as they do in the nonpregnant state (217).  Although not clear, it is thought that leptin may be utilized by the placenta to modulate maternal metabolism and partition energy supplies to the fetus (218). There is evidence that placental leptin is anti-apoptotic and promotes proliferation, protein synthesis and the expression of tolerogenic maternal response molecules such as HLA-G (219). Placental leptin expression is regulated by hCG, insulin, steroids, hypoxia and many other growth hormones, suggesting that it may have an important endocrine function in trophoblast cells (219). Additionally, the human placenta also expresses leptin receptors, and therefore can act in a paracrine manner to modulate placental function (220, 221).

Placental Ghrelin   

Ghrelin, is a gastric peptide isolated primarily from the stomach which is thought to stimulate GH release and participates in the regulation of energy homeostasis, increasing food intake, decreasing energy output, as well as exert a lipogenetic effect (222).  Ghrelin and its receptors have been isolated in the placenta, clearly indicating a role for ghrelin in reproduction. Circulating ghrelin levels peak at mid-gestation, then with advancing gestational age declining ghrelin levels are observed.  After delivery, near prepregnancy levels of ghrelin are seen (223).  It is thought that ghrelin may well be involved in regulation of energy intake during pregnancy (224), however its exact role is still unknown.

PLACENTAL MATURATION

As pregnancy advances, the relative numbers of trophoblasts increase as feto-maternal exchange begins to dominate the placenta's secretory functions. Later, throughout the second and third trimester, the placenta adapts its structure to reflect its function such that near term, the villi consist mainly of fetal capillaries with sparse supporting stroma beyond that which is required to maintain its anatomic integrity. In contrast to the early placental villus where trophoblasts are abundant as part of a continuous layer of basal cytotrophoblasts, the term placenta's membranous interface between the fetal and maternal circulation is extremely thin (65). Thus, as the gestation progresses toward term, the number of cytotrophoblasts declines and the remaining syncytial layer becomes thin and barely visible. This structural arrangement facilitates transport of compounds across the feto-maternal interface. Consistent with the cytologic changes that occur in the maternal fetal interface from mid-gestation to term, striking changes in the global gene expression profile of this tissue has been demonstrated over this interval (225).

FETAL COMPARTMENT

The endocrine system, a system that is functional from the time of intrauterine existence through old age, is one of the first systems to develop during fetal life. As in the placenta, the regulation of the fetal endocrine system relies, to some extent, on precursors secreted by the other compartments. As the fetus develops, its endocrine system matures and eventually becomes more independent, preparing the fetus for extrauterine life.

Fetal Hypothalamus and Pituitary

By the end of the fifth week of gestation, the primitive hypothalamus can be identified as a swelling on the inner surface of the diencephalic neural canal (226).  By the 9th to 10th week, the median eminence of the hypothalamus is evident.  By week 14 to 16 the hypophysiotropic hormones GnRH, TRH, CRH, GHRH and somatostatin appear in the fetal hypothalamus (227) .  The portal-vessel system that delivers the releasing hormones to the anterior pituitary is fully developed by 18 weeks of gestation (227).

The anterior pituitary cells that develop from those cells lining Rathke's pouch are capable of secreting growth hormone (GH), follicle-stimulating hormone (FSH), luteinizing hormone (LH) and adrenocorticotropic hormone (ACTH), in vitro, as early as 7 weeks of fetal life (Figure 12).

Figure 12. Fetal serum pituitary hormone levels. PrL indicates prolactin; TSH, thyroid-stimulating hormone; ACTH, corticotropin; GH, growth hormone; LH/FSH, luteinizing hormone/follicle stimulating hormone. (Modified from (228), with permission)

Fetal Thyroid Gland

The fetal thyroid gland develops initially in the absence of detectable TSH. By 12 weeks’ gestation, the thyroid is capable of iodine-concentrating activity and thyroid hormone synthesis (226) .  Prior to that time, the maternal thyroid appears to be the primary source for T4.  The levels of TSH and T4 are relatively low in fetal blood until mid-gestation. At 24-28 weeks' gestation, serum T4 and reverse tri-iodothyronine (rT3) concentrations begin to rise progressively until term while the TSH concentration peaks. At birth, there is an abrupt release of TSH, T4, and T3. The relative hyperthyroid state of the newborn is believed to facilitate thermoregulatory adjustments for extrauterine life.  The function of the fetal thyroid hormones is crucial for somatic growth and neonatal adaptation. 

Fetal Gonads

The internal genitalia in the embryo have the inherent tendency to feminize. The Wolffian (mesonephric) and Mullerian (paramesonephric) ducts are discrete primordia that temporarily coexist in all embryos during the ambisexual undifferentiated development period (up to 8 weeks). The critical factors in determining which of the duct structures stabilize or regress are the hormones secreted by the testes: Anti-Mullerian hormone (AMH) and testosterone. The testis is histologically identifiable at 6 weeks’ gestation. Primary testis differentiation begins with development of the Sertoli cells at 8 weeks’ gestation. SRY, the sex-determining region on the Y chromosome, determines male gonadal sex and directs the differentiation of the Sertoli cell (229).  Sertoli cells secrete AMH which triggers the resorption of the Mullerian ducts in males and prevents development of female internal structures (230). At approximately 8 weeks’ gestation Leydig cells differentiate and testosterone secretion commences.  Maximum levels of fetal testosterone are observed at about 15 – 18 weeks and decrease thereafter.

Differentiation of the ovaries occurs several weeks later than that of the testis.  If the primordial germ cells lack the SRY region on the Y chromosome, ovaries develop from the indifferent gonads.  Fetal ovarian function becomes apparent by 7 to 8 weeks gestation; the time when the ovary becomes morphologically recognizable. During this time ovarian differentiation is occurring with mitotic multiplication of germ cells, reaching 6-7 million oogonia, their maximal number, by 16-20 weeks’ gestation (231, 232). 

The pattern of luteinizing hormone (LH) levels in fetal plasma parallels that of follicle-stimulating hormone (FSH). The decline in pituitary gonadotropin content, and plasma concentration of gonadotropins after mid-gestation is believed to result from the maturation of the hypothalamic-pituitary-gonadal axis. The hypothalamus becomes progressively more sensitive to sex steroids originating from the placenta and circulating in fetal blood. Early secretion of fetal testosterone is important in initiating sexual differentiation in males. In the absence of testosterone, the Wolffian system regresses. Human chorionic gonadotropin (hCG), supplemented by fetal LH, is believed to be the primary stimulus effecting the early development and growth of Leydig cells as well as stimulating the subsequent peak of testosterone production. In females, the fetal ovary is involved primarily in the formation of follicles and germ cells and less involved in hormone secretion.

Fetal Adrenal Glands

The human fetal adrenal gland is a remarkable organ due to its incredible capacity for steroid biosynthesis in utero, and because of its unique morphologic features. The human fetal adrenals are disproportionately large, and at mid-pregnancy their size exceeds that of the fetal kidneys. At term, the adrenals are as large as those of adults, weighing 10 grams or more. The region that ultimately develops into the adult adrenal cortex, the outer or definitive zone, accounts for only about 15% of the fetal gland (Figure 13). The unique inner or fetal zone comprises 80-85% of the volume of the adrenal in utero, and is largely responsible for the tremendous secretory capacity of this organ. The fetal zone rapidly undergoes involution at parturition and by one year it has completely disappeared (233). Changes in the fetal adrenal volume throughout fetal life and into young adulthood are graphically depicted in Figure 14.

The adrenal function of 10 preterm infants of gestational age 27-34 weeks was assessed for up to 80 days after delivery. The changes in steroid excretion with time in preterm infants of gestation over 28 weeks reflect involution of the fetal adrenal zone at a similar rate to term infants. These findings are consistent with the removal at birth of the inhibitory effects of estrogen on the 3 beta-hydroxysteroid dehydrogenase enzyme. The continued function of the adrenal fetal zone beyond the first month in preterm infants of less than 28 weeks’ gestation may however be due to persistence of some other fetal regulatory adrenal mechanism. This suggests that it is gestation that determines fetal zone activity rather than birth (234).

The fetal adrenal gland secretes large quantities of steroid hormones (up to 200-mg daily) near term. The rate of steroidogenesis is 5-times that observed in the adrenal glands of adults at rest. The principal steroids secreted are C-19 steroids (mainly DHEAS), which serve as substrates for estrogen biosynthesis by the placenta (Figure 13).

The fetal adrenal gland contains a zone, unique to in-utero fetal life that accounts for the rapid growth of the adrenal gland; this zone regresses during the first few weeks after birth. In addition to the fetal zone, an outer layer of cells forms the adrenal cortex (definitive zone). The fetal zone differs not only histologically, but also biochemically from the cortex (i.e., the fetal zone is deficient in 3b-hydroxysteroid dehydrogenase enzyme activity and, therefore, secretes C-19 steroids (mainly DHEAS); the cortex secretes primarily cortisol).

Figure 14. An illustration demonstrating generalized pathways for steroid hormone formation in the fetal adrenal gland. DHA: dehydroepiandrosterone. DHAS: dehydroepiandrosterone sulfate. LDL: low-density lipoprotein cholesterol. (Modified from (235), with permission)

Figure 15. Changes in the fetal adrenal volume throughout fetal life and into young adulthood. (Modified from (236), with permission)

Research involving the fetal adrenal gland has attempted to determine the factors that stimulate and regulate fetal adrenal growth and steroidogenesis. Other work has focused on the mechanisms responsible for fetal zone atrophy after delivery. All investigations have shown that, in vitro, adrenocorticotropic (ACTH) stimulates steroidogenesis. Furthermore, there is clinical evidence that, in vivo, ACTH is the major trophic hormone of the fetal adrenal gland. For example, in anencephalic fetuses, the plasma levels of ACTH are very low and the fetal zone is markedly atrophic. Maternal glucocorticoid therapy suppresses fetal adrenal steroidogenesis by suppressing fetal ACTH secretion. Despite these observations, ACTH -related peptides, growth factors and other hormones have been proposed as possible trophic hormones for the fetal zone. After birth, the adrenal gland shrinks in size by more than 50% because of the regression of fetal zone cells.

Fetal Parathyroid Glands and Calcium Homeostasis

In the fetus, calcium concentrations are regulated by the movement of calcium across the placenta from the maternal compartment. In order to maintain fetal bone growth, the maternal compartment undergoes adjustments that provide a net transfer of sufficient calcium to the fetus. Maternal compartment changes that permit calcium accumulation include increases in maternal dietary intake, increases in maternal 1, 25-dihydroxyvitamin D3 levels, and increases in parathyroid hormone (PTH) levels.  The levels of total calcium and phosphorus decline in maternal serum, but ionized calcium levels remain unchanged. During pregnancy, the placenta forms a calcium pump in which a gradient of calcium and phosphorus is established which favors the fetus.  Thus, circulating fetal calcium and phosphorus levels increase steadily throughout gestation. Furthermore, fetal levels of total and ionized calcium, as well as phosphorus, exceed maternal levels at term.

By 10-12 weeks' gestation, the fetal parathyroid glands secrete PTH. Fetal plasma levels of PTH are low during gestation, but increase after delivery. In contrast to the unchanged maternal calcitonin levels, the fetal thyroid gland produces increasing levels of calcitonin. Since there is no transfer of parathyroid hormone across the placenta, changes noted in fetal calcium levels are related to fetal changes in these hormones (PTH and calcitonin). These adaptations are consistent with the need to conserve calcium and stimulate bone growth within the fetus. After birth, neonatal serum calcium and phosphorus levels fall. Parathyroid hormone levels start to rise within 48 hours after birth. Calcium and phosphorus levels steadily increase over the following several days, with some dependence on dietary intake of milk.

Fetal Endocrine Pancreas

The pancreas’ exocrine function begins after birth, while the endocrine function (hormone release) can be measured from 10 to 15 weeks onward.  The α-cells which contain glucagon, and the β-cells which contain somatostatin, can be recognized by 8 weeks’ gestation (234). Alpha cells are more numerous than β-cells in the early fetal pancreas and reach a peak at midgestaion; β-cells increase through the second half of gestation so that by term the ratio of α-cells to beta cells is approximately 1:1 (237, 238).  Human pancreatic insulin and glucagon concentrations increase with advancing fetal age, and are higher than concentrations found in the adult pancreas. In vivo studies of umbilical cord blood obtained at delivery and fetal scalp blood samples obtained at term show that fetal insulin secretion is low and tends to be relatively unresponsive to acute changes in glucose. In contrast, fetal insulin secretion in vitro is responsive to amino acids and glucagon as early as 14 weeks' gestation. In maternal diabetes mellitus, fetal islet cells undergo hypertrophy such that the rate of insulin secretion increases.

Fetal Alpha-Fetoprotein (AFP)

Alpha-fetoprotein is a glycoprotein synthesized first by the yolk sac, then the gastrointestinal tract, and lastly by the fetal liver (239, 240). After entering the fetal urine, it is readily detected in amniotic fluid. Amniotic fluid AFP (afAFP) peaks between 10-13 weeks’ gestation, and then declines from 14-32 weeks. In the fetus, AFP peaks at 12-14 weeks and steadily decreases until term (241). The fall in fetal plasma AFP (fpAFP) is most likely due to the combination of increasing fetal blood volume and a decline in fetal production. The concentration gradient between fpAFP and maternal serum AFP (msAFP) is approximately 150- to 200-fold. Detectable as early as 7 weeks' gestation, msAFP reaches peak concentrations between 28-32 weeks (241). The seemingly paradoxical rise in msAFP in association with decreasing afAFP and fetal serum levels can be accounted for by the increasing placental permeability to fetal plasma proteins that occurs with advancing gestational age (241). Alpha-fetoprotein acts as an osmoregulator to help adjust fetal intravascular volume (241). It may also be involved in certain immunoregulatory functions (242). Amniotic fluid AFP and maternal serum AFP are clinically important because they are elevated in association conditions such as neural tube defects (243). Additionally, msAFP is decreased in pregnancies in which the fetus has Down syndrome (trisomy 21) (244).

MATERNAL COMPARTMENT

Maternal Hypothalamus and Pituitary

Little information is definitively known about the endocrine alterations of the maternal hypothalamus during pregnancy. Thought to result from estrogen stimulation, the anterior pituitary undergoes a 2- to 3-fold enlargement during pregnancy, primarily because of hyperplasia and hypertrophy of lactotroph cells. Thus, plasma prolactin levels parallel the increase in pituitary size throughout gestation. In contrast to the lactotrophs, the size of the other pituitary cells decreases or remains unaltered during pregnancy. In line with these findings, maternal levels of somatotrophs and gonadotrophs are lower and the level of thyrotrophs and corticotrophs remains unchanged.  In contrast, adrenocorticotrophic hormone (ACTH) levels do increase with advancing gestation. Corticotrophin-releasing hormone (CRH) in the maternal plasma increases during pregnancy due to increased placental secretion, but alterations in binding-protein concentrations prevent increased biologic activity of this releasing hormone.

The size of the posterior pituitary gland diminishes during pregnancy (245).  Additionally, maternal plasma arginine vasopressin (AVP) levels remain low throughout gestation and are not believed to play a pivotal role in human pregnancy.  In contrast, maternal oxytocin levels progressively increase in the maternal blood and parallel the increase in maternal serum levels of estradiol and progesterone (246). Uterine oxytocin receptors also increase throughout pregnancy, resulting in a 100 fold increase in oxytocin binding at term in the myometrium (247).

Maternal Thyroid Gland

As a result of increased vascularity and glandular hyperplasia, the thyroid gland increases in size by 18% during pregnancy; however, true goiter is not usually present (248).  Enlargement is associated with an increase in the size of the follicles with increased amounts of colloid and enhanced blood volume.  This enlargement may be a response to the thyrotropic effect of hCG, which may account for some of the increase in serum thyroglobulin concentrations observed during pregnancy.  During gestation the mother remains in a euthyroid state. Total thyroxine (T4) and tri-iodothyronine (T3) levels increase but do not result in hyperthyroidism because there is a parallel increase in T4-binding globulin that results from estrogen exposure (Figure 15). The increase seen in binding-protein concentrations is similar to that observed in women who use oral contraceptives (OC). A modest increase in the basal metabolic rate (BMR) rate occurs during pregnancy secondary to increasing fetal requirements. Some T4 and T3, but no TSH, are transferred across the placenta.

Figure 15. Relative changes in maternal thyroid function during the course of human pregnancy from fertilization to term. (Modified from (249), with permission)

Maternal Adrenal Glands

The maternal adrenal gland does not change morphologically during pregnancy.  However, plasma adrenal steroid levels increase with advancing gestation. Total plasma cortisol concentrations increase to three times nonpregnant levels by the third trimester.  The hypoestrogenic state of pregnancy results in increased hepatic production of cortisol-binding globulin. This increase in cortisol-binding globulin results in decreased metabolic clearance of cortisol, resulting in an increase in plasma free cortisol and total free cortisol.  Additionally, cortisol production increases due to an increase in maternal plasma ACTH concentration and the hyperresponsiveness of the adrenal cortex to the ACTH stimulation (250).  Despite the elevated free cortisol levels, pregnant women do not exhibit any overt signs of hypercortisolism, likely due to the anti-glucocorticoid activities of the elevated levels of progesterone.

Plasma renin substrate levels are increased as a consequence of the effects of estrogen on the liver.  The higher levels of renin and angiotensin during pregnancy, lead to elevated angiotensin II levels and markedly elevated levels of aldosterone.  Similar to cortisol, the elevated aldosterone levels do not have a detrimental effect on maternal health.  The high level of progesterone is thought to displace aldosterone from its renal receptors.

Androgen levels are elevated during pregnancy, but the free androgen levels remain normal to low secondary to the estrogen-induced increase in hepatic synthesis of sex hormone-binding globulin. Dehydroepiandrosterone (DHEA) and DHEAS production is increased twofold during pregnancy. However, serum concentrations of DHEAS are reduced to less than nonpregnant levels secondary to enhanced 16 –hydroxylation and placental use of DHEAS in estrogen production (251).

Maternal Endocrine Pancreas

A dual-hormone secretion mechanism is partially responsible for the metabolic adaptation of pregnancy in which glucose is spared for the fetus by the maternal endocrine pancreas. Compared to the non-pregnant state, in response to a glucose load, there is a greater release of insulin from the β-cells and a greater suppression of glucagon release from the α-cells. Associated with the increased release of insulin, the maternal pancreas undergoes β-cell hyperplasia and islet-cell hypertrophy, with an accompanying increase in blood flow to the endocrine pancreas. During pregnancy, when fasting blood glucose levels fall, they rise to a greater extent in response to a glucose load than do levels in non-pregnant women. The increased release of insulin is related to insulin resistance due to hPL, which spares transfer of glucose to the fetus. Glucagon levels are also suppressed in response to a glucose load, with the greatest suppression occurring near term.

REGULATION OF FETO-MATERNAL STEROIDOGENESIS

Using in vitro investigations utilizing placental tissue explants as well as in vivo, catheterized primate models to study steroidogenic regulation in pregnancy, researchers have determined LDL-cholesterol, fetal pituitary hormones, intra-placental regulators, and intra-adrenal regulators act as the primary modulators of feto-placental steroid production (252-254).

Regulation by Low Density Lipoprotein Cholesterol (LDL)

A limiting factor in adrenal steroid output is the availability of LDL-cholesterol, the primary lipoprotein used in fetal adrenal steroid synthesis (Figure 16). Circulating LDL-cholesterol accounts for 50-70% of the cholesterol utilized for fetal adrenal steroidogenesis (255-257). The fetal adrenal is known to contain high affinity, low-capacity LDL binding sites. The presence of ACTH increases this binding capacity (256, 258, 259). Within the adrenal gland, hydrolysis of LDL makes cholesterol available for conversion to steroids. The majority of fetal LDL-cholesterol is made, de novo, in the fetal liver (260). In addition, cortisol from the fetal adrenal cortex and estradiol (aromatized from fetal DHEAS) augment this de novo synthesis within the fetal liver. These systems interact in a manner that is linked, self-perpetuating, and serves to increase steroid production to meet the needs of the maturing fetus (260).

Figure 16. Shown are the maternal, placental and fetal compartments for estrogen and progesterone synthesis in human pregnancy. The fetal adrenal gland lacks 3β-hydroxysteroid dehydrogenase, but has sulfation and 16α-hydroxylase capabilities. Likewise, the placenta lacks 17α-hydroxylase activity but contains sulfatase in order to cleave the sulfated fetal products. Modified from (261), with permission)

Regulation by Fetal Pituitary Hormones  

Fetal ACTH regulates steroidogenesis in both adrenal zones. Adrenocorticotropic hormone receptor activity is diminished in the fetal zone of the cortex during the early second trimester when other factors, such as hCG, are more important in the maintenance of this zone (260). In vitro studies in human fetal adrenal tissue, demonstrate that ACTH stimulates the release of D5 pregnenolone sulfate and DHEAS, whereas in adult adrenal cortex secretes only cortisol when stimulated by ACTH (260). Moreover, ACTH can act on its own adrenal-cell membrane receptor to express a direct stimulatory effect on steroidogenic enzymes (260).

Adrenocorticotropic hormone extracted from the human fetal pituitary gland has been shown, in vitro, to stimulate the production of DHEAS and cortisol (262, 263). Interestingly, concentrations of ACTH throughout gestation do not correlate with the increasing mass of the fetal adrenal cortex or the increasing steroidogenic function that are hallmarks of the third trimester (259). Fetal pituitary ACTH is detectable by 9 weeks’ gestation (263, 264). Thereafter, levels of ACTH increase steadily until 20 weeks’ gestation. The levels remain stable until approximately 34 weeks, when a significant decline is initiated and persists until term (259).

Prolactin may act as a co-regulator, along with ACTH, hCG and certain growth factors, in fetal adrenal steroid production (265, 266). Both in vitro and in vivo, prolactin augments ACTH-stimulated adrenal androgen production (253). Fetal pituitary prolactin is detectable at 10 weeks’ gestation (264). Umbilical cord prolactin levels increase with advancing gestational age and rise in parallel with increased fetal adrenal mass (267).

Regulation by Intra-Placental Mechanisms  

The placenta is an important co-regulator of the fetal adrenal zone due its ability to secrete hCG, placental CRH, progesterone and estradiol (233). In vitro and in vivo, hCG receptor activity is present in the fetal zone, and hCG stimulates fetal adrenal production of DHEAS (233, 268). However, after the 20th week of gestation ACTH primarily influences the fetal zone of the adrenal, and at this time hCG plays only a minor role. Placental CRH, acts in a paracrine relationship with placental ACTH, to complement the actions of the fetal hypothalamus and pituitary in producing the surge in fetal glucocorticoids notable in the late third trimester as fetal growth and maturity become increasingly important (125, 269).

Placental progesterone inhibits D5 to D4 steroid transformations in the fetal zone of the adrenal (101, 270). This effect is another explanation for fetal adrenal 3β-HSD deficiency. Placental estradiol modifies the production and metabolism of corticosteroids and progesterone. In vivo, the placenta regulates the inter-conversion of maternal cortisol to cortisone, and the fetal pituitary production of ACTH (264, 269). Modulation of the transfer of maternal cortisol across the placenta, into the fetus, is the primary mechanism through which this effect occurs.

Regulation by Intra-Adrenal Mechanisms  

With advancing gestational age, the fetal adrenal becomes more sensitive to circulating ACTH (253). Between 32 and 36 weeks’ gestation, the fetal adrenal mass increases (271-273). Blood flow to the fetal adrenal is affected by many factors that, in turn, affect the exposure of the fetal adrenal receptors of the different trophic stimuli. Growth factors modulate adrenal steroid pathways just as they do in the adult adrenal cortex. The fetal adrenal produces IGF-I and IGF-II; ACTH originating from either the fetal pituitary or the placenta can stimulate production of their respective mRNAs (274, 275).

PARTURITION

Parturition is a coordinated process of transition from a quiescent myometrium to an active rhythmically contractile state requiring elegant interplay between placental, fetal and maternal compartments. Though fetal maturity does not always predate the onset of labor, the two processes are related in primates. The timing of birth is a crucial determinant of perinatal outcome. Both preterm birth (<37wk) and post-term pregnancy (>42 wk) are associated with greater risk of adverse perinatal outcomes. The traditional dogma, supported by robust evidence from animal studies, has the fetoplacental unit as being in charge of the timing of parturition (276). While this is certainly true in some species, the presence of such a “placental clock” is not established in humans. Rather, it has become clear that the maternal endometrium/decidua also plays an important role in triggering the cascade of event leading to parturition (277).  

The precise mechanisms involved in human parturition are thought to involve CRH, functional progesterone withdrawal, increased estrogen bioavailability, and finally, increased responsiveness of the myometrium to prostaglandins and oxytocin. There is no simple chain of events as there are in other species.

Numerous lines of evidence support a role for CRH in human parturition. Studies have demonstrated increased CRH and decreased CRH-binding protein levels prior to the onset of both term and preterm labor (278, 279). CRH directly stimulates release of prostaglandins in decidua and myometrium (280). Interestingly, a paradoxical augmentation of placental CRH release by serum cortisol is maximal in the last ten weeks of pregnancy. This may be a function of cortisol competition with progesterone for placental glucocorticoid receptors, thereby blocking the inhibitory action of progesterone on CRH synthesis (281).

The ratios of estradiol and progesterone in various animal models are closely related to the stimulation of myometrial gap-junction formation (282). With decreasing progesterone relative to estradiol, gap junctions permit cell-cell communication for the synchronized myometrial smooth muscle contractions required for labor. Progesterone and the estrogens are antagonistic in the parturition process. Progesterone produces uterine relaxation, stabilizing lysosomal membranes and inhibiting prostaglandin synthesis and release. By contrast, estrogens destabilize lysosomal membranes and augment the synthesis of prostaglandin and their release (283). Although gradual increase in umbilical cord DHEAS and maternal estriol occurs toward term, there is no corresponding drop in either fetal or maternal progesterone concentrations (284).

Though a reduction in maternal or fetal progesterone levels during spontaneous labor has not been documented, functional progesterone withdrawal at the receptor level is believed to be involved in the process of parturition. This may occur via altered progesterone receptor isoform PR-A/PR-B levels in myometrium (285). Undoubtedly, progesterone is important in uterine quiescence because in the first trimester removal of the corpus luteum leads rapidly to myometrial contractions (84). Likewise, labor ensues following the administration of progesterone receptor antagonists in the third trimester (286). The anti-progesterone agents occupy progesterone receptors and inhibit the action of progesterone, which is clearly essential for maintenance of uterine quiescence. Consistent with these findings, pharmacologic treatment of women at risk for preterm labor with progesterone or synthetic progestational agents has demonstrated efficacy in the prevention of preterm labor (287-289).

A role for estrogen in the process of parturition is supported by the finding that pregnancies are often prolonged when estrogen levels in maternal blood and urine are low, as in placental sulfatase deficiency or when associated with anencephaly (290). In human studies, there is a correlation in uterine activity with circulating maternal estrogens and progesterone as labor approaches (291-293). Feto-placental estrogens are closely linked to myometrial irritability, contractility, and labor. In primates, estrogens ripen the cervix, initiate uterine activity, and established labor (294). Estrogens also increase the sensitivity of the myometrium to oxytocin by augmenting prostaglandin biosynthesis (283, 295). Because placental release of estrogens is linked to the fetal hypothalamus, pituitary, adrenals, and placenta the fetal pituitary adrenal axis appears to fine-tune parturition timing in part through its effect on estrogen production.

Prostaglandins (PG) are thought to play a central role in human parturition. For years, it has been known that rupture, stripping, or infection of the fetal membranes, as well as instillation of hypertonic solutions into the amniotic fluid results in the onset of labor. These facts have led to the hypothesis that a fetal-amniotic fluid-fetal membrane complex is a metabolically active unit that triggers the onset of labor. Evidence supporting a causative role of prostaglandins in the labor process is present since PGs induce myometrial contractions in all stages of gestation. While there is still no direct evidence relating endogenous PGs to labor,  there are several lines of evidence implicating PGs in this process; PG levels increase in maternal circulation and amniotic fluid in association with labor; indomethacin prevents the onset of labor in nonhuman primates and stops preterm labor in humans; stimuli that are known to induce labor (e.g. cervical ripening, rupture of membranes) are associated with PG release; the process of cervical ripening is mediated by PGs. Important to this hypothesis is the understanding that at least one mechanism in the onset of parturition is the release of stored precursors of PGs from the fetal membranes.

The major precursor for PGs is arachidonic acid, which is stored in glycerophospholipids. The fetal membranes are enriched with two major glycerophospholipids, phosphatidylinositol and phosphatidylethanolamine. As gestation advances, the progressively increasing levels of estrogen stimulate the storage, in fetal membranes, of these glycerophospholipids containing arachidonic acid.

A series of fetal membrane lipases, including phospholipase A2 and Phospholipase C control the release of arachidonic acid from storage in fetal membrane phospholipids. Once in a free state, arachidonic acid is available for conversion to PG. Additional factors that augment and accentuate the normal process of labor include the liberation of corticosteroid by the mother and fetus, resulting in a decrease in the production of myometrial prostacyclin, a smooth muscle relaxant.

Active labor is characterized by a dramatic increase in the number of oxytocin receptors in the myometrium. Once begun, the process appears to be self-perpetuating. The level of maternal catecholamines increases, resulting in the liberation of free fatty acids, including arachidonic acid; there is also an increase in the level of maternal or fetal cortisol, which decreases the production of uterine smooth muscle prostacyclin. It is unlikely that oxytocin is the initiator of labor despite the fact that oxytocin receptors are present in the myometrium and increase before labor, and it stimulates decidual prostaglandin E2 and prostaglandin F2a production. There is firm evidence of increasing, rhythmical fetal adrenal and placental steroid output over the 5 weeks just before term that is important in preparing human pregnancy for the final cascade of oxytocin and prostaglandins that stimulate labor (283, 291-293, 295, 296).

KEY POINTS

  • Synchrony between the development of the early embryo and establishment of a receptive endometrium is necessary to allow implantation and subsequent progression of pregnancy.
  • The placenta is a unique, dynamic organ with the inherent ability to produce, regulate, and inhibit factors that directly affect fetal growth and development.
  • During the luteal-placental transition period, between 6-10 weeks of gestation, corpus luteal function and progesterone production naturally declines and shifts to the developing placenta.
  • Steroidogenesis in pregnancy is characterized by enzymatic deficiencies within the placental and fetal compartments which foster interdependent transfer of precursors among compartments for the synthesis of steroid hormones. This process is modulated by LDL-cholesterol, fetal pituitary hormones, intra-placental regulators, and intra-adrenal regulators.
  • Redundancy in protein hormone – receptor interactions such as hPL and hPGH serve to ensure that adequate nutrition is supplied to the developing fetus.
  • A relatively insulin resistant state is generated within the maternal compartment to supply glucose and free fatty acids for fetal nutrition.
  • Human parturition exemplifies the interplay between placental, fetal, and maternal compartments, characterized by increased estrogen bioavailability, functional progesterone withdrawal, increased CRH synthesis and release, culminating in increased responsiveness of the myometrium to prostaglandins and oxytocin.

ACKNOWLEDGMENT

In addition to the journal and text references listed above, the following sources were used in the preparation of this chapter:

Taylor HS, Pal L, Seli E (eds.). Speroff’s Clinical Gynecologic Endocrinolofy & Infertility. Ninth edition, 2020. Wolters-Kluwer, Philadelphia.

Gabbe SG, Niebyl JR, Simpson JL [eds.]. Obstetrics: normal and problems pregnancies. Fifth edition, 2007. Churchill-Livingstone, New York.
Benirschke K, Kaufmann P, Baergen RN [eds.]. Pathology of the human placenta. Fifth edition, 2006. Springer, New York.Strauss JF,
Barbieri RL [eds.]. Yen and Jaffe’s Reproductive endocrinology: physiology, pathophysiology and clinical management. Fifth edition, 2004. Elsevier Saunders, Philadelphia.
Reece EA, Hobbins JC [eds.]. Clinical obstetrics: the fetus and mother. Third edition, 2007. Wiley-Blackwell, Malden, MA.

REFERENCES

  1. Mesino S: The Endocrinology of Human Pregnancy and Fetoplacental Neuroendocrine Development. ; in Yen & Jaffe Reproductive Endocrinology. Edited by Jaffe Y, 2009
  2. Strowitzki T, Germeyer A, Popovici R, et al.: The human endometrium as a fertility-determining factor. Human reproduction update 12:617-30, 2006
  3. Finn CA, Martin L: The control of implantation. Journal of reproduction and fertility 39:195-206, 1974
  4. Martin J, Dominguez F, Avila S, et al.: Human endometrial receptivity: gene regulation. Journal of reproductive immunology 55:131-9, 2002
  5. Gipson IK, Blalock T, Tisdale A, et al.: MUC16 is lost from the uterodome (pinopode) surface of the receptive human endometrium: in vitro evidence that MUC16 is a barrier to trophoblast adherence. Biology of reproduction 78:134-42, 2008
  6. Sharkey AM, Smith SK: The endometrium as a cause of implantation failure. Best practice & research Clinical obstetrics & gynaecology 17:289-307, 2003
  7. Chan RW, Schwab KE, Gargett CE: Clonogenicity of human endometrial epithelial and stromal cells. Biol Reprod 70:1738-50, 2004
  8. Taylor HS: Endometrial cells derived from donor stem cells in bone marrow transplant recipients. JAMA 292:81-5, 2004
  9. Tal R, Shaikh S, Pallavi P, et al.: Adult bone marrow progenitors become decidual cells and contribute to embryo implantation and pregnancy. PLoS Biol 17:e3000421, 2019
  10. Lima PD, Zhang J, Dunk C, et al.: Leukocyte driven-decidual angiogenesis in early pregnancy. Cellular & molecular immunology 11:522-37, 2014
  11. Hofmann AP, Gerber SA, Croy BA: Uterine natural killer cells pace early development of mouse decidua basalis. Molecular human reproduction 20:66-76, 2014
  12. Erlebacher A: Immunology of the maternal-fetal interface. Annu Rev Immunol 31:387-411, 2013
  13. DeMayo FJ, Lydon JP: 90 YEARS OF PROGESTERONE: New insights into progesterone receptor signaling in the endometrium required for embryo implantation. J Mol Endocrinol 65:T1-T14, 2020
  14. Lydon JP, DeMayo FJ, Funk CR, et al.: Mice lacking progesterone receptor exhibit pleiotropic reproductive abnormalities. Genes Dev 9:2266-78, 1995
  15. Mulac-Jericevic B, Lydon JP, DeMayo FJ, et al.: Defective mammary gland morphogenesis in mice lacking the progesterone receptor B isoform. Proc Natl Acad Sci U S A 100:9744-9, 2003
  16. Mulac-Jericevic B, Mullinax RA, DeMayo FJ, et al.: Subgroup of reproductive functions of progesterone mediated by progesterone receptor-B isoform. Science 289:1751-4, 2000
  17. Chwalisz K: The use of progesterone antagonists for cervical ripening and as an adjunct to labour and delivery. Human reproduction 9 Suppl 1:131-61, 1994
  18. Shaw KA, Topp NJ, Shaw JG, et al.: Mifepristone-misoprostol dosing interval and effect on induction abortion times: a systematic review. Obstetrics and gynecology 121:1335-47, 2013
  19. Lakha F, Ho PC, Van der Spuy ZM, et al.: A novel estrogen-free oral contraceptive pill for women: multicentre, double-blind, randomized controlled trial of mifepristone and progestogen-only pill (levonorgestrel). Human reproduction 22:2428-36, 2007
  20. Spitz IM, Croxatto HB, Lahteenmaki P, et al.: Effect of mifepristone on inhibition of ovulation and induction of luteolysis. Human reproduction 9 Suppl 1:69-76, 1994
  21. Giudice LC: Microarray expression profiling reveals candidate genes for human uterine receptivity. American journal of pharmacogenomics : genomics-related research in drug development and clinical practice 4:299-312, 2004
  22. Hsieh-Li HM, Witte DP, Weinstein M, et al.: Hoxa 11 structure, extensive antisense transcription, and function in male and female fertility. Development 121:1373-85, 1995
  23. Satokata I, Benson G, Maas R: Sexually dimorphic sterility phenotypes in Hoxa10-deficient mice. Nature 374:460-3, 1995
  24. Taylor HS, Arici A, Olive D, et al.: HOXA10 is expressed in response to sex steroids at the time of implantation in the human endometrium. The Journal of clinical investigation 101:1379-84, 1998
  25. Taylor HS, Igarashi P, Olive DL, et al.: Sex steroids mediate HOXA11 expression in the human peri-implantation endometrium. The Journal of clinical endocrinology and metabolism 84:1129-35, 1999
  26. Cakmak H, Taylor HS: Implantation failure: molecular mechanisms and clinical treatment. Human reproduction update 17:242-53, 2011
  27. Ochoa-Bernal MA, Fazleabas AT: Physiologic Events of Embryo Implantation and Decidualization in Human and Non-Human Primates. Int J Mol Sci 21, 2020
  28. Wilcox AJ, Baird DD, Weinberg CR: Time of implantation of the conceptus and loss of pregnancy. The New England journal of medicine 340:1796-9, 1999
  29. Morton H, Cavanagh AC, Athanasas-Platsis S, et al.: Early pregnancy factor has immunosuppressive and growth factor properties. Reproduction, fertility, and development 4:411-22, 1992
  30. Morton H, Rolfe BE, Cavanagh AC: Pregnancy proteins: basic concepts and clinical applications. Semin Reprod Endocrinol 10:72, 1992
  31. Cavanagh AC, Morton H, Rolfe BE, et al.: Ovum factor: a first signal of pregnancy? Am J Reprod Immunol 2:97-101, 1982
  32. Morton H, Rolfe BE, Cavanagh AC: Ovum factor and early pregnancy factor. Current topics in developmental biology 23:73-92, 1987
  33. Croxatto HB, Ortiz ME, Diaz S, et al.: Studies on the duration of egg transport by the human oviduct. II. Ovum location at various intervals following luteinizing hormone peak. American journal of obstetrics and gynecology 132:629-34, 1978
  34. Buster JE, Bustillo M, Rodi IA, et al.: Biologic and morphologic development of donated human ova recovered by nonsurgical uterine lavage. American journal of obstetrics and gynecology 153:211-7, 1985
  35. Macklon NS, Brosens JJ: The human endometrium as a sensor of embryo quality. Biology of reproduction 91:98, 2014
  36. Craciunas L, Gallos I, Chu J, et al.: Conventional and modern markers of endometrial receptivity: a systematic review and meta-analysis. Human reproduction update 25:202-23, 2019
  37. The morphological and functional development of the fetus. East Norwalk: Appleton & Lange, 1989
  38. Shutt DA, Lopata A: The secretion of hormones during the culture of human preimplantation embryos with corona cells. Fertility and sterility 35:413-6, 1981
  39. Laufer N, DeCherney AH, Haseltine FP, et al.: Steroid secretion by the human egg-corona-cumulus complex in culture. The Journal of clinical endocrinology and metabolism 58:1153-7, 1984
  40. Punnonen R, Lukola A: Binding of estrogen and progestin in the human fallopian tube. Fertility and sterility 36:610-4, 1981
  41. Hsueh AJ, Peck EJ, Jr., clark JH: Progesterone antagonism of the oestrogen receptor and oestrogen-induced uterine growth. Nature 254:337-9, 1975
  42. Ciarmela P, Islam MS, Reis FM, et al.: Growth factors and myometrium: biological effects in uterine fibroid and possible clinical implications. Human reproduction update 17:772-90, 2011
  43. Critchley HO, Brenner RM, Henderson TA, et al.: Estrogen receptor beta, but not estrogen receptor alpha, is present in the vascular endothelium of the human and nonhuman primate endometrium. The Journal of clinical endocrinology and metabolism 86:1370-8, 2001
  44. Albrecht ED, Robb VA, Pepe GJ: Regulation of placental vascular endothelial growth/permeability factor expression and angiogenesis by estrogen during early baboon pregnancy. The Journal of clinical endocrinology and metabolism 89:5803-9, 2004
  45. Albrecht ED, Aberdeen GW, Niklaus AL, et al.: Acute temporal regulation of vascular endothelial growth/permeability factor expression and endothelial morphology in the baboon endometrium by ovarian steroids. The Journal of clinical endocrinology and metabolism 88:2844-52, 2003
  46. Ma W, Tan J, Matsumoto H, et al.: Adult tissue angiogenesis: evidence for negative regulation by estrogen in the uterus. Molecular endocrinology 15:1983-92, 2001
  47. Bonduelle ML, Dodd R, Liebaers I, et al.: Chorionic gonadotrophin-beta mRNA, a trophoblast marker, is expressed in human 8-cell embryos derived from tripronucleate zygotes. Human reproduction (Oxford, England) 3:909-14, 1988
  48. Lopata A, Hay DL: The surplus human embryo: its potential for growth, blastulation, hatching, and human chorionic gonadotropin production in culture. Fertility and sterility 51:984-91, 1989
  49. Hay DL, Lopata A: Chorionic gonadotropin secretion by human embryos in vitro. The Journal of clinical endocrinology and metabolism 67:1322-4, 1988
  50. Enders AC: Embryo implantation, with emphasis on the rhesus monkey and the human. Reproduccion 5:163-7, 1981
  51. Tulchinsky D, Hobel CJ: Plasma human chorionic gonadotropin, estrone, estradiol, estriol, progesterone, and 17 alpha-hydroxyprogesterone in human pregnancy. 3. Early normal pregnancy. American journal of obstetrics and gynecology 117:884-93, 1973
  52. Chard T: Proteins of the human placenta: some general concepts; in Pregnancy Proteins: Biology, Chemistry and Clinical Application. Edited by Grudzinskas J, Teisner B, Sepala M. San Diego: Academic Press, 1982
  53. Saijonmaa O, Laatikainen T, Wahlstrom T: Corticotrophin-releasing factor in human placenta: localization, concentration and release in vitro. Placenta 9:373-85, 1988
  54. Khodr GS, Siler-Khodr TM: Placental luteinizing hormone-releasing factor and its synthesis. Science (New York, NY 207:315-7, 1980
  55. Shambaugh G, 3rd, Kubek M, Wilber JF: Thyrotropin-releasing hormone activity in the human placenta. The Journal of clinical endocrinology and metabolism 48:483-6, 1979
  56. Al-Timimi A, Fox H: Immunohistochemical localization of follicle-stimulating hormone, luteinizing hormone, growth hormone, adrenocorticotrophic hormone and prolactin in the human placenta. Placenta 7:163-72, 1986
  57. Hay DL: Placental histology and the production of human choriogonadotrophin and its subunits in pregnancy. British journal of obstetrics and gynaecology 95:1268-75, 1988
  58. Harada A, Hershman JM: Extraction of human chorionic thyrotropin (hCT) from term placentas: failure to recover thyrotropic activity. The Journal of clinical endocrinology and metabolism 47:681-5, 1978
  59. Steiner D: Peptide hormone precursors: biosynthesis, processing, and significance; in Peptide Hormones. Edited by Parson J. Baltimore: University Park Press, 1976
  60. Hoshina M, Hussa R, Pattillo R, et al.: The role of trophoblast differentiation in the control of the hCG and hPL genes. Advances in experimental medicine and biology 176:299-312, 1984
  61. Hoshina M, Boime I, Mochizuki M: [Cytological localization of hPL, hCG, and mRNA in chorionic tissue using in situ hybridization]. Nippon Sanka Fujinka Gakkai zasshi 36:397-404, 1984
  62. Kurman RJ, Young RH, Norris HJ, et al.: Immunocytochemical localization of placental lactogen and chorionic gonadotropin in the normal placenta and trophoblastic tumors, with emphasis on intermediate trophoblast and the placental site trophoblastic tumor. Int J Gynecol Pathol 3:101-21, 1984
  63. Kasai K, Aochi H, Shik SS, et al.: [Production and localization of human prolactin in the tissues associated with pregnancy (Report I) (author's transl)]. Nippon Naibunpi Gakkai zasshi 56:1574-80, 1980
  64. Watkins WB, Yen SS: Somatostatin in cytotrophoblast of the immature human placenta: localization by immunoperoxidase cytochemistry. The Journal of clinical endocrinology and metabolism 50:969-71, 1980
  65. Chard T, Grudzinskas JG: Pregnancy protein secretion. Semin Reprod Endocrinol 10:61, 1992
  66. Jones EE: Abnormal ovulation and implantation; in Medicine of the mother and fetus. Edited by Reece EA, Hobbins JC. Philadelphia: JB Lippincott Company, 1992
  67. Murphy BEP: Cortisol economy in the human fetus; in The Endocrine Function of the Human Adrenal Cortex. Edited by James VHT, Serio M, Gusti G. San Diego: Academic Press, 1978
  68. Murphy BE: Cortisol and cortisone in human fetal development. Journal of steroid biochemistry 11:509-13, 1979
  69. Handwerger S, Brar A: Placental lactogen, placental growth hormone, and decidual prolactin. Semin Reprod Endocrinol 10:106, 1992
  70. Maslar IA, Ansbacher R: Effects of progesterone on decidual prolactin production by organ cultures of human endometrium. Endocrinology 118:2102-8, 1986
  71. Raabe MA, McCoshen JA: Epithelial regulation of prolactin effect on amnionic permeability. American journal of obstetrics and gynecology 154:130-4, 1986
  72. Clements JA, Reyes FI, Winter JS, et al.: Studies on human sexual development. IV. Fetal pituitary and serum, and amniotic fluid concentrations of prolactin. The Journal of clinical endocrinology and metabolism 44:408-13, 1977
  73. Luciano AA, Varner MW: Decidual, amniotic fluid, maternal and fetal prolactin in normal and abnormal pregnancies. Obstetrics and gynecology 63:384-8, 1984
  74. Pullano JG, Cohen-Addad N, Apuzzio JJ, et al.: Water and salt conservation in the human fetus and newborn. I. Evidence for a role of fetal prolactin. The Journal of clinical endocrinology and metabolism 69:1180-6, 1989
  75. Golander A, Kopel R, Lazebnik N, et al.: Decreased prolactin secretion by decidual tissue of pre-eclampsia in vitro. Acta endocrinologica 108:111-3, 1985
  76. Healy DL, Herington AC, O'Herlihy C: Chronic polyhydramnios is a syndrome with a lactogen receptor defect in the chorion laeve. British journal of obstetrics and gynaecology 92:461-7, 1985
  77. McCoshen JA, Barc J: Prolactin bioactivity following decidual synthesis and transport by amniochorion. American journal of obstetrics and gynecology 153:217-23, 1985
  78. Rutanen E: Insulin-like growth factor binding protein-1. Semin Reprod Endocrinol 10:154, 1992
  79. Iwashita M, Kobayashi M, Matsuo A, et al.: Feto-maternal interaction of IGF-I and its binding proteins in fetal growth. Early Hum Dev 29:187-91, 1992
  80. Seppala M, Riittinen L, Kamarainen M: Placental protein 14/progesterone-associated endoemtrial protein revisited. Semin Reprod Endocrinol 10:164, 1992
  81. Julkunen M, Rutanen EM, Koskimies A, et al.: Distribution of placental protein 14 in tissues and body fluids during pregnancy. British journal of obstetrics and gynaecology 92:1145-51, 1985
  82. Stabile I, Olajide F, Chard T, et al.: Circulating levels of placental protein 14 in ectopic pregnancy. British journal of obstetrics and gynaecology 101:762-4, 1994
  83. Carr BR, MacDonald PC, Simpson ER: The role of lipoproteins in the regulation of progesterone secretion by the human corpus luteum. Fertility and sterility 38:303-11, 1982
  84. Csapo AI, Pulkkinen MO, Wiest WG: Effects of luteectomy and progesterone replacement therapy in early pregnant patients. American journal of obstetrics and gynecology 115:759-65, 1973
  85. Sauer MV, Paulson RJ, Lobo RA: A preliminary report on oocyte donation extending reproductive potential to women over 40. The New England journal of medicine 323:1157-60, 1990
  86. Yen SS: Endocrine-metabolic adaptations in pregnancy; in Reproductive endocrinology: physiology, pathophysiology and clinical management. Edited by Yen SSC, Jaffe RB, Barbieri RL. Philadelphia: WB Saunders Company, 1991
  87. Nygren KG, Johansson ED, Wide L: Evaluation of the prognosis of threatened abortion from the peripheral plasma levels of progesterone, estradiol, and human chorionic gonadotropin. American journal of obstetrics and gynecology 116:916-22, 1973
  88. Stovall TG, Ling FW, Carson SA, et al.: Serum progesterone and uterine curettage in differential diagnosis of ectopic pregnancy. Fertility and sterility 57:456-7, 1992
  89. Fields PA, Larkin LH: Purification and immunohistochemical localization of relaxin in the human term placenta. The Journal of clinical endocrinology and metabolism 52:79-85, 1981
  90. Lopez Bernal A, Bryant-Greenwood GD, Hansell DJ, et al.: Effect of relaxin on prostaglandin E production by human amnion: changes in relation to the onset of labour. British journal of obstetrics and gynaecology 94:1045-51, 1987
  91. Weiss G, O'Byrne EM, Hochman J, et al.: Distribution of relaxin in women during pregnancy. Obstetrics and gynecology 52:569-70, 1978
  92. Emmi AM, Skurnick J, Goldsmith LT, et al.: Ovarian control of pituitary hormone secretion in early human pregnancy. The Journal of clinical endocrinology and metabolism 72:1359-63, 1991
  93. Marnach ML, Ramin KD, Ramsey PS, et al.: Characterization of the relationship between joint laxity and maternal hormones in pregnancy. Obstetrics and gynecology 101:331-5, 2003
  94. Hwang JJ, Macinga D, Rorke EA: Relaxin modulates human cervical stromal cell activity. The Journal of clinical endocrinology and metabolism 81:3379-84, 1996
  95. MacLennan AH, Katz M, Creasy R: The morphologic characteristics of cervical ripening induced by the hormones relaxin and prostaglandin F2 alpha in a rabbit model. American journal of obstetrics and gynecology 152:691-6, 1985
  96. Garibay-Tupas JL, Maaskant RA, Greenwood FC, et al.: Characteristics of the binding of 32P-labelled human relaxins to the human fetal membranes. The Journal of endocrinology 145:441-8, 1995
  97. Bryant-Greenwood GD, Kern A, Yamamoto SY, et al.: Relaxin and the human fetal membranes. Reproductive sciences 14:42-5, 2007
  98. Tulchinsky D, Hobel CJ, Yeager E, et al.: Plasma estrone, estradiol, estriol, progesterone, and 17-hydroxyprogesterone in human pregnancy. I. Normal pregnancy. American journal of obstetrics and gynecology 112:1095-100, 1972
  99. Dicztalusy E: Steroid metabolism in the feto-placental unit; in The Feto-Placental Unit. Edited by Pecile A, Finzi C. Amsterdam: Excerpta Medica, 1969
  100. Pepe GJ, Albrecht ED: Actions of placental and fetal adrenal steroid hormones in primate pregnancy. Endocrine reviews 16:608-48, 1995
  101. Abraham GE, Odell WD, Swerdloff RS, et al.: Simultaneous radioimmunoassay of plasma FSH, LH, progesterone, 17-hydroxyprogesterone, and estradiol-17 beta during the menstrual cycle. The Journal of clinical endocrinology and metabolism 34:312-8, 1972
  102. Lindberg BS, Johansson ED, Nilsson BA: Plasma levels of nonconjugated oestrone, oestradiol-17beta and oestriol during uncomplicated pregnancy. Acta obstetricia et gynecologica Scandinavica 32:21-36, 1974
  103. Yen SS: Endocrine-metabolic adaptations in pregnancy; in Reproductive endocrinology: physiology, pathophysiology and clinical management. Edited by Yen SSC, Jaffe RB, Barbieri RL. Philadelphia: WB Saunders Company, 1991
  104. Mitchell BF, Challis JR, Lukash L: Progesterone synthesis by human amnion, chorion, and decidua at term. American journal of obstetrics and gynecology 157:349-53, 1987
  105. Siiteri PK, Febres F, Clemens LE, et al.: Progesterone and maintenance of pregnancy: is progesterone nature's immunosuppressant? Annals of the New York Academy of Sciences 286:384-97, 1977
  106. Moriyama I, Sugawa T: Progesterone facilitates implantation of xenogenic cultured cells in hamster uterus. Nature: New biology 236:150-2, 1972
  107. Partsch CJ, Sippell WG, MacKenzie IZ, et al.: The steroid hormonal milieu of the undisturbed human fetus and mother at 16-20 weeks gestation. The Journal of clinical endocrinology and metabolism 73:969-74, 1991
  108. Tulchinsky D, Simmer HH: Sources of plasma 17alpha-hydroxyprogesterone in human pregnancy. The Journal of clinical endocrinology and metabolism 35:799-808, 1972
  109. Siiteri PK, MacDonald PC: Placental estrogen biosynthesis during human pregnancy. The Journal of clinical endocrinology and metabolism 26:751-61, 1966
  110. Bradshaw KD, Carr BR: Placental sulfatase deficiency: maternal and fetal expression of steroid sulfatase deficiency and X-linked ichthyosis. Obstetrical & gynecological survey 41:401-13, 1986
  111. Simpson ER, Mahendroo MS, Means GD, et al.: Aromatase cytochrome P450, the enzyme responsible for estrogen biosynthesis. Endocrine reviews 15:342-55, 1994
  112. Yen SS: Endocrine-metabolic adaptations in pregnancy; in Reproductive endocrinology: physiology, pathophysiology and clinical management. Edited by Yen SSC, Jaffe RB, Barbieri RL. Philadelphia: WB Saunders Company, 1991
  113. Resnik R, Killam AP, Battaglia FC, et al.: The stimulation of uterine blood flow by various estrogens. Endocrinology 94:1192-6, 1974
  114. Henson MC, Pepe GJ, Albrecht ED: Regulation of placental low-density lipoprotein uptake in baboons by estrogen: dose-dependent effects of the anti-estrogen ethamoxytriphetol (MER-25). Biology of reproduction 45:43-8, 1991
  115. Tulchinsky D, Hobel CJ, Korenman SG: A radioligand assay for plasma unconjugated estriol in normal and abnormal pregnancies. American journal of obstetrics and gynecology 111:311-8, 1971
  116. Landon MB, Gabbe SG: Fetal surveillance in the pregnancy complicated by diabetes mellitus. Clinical obstetrics and gynecology 34:535-43, 1991
  117. Klopper A, Masson G, Campbell D, et al.: Estriol in plasma. A compartmental study. American journal of obstetrics and gynecology 117:21-6, 1973
  118. Solomon S: The placenta as an endocrine organ: steroids; in The physiology of reproduction. Edited by Knobil E, Neill JD. New York: Raven Press Ltd., 1988
  119. Barnea ER, Kaplan M: Spontaneous, gonadotropin-releasing hormone-induced, and progesterone-inhibited pulsatile secretion of human chorionic gonadotropin in the first trimester placenta in vitro. The Journal of clinical endocrinology and metabolism 69:215-7, 1989
  120. Petraglia F, Florio P, Nappi C, et al.: Peptide signaling in human placenta and membranes: autocrine, paracrine, and endocrine mechanisms. Endocrine reviews 17:156-86, 1996
  121. Chrousos GP, Calabrese JR, Avgerinos P, et al.: Corticotropin releasing factor: basic studies and clinical applications. Progress in neuro-psychopharmacology & biological psychiatry 9:349-59, 1985
  122. Stalla GK, Hartwimmer J, von Werder K, et al.: Ovine (o) and human (h) corticotrophin releasing factor (CRF) in man: CRF-stimulation and CRF-immunoreactivity. Acta endocrinologica 106:289-97, 1984
  123. Shibahara S, Morimoto Y, Furutani Y, et al.: Isolation and sequence analysis of the human corticotropin-releasing factor precursor gene. The EMBO journal 2:775-9, 1983
  124. Stalla GK, Bost H, Stalla J, et al.: Human corticotropin-releasing hormone during pregnancy. Gynecol Endocrinol 3:1-10, 1989
  125. Laatikainen TJ, Raisanen IJ, Salminen KR: Corticotropin-releasing hormone in amniotic fluid during gestation and labor and in relation to fetal lung maturation. American journal of obstetrics and gynecology 159:891-5, 1988
  126. Jones SA, Brooks AN, Challis JR: Steroids modulate corticotropin-releasing hormone production in human fetal membranes and placenta. The Journal of clinical endocrinology and metabolism 68:825-30, 1989
  127. Robinson BG, Emanuel RL, Frim DM, et al.: Glucocorticoid stimulates expression of corticotropin-releasing hormone gene in human placenta. Proceedings of the National Academy of Sciences of the United States of America 85:5244-8, 1988
  128. Linton EA, Perkins AV, Woods RJ, et al.: Corticotropin releasing hormone-binding protein (CRH-BP): plasma levels decrease during the third trimester of normal human pregnancy. The Journal of clinical endocrinology and metabolism 76:260-2, 1993
  129. Sug-Tang A, Bocking AD, Brooks AN, et al.: Effects of restricting uteroplacental blood flow on concentrations of corticotrophin-releasing hormone, adrenocorticotrophin, cortisol, and prostaglandin E2 in the sheep fetus during late pregnancy. Canadian journal of physiology and pharmacology 70:1396-402, 1992
  130. Clifton VL, Read MA, Leitch IM, et al.: Corticotropin-releasing hormone-induced vasodilatation in the human fetal placental circulation. The Journal of clinical endocrinology and metabolism 79:666-9, 1994
  131. Perkins AV, Linton EA, Eben F, et al.: Corticotrophin-releasing hormone and corticotrophin-releasing hormone binding protein in normal and pre-eclamptic human pregnancies. British journal of obstetrics and gynaecology 102:118-22, 1995
  132. Goland RS, Jozak S, Warren WB, et al.: Elevated levels of umbilical cord plasma corticotropin-releasing hormone in growth-retarded fetuses. The Journal of clinical endocrinology and metabolism 77:1174-9, 1993
  133. Ruth V, Hallman M, Laatikainen T: Corticotropin-releasing hormone and cortisol in cord plasma in relation to gestational age, labor, and fetal distress. American journal of perinatology 10:115-8, 1993
  134. Goland RS, Conwell IM, Warren WB, et al.: Placental corticotropin-releasing hormone and pituitary-adrenal function during pregnancy. Neuroendocrinology 56:742-9, 1992
  135. Youngblood WW, Humm J, Lipton MA, et al.: Thyrotropin-releasing hormone-like bioactivity in placenta: evidence for the existence of substances other than Pyroglu-His-Pro-NH2 (TRH) capable of stimulating pituitary thyrotropin release. Endocrinology 106:541-6, 1980
  136. Bajoria R, Babawale M: Ontogeny of endogenous secretion of immunoreactive-thyrotropin releasing hormone by the human placenta. The Journal of clinical endocrinology and metabolism 83:4148-55, 1998
  137. Taliadouros GS, Canfield RE, Nisula BC: Thyroid-stimulating activity of chorionic gonadotropin and luteinizing hormone. The Journal of clinical endocrinology and metabolism 47:855-60, 1978
  138. Kumasaka T, Nishi N, Yaoi Y, et al.: Demonstration of immunoreactive somatostatin-like substance in villi and decidua in early pregnancy. American journal of obstetrics and gynecology 134:39-44, 1979
  139. Tsalikian E, Foley TP, Jr., Becker DJ: Characterization of somatostatin specific binding in plasma cell membranes of human placenta. Pediatric research 18:953-7, 1984
  140. Ren SG, Braunstein GD: Human chorionic gonadotropin. Semin Reprod Endocrinol 10:95, 1992
  141. Mersol-Barg MS, Miller KF, Choi CM, et al.: Inhibin suppresses human chorionic gonadotropin secretion in term, but not first trimester, placenta. The Journal of clinical endocrinology and metabolism 71:1294-8, 1990
  142. Hanson FW, Powell JE, Stevens VC: Effects of HCG and human pituitary LH on steroid secretion and functional life of the human corpus luteum. The Journal of clinical endocrinology and metabolism 32:211-5, 1971
  143. Nisula BC, Ketelslegers JM: Thyroid-stimulating activity and chorionic gonadotropin. The Journal of clinical investigation 54:494-9, 1974
  144. Seron-Ferre M, Lawrence CC, Jaffee RB: Role of hCG in the regulation of the fetal adrenal gland. The Journal of clinical endocrinology and metabolism 46:834, 1978
  145. Huhtaniemi IT, Korenbrot CC, Jaffe RB: HCG binding and stimulation of testosterone biosynthesis in the human fetal testis. The Journal of clinical endocrinology and metabolism 44:963-7, 1977
  146. Hodgen GD, Itskovitz J: Recognition and maintenance of pregnancy; in The physiology of reproduction. Edited by Knobil E, Neill JD. New York: Raven Press Ltd., 1988
  147. Frankenne F, Closset J, Gomez F, et al.: The physiology of growth hormones (GHs) in pregnant women and partial characterization of the placental GH variant. The Journal of clinical endocrinology and metabolism 66:1171-80, 1988
  148. Eriksson L, Frankenne F, Eden S, et al.: Growth hormone secretion during termination of pregnancy. Further evidence of a placental variant. Acta Obstet Gynecol Scand 67:549-52, 1988
  149. Eriksson L: Growth hormone in human pregnancy. Maternal 24-hour serum profiles and experimental effects of continuous GH secretion. Acta obstetricia et gynecologica Scandinavica 147:1-38, 1989
  150. Eriksson L, Frankenne F, Eden S, et al.: Growth hormone 24-h serum profiles during pregnancy--lack of pulsatility for the secretion of the placental variant. British journal of obstetrics and gynaecology 96:949-53, 1989
  151. Mirlesse V, Frankenne F, Alsat E, et al.: Placental growth hormone levels in normal pregnancy and in pregnancies with intrauterine growth retardation. Pediatric research 34:439-42, 1993
  152. Takata K, Kasahara T, Kasahara M, et al.: Localization of erythrocyte/HepG2-type glucose transporter (GLUT1) in human placental villi. Cell and tissue research 267:407-12, 1992
  153. Hauguel-de Mouzon S, Leturque A, Alsat E, et al.: Developmental expression of Glut1 glucose transporter and c-fos genes in human placental cells. Placenta 15:35-46, 1994
  154. MacLeod JN, Lee AK, Liebhaber SA, et al.: Developmental control and alternative splicing of the placentally expressed transcripts from the human growth hormone gene cluster. The Journal of biological chemistry 267:14219-26, 1992
  155. de Zegher F, Vanderschueren-Lodeweyckx M, Spitz B, et al.: Perinatal growth hormone (GH) physiology: effect of GH-releasing factor on maternal and fetal secretion of pituitary and placental GH. The Journal of clinical endocrinology and metabolism 71:520-2, 1990
  156. Evain-Brion D, Alsat E, Mirlesse V, et al.: Regulation of growth hormone secretion in human trophoblastic cells in culture. Hormone research 33:256-9, 1990
  157. Han VK, Bassett N, Walton J, et al.: The expression of insulin-like growth factor (IGF) and IGF-binding protein (IGFBP) genes in the human placenta and membranes: evidence for IGF-IGFBP interactions at the feto-maternal interface. The Journal of clinical endocrinology and metabolism 81:2680-93, 1996
  158. Raben MS, Matsuzaki F, Minton PR: Growth-Promoting and Metabolic Effects of Growth Hormone. Metabolism: clinical and experimental 13:SUPPL:1102-7, 1964
  159. Salomon F, Cuneo RC, Hesp R, et al.: The effects of treatment with recombinant human growth hormone on body composition and metabolism in adults with growth hormone deficiency. The New England journal of medicine 321:1797-803, 1989
  160. Yen SS: Endocrine-metabolic adaptations in pregnancy; in Reproductive endocrinology: physiology, pathophysiology and clinical management. Edited by Yen SSC, Jaffe RB, Barbieri RL. Philadelphia: WB Saunders Company, 1991
  161. Niall HD, Hogan ML, Sauer R, et al.: Sequences of pituitary and placental lactogenic and growth hormones: evolution from a primordial peptide by gene reduplication. Proceedings of the National Academy of Sciences of the United States of America 68:866-70, 1971
  162. Braunstein GD, Rasor JL, Engvall E, et al.: Interrelationships of human chorionic gonadotropin, human placental lactogen, and pregnancy-specific beta 1-glycoprotein throughout normal human gestation. American journal of obstetrics and gynecology 138:1205-13, 1980
  163. Kim YJ, Felig P: Plasma chorionic somatomammotropin levels during starvation in midpregnancy. The Journal of clinical endocrinology and metabolism 32:864-7, 1971
  164. Furlanetto RW, Underwood LE, Van Wyk JJ, et al.: Serum immunoreactive somatomedin-C is elevated late in pregnancy. The Journal of clinical endocrinology and metabolism 47:695-8, 1978
  165. Kirwan JP, Hauguel-De Mouzon S, Lepercq J, et al.: TNF-alpha is a predictor of insulin resistance in human pregnancy. Diabetes 51:2207-13, 2002
  166. Navot D, Scott RT, Droesch K, et al.: The window of embryo transfer and the efficiency of human conception in vitro. Fertility and sterility 55:114-8, 1991
  167. Lenton EA, Neal LM, Sulaiman R: Plasma concentrations of human chorionic gonadotropin from the time of implantation until the second week of pregnancy. Fertility and sterility 37:773-8, 1982
  168. Kosasa T, Levesque L, Goldstein DP, et al.: Early detection of implantation using a radioimmunoassay specific for human chorionic gonadotropin. The Journal of clinical endocrinology and metabolism 36:622-4, 1973
  169. Cavanagh AC, Morton H: The purification of early-pregnancy factor to homogeneity from human platelets and identification as chaperonin 10. European journal of biochemistry / FEBS 222:551-60, 1994
  170. Di Trapani G, Orosco C, Perkins A, et al.: Isolation from human placental extracts of a preparation possessing 'early pregnancy factor' activity and identification of the polypeptide components. Human reproduction (Oxford, England) 6:450-7, 1991
  171. Zuo X, Su B, Wei D: Isolation and characterization of early pregnancy factor. Chinese medical sciences journal = Chung-kuo i hsueh k'o hsueh tsa chih / Chinese Academy of Medical Sciences 9:34-7, 1994
  172. Mehta AR, Eessalu TE, Aggarwal BB: Purification and characterization of early pregnancy factor from human pregnancy sera. The Journal of biological chemistry 264:2266-71, 1989
  173. Clarke FM: Identification of molecules and mechanisms involved in the 'early pregnancy factor' system. Reproduction, fertility, and development 4:423-33, 1992
  174. Chard T, Grudzinskas JG: Early pregnancy factor. Biological research in pregnancy and perinatology 8:53-6, 1987
  175. Mesrogli M, Schneider J, Maas DH: Early pregnancy factor as a marker for the earliest stages of pregnancy in infertile women. Human reproduction (Oxford, England) 3:113-5, 1988
  176. Shahani SK, Moniz CL, Bordekar AD, et al.: Early pregnancy factor as a marker for assessing embryonic viability in threatened and missed abortions. Gynecologic and obstetric investigation 37:73-6, 1994
  177. Straube W, Romer T, Zeenni L, et al.: [The early pregnancy factor (EPF) as an early marker of disorders in pregnancy]. Zentralblatt fur Gynakologie 117:32-4, 1995
  178. Hubel V, Straube W, Loh M, et al.: Human early pregnancy factor and early pregnancy associated protein before and after therapeutic abortion in comparison with beta-hCG, estradiol, progesterone and 17-hydroxyprogesterone. Experimental and clinical endocrinology 94:171-6, 1989
  179. Rees LH, Burke CW, Chard T, et al.: Possible placental origin of ACTH in normal human pregnancy. Nature 254:620-2, 1975
  180. Genazzani AR, Fraioli F, Hurlimann J, et al.: Immunoreactive ACTH and cortisol plasma levels during pregnancy. Detection and partial purification of corticotrophin-like placental hormone: the human chorionic corticotrophin (HCC). Clinical endocrinology 4:1-14, 1975
  181. Petraglia F, Sawchenko PE, Rivier J, et al.: Evidence for local stimulation of ACTH secretion by corticotropin-releasing factor in human placenta. Nature 328:717-9, 1987
  182. Nolten WE, Rueckert PA: Elevated free cortisol index in pregnancy: possible regulatory mechanisms. American journal of obstetrics and gynecology 139:492-8, 1981
  183. Prager D, Weber MM, Herman-Bonert V: Placental growth factors and releasing/inhibiting peptides. Semin Reprod Endocrinol 10:83, 1992
  184. Abe Y, Hasegawa Y, Miyamoto K, et al.: High concentrations of plasma immunoreactive inhibin during normal pregnancy in women. The Journal of clinical endocrinology and metabolism 71:133-7, 1990
  185. Tovanabutra S, Illingworth PJ, Ledger WL, et al.: The relationship between peripheral immunoactive inhibin, human chorionic gonadotrophin, oestradiol and progesterone during human pregnancy. Clinical endocrinology 38:101-7, 1993
  186. Petraglia F, Sawchenko P, Lim AT, et al.: Localization, secretion, and action of inhibin in human placenta. Science (New York, NY 237:187-9, 1987
  187. Muttukrishna S, Fowler PA, George L, et al.: Changes in peripheral serum levels of total activin A during the human menstrual cycle and pregnancy. The Journal of clinical endocrinology and metabolism 81:3328-34, 1996
  188. Bersinger NA, Smarason AK, Muttukrishna S, et al.: Women with preeclampsia have increased serum levels of pregnancy-associated plasma protein A (PAPP-A), inhibin A, activin A and soluble E-selectin. Hypertension in pregnancy 22:45-55, 2003
  189. Gagnon A, Wilson RD, Audibert F, et al.: Obstetrical complications associated with abnormal maternal serum markers analytes. Journal of obstetrics and gynaecology Canada : JOGC = Journal d'obstetrique et gynecologie du Canada : JOGC 30:918-49, 2008
  190. Petraglia F, Gallinelli A, Grande A, et al.: Local production and action of follistatin in human placenta. The Journal of clinical endocrinology and metabolism 78:205-10, 1994
  191. Mills NC, D'Ercole AJ, Underwood LE, et al.: Synthesis of somatomedin C/insulin-like growth factor I by human placenta. Molecular biology reports 11:231-6, 1986
  192. Grizzard JD, D'Ercole AJ, Wilkins JR, et al.: Affinity-labeled somatomedin-C receptors and binding proteins from the human fetus. The Journal of clinical endocrinology and metabolism 58:535-43, 1984
  193. Jonas HA, Harrison LC: The human placenta contains two distinct binding and immunoreactive species of insulin-like growth factor-I receptors. The Journal of biological chemistry 260:2288-94, 1985
  194. Napso T, Yong HEJ, Lopez-Tello J, et al.: The Role of Placental Hormones in Mediating Maternal Adaptations to Support Pregnancy and Lactation. Front Physiol 9:1091, 2018
  195. Levine RJ, Lam C, Qian C, et al.: Soluble endoglin and other circulating antiangiogenic factors in preeclampsia. The New England journal of medicine 355:992-1005, 2006
  196. Levine RJ, Maynard SE, Qian C, et al.: Circulating angiogenic factors and the risk of preeclampsia. The New England journal of medicine 350:672-83, 2004
  197. Venkatesha S, Toporsian M, Lam C, et al.: Soluble endoglin contributes to the pathogenesis of preeclampsia. Nature medicine 12:642-9, 2006
  198. Myatt L, Clifton RG, Roberts JM, et al.: First-trimester prediction of preeclampsia in nulliparous women at low risk. Obstetrics and gynecology 119:1234-42, 2012
  199. Kusanovic JP, Romero R, Chaiworapongsa T, et al.: A prospective cohort study of the value of maternal plasma concentrations of angiogenic and anti-angiogenic factors in early pregnancy and midtrimester in the identification of patients destined to develop preeclampsia. The journal of maternal-fetal & neonatal medicine : the official journal of the European Association of Perinatal Medicine, the Federation of Asia and Oceania Perinatal Societies, the International Society of Perinatal Obstet 22:1021-38, 2009
  200. Lenton EA, Grudzinskas JG, Gordon YB, et al.: Pregnancy specific beta 1 glycoprotein and chorionic gonadotrophin in early human pregnancy. Acta Obstet Gynecol Scand 60:489-92, 1981
  201. Chou JY, Plouzek CA: Pregnancy-specific 3t-glycoprotein. Semin Reprod Endocrinol 10:116, 1992
  202. Tatarinov YS: Trophoblast-specific beta1-glycoprotein as a marker for pregnancy and malignancies. Gynecologic and obstetric investigation 9:65-97, 1978
  203. Sinosich MJ, Teisner B, Folkersen J, et al.: Radioimmunoassay for pregnancy-associated plasma protein A. Clinical chemistry 28:50-3, 1982
  204. Bischof P: Pregnancy-associated plasma protein-A. Semin Reprod Endocrinol 10:127, 1992
  205. Westergaard JG, Teisner B, Sinosich MJ, et al.: Does ultrasound examination render biochemical tests obsolete in the prediction of early pregnancy failure? British journal of obstetrics and gynaecology 92:77-83, 1985
  206. Dumps P, Meisser A, Pons D, et al.: Accuracy of single measurements of pregnancy-associated plasma protein-A, human chorionic gonadotropin and progesterone in the diagnosis of early pregnancy failure. European journal of obstetrics, gynecology, and reproductive biology 100:174-80, 2002
  207. Obiekwe B, Pendlebury DJ, Gordeon YB, et al.: The radioimmunoassay of placental protein 5 and circulating levels in maternal blood in the third trimester of normal pregnancy. Clinica chimica acta; international journal of clinical chemistry 95:509-16, 1979
  208. Salem HT, Seppala M, Chard T: The effect of thrombin on serum placental protein 5 (PP5): is PP5 the naturally occurring antithrombin III of the human placenta? Placenta 2:205-9, 1981
  209. Flood JT, Hodgen GD: The physiology of fertilization, implantation and early human development; in Danforth's Obstetrics and Gynecology. Edited by Scott JR, Desaia PJ. Philadelphia: JB Lippincott Company, 1990
  210. Ahima RS, Flier JS: Leptin. Annual review of physiology 62:413-37, 2000
  211. Senaris R, Garcia-Caballero T, Casabiell X, et al.: Synthesis of leptin in human placenta. Endocrinology 138:4501-4, 1997
  212. Stock SM, Bremme KA: Elevation of plasma leptin levels during pregnancy in normal and diabetic women. Metabolism: clinical and experimental 47:840-3, 1998
  213. Tamas P, Sulyok E, Szabo I, et al.: Changes of maternal serum leptin levels during pregnancy. Gynecologic and obstetric investigation 46:169-71, 1998
  214. Tamura T, Goldenberg RL, Johnston KE, et al.: Serum leptin concentrations during pregnancy and their relationship to fetal growth. Obstetrics and gynecology 91:389-95, 1998
  215. Akerman F, Lei ZM, Rao CV: Human umbilical cord and fetal membranes co-express leptin and its receptor genes. Gynecological endocrinology : the official journal of the International Society of Gynecological Endocrinology 16:299-306, 2002
  216. Hardie L, Trayhurn P, Abramovich D, et al.: Circulating leptin in women: a longitudinal study in the menstrual cycle and during pregnancy. Clinical endocrinology 47:101-6, 1997
  217. Highman TJ, Friedman JE, Huston LP, et al.: Longitudinal changes in maternal serum leptin concentrations, body composition, and resting metabolic rate in pregnancy. American journal of obstetrics and gynecology 178:1010-5, 1998
  218. Hauguel-de Mouzon S, Lepercq J, Catalano P: The known and unknown of leptin in pregnancy. American journal of obstetrics and gynecology 194:1537-45, 2006
  219. Schanton M, Maymo JL, Perez-Perez A, et al.: Involvement of leptin in the molecular physiology of the placenta. Reproduction 155:R1-R12, 2018
  220. Chardonnens D, Cameo P, Aubert ML, et al.: Modulation of human cytotrophoblastic leptin secretion by interleukin-1alpha and 17beta-oestradiol and its effect on HCG secretion. Molecular human reproduction 5:1077-82, 1999
  221. Jansson N, Greenwood SL, Johansson BR, et al.: Leptin stimulates the activity of the system A amino acid transporter in human placental villous fragments. The Journal of clinical endocrinology and metabolism 88:1205-11, 2003
  222. Gnanapavan S, Kola B, Bustin SA, et al.: The tissue distribution of the mRNA of ghrelin and subtypes of its receptor, GHS-R, in humans. The Journal of clinical endocrinology and metabolism 87:2988, 2002
  223. Tanaka K, Minoura H, Isobe T, et al.: Ghrelin is involved in the decidualization of human endometrial stromal cells. The Journal of clinical endocrinology and metabolism 88:2335-40, 2003
  224. Fuglsang J: Ghrelin in pregnancy and lactation. Vitam Horm 77:259-84, 2008
  225. Winn VD, Haimov-Kochman R, Paquet AC, et al.: Gene expression profiling of the human maternal-fetal interface reveals dramatic changes between midgestation and term. Endocrinology 148:1059-79, 2007
  226. Jaffe RB: Neuroendocrine-metabolic regulation of pregnancy,; in Reproductive Endocrinology 4th ed Edited by Yen SSC JR, Barbieri RL (eds). Philadelphia: WB Saunders, 1999
  227. Fisher DA: Fetal and Neonatal Endocrinology; in Endocrinology. Edited by DeGroot LJ JJe. Philadelphia: WB Saunders, 2000
  228. Parker CR: The endocrinology of pregnancy; in Textbook of Reproductive Medicine. Edited by Carr BR, Blackwell RE. Norwalk: Appleton & Lange, 1993
  229. Tho SP, Layman LC, Lanclos KD, et al.: Absence of the testicular determining factor gene SRY in XX true hermaphrodites and presence of this locus in most subjects with gonadal dysgenesis caused by Y aneuploidy. American journal of obstetrics and gynecology 167:1794-802, 1992
  230. Jost A, Vigier B, Prepin J, et al.: Studies on sex differentiation in mammals. Recent progress in hormone research 29:1-41, 1973
  231. Baker TG: A Quantitative and Cytological Study of Germ Cells in Human Ovaries. Proceedings of the Royal Society of London Series B, Biological sciences 158:417-33, 1963
  232. Gondos B, Bhiraleus P, Hobel CJ: Ultrastructural observations on germ cells in human fetal ovaries. American journal of obstetrics and gynecology 110:644-52, 1971
  233. Johannison E: The foetal adrenal cortex in the human. Acta Endocrinol 58(Suppl. 130):7, 1968
  234. Honour JH, Wickramaratne K, Valman HB: Adrenal function in preterm infants. Biology of the neonate 61:214-21, 1992
  235. Hutchinson KA, DeCherney AH: The endocrinology of pregnancy; in Medicine of the mother and fetus. Edited by Reece EA, Hobbins JC. Philadelphia: JB Lippincott Company, 1999
  236. The morphological and functional development of the fetus. East Norwalk: Appleton & Lange, 1989
  237. Girard J: Control of fetal and neonatal glucose metabolism by pancreatic hormones. Bailliere's clinical endocrinology and metabolism 3:817-36, 1989
  238. MA. S: Carbohydrate metabolism: insulin and glucagon.; in Maternal Fetal Endocrinology 2nd ed Edited by Tulchinsky D LAe. Philadelpia: WB Saunders, 1994
  239. Alpert E, Drysdale JW, Isselbacher KJ, et al.: Human -fetoprotein. Isolation, characterization, and demonstration of microheterogeneity. The Journal of biological chemistry 247:3792-8, 1972
  240. Gitlin D, Perricelli A, Gitlin GM: Synthesis of -fetoprotein by liver, yolk sac, and gastrointestinal tract of the human conceptus. Cancer research 32:979-82, 1972
  241. Habib ZA: Maternal serum alpha-feto-protein: its value in antenatal diagnosis of genetic disease and in obstetrical-gynaecological care. Acta obstetricia et gynecologica Scandinavica 61:1-92, 1977
  242. Murgita RA, Tomasi TB, Jr.: Suppression of the immune response by alpha-fetoprotein on the primary and secondary antibody response. The Journal of experimental medicine 141:269-86, 1975
  243. Ferguson-Smith MA, Rawlinson HA, May HM, et al.: Avoidance of anencephalic and spina bifida births by maternal serum-alphafetoprotein screening. Lancet 1:1330-3, 1978
  244. Wald N, Cuckle H: AFP and age screening for Down syndrome. American journal of medical genetics 31:197-209, 1988
  245. Elster AD, Sanders TG, Vines FS, et al.: Size and shape of the pituitary gland during pregnancy and post partum: measurement with MR imaging. Radiology 181:531-5, 1991
  246. Leake RD, Weitzman RE, Glatz TH, et al.: Plasma oxytocin concentrations in men, nonpregnant women, and pregnant women before and during spontaneous labor. The Journal of clinical endocrinology and metabolism 53:730-3, 1981
  247. Zeeman GG, Khan-Dawood FS, Dawood MY: Oxytocin and its receptor in pregnancy and parturition: current concepts and clinical implications. Obstetrics and gynecology 89:873-83, 1997
  248. Glinoer D: The regulation of thyroid function in pregnancy: pathways of endocrine adaptation from physiology to pathology. Endocrine reviews 18:404-33, 1997
  249. Burrow GN, Fisher DA, Larsen PR: Maternal and fetal thyroid function. The New England journal of medicine 331:1072-8, 1994
  250. Lindsay JR, Nieman LK: The hypothalamic-pituitary-adrenal axis in pregnancy: challenges in disease detection and treatment. Endocrine reviews 26:775-99, 2005
  251. Rainey WE, Rehman KS, Carr BR: Fetal and maternal adrenals in human pregnancy. Obstetrics and gynecology clinics of North America 31:817-35, x, 2004
  252. Pepe GJ, Waddell BJ, Albrecht ED: The effects of adrenocorticotropin and prolactin on adrenal dehydroepiandrosterone secretion in the baboon fetus. Endocrinology 122:646-50, 1988
  253. Pepe GJ, Albrecht ED: Regulation of the primate fetal adrenal cortex. Endocrine reviews 11:151-76, 1990
  254. Albrecht ED, Pepe GJ: Placental steroid hormone biosynthesis in primate pregnancy. Endocrine reviews 11:124-50, 1990
  255. Carr BR, MacDonald PC, Simpson ER: The regulation of de novo synthesis of cholesterol in the human fetal adrenal gland by low density lipoprotein and adrenocorticotropin. Endocrinology 107:1000-6, 1980
  256. Carr BR, Porter JC, MacDonald PC, et al.: Metabolism of low density lipoprotein by human fetal adrenal tissue. Endocrinology 107:1034-40, 1980
  257. Parker CR, Jr., Carr BR, Winkel CA, et al.: Hypercholesterolemia due to elevated low density lipoprotein-cholesterol in newborns with anencephaly and adrenal atrophy. The Journal of clinical endocrinology and metabolism 57:37-43, 1983
  258. Simpson ER, Carr BR, Parker CR, Jr., et al.: The role of serum lipoproteins in steroidogenesis by the human fetal adrenal cortex. The Journal of clinical endocrinology and metabolism 49:146-8, 1979
  259. Winters AJ, Oliver C, Colston C, et al.: Plasma ACTH levels in the human fetus and neonate as related to age and parturition. The Journal of clinical endocrinology and metabolism 39:269-73, 1974
  260. Simpson ER, Parker CR, Jr., Carr BR: Role of lipoproteins in the regulation of steroidogenesis by the human fetal adrenal; in The Endocrine Physiology of Pregnancy and the Peripartal Period Vol 21 Serono Symposia Publications. Edited by Jaffe RB, Dell Acqua S. New York: Raven Press, 1985
  261. Nieman LK: The endocrinology of pregnancy; in Serono symposia in Reproductive Endocrinology
  262. Seron-Ferre M, Lawrence CC, Siiteri PK, et al.: Steroid production by definitive and fetal zones of the human fetal adrenal gland. The Journal of clinical endocrinology and metabolism 47:603-9, 1978
  263. Baird A, Kan KW, Solomon S: Role of pro-opiomelanocortin-derived peptides in the regulation of steroid production by human fetal adrenal cells in culture. The Journal of endocrinology 97:357-67, 1983
  264. Bugnon C, Lenys D, Bloch B, et al.: [Cyto-immunologic study of early cell differentiation phenomena in the human fetal anterior pituitary gland]. Comptes rendus des seances de la Societe de biologie et de ses filiales 168:460-5, 1974
  265. Katikineni M, Davies TF, Catt KJ: Regulation of adrenal and testicular prolactin receptors by adrenocorticotropin and luteinizing hormone. Endocrinology 108:2367-74, 1981
  266. Voutilainen R, Miller WL: Coordinate tropic hormone regulation of mRNAs for insulin-like growth factor II and the cholesterol side-chain-cleavage enzyme, P450scc [corrected], in human steroidogenic tissues. Proceedings of the National Academy of Sciences of the United States of America 84:1590-4, 1987
  267. Winters AJ, Colston C, MacDonald PC, et al.: Fetal plasma prolactin levels. The Journal of clinical endocrinology and metabolism 41:626-9, 1975
  268. Walsh SW, Norman RL, Novy MJ: In utero regulation of rhesus monkey fetal adrenals: effects of dexamethasone, adrenocorticotropin, thyrotropin-releasing hormone, prolactin, human chorionic gonadotropin, and alpha-melanocyte-stimulating hormone on fetal and maternal plasma steroids. Endocrinology 104:1805-13, 1979
  269. Liggins GC: Endocrinology of the foeto-maternal unit; in Human Reproductive Physiology. Edited by Sherman RP. Oxford: Blackwell Scientific Publications, 1972
  270. Baggia S, Albrecht ED, Pepe GJ: Regulation of 11 beta-hydroxysteroid dehydrogenase activity in the baboon placenta by estrogen. Endocrinology 126:2742-8, 1990
  271. Jost A: The fetal adrenal cortex; in Handbook of Physiology. Edited by Creep RO, Astwood WB. Washington, DC: Endocrinology Amer Physiol Soc, 1975
  272. Kondo S: Developmental studies on the Japanese human adrenals, I: ponderal growth. Bull Exp Biol 9:51, 1959
  273. Spector DVS: Handbook of Biological Data. WB Saunders, Philadelphia. 1956
  274. Fant M, Munro H, Moses AC: An autocrine/paracrine role for insulin-like growth factors in the regulation of human placental growth. The Journal of clinical endocrinology and metabolism 63:499-505, 1986
  275. Han VK, Lund PK, Lee DC, et al.: Expression of somatomedin/insulin-like growth factor messenger ribonucleic acids in the human fetus: identification, characterization, and tissue distribution. The Journal of clinical endocrinology and metabolism 66:422-9, 1988
  276. Sandman CA, Glynn L, Schetter CD, et al.: Elevated maternal cortisol early in pregnancy predicts third trimester levels of placental corticotropin releasing hormone (CRH): priming the placental clock. Peptides 27:1457-63, 2006
  277. Norwitz ER, Bonney EA, Snegovskikh VV, et al.: Molecular Regulation of Parturition: The Role of the Decidual Clock. Cold Spring Harb Perspect Med 5, 2015
  278. Berkowitz GS, Lapinski RH, Lockwood CJ, et al.: Corticotropin-releasing factor and its binding protein: maternal serum levels in term and preterm deliveries. American journal of obstetrics and gynecology 174:1477-83, 1996
  279. McGrath S, McLean M, Smith D, et al.: Maternal plasma corticotropin-releasing hormone trajectories vary depending on the cause of preterm delivery. American journal of obstetrics and gynecology 186:257-60, 2002
  280. Benedetto C, Petraglia F, Marozio L, et al.: Corticotropin-releasing hormone increases prostaglandin F2 alpha activity on human myometrium in vitro. American journal of obstetrics and gynecology 171:126-31, 1994
  281. Karalis K, Goodwin G, Majzoub JA: Cortisol blockade of progesterone: a possible molecular mechanism involved in the initiation of human labor. Nature medicine 2:556-60, 1996
  282. Case ML, MacDonald PC: Human parturition: distinction between the initiation of parturition and the onset of labor. Semin Reprod Endocrinol 11:272, 1993
  283. Olson DM, Zakar T: Intrauterine tissue prostaglandin synthesis: regulatory mechanisms. Semin Reprod Endocrinol 11:234, 1993
  284. Parker CR, Jr., Leveno K, Carr BR, et al.: Umbilical cord plasma levels of dehydroepiandrosterone sulfate during human gestation. The Journal of clinical endocrinology and metabolism 54:1216-20, 1982
  285. Mesiano S, Chan EC, Fitter JT, et al.: Progesterone withdrawal and estrogen activation in human parturition are coordinated by progesterone receptor A expression in the myometrium. The Journal of clinical endocrinology and metabolism 87:2924-30, 2002
  286. Csapo AI: Anti-progesterones in fertility control; in Pregnancy Termination: Procedures, Safety and New Developments. Edited by Zatuchn'i GI, Sciarra JJ, Speidel JJ. Hagerstown: Harper & Row, 1979
  287. Johnson JW, Austin KL, Jones GS, et al.: Efficacy of 17alpha-hydroxyprogesterone caproate in the prevention of premature labor. The New England journal of medicine 293:675-80, 1975
  288. Yemini M, Borenstein R, Dreazen E, et al.: Prevention of premature labor by 17 alpha-hydroxyprogesterone caproate. American journal of obstetrics and gynecology 151:574-7, 1985
  289. Meis PJ, Klebanoff M, Thom E, et al.: Prevention of recurrent preterm delivery by 17 alpha-hydroxyprogesterone caproate. The New England journal of medicine 348:2379-85, 2003
  290. Honnebier WJ, Swaab DF: The influence of anencephaly upon intrauterine growth of fetus and placenta and upon gestation length. The Journal of obstetrics and gynaecology of the British Commonwealth 80:577-88, 1973
  291. Ducsay CA, Seron-Ferre M, Germain AM, et al.: Endocrine and uterine activity rhythms in the perinatal period. Semin Reprod Endocrinol 11:285, 1993
  292. Honnebier MB, Nathanielsz PW: Primate parturition and the role of the maternal circadian system. European journal of obstetrics, gynecology, and reproductive biology 55:193-203, 1994
  293. Patrick J, Challis J, Campbell K, et al.: Circadian rhythms in maternal plasma cortisol and estriol concentrations at 30 to 31, 34 to 35, and 38 to 39 weeks' gestational age. American journal of obstetrics and gynecology 136:325-34, 1980
  294. Novy MJ: Hormonal regulation of parturition in primates; in Hormone Cell Interactions in Reproductive Tissues. Edited by Sciara J. New York: Masson Publishing, 1983
  295. Hirst JJ, Chibbar R, Mitchell BF: Role of oxytocin in the regulation of uterine activity during pregnancy and in the initiation of labor. Semin Reprod Endocrinol 11:219, 1993
  296. Haluska GJ, Novy MJ: Hormonal modulation of uterine activity during primate parturition. Semin Reprod Endocrinol 11:272, 1993

 

 

 

 

Non-Diabetic Hypoglycemia

ABSTRACT

 

Objective: To review the diagnosis, evaluation, and management of non-diabetic hypoglycemia in adults. Methods: A literature review using PubMed and Google Scholar was performed. In absence of data, clinical expert opinion was provided. Results: Hypoglycemia in an individual without diabetes is uncommon mainly because of a tightly regulated counterregulatory physiological response. A detailed medical history, review of medications and physical exam findings are critical first steps in providing guidance for further investigation in a non-diabetic person with documented hypoglycemia based on Whipple’s triad (presence of symptoms when plasma glucose concentrations are low and absence of symptoms with normalized glycemia). In this review, we highlight strategies to diagnose and treat hypoglycemic disorders in non-diabetic individuals based on underlying mechanisms. Conclusion: Evaluation and management of non-diabetic hypoglycemia should be individualized based on clinical presentation and suspected diagnoses.

 

INTRODUCTION

 

In healthy humans, glucose concentrations are efficiently maintained within a narrow range by the physiological mechanisms that respond to intermittent exogenous nutrient ingestion by enhancing glucose utilization and respond to intervals of nutrient deprivation by enhancing glucose production. Deviation of glucose from the normal range in both hyper or hypoglycemia only occurs when physiological mechanisms involved in maintaining the balance between the glucose utilization and the glucose production fail. As such, hypoglycemia is a manifestation of a heterogeneous group of underlying disorders that increase glucose utilization or reduce glucose production or a combination of both.

 

While hypoglycemia in persons without diabetes is relatively rare (1), the clinical relevance of this condition regarding patient safety, cognitive function, and quality of life is undeniable. Depending on severity and duration, hypoglycemia also can be fatal (2). Furthermore, hypoglycemia blunts defense against subsequent hypoglycemia leading to a vicious cycle of recurrent hypoglycemia (3), which in turn not only exaggerates related morbidities but also makes the diagnosis more complex.

 

Here, we review the current recommendations regarding diagnosis, pathophysiology, and management of hypoglycemia in non-diabetic individuals. Hypoglycemia in the pediatric population, hypoglycemia caused by anti-diabetic medications, and a comprehensive review of insulinomas can be found in the Endotext chapters entitled “Hypoglycemia in Neonates, Infants, and Children” (4), “Hypoglycemia During Therapy of Diabetes” (5) and “Insulinoma” (6), respectively.

 

PHYSIOLOGY / PATHOPHYSIOLOGY

 

Glucose is the main fuel for the brain since it cannot store glycogen or synthesize glucose (7,8). To minimize any disturbance in glucose supplies to the brain and cerebral function, therefore, redundant but very efficient physiological counter-regulatory responses are in place to prevent or correct hypoglycemia (8-10).

 

Prandial

 

In the prandial condition, the extent of glycemic excursion (difference between glycemic peak and nadir concentrations) is determined by the pace of food transition from the stomach into the gut (gastric emptying) as well as the net hepatic and extrahepatic glucose uptake (11). In healthy subjects, during the first 30-60 min of an oral glucose or mixed tolerance meal test, plasma glucose and insulin concentrations rise, shifting the hepatic net glucose output during fasting condition to net glucose uptake during the prandial state (12). As a result of changes in glucose kinetics in the early absorptive phase of glucose/mixed meal ingestion, plasma glucose concentrations start to decline in the latter absorptive phase falling below premeal levels in parallel with reduction in ingested glucose delivery to the gut. The regulatory mechanisms that are responsible for preventing hypoglycemia and restoring euglycemia during the transition of glucose flux from the ingested glucose delivery to the hepatic glucose production are not fully characterized, but both hormonal and non-hormonal factors play a role (13).

 

Fasting

 

In the fasting or postabsorptive state (4-6 hours after nutrient ingestion), plasma glucose  concentrations range from 80-90 mg/dl and rates of glucose utilization and production are equal (2mg/kg/min) (14). Glucose homeostasis during fasting is tightly regulated by a reciprocal bihormonal response, in which reduction in glucose concentrations below baseline reduces β-cell insulin secretion and stimulates α-cell glucagon release (15). While the full range of glucoregulatory effects of glucagon in the prandial state is unclear, the main function of glucagon in the fasting state is to counterbalance the action of insulin on hepatic glucose production (15-17). Glucose production is mainly (~80%) attributed to hepatic glycogenolysis with a smaller contribution (~20%) from hepatic gluconeogenesis primarily from amino acids and lactate (11). After an overnight fast, the liver contains ~ 50 g of glycogen storage, which can supply glucose for 24 hours after complete depletion. With prolonged fasting, hyperglucagonemia enhances gluconeogenesis and hypoinsulinemia promotes lipolysis.  Lipolysis releases glycerol, a gluconeogenic substrate, and free fatty acids that are converted to ketones, mainly beta hydroxybutyrate (BOHB) and acetoacetate, in the liver to be used as an alternate fuel by the brain.

 

Using a hyperinsulinemic hypoglycemic clamp in the fasting condition in normal humans, a decrement in plasma glucose concentration from the physiological range, in a hierarchical manner: (1) suppresses endogenous insulin secretion to lower glucose utilization, (2) increases glucagon response to increase glucose production, and (3) enhances epinephrine secretion (more relevant in absence of glucagon secretion) as well as cortisol and growth hormone release (more relevant during prolonged hypoglycemia) (8). Under physiological conditions during the fasting state, this counterregulatory response can reestablish euglycemia and prevent symptoms. A greater decline in plasma glucose concentration, though, would result in a symptomatic autonomic response to warn of low glucose and prompt the person to correct it by eating. However, if the individual did not (or could not) intervene, such as in the presence of overwhelming hyperinsulinemia) or blunted glucose counterregulatory responses, plasma glucose concentrations would drop further and neuroglycopenic symptoms and cognitive dysfunction would occur (Table 1) (18).

 

Table 1. Symptoms of Hypoglycemia

Autonomic (neurogenic)

Neuroglycopenic

Sweating

Anxiety

Tremor

Palpitation

Hunger

Tingling

Ill-defined symptoms

Warmth

Behavioral changes

Blurred vision

Confusion/difficulty speaking

Dizziness/lightheadedness

Lethargy and weakness

Seizure

Loss of consciousness/coma

 

It is unclear whether this hierarchy in hormonal responses or glycemic thresholds described during the fasting state would also apply to the prandial state.  Here, we classify the hypoglycemic disorders based on timing from meal ingestion (fasting versus prandial) given the differences in the regulatory factors involved in glucose metabolism between the two conditions.

 

DIAGNOSIS AND EVALUATION

 

Diagnosis of hypoglycemia should be made when symptoms accompany low plasma glucose concentrations (chemical hypoglycemia) but symptoms are absent when plasma glucose levels are normalized (Whipple’s triad (19)).

 

Chemical hypoglycemia has been defined based on a glucose threshold that can evoke a counterregulatory response. In the fasting state, a decline in glucose below 55 mg/dl causes neurogenic symptoms while insulin secretion is maximally suppressed (insulin < 3 uU/ml and C-peptide < 0.6 ng/ml) and glucagon response is maximized (9).

 

Awareness of hypoglycemia alerting individuals to correct hypoglycemia is mainly because of increased autonomic nervous system activity triggered by hypoglycemia. The autonomic (neurogenic) hypoglycemic symptoms can be adrenergic (such as palpitation, tremor, and anxiety) or cholinergic (such as sweating, hunger, and paresthesia) (18). As glucose concentrations drop below 48-50 mg/dl during the fasting state, the neuroglycopenic symptoms (caused by brain glucose deprivation) manifest; these symptoms range from behavioral changes, fatigue, and confusion to loss of consciousness or seizure (18,20-22).

 

Diagnosis of hypoglycemia in the prandial state is much more complex since the glycemic threshold to define hypoglycemia in the prandial state has not been well characterized. Using an oral glucose challenge in 650 healthy individuals in a previous study (23), 10% of subjects developed postprandial nadir glucose concentrations below 47 mg/dl without associated symptoms, suggesting that asymptomatic low glucose events are relatively common following an oral glucose load in normal humans. Hence, the mixed meal test (described below) is the preferred provocative test to diagnose prandial hypoglycemia. Furthermore, recurrent postprandial symptoms suggestive of hypoglycemia but not associated with low glucose concentrations have also been observed in normal individuals (24), indicating that other factors beyond hypoglycemia play a role in provoking autonomic symptoms.

 

Therefore, after obtaining a detailed medical history and physical exam, diagnosis of hypoglycemia should be confirmed by verification of low glucose concentration associated with symptoms or signs that are relieved by raising glucose values (Whipple’s triad).

 

A careful history of nutritional status, current medication use, and concurrent multisystem illnesses such as liver, heart, kidney failure, or sepsis, as well as a thorough physical exam and laboratory data, can point to existing primary conditions that predispose to hypoglycemia. This is especially crucial in patients who are often too ill to be subjected to extensive evaluation. For healthy subjects who lack any background predisposing illnesses, the details about timing (relationship to food ingestion, physical activities, day versus nocturnal time), severity (frequency, presence of neuroglycopenia, and requiring assistance to treat), and time of onset of hypoglycemic episodes are critical in differential diagnosis. To understand the pattern of hypoglycemic episodes, reviewing the records of symptoms, activity, food intake along with capillary blood or interstitial glucose levels measured by glucometer or continuous glucose monitoring (CGM), respectively, may be helpful. However, the accuracy of glucometer and CGM is low in the hypoglycemic range, and they should not be used for diagnostic purposes.  On the other hand, masked (blinded) monitoring by CGM can provide insights into patterns of hypoglycemic episodes and triggering factors during patients’ daily routine (25).

 

The flowchart in Figure 1 demonstrates the suggested approach for evaluation in healthy appearing patients after a careful medical history, physical exam, and laboratory data excludes an underlying illness that can predispose to hypoglycemia. Disorders that may cause hypoglycemia are listed in Table 2.

 

To confirm the diagnosis and explore etiology, it is necessary to collect blood samples during hypoglycemia, whether it occurs spontaneously or by provoked testing that can be selected based on clues from the medical history.

 

In asymptomatic patients with documented chemical hypoglycemia, artifactual hypoglycemia due to conditions such as reticulocytosis (polycythemia, sickle cell anemia), leukocytosis (leukemia), and thrombocytosis that increase in vitro glycolysis in the blood sample while awaiting laboratory analysis should be considered (26).  Also, nadir glucose levels in the prandial state can be low without any associated symptoms, particularly in persons with a history of upper gastrointestinal (GI) surgery (25). A potential diagnostic challenge in using clinical criteria remains in patients who are adapted to recurrent hypoglycemia by blunted autonomic response, so called hypoglycemic unawareness (27).  It has been well recognized that antecedent insulin-induced hypoglycemia impairs counterregulatory glucose responses and blunts hypoglycemia symptoms (mainly autonomic symptoms) in normal humans (28). Therefore, in patients with a high index of clinical suspicion, monitoring of symptoms and signs of neuroglycopenia, which is less likely to be affected by recurrent hypoglycemia, and reevaluation over time should be considered.

 

Figure 1. Evaluation of non-diabetic hypoglycemia in healthy appearing adults.

 

 

Table 2. Causes of Hypoglycemia

Artifactual Hypoglycemia (without symptoms)

Reticulocytosis (polycythemia, sickle cell anemia)

Leukocytosis (leukemia)

Thrombocytosis

Fasting Hypoglycemia (> 5 hour from the last meal)

High Insulin, Low beta-hydroxy butyrate, High glucagon response

Insulinoma

Auto immune syndrome (antibodies to insulin or the insulin receptor)

Factitial due to exogenous insulin 

Factitial due to insulin secretagogues

Induced by non-diabetic medications

Low Insulin, High beta-hydroxy butyrate, Low glucagon response

Ketotic hypoglycemia

Prolonged exercise

Alcohol induced

Glycogen storage diseases

Post Prandial Hypoglycemia (within 5 hours from the last meal)

Bariatric surgery

Nesidioblastosis

Hereditary fructose intolerance

Associated with Other Disorder

Critical illness (liver failure, congestive heart failure, sepsis, renal failure, etc.)

Malnutrition

Adrenal insufficiency

Non-islet cell tumors

 

Fasting Hypoglycemia

 

In patients with concern for fasting hypoglycemia, confirmation of Whipple’s triad and exploration of the cause is recommended during an episode of spontaneous hypoglycemia or with a supervised fast of up to 72 hours (9)(Figure.1).  During the fast, patients can consume non-caloric caffeine-free beverages with all non-essential medications discontinued.  Plasma glucose, insulin, c-peptide, and BOHB are collected every 6 hours until plasma glucose is < 60 mg/dL; at that time, frequency of blood collection should be increased to every 1-2 hours. The fast is terminated after collecting the last blood sample when the plasma glucose is < 45 mg/dL and the patient has signs and/or symptoms of hypoglycemia or if the patient has not exhibited symptoms after 72 hours have elapsed.  Alternatively, the fast can be terminated when plasma glucose  is < 55 mg/dL in men and < 35 mg/dl in women, given the sex differences in abnormal fasting glycemic concentrations (29), without signs/symptoms if Whipple’s triad was documented previously but blood samples were not collected (9). At the end of the fast, glucose response to 1 mg of glucagon IV bolus injection will be measured every 10 minutes for a 30-minute period and then the patient is fed. Insulin antibodies from baseline blood samples as well as hypoglycemic anti-diabetic medications (sulfonylureas and meglitinides) screening from baseline blood and urine samples are also collected. After confirmation of diagnosis, the results of the fasting test will help to differentiate hypoglycemia mediated by insulin- versus non-insulin factors.  

 

INSULIN-DEPENDENT HYPOGLYCEMIA (HIGH PLASMA INSULIN CONCENTRATION)

 

C-peptide is secreted from β-cells at an equimolar ratio to insulin (30). Approximately half of the insulin which is secreted into the portal vein is removed by the liver (31). Therefore, plasma insulin concentration reflects not only insulin secretion or exogenous insulin administration, but also hepatic insulin degradation. In contrast to insulin, c-peptide undergoes minimal extraction by the liver and other organs (32), therefore c-peptide concentration represents endogenous insulin secretion (32,33). It has been well documented that exogenous insulin administration during euglycemia or hypoglycemia inhibits endogenous insulin secretion (34,35). In fact, this physiological phenomenon has been used to support the diagnosis of insulinoma using hyperinsulinemic hypoglycemic or euglycemic clamp (36,37).

 

Therefore, low plasma concentration of c-peptide in presence of elevated insulin values during hypoglycemia indicates factitial hypoglycemia due to exogenous insulin administration, whereas elevated c-peptide and insulin represents inappropriately greater endogenous insulin secretion due to insulin secretagogues (sulfonylurea or meglitinides), autoimmune syndromes (insulin antibody syndrome or type B insulin resistance) or an insulin-producing tumor (insulinoma).

 

Further, hypoglycemia induced by hyperinsulinemia is associated with low BOHB and glycemic response >25 mg/dl to glucagon injection. Insulin increases fatty acid synthesis and esterification and decreases fatty acid oxidation and ketogenesis in the liver, leading to lower plasma concentrations of BOHB. Enhanced insulin signaling in the liver activated by hyperinsulinemia or any non-insulin ligands, such as insulin-like growth factor—2 (IGF-2), also results in higher glycogen storage, hence a larger glycemic response to glucagon injection.

 

Factitial Hypoglycemia

 

Factitial hypoglycemia due to exogenous insulin (high insulin and low c-peptide) or insulin secretagogues medications (high insulin and c-peptide) remains a diagnostic challenge and often leads to extensive and costly investigation to rule out other causes because of limitations in biochemical assays (38), as well as patients’ denial of medication misuse. Factitial hypoglycemia is observed more often in patients who work in the medical health care system, have relatives with diabetes living in the same household, and those with underlying mental illness such as major depression. Therefore, obtaining detailed information regarding the patient’s medication list including herbal preparations that can be contaminated with sulfonylurea as well as family history is essential (39).

 

Hypoglycemia due to exogenous insulin is characterized by elevated plasma insulin, suppressed c-peptide and low BOHB, as well as an increase in plasma glucose > 25mg/dL after glucagon challenge (9).  Insulin antibodies may also be positive (40). 

 

Insulin-induced hypoglycemia due to insulin secretagogues (sulfonylurea or meglitinide) has a similar biochemical profile except that the c-peptide is elevated.  Therefore, the only way to differentiate anti-diabetic factitial hypoglycemia from insulinoma is by detecting the drug in blood or urine.

 

In a single-center retrospective study, factitious hypoglycemia accounted for 11 of 70 (16%) of admissions for evaluation of hypoglycemia. (41).  Prognosis is poor based on a small-size study, in which only 30% of affected patients during several years of follow-up recovered (42). Treatment requires a multisystemic treatment team led by a psychiatrist (42).

 

Autoimmune Syndromes

 

Autoimmune syndromes are a rare cause of hypoglycemia characterized by high concentrations of insulin autoantibodies (insulin autoimmune syndrome [IAS]) or anti-insulin receptor antibodies (type B insulin resistance) (43,44). While IAS is the third leading cause of hypoglycemia in Japan, it is very uncommon in the non-Asian population; type B insulin resistance is even less common (43). 

 

Insulin autoantibodies (IAAs) are mainly immunoglobulins (Ig) directed against endogenously released insulin in response to nutrient ingestion with a high binding capacity but low affinity to insulin. As a result, patients may manifest hyperglycemia in the early absorptive phase of meal or oral glucose intake, when exogenous glucose appearance into circulation is maximal, followed by hypoglycemia in a few hours during the late prandial condition or postabsorptive state. Hypoglycemia, in IAS, is caused by the binding and release of insulin from the antigen-antibody complex independently of changes in glucose concentrations. Therefore, insulin and c-peptide are both elevated at the time of low plasma glucose concentrations.  IAAs are different from insulin antibodies produced against exogenous insulin that are generally low binding and high affinity, thus, unable to cause hypoglycemia.

 

In non-Asian patients, IAS is mainly reported in individuals with autoimmune (lupus, rheumatoid arthritis) /hematological diseases (multiple myeloma, benign monoclonal gammopathy), who are exposed to triggering factors, such as medications (captopril, propylthiouracil, penicillin G) and viral infections (measles, mumps, rubella, varicella zoster, coxsackie B, and hepatitis C) (43,44).  In Japanese patients, IAS is commonly associated with exposure to medications with a sulphydryl group (methimazole) (43). The prognosis is relatively good with self-remission reported in 82% of patients (44). Treatment is often dietary modification (small, frequent low-carbohydrate meals and uncooked cornstarch) and occasionally requires medications to decrease insulin secretion (somatostatin analogues, diazoxide) or immunosuppressants (high -dose corticosteroids, azathioprine, rituximab) (43,44).

 

Type B insulin resistance is caused by anti-insulin receptor antibodies. Affected patients tend to be middle-aged women of Black race with obesity, acanthosis nigricans, and hyperandrogenism.  Co-occurrence of systemic autoimmune disease (i.e., lupus) is common (43). Patients typically present with hyperglycemia; however, a subset of patients (8 of 34 patients in an NIH cohort) experience fasting or postprandial hypoglycemia after period of hyperglycemia or without a history of hyperglycemia (43). The autoantibodies are believed to be partial agonists for the insulin receptor. Hyperglycemia or hypoglycemia ensues depending on the antibody titer: high titers antagonize the receptor, resulting in hyperglycemia and high insulin and c-peptide levels to compensate for the resistance; low titers activate the receptor, leading to hypoglycemia. There are diagnostic challenges as immunoprecipitation, the gold standard method to detect insulin receptor autoantibodies, is generally not commercially available (45).  Unlike IAS, prognosis of type B insulin resistance is poor with high mortality especially in patients that transition from a hyperglycemic to hypoglycemic phase.  Deaths are related to hypoglycemia and other causes (lupus, renal failure, cancer, cardiovascular events).  Therapy can include immunosuppressants, but response is variable or poor (43).

 

Insulinoma

 

Neuroendocrine insulin-producing tumors (insulinoma) are relatively rare with an estimated incidence of ~ 1 – 4 new cases per million people/year (46). Less than 10% of insulinomas are malignant, 10% multiple, and 4% associated with multiple endocrine neoplasia type 1 (MEN-1) syndrome (47).  They primarily manifest in the 5th decade of life and are slightly more common in females (48).  Typically, patients experience episodes of hypoglycemia fasting or after exercise, but some individuals may experience fasting and prandial hypoglycemia (49).  Less frequently (6% of 237 patients in a Mayo Clinic cohort), patients present solely with prandial hypoglycemia (50).  Diagnosis relies on biochemical testing. The 72-hour fast is usually successful in capturing hypoglycemia in patients with insulinoma, as 65% of patients will experience hypoglycemia within 24 hours, 93% within 48 hours, and 99% within 72-hours (51).

 

Individuals with confirmed fasting (or postprandial) hyperinsulinemic hypoglycemia, negative screening for oral hypoglycemic medications, and negative insulin autoantibody testing should undergo diagnostic tests to locate the insulinoma prior to surgery (9).  Non-invasive imaging can include transabdominal and endoscopic ultrasonography, abdominal computed tomography (CT), and magnetic resonance imaging (MRI) (52). CT has been shown to detect ~70-80% of tumors and MRI 85% (47). Given that insulinomas tend to be small in size (< 1 cm in diameter in 40% of diagnosed cases) (47), negative imaging does not rule out the diagnosis (9). If non-invasive imaging cannot determine preoperative localization, selective arterial calcium stimulation with hepatic venous sampling can been utilized (53). Calcium is injected into arteries supplying the pancreas, which stimulates insulin secretion from insulinomas; a ≥ 2-fold increase in insulin concentrations from baseline localizes the site of an insulinoma with a > 90% sensitivity (54,55).

 

Treatment is surgical enucleation of the tumor, performed more commonly via open surgery than laparoscopic (48). If not localized prior to surgery, intraoperative palpation by an experienced surgeon coupled with intraoperative ultrasound detects > 80% of tumors (48). Pancreatectomy (distal or central depending on location of tumor) is preferred over enucleation if the insulinoma is large or there is concern for malignancy or metastases. Pancreatic fistula is the most common complication in both open and laparoscopic surgeries.  As noted earlier, the hyperinsulinemic hypoglycemic or euglycemic clamp has also be utilized to differentiate insulinoma from other causes when conventional evaluation (prolonged fasting, imaging) were equivocal (37) or when surgery failed to detect an insulinoma but clinical suspicion was high (56).

 

Benign insulinomas have a high 5yr survival rate of 95-100% post-resection with relapse frequency 6% at 10yr and 8% at 20 yrs (48). Malignant insulinomas carry a poor prognosis with median life expectancy of 2 years (48). Inoperable cases may benefit from medical management such as diazoxide, streptozocin, verapamil, and phenytoin by reducing insulin secretion and corticosteroids by diminishing insulin action (48,57-59). Non-surgical procedures such as CT guided radiofrequency ablation, US-guided ablation with ethanol, peptide receptor radionuclide therapy, and robotic radiosurgery have also been utilized (48).

 

Non-Diabetic Medications

 

Non-diabetic medications such as quinolones, non-steroidal anti-inflammatory drugs, antipsychotics, and α and β blockers have been implicated in inducing hypoglycemia by stimulating insulin secretion (60).  Other medications, such as pentamidine, damage the β cells, resulting in transient hypoglycemia induced by cytolytic insulin release (60).  Risk factors related to hypoglycemia include higher medication doses, concomitant renal failure, older age, and poor nutrition  (60).  Treatment may include discontinuation of the medication and supportive care (60).  

 

INSULIN-INDEPENDENT HYPOGLYCEMIA (LOW PLASMA INSULIN CONCENTRATION)

 

In this group, plasma BOHB is elevated and glucose response to glucagon is small. In patients who appear healthy the following conditions should be considered and ruled out: 

 

Ketotic Hypoglycemia

 

Ketotic hypoglycemia is a relatively rare condition that can occur during extended periods of carbohydrate deprivation, as during fasting or starvation. Prolonged adherence to a ketogenic diet, which severely restricts carbohydrate intake to 20-50g/day to promote weight loss and increase plasma ketone bodies (61), combined with other factors interfering with counterregulatory response, such as alcohol intake, also can result in ketotic hypoglycemia (62). Hypoglycemia in this condition is mainly caused by reduction in hepatic glucose production by hyperketonemia, which outweighs the diminished glucose utilization (63).  Management is largely supportive depending on the severity of the presentation.

 

Prolonged Exercise

 

Prolonged exercise can result in plasma glucose to decline to the hypoglycemic range in 30-40% of healthy subjects, but these events are not associated with symptoms (Whipple’s triad) nor incompatible with continued exercise (64). While the mechanisms for lack of symptoms is not completely understood, studies in non-diabetic dogs using a hypoglycemic clamp with and without exercise have shown that (a) counterregulatory hormonal responses to exercise and hypoglycemia combined are greater than the response to either conditions alone, and (b) larger insulin action during exercise are negated by the counterregulatory response during hypoglycemia (both enhanced endogenous glucose production and reduced glucose utilization) (65). Thus, disturbed counterregulatory response in prolonged exercise combined with inadequate carbohydrate ingestion may result in clinical hypoglycemia (66).  Management is supportive.

 

Alcohol-Induced Hypoglycemia

 

Hypoglycemia due to alcohol has been attributed to inhibition of gluconeogenesis and blunting of growth hormone response to hypoglycemia (67). Management is mainly supportive depending on the severity and length of hypoglycemia. 

 

Glycogen Storage Diseases

 

Glycogen storage diseases (GSD) are rare genetic disorders that impair the breakdown of glycogen.  Although these diseases are commonly diagnosed in infancy, GSDs Type I (deficiency in glucose-6-phosphatase), III (deficiency in amylo-1,6-glucosidase), and 0 (lack of glycogen synthase) can present in adulthood or continue to persist in adulthood (68).  Adults with Type 1 GSD (most common form, annual incidence 1/100,000) may present with hypoglycemia, lactic acidosis, hyperuricemia, hypertriglyceridemia, and hepatomegaly (68).  Patients with glycogen storage disease are generally managed by frequent feeding with complex carbohydrates and cornstarch to prevent hypoglycemia.  Patients need to be followed long-term by a metabolic specialist.

 

Fatty Acid Oxidation (FAO) Disorders

 

FAO disorders are a rare group of autosomal recessive conditions characterized by impaired breakdown of fatty acids, leading to hypoketotic hypoglycemia and myopathy. FAO disorders also typically manifest in childhood but can continue through adulthood. Prognosis depends on the specific condition and severity. Treatment typically includes avoidance of fasting and high carbohydrate/low fat diets (68). Patients are managed by metabolic specialists. 

 

ASSOCIATED WITH OTHER DISORDERS

 

In ill-appearing patients with hypoglycemia but low insulin concentrations the following conditions should be considered:

 

Critical Illness

 

Critical illness including organ failure such as acute liver failure and congestive heart failure with hepatic congestion have been associated with hypoglycemia, likely due to impaired gluconeogenesis and depletion of hepatic glycogen stores (69,70).  Sepsis-induced hypoglycemia has been appreciated in humans and animal models with depleted glycogen stores, impaired gluconeogenesis, and increased peripheral glucose utilization implicated as contributing factors (71,72).  Hypoglycemia in non-diabetic people with end stage renal disease is attributed to concomitant adrenal insufficiency, certain medications, malnutrition, and infection (73,74).  Management of Ill-appearing individuals due to sepsis or organ failure is mainly treatment of underlying disorders and treatment of severe hypoglycemia by intravenous glucose administration.

 

Addison’s Disease

 

Addison’s disease is a rare disease that results in primary adrenal insufficiency characterized by glucocorticoid deficiency with or without mineralocorticoid deficiency.  Most cases are caused by autoimmune damage to the adrenal cortex by 21-hydroxylase antibodies (75).  Hypoglycemia, although rare, is likely due to cortisol deficiency that interferes with counterregulatory response during times of stress (76).  Addison’s disease should be considered in a hypoglycemic individual with hyperpigmentation, hyponatremia, hyperkalemia and acidosis (75). Further testing, such as ACTH stimulation test, may be warranted to confirm diagnosis (77).  Treatment for patients with primary adrenal insufficiency in adrenal crisis should be initiated by volume replacement and immediate treatment with intravenous or intramuscular hydrocortisone 100 mg followed by 100 mg every 6-8 hours until clinically stable.  Etiology of the precipitating adrenal crisis should be identified and treated (i.e., infection, hemorrhage, etc.) (75).

 

Non-Islet Cell Tumors

 

Nonislet cell tumors are rare mesenchymal and epithelial tumors that can be benign or malignant, are often large (> 10 cm), and clinically apparent.  Hypoglycemia results when the tumor overproduces incompletely processed IGF-2 (9,78-80). Tumors can secrete IGF-2 or its posttranslational precursor “big IGF” (81).  IGF-2 structurally is similar to insulin; high levels of IGF-2 can bind to the insulin receptor and mimic the action of insulin, resulting in hypoglycemia (82). Insulin and c-peptide levels are appropriately suppressed in response to the hypoglycemia.  Management of non-islet cell tumors may include surgery, radiotherapy, chemotherapy and medical therapy with glucocorticoids, GH, or octreotide (9).

 

Prandial Hypoglycemia

 

The glycemic threshold to define hypoglycemia after meal ingestion is unknown. However, symptoms associated with plasma glucose less than 50-55 mg/dL during mixed meal test that is relieved by normalization of glucose has been used to confirm meal-induced hypoglycemia (11,25).  In the prandial state, provocative testing should use a mixed meal containing protein, carbohydrates, and fat and not oral glucose. This is mainly because the oral glucose challenge has low specificity for detecting clinical hypoglycemia by causing asymptomatic low glucose nadirs as well as hypoglycemia symptoms (mainly autonomic) that do not correlate with low glucose concentrations (23,24).

 

Currently meal tests are not standardized as both solid and liquid mixed meals as well as variable carbohydrate content from 45 to 105 grams have been used (83). Regardless of approach, meal studies can increase the risk of inducing hypoglycemia, thus, these tests need to be done under supervision by personnel trained in a safe environment.

 

HYPOGLYCEMIA AFTER BARIATRIC SURGERY

 

Meal-induced hypoglycemia after upper GI tract (gastrectomy and pyloroplasty) (84) or bariatric surgery (85-87) (Roux-en y gastric bypass surgery [RYGB] and sleeve gastrectomy [SG]) are well documented. One in 10 bariatric subjects develop a late-complication of hypoglycemia (88,89), and one in 150 suffer from severe hypoglycemia requiring an emergency room visit or hospitalization (88). Hypoglycemia in this population is postprandial, progressive, often associated with cognitive impairment and occasionally with loss of consciousness or seizures and is only partially responsive to diet modification or available therapeutic options (25,86,90-92). Despite sporadic case reports of postprandial hypoglycemia after SG (85), this condition, in our experience, is less prevalent, and likely to be of lesser severity than RYGB. Severe hypoglycemia after bariatric surgery is debilitating as it compromises patient safety, cognition, and quality of life (both professional and personal). The long-term health outcomes of this debilitating complication are largely unknown. A recent study (93) using a driver simulator has demonstrated that driving performance and cognitive function is impaired following RYGB during prandial hypoglycemia without any changes in perception of symptoms (94,95).

 

Differentiating true hypoglycemia from those with prandial asymptomatic low glucose concentration or prandial symptoms without low glucose levels is more challenging in patients after GI surgery than non-operated individuals because of higher frequency of both conditions after bariatric surgery. Using CGM for 5 days has demonstrated that 70% of non-diabetic subjects after RYGB (n=40) had at least one episode of low interstitial glucose concentration (<55 mg/dl) (94). However, 80% of these low glucose events have been shown to be asymptomatic. Furthermore, it is well documented that a large proportion of bariatric patients experience dumping symptoms (91), which are almost identical with autonomic symptoms of hypoglycemia, but not associated with low glucose concentrations. Therefore, it is critical to document Whipple’s triad (neuroglycopenic rather than autonomic symptoms associated with low glucose) during free-living conditions or using mixed meal test to confirm hypoglycemia in this population.

 

Additional testing should be considered in ill-appearing patients after bariatric surgery to exclude adrenal insufficiency, other critical illnesses, and malnutrition (25). Post bariatric patients who experience fasting hypoglycemia (beyond 5 hours from previous meal ingestion) or hypoglycemia within 6-12 months from surgery should be evaluated for other causes of hypoglycemia such as insulinoma (25).

 

Underlying mechanisms by which rerouted gut after GI surgeries cause hypoglycemia is not completely understood. However, it is well documented that following RYGB, and to a smaller extent after SG, meal ingestion enhances glucose excursion leading to higher glucose peaks and lower nadir glucose concentrations mainly due to faster nutrient emptying from the stomach pouch/stomach to the gut (96) (Figure 2). Increased glucose delivery from the stomach pouch/ tube-like stomach to the gut after bariatric surgery is associated with hyperinsulinemia, which is exaggerated in RYGB patients with hypoglycemia compared to asymptomatic RYGB subjects (92) (Figure 2). Enhanced meal-induced beta-cell secretion in patients with hypoglycemia after RYGB has been attributed to not only a greater beta-cell sensitivity to increasing glycemia in the first absorptive phase, but also a lower insulin suppression during glycemic decline from peak to glucose nadir (87).

 

Prandial hyperinsulinemia after RYGB, particularly in patients with hypoglycemia, has been shown to be associated with greater prandial plasma concentration of glucagon-like peptide 1 (GLP-1), an insulinotropic gut hormone (87,90,97)(Figure 2). These observations hinted towards a key role for GLP-1 signaling beyond glycemic stimuli in meal-stimulated hyperinsulinemia after RYGB. In fact, we and others have shown that blocking the GLP-1 receptor (GLP-1R) corrects post-RYGB hypoglycemia (87,98).

 

Figure 2. The prandial glycemic effects of RYGB are exaggerated in patients with late-complication of hypoglycemia. RYGB enhances prandial glycemic excursion and increases insulin secretion rate (ISR) along with plasma GLP-1 concentrations. Patients with documented hypoglycemia after RYGB have greater insulin and GLP-1 secretin compared to those without. Following RYGB, glucagon response to meal ingestion is enhanced but there is no further increase in response to hypoglycemia. Adapted with permission from Salehi, JCEM, 2014.

 

Despite a larger meal-induced glucagon response after RYGB compared to non-operated individuals, there is no further increase in plasma glucagon concentration during prandial hyperglycemia (Figure 2), suggestive of dysregulated pancreatic α-cell response. In fact, we have shown that patients with RYGB and SG, glucagon response to insulin-induced hypoglycemia is smaller than healthy individuals without GI surgery (99,100). This data is aligned with a report demonstrating that counterregulatory hormonal response (glucagon, cortisol, and catecholamines) to hypoglycemia is significantly reduced after RYGB compared with before surgery (101). In prandial state using tracer technique, we also have shown that despite a larger prandial plasma glucagon concentration, endogenous glucose production response to hypoglycemia is smaller after RYGB compared to non-operated controls (102), suggestive of diminished liver sensitivity to glucagon. However, we have demonstrated that blocking the GLP-1R increases the prandial hepatic glucose production response to insulin-induced hypoglycemia in RYGB subjects but not in non-operated controls, suggesting that enhanced GLP-1 signal due to rerouted gut can potentially contribute to the impaired counterregulatory response to hypoglycemia (103).

 

Based on current pathophysiologic understanding treatment strategies that selectively reduce the pace of nutrient delivery to the gut and prandial insulin secretion or improve counterregulatory response are the most effective options. Because of limited therapeutic options at this point, dietary modification remains the cornerstone of management. The goal of dietary modification is to lower prandial glucose spikes while increasing glucose nadirs by (a) lowering the amount of carbohydrates for every meal (<30g) or snacks (<15 g), (b) avoiding simple carbohydrates with high-glycemic index, (c) adding fats and proteins to every meal and snack, and (d) changing the composition of carbohydrate from glucose to fructose (25). Uncooked starch has also been used in this population based on effectiveness in reducing hypoglycemic episodes in patients with diabetes mellitus (104).

 

The current medical interventions rely on drugs that had previously been used for treatment of other hypoglycemic conditions.  Acarbose, an antidiabetic medication, has been utilized as the first drug started with dietary modification. The effect of this intestinal alpha-glucosidase blocker is to block carbohydrate absorption and reduce prandial glycemic excursion after RYGB {Valderas, 2012 #6617}. Adverse effects include flatulence and bloating, especially if the dose is not gradually titrated. Other medications such as somatostatin analogues, diazoxide, and GLP-1R agonists also are used based on sporadic case reports (25). Unblinded CGM in a small size study of patients with post-RYGB hypoglycemia has also been shown to reduce hypoglycemic episodes, likely due to better self-assessment of glycemic excursion and the need for treatments (105). The investigational drugs that are in various phases of development include exendin-(9-30) (Eiger Biopharmaceutical, Paol Alto, CA, USA), a potent GLP-1R antagonist (87,106), glucagon-based drugs (107,108), somatostatin analogues (109) and sodium-glucose cotransporter-1 inhibitor (110).

 

OTHER PRANDIAL HYPOGLYCEMIC CONDITIONS (RARE)

 

Nesidioblastosis

 

Non-insulinoma pancreatogenous hypoglycemia is a rare condition that typically causes hypoglycemia in the postprandial state due to diffuse nesidioblastosis (β cell hypertrophy, islet hyperplasia, increase in β cell mass) (111). The pathogenesis in adults is largely unknown, but likely differs from nesidioblastosis that occurs in congenital hyperinsulinism that is caused by a genetic mutation (112,113). This etiology should be considered in a hypoglycemia patient with a negative 72 hour fast, positive mixed meal test concerning for endogenous hyperinsulinism without a history of GI surgery, and negative imaging for insulinoma. Selective arterial calcium stimulation demonstrates diffuse insulin secretion. Histopathology reveals nesidioblastosis (114-116). Ideal management is difficult to determine as the condition is rare; a majority of the publications are from case reports or case-series, and there is a lack of long-term follow up (82,117).  Management can include dietary interventions (low-carbohydrate frequent meals) or medical interventions with diazoxide, acarbose, verapamil or octreotide (82).  When these interventions fail, partial pancreatectomy can be performed in patients with severe neuroglycopenic symptoms (118).

 

Hereditary Fructose Intolerance

 

Hereditary fructose intolerance is a rare autosomal recessive disorder (<1-9/100,000 annual incidence) caused by fructose-1-phoshate deficiency that results in postprandial hypoglycemia after ingestion of fructose (fruits) or sucrose (sweet foods) that is usually diagnosed in childhood. The diagnosis should be suspected in ill-appearing adults with hypoglycemia associated with GI symptoms (nausea, vomiting, diffuse abdominal pain) after eating fruits or sugar. Clinical symptom resolution within days of elimination suggests hereditary fructose intolerance and can be confirmed by molecular diagnosis on DNA obtained from peripheral leukocytes (68).

 

CONCLUSION

 

Non-diabetic hypoglycemia is a rare phenomenon since in healthy individuals counterregulatory mechanisms prevent and correct hypoglycemia by reducing glucose uptake and by enhancing hepatic glucose production. These mechanisms are less well characterized in the prandial state compared to the fasting state. Nonetheless, hypoglycemia only occurs when impaired physiological responses offset the balance between glucose utilization and production. Evaluation of hypoglycemia starts with a detailed history, comprehensive review of medications and clinical presentation, and a thorough physical exam that guides the diagnostic approach in patients with documented hypoglycemia based on Whipple’s triad. In absence of confirmation of hypoglycemia during free living condition provoked testing, prolonged fast or mixed meal test for hypoglycemic conditions reported during fasting or prandial state, respectively, are indicated. Diagnostic and management strategies for non-diabetic hypoglycemia is individualized depending on specific pathophysiology and can include interventions that are dietary, medical, or surgical.

 

REFERENCES

 

  1. Cryer PE, Davis SN, Shamoon H. Hypoglycemia in diabetes. Diabetes Care. 2003;26(6):1902-1912.
  2. Cryer PE. Severe hypoglycemia predicts mortality in diabetes. Diabetes Care. 2012;35(9):1814-1816.
  3. Davis MR, Shamoon H. Counterregulatory adaptation to recurrent hypoglycemia in normal humans. J Clin Endocrinol Metab. 1991;73(5):995-1001.
  4. Rosenfeld E, Thornton PS. Hypoglycemia in Neonates, Infants, and Children. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  5. Davis HA, Spanakis EK, Cryer PE, Davis SN. Hypoglycemia During Therapy of Diabetes. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2021.
  6. de Herder WW, Hofland J. Insulinoma. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  7. Lopez-Gambero AJ, Martinez F, Salazar K, Cifuentes M, Nualart F. Brain Glucose-Sensing Mechanism and Energy Homeostasis. Mol Neurobiol. 2019;56(2):769-796.
  8. Cryer PE. Hypoglycemia, functional brain failure, and brain death. J Clin Invest. 2007;117(4):868-870.
  9. Cryer PE, Axelrod L, Grossman AB, Heller SR, Montori VM, Seaquist ER, Service FJ, Endocrine S. Evaluation and management of adult hypoglycemic disorders: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2009;94(3):709-728.
  10. Service FJ. Hypoglycemic disorders. N Engl J Med. 1995;332(17):1144-1152.
  11. Service FJ. Hypoglycemias. West J Med. 1991;154(4):442-454.
  12. Petersen MC, Vatner DF, Shulman GI. Regulation of hepatic glucose metabolism in health and disease. Nat Rev Endocrinol. 2017;13(10):572-587.
  13. Tse TF, Clutter WE, Shah SD, Cryer PE. Mechanisms of postprandial glucose counterregulation in man. Physiologic roles of glucagon and epinephrine vis-a-vis insulin in the prevention of hypoglycemia late after glucose ingestion. J Clin Invest. 1983;72(1):278-286.
  14. DeFronzo RA. Lilly lecture 1987. The triumvirate: beta-cell, muscle, liver. A collusion responsible for NIDDM. Diabetes. 1988;37(6):667-687.
  15. Unger RH, Cherrington AD. Glucagonocentric restructuring of diabetes: a pathophysiologic and therapeutic makeover. J Clin Invest. 2012;122(1):4-12.
  16. Finan B, Capozzi ME, Campbell JE. Repositioning Glucagon Action in the Physiology and Pharmacology of Diabetes. Diabetes. 2020;69(4):532-541.
  17. Sandoval DA, D'Alessio DA. Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease. Physiol Rev. 2015;95(2):513-548.
  18. Towler DA, Havlin CE, Craft S, Cryer P. Mechanism of awareness of hypoglycemia. Perception of neurogenic (predominantly cholinergic) rather than neuroglycopenic symptoms. Diabetes. 1993;42(12):1791-1798.
  19. Whipple AO, Frantz VK. Adenoma of Islet Cells with Hyperinsulinism: A Review. Ann Surg. 1935;101(6):1299-1335.
  20. Mitrakou A, Ryan C, Veneman T, Mokan M, Jenssen T, Kiss I, Durrant J, Cryer P, Gerich J. Hierarchy of glycemic thresholds for counterregulatory hormone secretion, symptoms, and cerebral dysfunction. Am J Physiol. 1991;260(1 Pt 1):E67-74.
  21. Stanley S, Moheet A, Seaquist ER. Central Mechanisms of Glucose Sensing and Counterregulation in Defense of Hypoglycemia. Endocr Rev. 2019;40(3):768-788.
  22. DeRosa MA, Cryer PE. Hypoglycemia and the sympathoadrenal system: neurogenic symptoms are largely the result of sympathetic neural, rather than adrenomedullary, activation. Am J Physiol Endocrinol Metab. 2004;287(1):E32-41.
  23. Lev-Ran A, Anderson RW. The diagnosis of postprandial hypoglycemia. Diabetes. 1981;30(12):996-999.
  24. Charles MA, Hofeldt F, Shackelford A, Waldeck N, Dodson LE, Jr., Bunker D, Coggins JT, Eichner H. Comparison of oral glucose tolerance tests and mixed meals in patients with apparent idiopathic postabsorptive hypoglycemia: absence of hypoglycemia after meals. Diabetes. 1981;30(6):465-470.
  25. Salehi M, Vella A, McLaughlin T, Patti ME. Hypoglycemia After Gastric Bypass Surgery: Current Concepts and Controversies. J Clin Endocrinol Metab. 2018;103(8):2815-2826.
  26. Wang LR, Morein J, McCudden C, Sorisky A. Artifactual hypoglycemia in a patient with sickle cell anemia. CMAJ. 2021;193(43):E1660-E1662.
  27. Cryer PE. Symptoms of hypoglycemia, thresholds for their occurrence, and hypoglycemia unawareness. Endocrinol Metab Clin North Am. 1999;28(3):495-500, v-vi.
  28. Davis SN, Galassetti P, Wasserman DH, Tate D. Effects of antecedent hypoglycemia on subsequent counterregulatory responses to exercise. Diabetes. 2000;49(1):73-81.
  29. Merimee TJ, Tyson JE. Stabilization of plasma glucose during fasting; Normal variations in two separate studies. N Engl J Med. 1974;291(24):1275-1278.
  30. Laurenti MC, Matveyenko A, Vella A. Measurement of Pulsatile Insulin Secretion: Rationale and Methodology. Metabolites. 2021;11(7).
  31. Mittendorfer B, Patterson BW, Smith GI, Yoshino M, Klein S. beta Cell function and plasma insulin clearance in people with obesity and different glycemic status. J Clin Invest. 2022;132(3).
  32. Polonsky KS, Rubenstein AH. C-peptide as a measure of the secretion and hepatic extraction of insulin. Pitfalls and limitations. Diabetes. 1984;33(5):486-494.
  33. Polonsky KS, Pugh W, Jaspan JB, Cohen DM, Karrison T, Tager HS, Rubenstein AH. C-peptide and insulin secretion. Relationship between peripheral concentrations of C-peptide and insulin and their secretion rates in the dog. J Clin Invest. 1984;74(5):1821-1829.
  34. Boden G, Chen X, Desantis RA, Kendrick Z. Effects of insulin on fatty acid reesterification in healthy subjects. Diabetes. 1993;42(11):1588-1593.
  35. Luzi L, Battezzati A, Perseghin G, Bianchi E, Vergani S, Secchi A, La Rocca E, Staudacher C, Spotti D, Ferrari G, et al. Lack of feedback inhibition of insulin secretion in denervated human pancreas. Diabetes. 1992;41(12):1632-1639.
  36. Gin H, Brottier E, Dupuy B, Guillaume D, Ponzo J, Aubertin J. Use of the glucose clamp technique for confirmation of insulinoma autonomous hyperinsulinism. Arch Intern Med. 1987;147(5):985-987.
  37. Nauck MA, Baum F, Seidensticker F, Roder M, Dinesen B, Creutzfeldt W. A hyperinsulinaemic, sequentially eu- and hypoglycaemic clamp test to characterize autonomous insulin secretion in patients with insulinoma. Eur J Clin Invest. 1997;27(2):109-115.
  38. Egan AM, Galior KD, Maus AD, Fatica E, Simha V, Shah P, Singh RJ, Vella A. Pitfalls in Diagnosing Hypoglycemia Due to Exogenous Insulin: Validation and Utility of an Insulin Analog Assay. Mayo Clin Proc. 2022;97(11):1994-2004.
  39. Awad DH, Gokarakonda SB, Ilahi M. Factitious Hypoglycemia. StatPearls. Treasure Island (FL)2023.
  40. Fineberg SE, Kawabata TT, Finco-Kent D, Fountaine RJ, Finch GL, Krasner AS. Immunological responses to exogenous insulin. Endocr Rev. 2007;28(6):625-652.
  41. Oueslati I, Terzi A, Yazidi M, Kamoun E, Chihaoui M. Prevalence and characteristics of factitious hypoglycaemia in non-diabetic patients in a department of endocrinology. Endocrinol Diabetes Metab. 2022;5(6):e375.
  42. Grunberger G, Weiner JL, Silverman R, Taylor S, Gorden P. Factitious hypoglycemia due to surreptitious administration of insulin. Diagnosis, treatment, and long-term follow-up. Ann Intern Med. 1988;108(2):252-257.
  43. Lupsa BC, Chong AY, Cochran EK, Soos MA, Semple RK, Gorden P. Autoimmune forms of hypoglycemia. Medicine (Baltimore). 2009;88(3):141-153.
  44. Cappellani D, Macchia E, Falorni A, Marchetti P. Insulin Autoimmune Syndrome (Hirata Disease): A Comprehensive Review Fifty Years After Its First Description. Diabetes Metab Syndr Obes. 2020;13:963-978.
  45. Jialal I, Basheer H. Syndromes of autoantibodies to the insulin receptor. Int J Biochem Mol Biol. 2022;13(6):87-91.
  46. Zhuo F, Anastasopoulou C. Insulinoma. StatPearls. Treasure Island (FL)2023.
  47. Noone TC, Hosey J, Firat Z, Semelka RC. Imaging and localization of islet-cell tumours of the pancreas on CT and MRI. Best Pract Res Clin Endocrinol Metab. 2005;19(2):195-211.
  48. Giannis D, Moris D, Karachaliou GS, Tsilimigras DI, Karaolanis G, Papalampros A, Felekouras E. Insulinomas: from diagnosis to treatment. A review of the literature. J BUON. 2020;25(3):1302-1314.
  49. Okabayashi T, Shima Y, Sumiyoshi T, Kozuki A, Ito S, Ogawa Y, Kobayashi M, Hanazaki K. Diagnosis and management of insulinoma. World J Gastroenterol. 2013;19(6):829-837.
  50. Placzkowski KA, Vella A, Thompson GB, Grant CS, Reading CC, Charboneau JW, Andrews JC, Lloyd RV, Service FJ. Secular trends in the presentation and management of functioning insulinoma at the Mayo Clinic, 1987-2007. J Clin Endocrinol Metab. 2009;94(4):1069-1073.
  51. Service FJ, Natt N. The prolonged fast. J Clin Endocrinol Metab. 2000;85(11):3973-3974.
  52. Tucker ON, Crotty PL, Conlon KC. The management of insulinoma. Br J Surg. 2006;93(3):264-275.
  53. Goh BK, Ooi LL, Cheow PC, Tan YM, Ong HS, Chung YF, Chow PK, Wong WK, Soo KC. Accurate preoperative localization of insulinomas avoids the need for blind resection and reoperation: analysis of a single institution experience with 17 surgically treated tumors over 19 years. J Gastrointest Surg. 2009;13(6):1071-1077.
  54. Zhao K, Patel N, Kulkarni K, Gross JS, Taslakian B. Essentials of Insulinoma Localization with Selective Arterial Calcium Stimulation and Hepatic Venous Sampling. J Clin Med. 2020;9(10).
  55. Thompson SM, Vella A, Thompson GB, Rumilla KM, Service FJ, Grant CS, Andrews JC. Selective Arterial Calcium Stimulation With Hepatic Venous Sampling Differentiates Insulinoma From Nesidioblastosis. J Clin Endocrinol Metab. 2015;100(11):4189-4197.
  56. Ritzel RA, Isermann B, Schilling T, Knaebel HP, Buchler MW, Nawroth PP. Diagnosis and localization of insulinoma after negative laparotomy by hyperinsulinemic, hypoglycemic clamp and intra-arterial calcium stimulation. Rev Diabet Stud. 2004;1(1):42-46.
  57. Gill GV, Rauf O, MacFarlane IA. Diazoxide treatment for insulinoma: a national UK survey. Postgrad Med J. 1997;73(864):640-641.
  58. Grant CS. Insulinoma. Best Pract Res Clin Gastroenterol. 2005;19(5):783-798.
  59. Mele C, Brunani A, Damascelli B, Ticha V, Castello L, Aimaretti G, Scacchi M, Marzullo P. Non-surgical ablative therapies for inoperable benign insulinoma. J Endocrinol Invest. 2018;41(2):153-162.
  60. Maines E, Urru SAM, Leonardi L, Fancellu E, Campomori A, Piccoli G, Maiorana A, Soffiati M, Franceschi R. Drug-induced hyperinsulinemic hypoglycemia: An update on pathophysiology and treatment. Rev Endocr Metab Disord. 2023.
  61. Paoli A. Ketogenic diet for obesity: friend or foe? Int J Environ Res Public Health. 2014;11(2):2092-2107.
  62. Spoke C, Malaeb S. A Case of Hypoglycemia Associated With the Ketogenic Diet and Alcohol Use. J Endocr Soc. 2020;4(6):bvaa045.
  63. Mebane D, Madison LL. Hypoglycemic Action of Ketones. I. Effects of Ketones on Hepatic Glucose Output and Peripheral Glucose Utilization. J Lab Clin Med. 1964;63:177-192.
  64. Felig P, Cherif A, Minagawa A, Wahren J. Hypoglycemia during prolonged exercise in normal men. N Engl J Med. 1982;306(15):895-900.
  65. Zinker BA, Allison RG, Lacy DB, Wasserman DH. Interaction of exercise, insulin, and hypoglycemia studied using euglycemic and hypoglycemic insulin clamps. Am J Physiol. 1997;272(4 Pt 1):E530-542.
  66. Field JB. Exercise and deficient carbohydrate storage and intake as causes of hypoglycemia. Endocrinol Metab Clin North Am. 1989;18(1):155-161.
  67. Tetzschner R, Norgaard K, Ranjan A. Effects of alcohol on plasma glucose and prevention of alcohol-induced hypoglycemia in type 1 diabetes-A systematic review with GRADE. Diabetes Metab Res Rev. 2018;34(3).
  68. Douillard C, Mention K, Dobbelaere D, Wemeau JL, Saudubray JM, Vantyghem MC. Hypoglycaemia related to inherited metabolic diseases in adults. Orphanet J Rare Dis. 2012;7:26.
  69. Gill RQ, Sterling RK. Acute liver failure. J Clin Gastroenterol. 2001;33(3):191-198.
  70. Mellinkoff SM, Tumulty PA. Hepatic hypoglycemia; its occurrence in congestive heart failure. N Engl J Med. 1952;247(20):745-750.
  71. Miller SI, Wallace RJ, Jr., Musher DM, Septimus EJ, Kohl S, Baughn RE. Hypoglycemia as a manifestation of sepsis. Am J Med. 1980;68(5):649-654.
  72. Maitra SR, Wojnar MM, Lang CH. Alterations in tissue glucose uptake during the hyperglycemic and hypoglycemic phases of sepsis. Shock. 2000;13(5):379-385.
  73. Gosmanov AR, Gosmanova EO, Kovesdy CP. Evaluation and management of diabetic and non-diabetic hypoglycemia in end-stage renal disease. Nephrol Dial Transplant. 2016;31(1):8-15.
  74. Arem R. Hypoglycemia associated with renal failure. Endocrinol Metab Clin North Am. 1989;18(1):103-121.
  75. Husebye ES, Allolio B, Arlt W, Badenhoop K, Bensing S, Betterle C, Falorni A, Gan EH, Hulting AL, Kasperlik-Zaluska A, Kampe O, Lovas K, Meyer G, Pearce SH. Consensus statement on the diagnosis, treatment and follow-up of patients with primary adrenal insufficiency. J Intern Med. 2014;275(2):104-115.
  76. Rushworth RL, Torpy DJ, Falhammar H. Adrenal Crisis. N Engl J Med. 2019;381(9):852-861.
  77. Bornstein SR, Allolio B, Arlt W, Barthel A, Don-Wauchope A, Hammer GD, Husebye ES, Merke DP, Murad MH, Stratakis CA, Torpy DJ. Diagnosis and Treatment of Primary Adrenal Insufficiency: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2016;101(2):364-389.
  78. Fukuda I, Hizuka N, Ishikawa Y, Yasumoto K, Murakami Y, Sata A, Morita J, Kurimoto M, Okubo Y, Takano K. Clinical features of insulin-like growth factor-II producing non-islet-cell tumor hypoglycemia. Growth Horm IGF Res. 2006;16(4):211-216.
  79. Tietge UJ, Schofl C, Ocran KW, Wagner S, Boker KH, Brabant G, Zapf J, Manns MP. Hepatoma with severe non-islet cell tumor hypoglycemia. Am J Gastroenterol. 1998;93(6):997-1000.
  80. Schovanek J, Cibickova L, Ctvrtlik F, Tudos Z, Karasek D, Iacobone M, Frysak Z. Hypoglycemia as a Symptom of Neoplastic Disease, with a focus on Insulin-like Growth Factors Producing Tumors. J Cancer. 2019;10(26):6475-6480.
  81. Khowaja A, Johnson-Rabbett B, Bantle J, Moheet A. Hypoglycemia mediated by paraneoplastic production of Insulin like growth factor-2 from a malignant renal solitary fibrous tumor - clinical case and literature review. BMC Endocr Disord. 2014;14:49.
  82. Martens P, Tits J. Approach to the patient with spontaneous hypoglycemia. Eur J Intern Med. 2014;25(5):415-421.
  83. Lages M, Barros R, Moreira P, Guarino MP. Metabolic Effects of an Oral Glucose Tolerance Test Compared to the Mixed Meal Tolerance Tests: A Narrative Review. Nutrients. 2022;14(10).
  84. Holdsworth CD, Turner D, McIntyre N. Pathophysiology of post-gastrectomy hypoglycaemia. Br Med J. 1969;4(5678):257-259.
  85. Capristo E, Panunzi S, De Gaetano A, Spuntarelli V, Bellantone R, Giustacchini P, Birkenfeld AL, Amiel S, Bornstein SR, Raffaelli M, Mingrone G. Incidence of Hypoglycemia After Gastric Bypass vs Sleeve Gastrectomy: A Randomized Trial. J Clin Endocrinol Metab. 2018;103(6):2136-2146.
  86. Goldfine AB, Mun EC, Devine E, Bernier R, Baz-Hecht M, Jones DB, Schneider BE, Holst JJ, Patti ME. Patients with neuroglycopenia after gastric bypass surgery have exaggerated incretin and insulin secretory responses to a mixed meal. J Clin Endocrinol Metab. 2007;92(12):4678-4685.
  87. Salehi M, Gastaldelli A, D'Alessio DA. Blockade of glucagon-like peptide 1 receptor corrects postprandial hypoglycemia after gastric bypass. Gastroenterology. 2014;146(3):669-680 e662.
  88. Lee CJ, Wood GC, Lazo M, Brown TT, Clark JM, Still C, Benotti P. Risk of post-gastric bypass surgery hypoglycemia in nondiabetic individuals: A single center experience. Obesity (Silver Spring). 2016;24(6):1342-1348.
  89. Raverdy V, Baud G, Pigeyre M, Verkindt H, Torres F, Preda C, Thuillier D, Gele P, Vantyghem MC, Caiazzo R, Pattou F. Incidence and Predictive Factors of Postprandial Hyperinsulinemic Hypoglycemia After Roux-en-Y Gastric Bypass: A Five year Longitudinal Study. Ann Surg. 2016;264(5):878-885.
  90. Salehi M, Gastaldelli A, D'Alessio DA. Altered islet function and insulin clearance cause hyperinsulinemia in gastric bypass patients with symptoms of postprandial hypoglycemia. J Clin Endocrinol Metab. 2014;99(6):2008-2017.
  91. Yaqub A, Smith EP, Salehi M. Hyperinsulinemic hypoglycemia after gastric bypass surgery: what's up and what's down? Int J Obes (Lond). 2017.
  92. Honka H, Salehi M. Postprandial hypoglycemia after gastric bypass surgery: from pathogenesis to diagnosis and treatment. Curr Opin Clin Nutr Metab Care. 2019;22(4):295-302.
  93. Lehmann V, Tripyla A, Herzig D, Meier J, Banholzer N, Maritsch M, Zehetner J, Giachino D, Nett P, Feuerriegel S, Wortmann F, Bally L. The impact of postbariatric hypoglycaemia on driving performance: A randomized, single-blind, two-period, crossover study in a driving simulator. Diabetes Obes Metab. 2021;23(9):2189-2193.
  94. Kefurt R, Langer FB, Schindler K, Shakeri-Leidenmuhler S, Ludvik B, Prager G. Hypoglycemia after Roux-En-Y gastric bypass: detection rates of continuous glucose monitoring (CGM) versus mixed meal test. Surg Obes Relat Dis. 2015;11(3):564-569.
  95. Lazar LO, Sapojnikov S, Pines G, Mavor E, Ostrovsky V, Schiller T, Knobler H, Zornitzki T. Symptomatic and Asymptomatic Hypoglycemia Post Three Different Bariatric Procedures: A Common and Severe Complication. Endocr Pract. 2019.
  96. Nguyen NQ, Debreceni TL, Bambrick JE, Bellon M, Wishart J, Standfield S, Rayner CK, Horowitz M. Rapid gastric and intestinal transit is a major determinant of changes in blood glucose, intestinal hormones, glucose absorption and postprandial symptoms after gastric bypass. Obesity (Silver Spring). 2014;22(9):2003-2009.
  97. Patti ME, Goldfine AB. Hypoglycemia after gastric bypass: the dark side of GLP-1. Gastroenterology. 2014;146(3):605-608.
  98. Craig CM, Liu LF, Deacon CF, Holst JJ, McLaughlin TL. Critical role for GLP-1 in symptomatic post-bariatric hypoglycaemia. Diabetologia. 2017;60(3):531-540.
  99. Salehi M, Gastaldelli A, DeFronzo R. Prandial hepatic glucose production during hypoglycemia is altered after gastric bypass surgery and sleeve gastrectomy. Metabolism. 2022:155199.
  100. Salehi M, Woods SC, D'Alessio DA. Gastric bypass alters both glucose-dependent and glucose-independent regulation of islet hormone secretion. Obesity (Silver Spring). 2015;23(10):2046-2052.
  101. Abrahamsson N, Borjesson JL, Sundbom M, Wiklund U, Karlsson FA, Eriksson JW. Gastric Bypass Reduces Symptoms and Hormonal Responses in Hypoglycemia. Diabetes. 2016;65(9):2667-2675.
  102. Salehi M, Gastaldelli A, DeFronzo R. Prandial hepatic glucose production during hypoglycemia is altered after gastric bypass surgery and sleeve gastrectomy. Metabolism. 2022;131:155199.
  103. Honka H, Gastaldelli A, Pezzica S, Peterson R, DeFronzo R, Salehi M. Endogenous glucagon-like peptide 1 diminishes prandial glucose counterregulatory response to hypoglycemia after gastric bypass surgery. medRxiv. 2023.
  104. Axelsen M, Wesslau C, Lonnroth P, Arvidsson Lenner R, Smith U. Bedtime uncooked cornstarch supplement prevents nocturnal hypoglycaemia in intensively treated type 1 diabetes subjects. J Intern Med. 1999;245(3):229-236.
  105. Cummings C, Jiang A, Sheehan A, Ferraz-Bannitz R, Puleio A, Simonson DC, Dreyfuss JM, Patti ME. Continuous glucose monitoring in patients with post-bariatric hypoglycaemia reduces hypoglycaemia and glycaemic variability. Diabetes Obes Metab. 2023;25(8):2191-2202.
  106. Tan M, Lamendola C, Luong R, McLaughlin T, Craig C. Safety, efficacy and pharmacokinetics of repeat subcutaneous dosing of avexitide (exendin 9-39) for treatment of post-bariatric hypoglycaemia. Diabetes Obes Metab. 2020;22(8):1406-1416.
  107. Mulla CM, Zavitsanou S, Laguna Sanz AJ, Pober D, Richardson L, Walcott P, Arora I, Newswanger B, Cummins MJ, Prestrelski SJ, Doyle FJ, Dassau E, Patti ME. A Randomized, Placebo-Controlled Double-Blind Trial of a Closed-Loop Glucagon System for Postbariatric Hypoglycemia. J Clin Endocrinol Metab. 2020;105(4):e1260-1271.
  108. Nielsen CK, Ohrstrom CC, Houji IJK, Helsted MM, Krogh LSL, Johansen NJ, Hartmann B, Holst JJ, Vilsboll T, Knop FK. Dasiglucagon Treatment for Postprandial Hypoglycemia After Gastric Bypass: A Randomized, Double-Blind, Placebo-Controlled Trial. Diabetes Care. 2023;46(12):2208-2217.
  109. Pasireotide s.c. in Patients With Post-Bariatric Hypoglycemia (PASIPHY).
  110. Lawler HM. Inhibition of Intestinal SGLT1 with Mizagliflozin for the Treatment of Post-bariatric Hypoglycemia J Endocr Soc. 2023;7:A446.
  111. Kloppel G, Anlauf M, Raffel A, Perren A, Knoefel WT. Adult diffuse nesidioblastosis: genetically or environmentally induced? Hum Pathol. 2008;39(1):3-8.
  112. Dravecka I, Lazurova I. Nesidioblastosis in adults. Neoplasma. 2014;61(3):252-256.
  113. Service FJ, Natt N, Thompson GB, Grant CS, van Heerden JA, Andrews JC, Lorenz E, Terzic A, Lloyd RV. Noninsulinoma pancreatogenous hypoglycemia: a novel syndrome of hyperinsulinemic hypoglycemia in adults independent of mutations in Kir6.2 and SUR1 genes. J Clin Endocrinol Metab. 1999;84(5):1582-1589.
  114. Thompson GB, Service FJ, Andrews JC, Lloyd RV, Natt N, van Heerden JA, Grant CS. Noninsulinoma pancreatogenous hypoglycemia syndrome: an update in 10 surgically treated patients. Surgery. 2000;128(6):937-944;discussion 944-935.
  115. Won JG, Tseng HS, Yang AH, Tang KT, Jap TS, Lee CH, Lin HD, Burcus N, Pittenger G, Vinik A. Clinical features and morphological characterization of 10 patients with noninsulinoma pancreatogenous hypoglycaemia syndrome (NIPHS). Clin Endocrinol (Oxf). 2006;65(5):566-578.
  116. Witteles RM, Straus IF, Sugg SL, Koka MR, Costa EA, Kaplan EL. Adult-onset nesidioblastosis causing hypoglycemia: an important clinical entity and continuing treatment dilemma. Arch Surg. 2001;136(6):656-663.
  117. Then C, Nam-Apostolopoulos YC, Seissler J, Lechner A. Refractory idiopathic non-insulinoma pancreatogenous hypoglycemia in an adult: case report and review of the literature. JOP. 2013;14(3):264-268.
  118. Vanderveen KA, Grant CS, Thompson GB, Farley DR, Richards ML, Vella A, Vollrath B, Service FJ. Outcomes and quality of life after partial pancreatectomy for noninsulinoma pancreatogenous hypoglycemia from diffuse islet cell disease. Surgery. 2010;148(6):1237-1245; discussion 1245-1236.

 

Triglyceride Lowering Drugs

ABSTRACT

 

The two major goals of the treatment of hypertriglyceridemia are the prevention of cardiovascular disease and pancreatitis. Here we discuss the drugs used for the treatment of hypertriglyceridemia: (niacin, fibrates, omega-3-fatty acids, volanesorsen (available in Europe) and lipoprotein lipase gene therapy (alipogene tiparvovec- no longer available). Niacin decreases total cholesterol, TGs (20-50% decrease), LDL-C, and Lp(a). Additionally, niacin decreases small dense LDL resulting in a shift to large, buoyant LDL particles. Moreover, niacin increases HDL-C. Skin flushing, insulin resistance, and other side effects have limited the use of niacin. The enthusiasm for niacin has greatly decreased with the failure of AIM-HIGH and HPS-2 Thrive to decrease cardiovascular events when niacin was added to statin therapy. The omega-3-fatty acids eicosapentaenoic acid (C20:5n-3) (EPA) and docosahexaenoic acid (C22:6n-3) (DHA) lower TGs by 10-50% but do not affect total cholesterol, HDL-C, or Lp(a). LDL-C may increase with EPA + DHA when the TG levels are markedly elevated (>500mg/dL). EPA alone does not increase LDL-C. Omega-3-fatty acids have few side effects, drug interactions, or contraindications. Numerous studies of low dose omega-3-fatty acids on cardiovascular outcomes have failed to demonstrate a benefit. However, in the JELIS trial and REDUCE-IT trial high doses of EPA alone reduced cardiovascular events while in the STRENGTH trial high dose EPA+DHA did not reduce cardiovascular events. Fibrates reduce TG levels by 25-50% and increase HDL-C by 5-20%. The effect on LDL-C is variable. If the TG levels are very high (>500mg/dL), fibrate therapy may result in an increase in LDL-C, whereas if TGs are not markedly elevated fibrates decrease LDL-C by 10-30%. Fibrates also reduce apolipoprotein B, LDL particle number, and non-HDL-C and there may be a shift from small dense LDL towards large LDL particles. Fibrates do not have any major effects on Lp(a). Monotherapy with fibrates appears to reduce cardiovascular events in patients with high TG and low HDL-C levels. Whether the addition of fibrates to statin therapy will reduce cardiovascular disease is uncertain. In patients with diabetes fibrates appear to slow the progression of microvascular disease. Volanesorsen is an antisense oligonucleotide that inhibits the production of apolipoprotein C-III. In patients with the familial chylomicronemia syndrome (FCS) volanesorsen decreases TG by 77% (mean decrease of 1712 mg/dL) with 77% of the patients having TG levels less than 750 mg/dL. In addition, volanesorsen treatment resulted in decreases in non–HDL-C by 46%, and VLDL-C by 58% and increases in HDL-C by 46%, LDL-C by 136%, (LDL-C increased from 28 to 61 mg/dL), and total apolipoprotein B by 20%. Studies have suggested that volanesorsen may reduce episodes of pancreatitis. Patients with FCS have also reported that volanesorsen improved symptoms and reduced interference of FCS with work/school responsibilities. Of concern has been decreases in platelet levels with 47% of patients treated with volanesorsen developing platelet counts below100 x 109/L. Thus, a number of drugs are available for the treatment of hypertriglyceridemia and may be employed when lifestyle changes are not sufficient. 

 

INTRODUCTION

 

The two primary goals of the treatment of hypertriglyceridemia are the prevention of cardiovascular disease and the prevention of pancreatitis. The evaluation and guidelines for the management of hypertriglyceridemia are discussed in detail in the Endotext chapter “Risk of Fasting and Non-Fasting Hypertriglyceridemia in Coronary Vascular Disease and Pancreatitis” (1) and the approach to evaluating a patient with hypertriglyceridemia is discussed in the Endotext chapter “Approach to the Patient with Dyslipidemia” (2). The treatment of hypertriglyceridemia by diet and weight loss are discussed in detail in the Endotext chapter “The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels” and “Obesity and Dyslipidemia” (3,4). Lifestyle changes are recommended as the first line for therapy of hypertriglyceridemia, but drug therapy is often required. In this chapter we will discuss the drugs used for the treatment of elevated plasma TG levels. Statins, ezetimibe, PCSK9 inhibitors, bempedoic acid, lomitapide, mipomersen, and evinacumab, which are primarily used to lower LDL-C, are discussed in the chapter “Cholesterol Lowering Drugs” (5).  

 

NIACIN

 

Introduction

 

Niacin was the first drug approved to treat dyslipidemia. In 1955, Altschul et al showed that pharmacologic doses of niacin decreased plasma cholesterol levels (6). Several forms of niacin are available for clinical use. Immediate release niacin has a short duration of action and is typically given two or three times per day with meals, whereas sustained release niacin and extended-release niacin are once a day drugs usually given at bedtime. The extended release form of niacin exhibits release rates that are intermediate between immediate release niacin and sustained release niacin (7). While the effects of the various forms of niacin on plasma lipid levels are similar, the side effect profiles are different. Because of an increased risk of serious liver toxicity with sustained release niacin this preparation is no longer widely used to treat dyslipidemia. Over-the- counter “No flush” niacin is also available but is generally ineffective as a lipid-modifying agent because most of these preparations do not contain active nicotinic acid.    

 

Effect of Niacin on Lipid and Lipoprotein Levels

 

Table 1. Effect of Niacin on Lipid and Lipoproteins

Decreases Total Cholesterol

Decreases LDL-C

Decreases TGs

Decreases Non-HDL-C

Decreases Lp(a)

Increases HDL-C

Decreases Apolipoprotein B

Shifts Small Dense LDL to Large Buoyant LDL

 

Niacin decreases all the pro-atherogenic lipid and lipoprotein particles including total cholesterol, TG, LDL-C, and Lp(a) levels (Table 1) (8,9). Additionally, niacin has been shown to decrease small dense LDL resulting in a shift to large, buoyant LDL particles (10). Moreover, niacin increases HDL-C levels (8,9).

 

In a meta-analysis of 30 trials with 4,749 subjects treatment with immediate release, sustained release, or extended release niacin decreased total cholesterol by 10%, decreased TGs by 20%, decreased LDL-C by 14%, and increased HDL-C by 16% (11). All three niacin preparations were effective in decreasing total cholesterol, TG, and LDL-C levels and increasing HDL-C levels (11). At a dose of 1.5 grams per day, immediate release niacin and extended release niacin produced similar decreases in total cholesterol, TGs, and LDL-C and a similar increase in HDL-C (12). A meta-analysis of 14 studies with 9,013 subjects reported a 23% decrease in Lp(a) with extended release niacin treatment (13).

 

A small meta-analysis of 5 trials in 432 subjects compared the response to extended release niacin in men and women (14). The effect of niacin on LDL-C was greater in women than men at all niacin doses (1,000mg 6.8% decrease in women vs 0.2% in men, p = 0.006; 1,500mg 11.3% decrease vs 5.6% decrease, p = 0.013; 2,000 mg 14.8% decrease vs 6.9% decrease, p = 0.010; 3,000mg 28.7% decrease vs 17.7% decrease, p = 0.006). The effect of niacin on plasma TG levels also tended to be greater in women but the difference only reached statistical significance at the 1,500mg dose (28.6% vs 20.4%, p = 0.040). The mechanism for the more robust decrease in LDL-C and TGs in women is unknown but might be due to a smaller body mass in women leading to increased circulating niacin levels and hence a greater response. However, the effect of niacin on HDL-C and Lp(a) levels were similar in males and females. Not unexpectedly the effect of niacin is dose dependent with higher doses having a greater effect on plasma lipid and lipoprotein levels (Table 2) (14).

 

Table 2. Effect of Niacin Dose on Lipid and Lipoprotein Response in Women (percent change)

Niacin Dose

LDL-C

TG

HDL-C

Lp(a)

500mg

-5.2

-9.5

7.7

-2.6

1000mg

-6.8

-14.5

17.6

-11.5

1500mg

-11.3

-28.6

21.1

-4.0

2000mg

-14.8

-37.3

25.2

-24.7

2500mg

-28.7

-45.6

34.5

-28.6

3000mg

-28.7

-51.0

28.7

-29.9

 

Numerous studies have examined the effect of the addition of niacin to statin therapy. Combination therapy typically results in further reductions in atherogenic lipoprotein particles and an increase in HDL-C levels. An example of such a study is shown in Table 3 (15).

 

Table 3. Effect of the Addition of Niacin to Statin Therapy on Lipid and Lipoprotein Levels

 

 Change in Lipids with Addition of Extended-Release Niacin 2000mg/day to Simvastatin 20mg/day

LDL-C

7.1% Decrease

HDL-C

18.2% Increase

TG

22.7% Decrease

Non-HDL-C

15.1% Decrease

Lp(a)

17.4% Decrease

 

While a literature search did not find any studies comparing the combination of ezetimibe + niacin vs. monotherapy there is a large trial that has examined the effect of adding 2 grams niacin to ezetimibe/simvastatin 10/20 (16). In this study the addition of niacin improved the lipid profile with a marked decrease in TGs and an increase in HDL-C levels (table 4).

 

Table 4. Effect of the Addition of Niacin to Ezetimibe/Statin Therapy on Lipid and Lipoprotein Levels

 

 Change in Lipids with Addition of Niacin 2000mg/day to Ezetimibe/Simvastatin 10/20mg/day

LDL-C

4.8% Decrease

HDL-C

21.5% Increase

TG

17.6% Decrease

Non-HDL-C

7.3% Decrease

 

In patients with marked hypertriglyceridemia combining niacin with other drugs that also lower plasma TGs can be considered. Sixty patients with the metabolic syndrome were randomized to 16 weeks of treatment with placebo, omega-3-fatty acids (Lovaza 4 g/day), extended release niacin (2 g/day), or both drugs in combination (17). In the niacin group TGs were decreased by 30%, in the omega-3-fatty acids group by 22%, and in the combination group by 42% compared to the placebo group. Of note the beneficial effects of niacin on decreasing LDL and non-HDL-C levels were blunted by omega-3-fatty acids, which are known to raise LDL-C levels in patients with marked hypertriglyceridemia (see below). These results show that the combination of niacin and fish oil will lower TG levels more than either drug individually but at the expense of diminishing the effect of niacin on LDL and non-HDL-C levels.   

 

Surprisingly there are few large randomized trials examining the effect of combination therapy with niacin + fibrate vs. monotherapy. One very small trial reported that while both niacin monotherapy and bezafibrate monotherapy were effective in lowering serum TGs there was no statistically significant added benefit of combination therapy in reducing serum TG levels (18). However, a larger trial in HIV+ patients reported that the combination of niacin and fenofibrate was better at lowering TGs and non-HDL-C and increasing HDL-C levels than monotherapy with either niacin or fenofibrate (19). It would be informative if additional trials of combination therapy were carried out in patients with marked hypertriglyceridemia that can often be difficult to control with lifestyle changes and monotherapy.

 

Mechanisms Accounting for the Niacin Induced Lipid Effects

 

TRIGLYCERIDES

 

Early studies demonstrated that niacin inhibited the release of free fatty acids from cultured adipocytes and decreased circulating free fatty acid levels (20-22). The ability of niacin to inhibit adipose tissue lipolysis is mediated by the activation of GPR109A (hydroxycarboxylic acid 2 receptor), a G protein-coupled receptor that is highly expressed in adipose tissue (22-24). It was initially thought that the decrease in plasma TGs induced by niacin therapy was due to niacin inhibiting lipolysis in adipose tissue resulting in a decrease in the transport of fatty acids to the liver leading to the decreased availability of fatty acids for hepatic TG synthesis. However, studies have shown that while niacin acutely decreases plasma free fatty acid levels this inhibition is not sustained (25). Additionally, studies in mice lacking GPR109A have shown that niacin does not inhibit lipolysis but still decreases plasma TG and LDL-C levels (26). Moreover, studies in humans using GPR109A agonists lowered plasma free fatty acid levels but did not cause the expected effects on plasma TGs and LDL-C (26). Thus, the effects of niacin on adipose tissue lipolysis are no longer thought to mediate the niacin induced decrease in plasma TG levels.

 

Niacin has been shown to inhibit diglycerol acyltransferase 2 (DGAT2) activity in the liver (22,27). DGAT2 is the key enzyme that catalyzes the final step in TG synthesis. Inhibition of DGAT2 will reduce hepatic TG synthesis and the availability of TG for VLDL assembly and secretion (22). A decrease in TG will result in an increase in apolipoprotein B degradation in the liver. Kinetic studies in humans have shown that treatment with niacin decreases VLDL TG production (28,29).

 

In addition, in animal models, niacin reduces the hepatic expression of apolipoprotein C-III, which could result in the accelerated clearance of TG rich lipoproteins (30). Whether this plays a significant role in mediating the decrease in plasma TG levels induced by niacin therapy remains to be determined.

 

LOW DENSITY LIPOPROTEIN

 

The decrease in plasma LDL-C with niacin therapy is thought to be secondary to a reduction in VLDL and LDL formation and secretion by the liver (22).

 

HIGH DENSITY LIPOPROTEIN

 

There are multiple potential mechanisms by which niacin may increase HDL-C levels. Some of these changes may be anti-atherogenic while others may be pro-atherogenic. One hypothesis for the increase in HDL induced by niacin therapy is a decrease in the surface expression of hepatocyte beta chain ATP synthase, a receptor that has been proposed to be involved in the uptake of HDL particles by the liver (31). Studies have further shown that niacin inhibits HDL protein degradation by cultured hepatocytes but does not inhibit the selective uptake of cholesterol esters carried in HDL (22,32).

 

Some kinetic studies have shown that niacin decreases HDL and apolipoprotein A1 fractional catabolic rate (33,34). In contrast, other kinetic studies have shown that niacin increase apolipoprotein AI production (35).

 

In addition, in monocytes, niacin also increased the expression of ABCA1 and CD36 resulting in an increase in cholesterol efflux to HDL, which would increase HDL-C levels and likely have anti-atherogenic effects (36). Similarly, in vitro studies suggest that niacin may increase the transport of cholesterol and phospholipids via ABCA1 from the liver to lipid poor apolipoprotein A1 particles thereby decreasing the clearance of apolipoprotein A1, which might not be anti-atherogenic (22,37).

 

Finally, decreasing plasma TG levels may result in a reduction in CETP mediated exchange of TGs on VLDL for cholesterol on HDL leading to an increase in HDL-C levels. Additionally, studies have shown that niacin decreases the expression of CETP (38).

   

LIPOPROTEIN(a)

 

Niacin decreases the synthetic rate of Lp(a) but does not increase Lp(a) catabolism (39,40). In cell culture and animal studies niacin has been shown to decrease the expression of apo (a) (41).

 

Pharmacokinetics

 

Oral niacin is well absorbed with immediate release niacin resulting in a rapid increase in plasma levels while extended release and sustained release niacin result in a delayed peak in plasma levels. Niacin undergoes metabolism in the liver by two primary pathways; conjugation or amidation (7,42). The conjugative pathway is low affinity and high capacity that metabolizes niacin to nicotinuric acid while the amidation pathway is high affinity and low capacity that converts niacin into several oxidative-reductive intermediates, which can induce hepatic toxicity (7,42) (Figure 1). The clinical importance is that immediate release niacin results in high levels of niacin and therefore is primarily metabolized by the conjugative pathway (low affinity, high capacity), which does not result in toxic intermediates that can cause liver damage. In contrast, sustained release niacin results in lower levels of niacin for a longer period and therefore metabolism via the amidation pathway (high affinity, low capacity) is dominant leading to an increase in the formation of toxic intermediates that can induce hepatic injury (7,42). Extended-release niacin would be metabolized midway between immediate release and sustained release niacin (42).

 

Figure 1. Pathways of Niacin Metabolism.

 

Effect of Niacin on Cardiovascular Outcomes

 

MONOTHERAPY

 

The Coronary Drug Project, conducted between 1966 and 1975, was the first large randomized, double-blind clinical trial to show that lowering lipids reduced cardiovascular disease (43). This trial determined the effect of clofibrate (1.8g/day), dextrothyroxine (6mg/day), two doses of oral estrogen (2.5 or 5mg per day), or immediate release niacin (3 grams/day) vs. placebo in 8,341 men 30 to 64 years of age with an electrocardiogram documented myocardial infarction. The mean baseline total cholesterol level was 251mg/dL and the TG level was 183mg/dL. The two estrogen regimens and dextrothyroxine treatment groups were discontinued early because of increased adverse effects. Clofibrate treatment did not demonstrate clinical benefit. In the niacin treated patients there was an average 10% decrease in serum cholesterol and 26% decrease in serum TGs despite modest compliance with the study medication. Moreover, niacin treatment (n=1,119) decreased recurrent myocardial infarctions by 26%, stroke by 24%, and revascularization by 67% compared to placebo (n=2,789) but did not decrease total mortality, which was the primary endpoint. Long term follow-up (6.2 years during the study and 8.8 years post study after niacin was discontinued in most participants) demonstrated an 11% decrease in mortality in the niacin group vs. the placebo group (52.0 versus 58.2%; p = 0.0004) (44). The majority of this difference in mortality was accounted for by a decrease in coronary heart disease mortality (36.5% vs. 41.3%; p=0.005). Further analysis revealed that niacin reduced the risk of 6-year recurrent myocardial infarction and coronary heart disease death and 15-year total mortality similarly in patients at all levels of baseline fasting plasma glucose, including those with glucose levels ≥126mg/dL (i.e. patients with diabetes) (45). Additionally, the beneficial effect of niacin on cardiovascular events and total mortality was not diminished, even among those with one hour plasma glucose levels > 220mg/dL (45). Moreover, the beneficial effects of niacin on recurrent myocardial infarction and total mortality were similar in patients with or without the metabolic syndrome at baseline (46). These results demonstrate that immediate release niacin monotherapy decreases recurrent atherosclerotic cardiovascular events in a broad spectrum of patients with pre-existing cardiovascular disease (secondary prevention).

 

COMBINATION WITH FIBRATES

 

In the Stockholm Ischemic Heart Disease Secondary Prevention Study survivors of a myocardial infarction below 70 years of age were randomized to a control group (n = 276) (no placebo) and a group treated with clofibrate (2 grams) and immediate release nicotinic acid (up to 3 grams) (n = 279) (47). Serum cholesterol and TG was lowered by 13% and 19%, respectively, in the treatment group compared to the control group. Recurrent myocardial infarction was reduced by 50% within one year (48). Total mortality was decreased by 26% in the group treated with clofibrate + niacin (p< 0.05) while ischemic heart disease mortality was decreased by 36% (p< 0.01). Notably, the benefit of clofibrate + niacin was only observed in patients with a baseline TG level > 143mg/dL. In the age of statins, the clinical implications of this early study are unclear. 

 

COMBINATION WITH STATINS

 

The AIM-HIGH trial was designed to determine if the addition of Niaspan, an extended-release form of niacin, to aggressive statin therapy would result in a further reduction in cardiovascular events in patients with pre-existing cardiovascular disease (49). In this trial 3,314 patients were randomized to extended-release Niaspan (1500-2000mg/day) vs. placebo that contained 100-150mg of immediate release niacin. On trial, LDL-C levels were in the 60-70mg/dL range in both groups. As expected, HDL-C levels were increased in the Niaspan treated group (approximately 44mg/dL vs. 38mg/dL), while TGs were decreased (approximately 121mg/dL vs. 155mg/dL). However, there were no differences in the primary endpoint between the control and Niaspan treated groups (Primary endpoint consisted of the first event of death from coronary heart disease, nonfatal myocardial infarction, ischemic stroke, hospitalization for an acute coronary syndrome, or symptom-driven coronary or cerebral revascularization). There were also no differences in secondary endpoints except for a possible increase in strokes in the Niaspan treated group. The addition of Niaspan to statin therapy did not result in a significant increase in either muscle or liver toxicity. Thus, this study does not provide support for the addition of niacin to statins. However, most of the patients included in this study did not have a lipid profile that one would typically consider treating with niacin therapy. In the subset of patients with TG > 198mg/dL and HDL-C < 33mg/dL Niaspan treatment showed a trend towards benefit (hazard ratio 0.74; p=0.073), suggesting that if the appropriate patient population was studied the results may have been different (50).

 

HPS 2 Thrive also studied the effect of niacin added to statin therapy (51). This trial utilized extended-release niacin (2000mg/day) combined with laropiprant, a prostaglandin D2 receptor antagonist, which reduces the flushing side effect of niacin treatment. HPS 2 Thrive was a very large trial with over 25,000 patients randomized to either niacin therapy or placebo. As in the AIM HIGH study, the baseline LDL-C levels were low at 63mg/dL, the HDL-C levels were 44mg/dL, and the TGs were 125mg/dL at baseline. As expected, niacin therapy resulted in a modest reduction in LDL-C (10mg/dL), a modest increase in HDL-C (6mg/dL), and a marked reduction in TGs (33mg/dL) compared to placebo. However, despite these lipid changes there were no significant differences in major cardiovascular events between the niacin and control group (risk ratio 0.96 CI 0.90- 1.03). It is unknown whether laropiprant, the prostaglandin D2 receptor antagonist, might have effects that worsen atherosclerosis and increase event rates. Mice deficient in the prostaglandin D2 receptor have been noted to have an increase in atherogenesis in response to angiotensin II (52). Similar to the AIM-HIGH study, the group of patients included in the HPS 2 Thrive trial may not have been the ideal patient population to study for the beneficial effects of niacin treatment added to statin therapy. Ideally, patients with high TGs and high non-HDL-C levels coupled with low HDL-C levels should be studied.

 

Thus, these two studies have failed to demonstrate that adding niacin to statin therapy results in a decrease in cardiovascular events. It should be recognized that both the AIM-HIGH study and the HPS-2 Thrive study had limitations. First, the patient populations that were included in these studies were not ideal as the TG and non-HDL-C levels were not elevated in a range that one would usually consider adding niacin therapy. Second, in both trials a significant percentage of patients stopped niacin therapy (AIM-HIGH 25.4% discontinued niacin; HPS-2 Thrive 25.4% discontinued niacin). Third, the duration of these studies was relatively short and it is possible that the beneficial effects of niacin take longer to occur (AIM-HIGH 3 years; HPS-2 Thrive 3.9 years). Fourth, in the HPS-2 Thrive it is possible, as noted earlier, that laropiprant had adverse effects that increased the risk of cardiovascular events. Fifth, in the AIM-HIGH study the placebo contained a low dose of niacin, which may have resulted in beneficial effects. Finally, both of these trials used extended-release niacin, whereas the Coronary Drug Project and the Stockholm Ischemic Heart Disease Secondary Prevention Study used immediate release niacin. It is possible that these different formulations of niacin have different effects on cardiovascular events. Additional studies are required to definitively determine the effect of niacin added to a statin therapy on cardiovascular events.

 

Effect of Niacin on Atherosclerosis

 

Many of the initial niacin therapy imaging studies combined niacin with other drugs and compared these combinations vs. placebo. These studies showed that niacin in combination with other drugs reduced the progression and/or increased the regression of atherosclerosis. However, because of the use of other drugs it is impossible to determine if niacin therapy per se was beneficial (Table 5).

 

Table 5. Niacin Angiography Imaging Studies

Combination Studies

Drugs

Cholesterol Lowering Atherosclerosis Study (CLAS) (53)

Niacin + colestipol vs. placebo

Familial Atherosclerosis Treatment Study (FATS) (54)

Niacin + colestipol or lovastatin + colestipol vs. placebo

UCSF-SCORE (55)

Niacin + colestipol +/- lovastatin vs. placebo +/- low dose colestipol

HDL Atherosclerosis Study (HATS) (56)

Niacin + simvastatin vs. placebo

Armed Forces Regression Study (57)

Niacin + gemfibrozil + cholestyramine vs. placebo

Harvard Atherosclerosis Reversibility Project (HARP)  (58)

Niacin + pravastatin + cholestyramine + gemfibrozil as needed vs. placebo

 

However, there are studies that compared niacin to placebo or other drugs added to standard statin therapy that do provide useful insights (Table 6).

 

Table 6. Effect of Niacin Added to Statin Therapy on Atherosclerosis

ARBITER 2/3

(59,60)

ER niacin vs. placebo

Decrease in CIMT vs. placebo

ARBITER 6 (61)

ER niacin vs. ezetimibe

Decrease in CIMT vs. ezetimibe

Thoenes (62)

ER niacin vs. placebo

Decrease in CIMT vs. placebo

Lee (63)

Modified release niacin vs. placebo

Decrease in carotid wall area on MRI vs. placebo

 

The ARBITER 2 Trial was a double-blind randomized study of extended-release niacin (1000mg) vs. placebo added to background statin therapy in 167 patients with coronary heart disease and low HDL-C levels (<45mg/dL) (60). At the initiation of the study mean LDL-C levels were < 100mg/dL. The primary end point was the change in common carotid intima-media thickness (CIMT). As expected, plasma TGs decreased and HDL-C levels increased with niacin therapy. LDL-C levels were unchanged. After 12 months, mean CIMT increased significantly in the placebo group (P<0.001) and was unchanged in the niacin group (P=0.23). The overall difference in CIMT progression between the niacin and placebo groups was almost statistically significant (P=0.08). Cardiovascular events occurred in 3 patients treated with niacin (3.8%) and 7 patients treated with placebo (9.6%; P=0.20). ARBITER 3 was a 12-month extension and in the 57 patients that continued on niacin therapy there was an additional regression of CIMT (p = 0.001 vs. placebo) (59).

 

In ARBITER 6, patients with coronary heart disease or a coronary heart disease risk equivalent on long-term statin therapy with LDL-C level < 100mg/dL and an HDL-C level < 50mg/dL for men or 55mg/dL for women were randomly assigned to receive either extended-release niacin (target dose, 2000mg per day) or ezetimibe (10mg per day) (61). The primary end point was the change from baseline in the mean CIMT. LDL-C levels decreased in the ezetimibe group by −18mg/dL (~ 20%) and by −10.0mgdl (~ 12%) in the niacin group (P=0.01) while HDL-C levels were slightly decreased in the ezetimibe group −2.8mg/dL and increased by 7.5mg/dL (~18%) in the niacin group (P<0.001). TG levels were not markedly altered in the ezetimibe group but decreased by ~ 15-20% in the niacin group.  Notably niacin therapy resulted in a significant reduction of both mean (P = 0.001) and maximal CIMT (P < 0.001) while ezetimibe therapy significantly increased CIMT (P < 0.001). The incidence of major cardiovascular events was lower in the niacin group than in the ezetimibe group (1% vs. 5%, P = 0.04).

 

In a trial by Thoenes and colleagues fifty patients with the metabolic syndrome not on statin therapy were randomized to either extended-release niacin (1000mg/day) or placebo (62). Treatment with niacin decreased LDL-C by 17% and TGs by 23% and increased HDL-C levels by 24% without significant changes in the placebo group. After 52 weeks of treatment, there was an increase in CIMT of +0.009 +/- 0.003 mm in the placebo group and a decrease in CIMT of -0.005 +/- 0.002 mm in the niacin group (p = 0.021 between groups).

 

Finally, Lee and colleagues performed a double-blind, randomized study of 2 g daily modified-release niacin or placebo added to statin therapy in 71 patients with low HDL-C (<40mg/dL) and either: 1) type 2 diabetes with coronary heart disease; or 2) carotid/peripheral atherosclerosis (63). The primary end point was the change in carotid artery wall area, quantified by magnetic resonance imaging, after 1 year. Treatment with niacin increased HDL-C by 23% and decreased LDL-C by 19% and TGs by 11%. At 12 months, niacin significantly reduced carotid wall area compared with placebo (Mean change in carotid wall area was -1.1 +/- 2.6 mm2 for niacin vs +1.2 +/- 3.0 mm2 for placebo).

 

While these imaging studies provide data suggesting that niacin therapy when added to statin therapy may reduce atherosclerotic cardiovascular disease, one must recognize that the studies described above were relatively small studies and that decreases or the lack of progression in CIMT or carotid wall area are surrogate markers, which may not necessarily indicate that cardiovascular events will be decreased.  

 

Side Effects

 

Treatment with niacin frequently results in side effects and these side effects are a major limitation of niacin therapy.

 

SKIN FLUSHING

 

This is a very common side effect and is characterized by redness and warmth due to vasodilation of the blood vessels in the skin (8,64). It is often most apparent in the head and neck region. Itching can occur and a tingling and burning sensation may also be noted. Niacin induced flushing is usually not accompanied by diaphoresis. The cutaneous flushing usually lasts for approximately one hour and in some patients is extremely annoying. In a review of 30 studies, it was noted that flushing occurred in 85% of participants treated with immediate release niacin, 66% of participants treated with extended release niacin, and 26% of participants treated with slow release niacin (11).  The occurrence of flushing is related to a rapid increase in plasma nicotinic acid levels, which differs depending upon the niacin preparation. Flushing was the primary reason that subjects discontinued niacin therapy during studies and with either immediate release or extended release niacin approximately 20% of study participants discontinue niacin, which is twice the rate of discontinuation observed in the placebo groups (11). Continuous administration of niacin for approximately one- week results in tachyphylaxis and the flushing decreases. Unfortunately, if a patient skips taking niacin for a few days this tachyphylaxis is lost and the flushing returns.

 

The mechanism for the niacin induced skin flushing has been partially elucidated (8,64). Niacin activates GPR109A in dermal Langerhan cells (macrophages in the skin), which leads to the increased production of prostaglandin D2.  Additionally, niacin activates GPR 109A in keratinocytes, which leads to the production of prostaglandin E2.  The prostaglandins then interact with prostaglandin receptors on blood vessels resulting in vasodilation and the flushing phenomena. Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDS) taken prior to niacin administration can decrease flushing by inhibiting the synthesis of prostaglandins (8,65). Laropiprant decreases flushing by blocking the D prostanoid receptor (8). Since flushing is related to rapid increases in plasma nicotinamide levels taking immediate release niacin with food slows absorption and thereby reduces flushing. Extended-release niacin is typically taken at bedtime so that the flushing will occur when the patient is asleep. Conditions that predispose to cutaneous vasodilatation such as alcohol intake, hot liquids, spicy foods, or hot showers should be avoided. One should increase the dose of niacin slowly to reduce the severity of flushing reactions and allow tolerance to develop.

 

HEPATIC TOXICITY

 

Sustained release niacin has a much greater propensity to induce hepatic toxicity than other niacin preparations and therefore is no longer widely used (7,42,66). The explanation for this difference is due to the increased metabolism of sustained release niacin by the amidation pathway described in the pharmacokinetics section, which results in toxic compounds that injure the liver (7,42). Patients who have developed signs of liver toxicity on sustained release niacin can often be treated with immediate release niacin without developing liver problems (67). Extended-release niacin can induce liver dysfunction but the rate is much lower than sustained release niacin. Because of the potential for liver disease, serum transaminase levels (SGOT and SGPT) should be monitored before treatment begins, every 6 to 12 weeks for the first year, and periodically thereafter (e.g., at approximately 6-month intervals).

 

It should be noted that there is some evidence that niacin may be beneficial for non-alcoholic fatty liver disease (NAFLD) but further studies are required (68).

 

MUSCLE SYMPTOMS

 

Myalgias and myopathy have not been a significant adverse effect with niacin monotherapy (11). In combination with statins, an increased risk of muscle symptoms has been observed in some studies. In the HPS-2 Thrive study the combination of simvastatin and extended-release niacin increased the risk of myopathy four-fold (1.2% of patients on combined therapy) (51). Of note, this increase occurred predominantly in Chinese participants. In contrast, in the AIM-HIGH trial muscle related symptoms were not increased with the simvastatin + niacin combination (49,69).

 

HYPERGLYCEMIA

 

It has been recognized for many years that niacin induces insulin resistance (70). The mechanisms by which niacin induces insulin resistance are unknown but possible mechanisms include a rebound increase in free fatty acids with niacin therapy or the accumulation of diacylglycerol (29,71). A recent analysis of the AIM-HIGH trial demonstrated that in subjects with normal glucose metabolism, subjects with impaired fasting glucose, and subjects with diabetes, treatment with extended release niacin resulted in only small increases in fasting glucose levels but increased serum insulin levels due to an increase in insulin resistance (72). Additionally, there was an increased risk of progressing from normal to impaired fasting glucose in subjects treated with niacin in the AIM-HIGH trial (niacin 58.6% vs placebo 41.5%; P < .001) (72).

 

A meta-analysis examined the effect of niacin therapy on the development of new onset diabetes (73). In 11 trials with 26,340 non-diabetic participants, niacin therapy was associated with a 34% increased risk of developing diabetes (RR of 1.34; 95% CIs 1.21 to 1.49). This increased risk results in one additional case of diabetes per 43 initially non-diabetic individuals who are treated with niacin for 5 years (0.47% ten-year risk or 4.7 per 1000 patient years). Results were similar in patients who were receiving niacin therapy in combination with statin therapy.

 

Studies have shown that niacin is usually well tolerated in diabetic subjects who are in good glycemic control (74,75). In patients with poor glycemic control, niacin is more likely to adversely impact glucose levels. A meta-analysis of 7 studies with 838 patients with diabetes found that niacin therapy did not result in a significant increase in fasting glucose levels in short term studies but in long term studies there was a very small increase in fasting glucose levels (1.5mg/dL) that was not clinically significant (76). An important caveat is that in most of these trials adjustments in diabetes therapy was permitted, which could blunt worsening of glycemic control. In contrast to these findings, the HPS-2 Thrive Trial reported that in the 8,299 participants who had diabetes at the time of randomization, treatment with niacin–laropiprant was associated with a 55% increase in serious disturbances in diabetes control, most of which led to hospitalization (11.1% vs. 7.5%, P<0.001) (51). The extent to which the latter was due to laropiprant is unknown. Thus, care must be used in treating patients with diabetes with niacin. In patients in whom adjustments in diabetic therapy can easily be carried out the risk of adverse effects will likely be limited whereas in patients in whom adjustments in diabetic therapy will be difficult the risks of niacin therapy are likely to be increased. Careful patient selection and education are important steps to reduce the risks of niacin therapy in patients with diabetes.

 

Thus, while niacin therapy may adversely affect glucose homeostasis one needs to balance these adverse effects with the potential benefits of niacin therapy. One should note that in the Coronary Drug Project participants with abnormal glucose metabolism also demonstrated a decrease in cardiovascular events with niacin therapy (45).  

 

URIC ACID  

 

Niacin may increase uric acid levels by inhibiting the secretion of uric acid (8,77). In susceptible patients niacin therapy can precipitate gouty attacks (8).   

 

GASTROINTESTINAL SYMPTOMS  

 

Niacin therapy can induce heartburn, indigestion, nausea, diarrhea, and abdominal discomfort (8). High dose niacin is more likely to cause these gastrointestinal disturbances. The mechanism for these symptoms is not clear. 

 

MISCELLANEOUS  

 

Recent trials have reported an increased incidence of infections with niacin therapy (51,69). A trial of niacin in combination with laropiprant found increased bleeding (51). The increased bleeding could be due to the approximately 10% decrease in platelet levels that can occur with niacin (see Niaspan Package Insert). However, a very large observational study that compared rates of major gastrointestinal bleeding and intracranial hemorrhage in patients treated with niacin (>200,000 subjects) to propensity matched subjects on fenofibrate did not observe an increase in bleeding (78). Niacin has been reported to induce cystoid macular edema, which resolves when the drug is stopped (79).

 

Contraindications

 

There are a number of contraindications to niacin therapy (Table 7).

 

Table 7. Contraindications for Niacin Therapy

Active gastritis or peptic ulcer disease

Impaired liver function (elevated transaminases 2-3X the upper limit or cholestasis)

Uncontrolled gout

Pregnancy

Lactation

Poorly controlled diabetes

Active bleeding

 

Summary

 

The enthusiasm for the use of niacin has greatly decreased with the failure of AIM-HIGH and HPS-2 Thrive to show a decrease in cardiovascular events when niacin was added to statin therapy. In the absence of definitive data showing benefits from niacin therapy when added to a statin it is hard to justify the use of this drug given the frequent side effects. The availability of ezetimibe, bempedoic acid, and PCSK9 inhibitors has greatly reduced the need to use niacin to lower LDL-C levels. Additionally, in patients with markedly elevated TG levels (>500mg/dL), niacin can be employed in combination with other drugs to reduce the risk of pancreatitis but fibrates and omega-3-fatty acids are the initial choices.

 

OMEGA-3-FATTY ACIDS (FISH OIL)

 

Introduction

 

The lipid lowering effects of fish oil are mediated by two omega-3-fatty acids; eicosapentaenoic acid (C20:5n-3) (EPA) and docosahexaenoic acid (C22:6n-3) (DHA). There are four prescription products approved by the FDA which contain various amounts of EPA and DHA (Table 8). Lovaza and Omacor contain a mixture of EPA and DHA fatty acid esters (ethyl esters), Vascepa contains only EPA fatty acid esters (ethyl esters), and Epanova contains a mixture of EPA and DHA free fatty acids (Epanova is currently not available in the US).

 

Table 8. Prescription Omega-3-fatty acid products (data from package inserts)

Generic Name

Omega-3-ethyl esters

Icosapent ethyl

Omega-3-carboxylic acid

Brand Name

Lovaza or Omacor

Vascepa

Epanova

EPA/capsule

0.465g

1.0g

See below

DHA/capsule

0.375g

---

See below

Daily Dose

4 capsules/day

4 capsules/day

2-4 capsules/day

1-gram capsules of Epanova contain at least 850mg of fish oil derived fatty acids including multiple omega-3-fatty acids with EPA and DHA being the most abundant

 

Fish oil is also sold as a food supplement. It should be recognized that dietary fish oil supplements are not approved by the FDA and quality control will not meet the same rigorous standards as prescription or over the counter drugs. The amount of EPA and DHA can vary greatly in these supplements and one needs to read the labels carefully, as products can contain less than 100mg of EPA/DHA per 1 gram capsule (80). It is helpful to have the patient bring their fish oil supplements to the clinic for verification of the actual amount of EPA and DHA in the product. Moreover, the amount of EPA and DHA indicated on the label may not be accurate (81). One needs to take a sufficient number of capsules to provide 2-4 grams of EPA/DHA per day to effectively lower plasma TG levels. Depending upon the fish oil supplement, the patient may be required to take a large number of capsules to obtain 2-4 grams of EPA/DHA per day. Furthermore, these supplements may contain other compounds in addition to omega-3-fatty acids, such as cholesterol, oxidized lipids, and saturated fatty acids. The major advantage of fish oil supplements is that they are much less expensive than prescription omega-3-fatty acid drugs. If one elects to use fish oil supplements, one should have the patient use a single brand to try to ensure as much consistency as possible.

 

Some omega-3 supplements contain alpha linolenic acid (C18:3n-3) (ALA), a plant omega-3-fatty acid rather than EPA/DHA. ALA can be converted to EPA and DHA but the conversion is limited and hence it is ineffective in lowering plasma TG levels or altering other lipid or lipoprotein levels (82).

 

Effect of Omega-3-Fatty Acids on Lipid and Lipoprotein Levels

 

Table 9. Effect of Fish Oil Supplements on Lipids and Lipoproteins

Decreases TGs

No Change in Total Cholesterol

No Change in LDL-C; if TGs are very high may increase LDL-C

No Change in HDL-C

No Change in Lp(a); modest decrease in some studies

Shift from Small Dense LDL to Large Buoyant LDL

 

Several meta-analyses have examined the effect of fish oil supplements on lipid and lipoprotein levels. A meta-analysis by Eslick and colleagues of 47 studies with 16,511 participants found that fish oil supplements significantly decreased plasma TG levels by approximately 14% without resulting in clinically significant changes in total, LDL-C, or HDL-C levels (83). These authors also reported that the reduction in plasma TG levels was directly related to baseline plasma TG levels (i.e., the higher the baseline TG level the greater the reduction in TGs with fish oil). Additionally, the higher the dose of EPA/DHA, the greater the reduction in plasma TGs, with clinically significant reductions occurring with approximately 3.25 grams per day. A meta-analysis by Balk and colleagues of 21 studies also found minimal effects of fish oil supplements on total, LDL-C, and HDL-C levels (< 5% change) with significant decreases in plasma TG levels (most of the studies in this meta-analysis had at least a 15% decrease) (84). Similar to the meta-analysis by Eslick et al, the higher the baseline TG levels the greater the reduction in TG levels. 

 

Several meta-analyses have focused on specific patient populations. In a meta-analysis of patients with diabetes, twenty three trials with1075 participants were analyzed and similar to patients without diabetes the major effect of fish oil supplements was a reduction in plasma TG levels with no change in total cholesterol or HDL-C (85). A small increase in LDL-C was observed (4.3mg/dL). Of note, fish oil supplementation did not alter fasting glucose or glycated hemoglobin levels indicating that fish oil supplementation does not adversely affect glycemic control. In a meta-analysis that included patients with type 2 diabetes or impaired glucose metabolism a decrease in TGs was observed without significant changes in total cholesterol, LDL-C, or HDL-C levels (86). Again, no adverse effects on glycemic control were observed.

 

In patients with end stage renal disease several meta-analyses have consistently shown a decrease in plasma TGs with fish oil administration but the effect on total, LDL-C, and HDL-C has been variable (87-89). This variability was likely due to the small changes that were observed. In patients with nephrotic syndrome a study has shown a reduction in plasma TGs and an increase in LDL-C levels without a change in total cholesterol or HDL-C levels (90). In patients with non-alcoholic fatty liver disease, omega-3-fatty acids have also been shown to decrease plasma TG levels (91). Finally, In HIV infected subjects, fish oil supplementation was also effective in lowering plasma TG levels (92,93).

 

Thus, fish oil supplementation in a variety of different patient populations lowers plasma TG levels. In patients with elevated TG levels treated with 3-4 grams of EPA/DHA one can expect an approximate 25% decrease. Total plasma cholesterol levels are usually not altered by fish oil supplementation. The exceptions are patients with high chylomicron and/or VLDL levels where a substantial portion of the plasma cholesterol is carried on these TG rich lipoproteins. Fish oil supplementation will decrease the levels of these TG rich lipoproteins and thereby result in a decrease in total plasma cholesterol levels. LDL-C levels are not markedly affected by fish oil supplementation except in patients with very high TG levels (>500mg/dL) where increases in LDL-C levels have been observed (94-96). If there are sufficient reductions in plasma TG levels a shift from small dense LDL to large buoyant LDL may be observed (97,98). The effect of fish oil supplements on HDL-C levels is minimal except if the patient has very high TG levels where significant elevations (>10%) have been reported (94-96). Finally, some but not all studies have shown that the administration of fish oil modestly lowers Lp(a) levels (99-103)

 

During the development of pharmacological omega-3-fatty acid drugs for approval by the FDA, extensive clinical trials were carried out and will be reviewed below (Tables 10 and 11). It should be noted that these studies are not directly comparable as they studied different patient populations at different times.

 

EPA + DHA FATTY ACID ESTERS (LOVAZA)  

 

In patients with marked elevations in plasma TG levels (500-2000mg/dL) a 6 week trial of EPA + DHA esters resulted in a 31% decrease in plasma TGs, a 21% increase in LDL-C levels, and a 12% increase in HDL-C levels compared to the placebo group (96). In a 16 week trial TG concentrations were decreased by 45% and LDL-C and HDL-C were increased by 31% and 13%, respectively (94). Studies have also been carried out in patients with moderate hypertriglyceridemia (200-500mg/dL) who were on statin therapy (104). EPA + DHA esters resulted in a 23% decrease in plasma TGs and a 7% decrease in non-HDL-C levels, and a 4.6% increase in HDL-C levels (104).

 

EPA FATTY ACID ESTER ALONE (VASCEPA)  

 

In patients with marked elevations in plasma TGs (500-2000mg/dL), 4 grams of EPA ester alone significantly decreased TG levels by 33.1% and non-HDL-C levels by 17.7% (105). In contrast to EPA and DHA fatty acid esters, LDL-C and HDL-C levels were not significantly altered by EPA fatty acid esters alone (105). Studies have also been carried out in patients with moderate hypertriglyceridemia (200-500mg/dL) who were on statin therapy. EPA esters resulted in a 21.5% decrease in plasma TGs, 13.6% decrease in non-HDL-C, 6.2% decrease in LDL-C, and a 4.5% decrease in HDL-C levels (106)

 

EPA + DHA FATTY ACIDS (EPANOVA)  

 

In patients with marked elevations in plasma TGs (500-2000mg/dL), 4 grams of EPA + DHA fatty acids decreased plasma TGs by 31% and non-HDL-C by 9.6% and increased LDL-C by 19% and HDL-C by 5.8% (107). Studies have also been carried out in patients with moderate hypertriglyceridemia (200-500mg/dL) who were on statin therapy. EPA + DHA fatty acids resulted in a 20.6% decrease in plasma TGs, 6.9% decrease in non-HDL-C with no significant changes in LDL-C or HDL-C levels (95).

 

These studies demonstrate that in patients on statin therapy with moderate elevations in plasma TG levels the effects of these three pharmaceutical products on lipids and lipoprotein levels are similar (table 11). However, in patients with marked elevations in plasma TG levels EPA ethyl esters alone do not increase LDL-C levels whereas products containing EPA and DHA result in a substantial increase in LDL-C levels (table 10). It should also be noted that the ability of omega-3-fatty acids to reduce plasma TGs and increase HDL-C levels is enhanced if baseline TG levels are markedly elevated.

 

Table 10: Effect of Omega-3-Fatty Acids on Lipids and Lipoprotein in Patients with Marked Hypertriglyceridemia (500-2000mg/dL)

 

TGs

Non-HDL-C

LDL-C

HDL-C

EPA+DHA ethyl esters-

6 weeks

31% decrease

ND

21% increase

12% increase

EPA+DHA ethyl esters

12 weeks

45% decrease

ND

31% increase

13% increase

EPA ethyl esters

33% decrease

18% decrease

NS

NS

EPA+DHA fatty acids

31% decrease

9.6% decrease

19% increase

5.8% increase

ND- not determined; NS- no significant change

 

Table 11: Effect of Omega-3-Fatty Acids on Lipids and Lipoprotein in Patients with Moderate Hypertriglyceridemia (200-500mg/dL) on Statin Therapy

 

TGs

Non-HDL-C

LDL-C

HDL-C

EPA+DHA ethyl esters

23% decrease

7% decrease

__

4.6% increase

EPA ethyl esters

22% decrease

14% decrease

6.2% increase

4.5% decrease

EPA+DHA fatty acids

21% decrease

6.9% decrease

NS

NS

NS- no significant change

 

HEAD-TO-HEAD COMPARISONS  

 

A meta-analysis of six studies has compared the effect of EPA alone vs. DHA alone on plasma lipids and lipoproteins (108). Administration of DHA increased LDL-C by 4.6mg/dL compared to EPA (95% CI 2.2- 7.1). In contrast, DHA reduced plasma TG levels to a greater extent than EPA (6.1mg/dL; 95% CI 2.5- 9.8). Finally, DHA increased HDL-C levels more than EPA (3.7mg/dL; 95% CI: 2.4- 5.1). Whether these very modest differences are clinically significant is unknown.

 

Tatsuno et al compared the effect of DHA + EPA ethyl esters vs. EPA ethyl esters alone on lipid and lipoprotein levels in patients with mean baseline plasma TG of 250-270mg/dL and mean LDL-C levels of 125-135mg/dL (109,110). These authors found that at equivalent doses there were no differences in effect on plasma TG, LDL-C, or HDL-C levels between DHA + EPA ethyl ester or EPA ethyl ester treatment.

 

These head-to-head studies indicate that in subjects with moderate hypertriglyceridemia the effects of EPA and DHA on lipid and lipoprotein levels are similar. Perhaps if the baseline TGs were markedly elevated differences in response might have been observed.

 

IN COMBINATION WITH FENOFIBRATE  

 

In patients with marked hypertriglyceridemia a single drug is often not sufficient to lower TGs into the desired range. In patients with TG levels > 500mg/dL the combination of fenofibrate 130mg per day and 4 grams of Lovaza reduced TG levels to a greater extent than monotherapy with fenofibrate alone (7% decrease; P = 0.059) (111). Not unexpectedly, LDL-C levels were increased to a greater extent with combination therapy (9% increase; p= 0.03). When subjects who had received 8 weeks of fenofibrate monotherapy were treated with Lovaza during the 8-week, open-label extension study, TG levels were reduced by an additional 17.5% (P = 0.003). These results indicate that the addition of omega-3-fatty acids to fenofibrate will further decrease TG levels.

 

IN COMBINATION WITH NIACIN

 

Sixty patients with the metabolic syndrome were randomized to 16 weeks of treatment with placebo, Lovaza (4 g/day), extended release niacin (2 g/day), or both drugs in combination (17). In the niacin group TGs were decreased by 30%, in the omega-3-fatty acids group by 22%, and in the combination group by 42% compared to the placebo group. Of note, the beneficial effects of niacin on decreasing LDL and non-HDL-C were blunted by omega-3-fatty acids. These results show that the combination of niacin and fish oil will lower TG levels more than either drug individually but at the expense of diminishing the effect of niacin on LDL and non-HDL-C levels.

 

Mechanism Accounting for the Omega-3-Fatty Acid Induced Lipid Effects

 

As noted above, the major effect of fish oil is to lower plasma TG levels. The predominant cause of the reduction in plasma TG levels is a decrease in the hepatic production and secretion of TG rich lipoproteins (112-115). In cultured hepatocytes, omega-3-fatty acids inhibit the assembly and secretion of VLDL and apolipoprotein B 100 (113,115-117).  The incorporation of TGs into VLDL is a key regulatory step in determining the rate of formation and secretion of VLDL and there are a number of mechanisms by which omega-3 fatty acids reduce the level of hepatic TGs available for VLDL formation (112,113,115). Studies in animal models have demonstrated that omega-3-fatty acids inhibit fatty acid synthesis and stimulate fatty acid oxidation in the liver, which would reduce the availability of fatty acids for TG synthesis (112-115). The increase in fatty acid oxidation is due to omega-3-fatty acids activating PPAR alpha, which stimulates fatty acid oxidation in the liver and other tissues (112,114,115,118). The decrease in fatty acid synthesis is due to omega-3-fatty acids inhibiting the expression of SREBP-1c, a key transcription factor that regulates fatty acid synthesis (114,115,118). In addition, omega-3-fatty acids decrease TG synthesis, which may be due to the decreased availability of fatty acids and an inhibition of the activity of DGAT, a key enzyme required for TG synthesis (112,114,115). Finally, omega-3-fatty acids also decrease the flux of free fatty acids from adipose tissue to the liver, which will lead to a decreased quantity of fatty acids available for TG synthesis in the liver (112). This decrease in flux of free fatty acids is due to omega-3-fatty acids reducing hormone sensitive lipase mediated intracellular lipolysis in adipose tissue (112). It is likely that these and perhaps other factors lead to the decreased availability of TGs resulting in a reduction in VLDL formation and secretion. In addition, the peroxidation of omega-3-fatty acids may stimulate the degradation of apolipoprotein B-100, which would provide another pathway that could contribute to a decrease in VLDL formation and secretion (115).

 

While not the primary mechanism for the decrease in plasma TGs, studies have shown that omega-3-fatty acids may increase the clearance of TG rich lipoproteins (112,119). Post heparin lipoprotein lipase activity is not increased by omega-3-fatty acid administration but the lipolytic activity of non-stimulated plasma is enhanced (112,119).  Additionally, apolipoprotein C-III levels are decreased with omega-3-fatty acid administration which could also contribute to an increase in the clearance of TG rich lipoproteins (120-123).

 

The increase in LDL-C levels that occurs in patients with marked hypertriglyceridemia treated with omega-3-fatty acids is thought to be due to the enhanced conversion of VLDL to LDL (114). The increase in HDL-C observed in studies in patients with very high TG levels may be due to the increased clearance of TG rich lipoproteins.    

 

Pharmacokinetics and Drug Interactions

 

Omega-3 ethyl esters and fatty acids are absorbed by the GI tract similar to other dietary lipids. It is worth noting that omega-3-free fatty acids (Epanova) are directly absorbed by the small intestine and are not dependent on pancreatic lipases for absorption. Thus, absorption of omega-3-fatty acids is not decreased in patients with pancreatic insufficiency and therefore may be preferred in patients with pancreatic disease. Additionally, the bioavailability of omega-3-fatty acids with a low fat diet was greater than omega-3-ethyl esters while there was little difference between these different formulations with a high fat diet (124,125).

 

Drug interactions have not been seen with omega-3-fatty acids (Package Inserts for Lovaza, Vascepa, and Epanova).

 

Effect of Low Dose Omega-3-Fatty Acids on Clinical Outcomes

 

Initial studies of the effect of low dose fish oil administration on cardiovascular outcomes were favorable, demonstrating a reduction in events including all-cause mortality. However, more recent studies have failed to confirm these favorable results. In these more recent studies the use of other drugs, such as statins, that reduce cardiovascular disease were more intensively utilized. The outcomes studies that will be described below were carried out with doses of EPA and DHA that are lower than the doses used to lower plasma TGs. We will limit our discussion to the administration of fish oil as a drug and not discuss diet studies, such as DART, which had patients increase fatty fish intake (126,127).

 

  • GISSI-Prevenzione trial was a randomized trial of 850-882mg of EPA and DHA ethyl esters per day in 11,323 participants with a recent myocardial infarction (< 3 months) for 3.5 years (128). The primary endpoint was death, non-fatal myocardial infarction, and stroke. No change in total cholesterol, LDL-C, or HDL-C was observed but plasma TG levels were decreased by 5%. Patients treated with EPA/DHA had a significant decreased risk of major cardiovascular events (RR 0.90), cardiac death (RR 0.78), and sudden death (RR 0.74). The decrease in sudden death occurred very quickly and was noted as early as 4 months after initiation of therapy. Interestingly, non-fatal cardiovascular events were not affected by EPA/DHA treatment (RR 0.98). The decrease in total mortality was driven by a reduction in sudden death suggesting an anti-arrhythmic effect of EPA/DHA.

 

  • GISSI-Heart Failure (GISSI-HF) trial was a randomized, double-blind, placebo-controlled trial in patients with chronic heart failure who were randomly assigned to 850-882mg of EPA and DHA ethyl esters per day (n=3,494) or placebo (n=3,481) (129). Patients were followed for a median of 3.9 years. Primary endpoints were time to death, and time to death or admission to the hospital for cardiovascular reasons. Omega-3-fatty acid treatment at these low doses resulted in a slight decrease in plasma TG levels with no change in total, LDL-C or HDL-C levels. In the omega-3-fatty acid group 27% patients died from any cause vs. 29% in the placebo group (HR 0.91; p=0.041). In the omega-3-fatty acid group 57% of patients died or were admitted to hospital for cardiovascular reasons vs. 59% in the placebo group (HR 0.92; p=0.009). No significant differences were observed in fatal or non-fatal myocardial infarctions or strokes. In this trial, similar to the GISSI-Prevenzione trial, the benefit was primarily due to a reduction in arrhythmic events and little benefit on atherothrombotic events was noted.

 

  • OMEGA was a randomized, placebo-controlled, double-blind, trial in 3,851 survivors of an acute myocardial infarction (130). Patients were randomized 3 to 14 days after an acute myocardial infarction to omega-3-acid ethyl esters, 1 gram/day (460mg EPA and 380mg DHA) or placebo capsules containing 1 gram of olive oil and followed for one year. The primary endpoint was rate of sudden death and secondary end points were total mortality and nonfatal clinical events. No significant differences were seen in the primary or secondary endpoints.

 

  • Alpha Omega was a double-blind, placebo-controlled trial in 4,837 patients between 60 and 80 years of age (78% men) who had had a myocardial infarction (131). Patients were randomized to receive for 40 months one of four trial margarines: a margarine supplemented with a combination of EPA and DHA (with a targeted additional daily intake of 400mg of EPA-DHA; actual intake 226mg EPA and 150mg DHA), a margarine supplemented with alpha-linolenic acid (ALA) (with a targeted additional daily intake of 2g of ALA), a margarine supplemented with EPA-DHA and ALA, or a placebo margarine. The primary end point was the rate of major cardiovascular events, which comprised fatal and nonfatal cardiovascular events and cardiac interventions. Neither low dose EPA-DHA, ALA, nor the combination of EPA/DHA and ALA significantly reduced the rate of major cardiovascular events or cardiac interventions.

 

  • FOL.OM3 Study was a double blind, randomized, placebo-controlled trial in 2,501 patients with a history of a myocardial infarction, unstable angina, or ischemic stroke in the past 12 months (132). Patients were randomized to a daily dietary supplement containing 5-methyltetrahydrofolate (560μg), vitamin B-6 (3mg), and vitamin B-12 (20μg) or placebo; and a dietary supplement containing omega 3 fatty acids (600mg of EPA and DHA) or placebo. Median duration of treatment was 4.7 years. The primary outcome was a composite of non-fatal myocardial infarction, stroke, or death from cardiovascular disease. Treatment with B vitamins or omega 3 fatty acids had no significant effect on major vascular events.

 

  • Origin was a double-blind study in 12,536 patients at high risk for cardiovascular disease who had impaired fasting glucose, impaired glucose tolerance, or diabetes (133). Patients were randomized to receive a 1-gram capsule containing at least 900mg of ethyl esters of omega-3 fatty acids (EPA 465mg and DHA 375mg) or placebo for approximately 6 years. The primary outcome was death from cardiovascular causes. TG levels were reduced by 14.5mg/dL in the group receiving omega-3-fatty acids compared to the placebo group (P<0.001), without a significant effect on other lipids. The incidence of the primary outcome was not significantly decreased among patients receiving omega-3-fatty acids as compared with those receiving placebo. The use of omega-3-fatty acids also had no significant effect on the rates of major vascular events, death from any cause, or death from arrhythmia.

 

  • Risk and Prevention Study was a double-blind, placebo-controlled trial in 12,513 men and women with multiple cardiovascular risk factors or atherosclerotic vascular disease but not myocardial infarctions (134). Patients were randomly assigned to 1-gram daily omega-3 fatty acids (EPA and DHA content not <85 %,) or placebo (olive oil) for 5 years. The initially specified primary end point was the rate of death, nonfatal myocardial infarction, and nonfatal stroke. At 1 year, after the event rate was found to be lower than anticipated, the primary end point was revised as time to death from cardiovascular causes or admission to the hospital for cardiovascular causes. Plasma TG levels decreased slightly more in the omega−3-fatty acid group than in those who received placebo (−28.2±1.3mg/dL vs. −20.1±1.3mg/dL; P<0.001). Total, LDL, and HDL-C levels were similar in the omega-3-fatty acid and placebo groups. No significant differences were observed between the omega-3-fatty acid group and placebo group for the primary endpoint or any of the secondary endpoints.

 

  • A Study of Cardiovascular Events in Diabetes (ASCEND) was a randomized, placebo controlled, double blind trial of 1-gram omega-3-fattys acids (400mg EPA and 300mg DHA ethyl esters) vs. olive oil placebo in 15,480 patients with diabetes without a history of cardiovascular disease (primary prevention trial) (135). The primary end point was serious vascular events (non-fatal myocardial infarction, non-fatal stroke, transient ischemic attack, or vascular death). Total cholesterol, HDL-C, and non-HDL-C levels were not significantly altered by omega-3-fatty acid treatment (changes in TG levels were not reported). After a mean follow-up of 7.4 years the composite outcome of a serious vascular event or revascularization occurred in 882 patients (11.4%) on omega-3-fatty acids and 887 patients (11.5%) on placebo (rate ratio, 1.00; 95% CI, 0.91 to 1.09). Serious adverse events were similar in placebo and omega-3-fatty acid treated groups.

 

  • The Vitamin D and Omega-3 Trial (Vital) was a randomized, double blind, placebo-controlled trial of 1-gram omega-3 fatty acids (465mg EPA and 375mg DHA ethyl esters) vs. placebo in 25,875 men (>50 years of age) and women (>55 years of age) that were not selected on the basis of an elevated risk (primary prevention) (136). Changes in lipid levels were not reported. The primary end point was major cardiovascular events, a composite of myocardial infarction, stroke, or death from cardiovascular causes. After a median follow-up of 5.3 years, major cardiovascular event occurred in 386 participants in the omega-3 fatty acid group and in 419 in the placebo group (hazard ratio, 0.92; 95% confidence interval (CI), 0.80 to 1.06; P=0.24). Serious adverse events were similar in placebo and omega-3-fatty acid treated groups.

 

  • Summary: The above results indicate that low dose fish oil (doses that do not greatly affect lipid levels) do not consistently reduce the risk of cardiovascular disease.

 

Effect of High Dose Omega-3-Fatty Acids on Clinical Outcomes

 

  • Japan EPA Lipid Intervention Study (JELIS) was an open label study without a placebo in patients with total cholesterol levels > 254mg/dL with (n= 3,664) or without cardiovascular disease (n=14,981) who were randomly assigned to be treated with 1800 mg of EPA (Vascepa) + statin (n=9,326) or statin alone (n= 9,319) with a 5-year follow-up (130). The primary endpoint was any major coronary event, including sudden cardiac death, fatal and non-fatal myocardial infarction, and other non-fatal events including unstable angina pectoris, angioplasty, stenting, or coronary artery bypass grafting. Total, LDL-C, and HDL-C levels were similar in the two groups but plasma TGs were modestly decreased in the EPA treated group (5% decrease in EPA group compared to controls; p = 0.0001). In the EPA group the primary endpoint occurred in 2.8% of the patients vs. 3.5% of the patients in the statin alone group (19% decrease; p = 0.011). Unstable angina and non-fatal coronary events were also significantly reduced in the EPA group but in this study sudden cardiac death and coronary death did not differ between groups. Unstable angina was the main component contributing to the primary endpoint and this is a more subjective endpoint than other endpoints such as a myocardial infarction, stroke, or cardiovascular death. In patients with high TG levels (>150 mg/dL) and low HDL-C levels (<40 mg/dL EPA treatment decreased the risk of CAD by 53% (HR: 0.47; P=0.043) (137). A subjective endpoint has the potential to be an unreliable endpoint in an open label study and is a limitation of the JELIS Study.

 

  • The Reduction of Cardiovascular Events with EPA – Intervention Trial (REDUCE-IT) was a randomized, double blind trial of 2 grams twice per day of EPA ethyl ester (icosapent ethyl) (Vascepa) vs. mineral oil placebo in 8,179 patients with hypertriglyceridemia (135mg/dL to 499mg/dL) and established cardiovascular disease or high cardiovascular disease risk (diabetes plus one risk factor) who were on stable statin therapy (138). The primary end point was a composite of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, coronary revascularization, or unstable angina. The key secondary end point was a composite of cardiovascular death, nonfatal myocardial infarction, or nonfatal stroke. At baseline, the median LDL-C level was 75.0 mg/dL, HDL-C level was 40.0 mg/dL, and TG level was 216.0 mg/dL. The median change in TG level from baseline to 1 year was a decrease of 18.3% (−39.0 mg/dL) in the EPA group and an increase of 2.2% (4.5 mg/dL) in the placebo group. After a median of 4.9 years the primary end-point occurred in 17.2% of the patients in the EPA group vs. 22.0% of the patients in the placebo group (hazard ratio, 0.75; P<0.001), indicating a 25% decrease in events. The number needed to treat to avoid one primary end-point event was 21. The reduction in cardiovascular events was noted after approximately 2 years of EPA treatment. Additionally, the rate of cardiovascular death was decreased by 20% in the EPA group (4.3% vs. 5.2%; hazard ratio, 0.80; P=0.03). The cardiovascular benefits of EPA were similar across baseline levels of TGs (<150, ≥150 to <200, and ≥200 mg per deciliter). Moreover, the cardiovascular benefits of EPA appeared to occur irrespective of the attained TG level at 1 year (≥150 or <150 mg/dL), suggesting that the cardiovascular risk reduction was not associated with attainment of a normal TG level. An increase in hospitalization for atrial fibrillation or flutter (3.1% vs. 2.1%, P=0.004) occurred in the EPA group. In addition, serious bleeding events occurred in 2.7% of the patients in the EPA group and in 2.1% in the placebo group (P=0.06). There were no fatal bleeding events in either group and the rates of hemorrhagic stroke, serious central nervous system bleeding, and serious gastrointestinal bleeding were not significantly higher in the EPA group than in the placebo group.

 

It should be noted that in this trial mineral oil was used as the placebo. In the placebo group the LDL-C, non-HDL-C, and CRP levels were increased compared to the EPA group during the trial (LDL-C 96mg/dL vs 85mg/dL; non-HDL-C 130mg/dL vs. 113mg/dL; hsCRP 2.8mg/L vs. 1.8mg/L). The impact of these adverse changes on clinical outcomes is uncertain and whether they contributed to the apparent beneficial effects observed in the individuals treated with EPA is unknown.

 

  • The STRENGTH Trial was a double-blind, randomized, trial comparing 4 grams per day of a carboxylic acid formulation of omega-3 fatty acids (EPA and DHA; Epanova) (n = 6,539)) vs. corn oil placebo (n = 6539) in statin-treated participants with high cardiovascular risk, hypertriglyceridemia, and low levels of HDL-C (139). Approximately 55% of patients had established cardiovascular disease and approximately 70% had diabetes. Median LDL-C level was 75.0 mg/dL, median TG level was 240 mg/dL and median HDL-C level was 36 mg/dL. There were minimal differences in the change in LDL-C and HDL-C levels between the treated and placebo groups after treatment for 12 months but as expected there was a greater reduction in TG levels in the group treated with omega-3-fatty acids (−19.0% vs −0.9%). The primary endpoint was a composite of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, coronary revascularization, or unstable angina requiring hospitalization which occurred in 12.0% of individuals treated with omega-3 CA vs. 12.2% treated with corn oil (hazard ratio, 0.99; P = .84). There were no significant differences between the treatment groups with regard to the risk of the individual components of the primary end point over the 3-4 years of the study. Similar to the REDUCE-IT trial atrial fibrillation was increased with EPA + DHA treatment (HR 1.69 CI 1.29- 2.21). Thus, in contrast the JELIS and REDUCE-IT trials the STRENGTH trial did not demonstrate a benefit of treatment with a mixture of omega-3-fatty acids (EPA + DHA).

 

  • The OMEMI trial was a randomized trial of 1.8 grams per day of omega-3-fatty acids (930 mg EPA and 660 mg DHA) (n= 505) vs. corn oil placebo (509) in patients aged 70 to 82 years with a recent myocardial infarction (2-8 weeks) (140). Baseline LDL-C was approximately 76mg/dL, HDL-C was 49mg/dL, and TGs 110mg/dL. The primary endpoint was a composite of nonfatal myocardial infarction, unscheduled revascularization, stroke, all-cause death, and heart failure hospitalization after 2 years of follow-up. The primary endpoint occurred in 21.4% of patients on omega-3-fatty acids vs. 20.0% on placebo (hazard ratio, 1.08; P=0.60). TGs levels decreased 8.1% in the omega-3-fatty acid group and increased 5.1% in the placebo group (between group difference 13.2%; P<0.001) while changes in LDL-C were minimal in both groups. Thus, similar to the STRENGTH trial no benefits on cardiovascular disease were observed with EPA + DHA treatment.

 

Summary of Omega-3-Fatty Acid Clinical Outcome Trials

 

  • Low dose omega-3-fatty acids are not effective at decreasing cardiovascular outcomes.
  • High dose EPA (JELIS and REDUCE-IT) reduced cardiovascular outcomes while high dose EPA+DHA (STENGTH and OMEMI) did not decrease cardiovascular outcomes.
  • The decrease in TG levels is not a major contributor to the beneficial effect of high dose EPA as the combination of high EPA+DHA lowers TG levels to the same degree as EPA alone without benefit. Additionally, the JELIS trial only lowered TG levels by 5% but nevertheless reduced cardiovascular events. It is likely that the beneficial effects of EPA seen in the JELIS and REDUCE-IT trials are multifactorial with TG lowering making only a small contribution to the decrease in cardiovascular disease. Other actions of EPA, such as decreasing platelet function, anti-inflammation, decreasing lipid oxidation, stabilizing membranes, etc. could account for or contribute to the reduction in cardiovascular events (141). A large meta-analysis, excluding the REDUCE-IT trial, demonstrated that a 40mg/dL decrease in triglyceride levels resulted in a relative risk reduction of only 0.96 (4% decrease) indicating that one needs to markedly lower triglyceride levels to reduce cardiovascular events (142).
  • Whether EPA has special properties that resulted in the reduction in cardiovascular events in the REDUCE-IT trial or there were flaws in the trial design (the use of mineral oil as the placebo) is uncertain and debated. It should be noted that in the REDUCE-IT trial LDL-C and non-HDL-C levels were increased by approximately 10% in the mineral oil placebo group (138). Additionally, Apo B levels were increased by 7% (6mg/dL) by mineral oil (138). Finally, an increase in hsCRP (20-30%) and other biomarkers of atherosclerosis (oxidized LDL-C, IL-6, IL-1 beta, and lipoprotein-associated phospholipase A2) were noted in the mineral oil group (138,143). In the STRENGTH trial there were no differences in LDL-C, Non-HDL-C, HDL-C, Apo B, or hsCRP levels between the treated vs. placebo groups (139). Whether EPA has special properties compared to DHA leading to a reduction in cardiovascular events or the mineral oil placebo resulted in adverse changes increasing ASCVD in the placebo resulting in an artifactual decrease in the EPA group is debated (144,145). Ideally, another large randomized cardiovascular trial with EPA ethyl ester (icosapent ethyl) (Vascepa) using a placebo other than mineral oil would help resolve this controversy. In the meantime, clinicians will need to use their clinical judgement on whether to treat patients with modest elevations in TG levels with EPA (icosapent ethyl; Vascepa) balancing the potential benefits of treatment vs. the potential side effects.

 

Side Effects

 

Gastrointestinal side effects such as diarrhea, nausea, dyspepsia, abdominal discomfort, and eructation have been observed with fish oil therapy (Package Inserts for Lovaza, Vascepa, and Epanova).

 

At very high doses, omega-3-fatty acids can inhibit platelets and prolong bleeding time. However, at the recommended doses this has not been a major clinical problem but nevertheless when patients are on anti-platelet drugs one should be alert for the possibility of bleeding problems (Package Inserts for Lovaza, Vascepa, and Epanova). Increased bleeding was noted in the REDUCE-IT trial in the patients treated with icosapent ethyl 4 grams/day (EPA) (see above discussion of this trial). A recent review found no evidence for discontinuing the use of omega-3 fatty acid treatment before invasive procedures or when given in combination with other agents that affect bleeding (146).

 

As noted above an increase in atrial fibrillation was observed in the REDUCE-IT trial in the patients treated with icosapent ethyl 4 grams/day (EPA) and in the STRENGTH trial in the patients treated with EPA + DHA.

 

Contraindications

 

There are no contraindications to the use of omega-3-fatty acids. Lovaza, Omacor, and Vascepa are pregnancy category C drugs and they should only be used if the benefits to the mother outweigh the potential risks to the fetus.

 

Conclusions

 

Omega-3-fatty acids are effective drugs in reducing TG levels with few significant side effects, drug interactions, or contraindications.  High dose EPA (4 grams/day) reduced cardiovascular disease events in the REDUCE-IT trial and a moderate dose of EPA (1.8 grams/day) reduced cardiovascular events in the JELIS trial but trials of EPA and DHA have not produced cardiovascular benefits. The basis for these differences is debated and discussed in the “Summary of Omega-3-Fatty Acid Clinical Outcome Trials” section above. Finally, omega-3-fatty acids are effective in lowering TGs in patients with marked hypertriglyceridemia and while not proven will likely reduce the risk of development of pancreatitis.

 

FIBRATES

 

Introduction

 

The fibrate drug class includes clofibrate, gemfibrozil, fenofibrate, bezafibrate, and ciprofibrate. Clofibrate was developed in the 1960s and was the first member of this class. Clofibrate is no longer available because of an increased risk of adverse effects. Gemfibrozil and fenofibrate are available in the United States while gemfibrozil, fenofibrate, bezafibrate, and ciprofibrate are available in Europe. All of the fibrates work via activation of the nuclear hormone receptor PPAR alpha.

 

Effect of Fibrates on Lipid and Lipoprotein Levels

 

Table 12. Effect of Fibrates on Lipids and Lipoproteins

Decreases TG

Increases HDL-C

Decreases LDL-C; if TGs Very High can Increase LDL-C

Decreases Non-HDL-C

Decreases Apolipoprotein B

Decreases LDL Particle Number

Shift Small Dense LDL to Large Buoyant LDL

No Effect on Lp(a)

 

Fibrates reduce fasting TG levels by 25-50% (147-149). The magnitude of the reduction in TGs is dependent on the baseline TG levels. Patients with marked elevations in TGs have a greater reduction in TG levels (147,149,150). In addition, fibrates increase HDL-C levels by 5-20% (148,149). The increase in HDL-C levels is more robust if the TG levels are elevated and/or if the HDL-C levels are low (150). The effect on LDL-C is more variable (149). If the TG levels are very high (>400-500mg/dL), fibrate therapy may result in an increase in LDL-C levels whereas if TGs are not elevated fibrates decrease LDL-C by 10-30% (147). Given the decrease in plasma TGs and LDL-C levels, fibrates also reduce apolipoprotein B, LDL particle number, and non-HDL-C levels (149). Depending upon the TG level there may be a shift from small dense LDL towards large LDL particles (149). Fibrates do not have any major or consistent effects on Lp(a) levels (151). Table 13 below shows the effect of fenofibrate on lipid and lipoprotein levels in patients with different lipid profiles and illustrates some of the principles outlined above.

 

Table 13. Effect of Fenofibrate on Lipid and Lipoprotein Levels

 

TGs

LDL-C

HDL-C

Elevated TG Levels

 

 

 

Baseline Levels

~404mg/dL

~125mg/dL

~35mg/dL

Change with Fenofibrate

45% Decrease

2.5% Increase

16% Increase

Elevated LDL-C and TG Levels

 

 

 

Baseline Levels

232mg/dL

220mg/dL

46.7mg/dL

Change with Fenofibrate

37% Decrease

13% Decrease

12% Increase

Elevated LDL-C and Normal TG Levels

 

 

 

Baseline Levels

102mg/dL

228mg/dL

58.1mg/dL

Change with Fenofibrate

35% Decrease

29% Decrease

7% Increase

The values are adjusted for changes in the placebo group. Data modified from Tricor Package Insert.

 

In large, randomized, fibrate outcome trials similar changes in lipid and lipoprotein levels were noted (Table 14). These trials are discussed in detail in the effect of fibrates on cardiovascular outcomes section presented below.

 

Table 14. Effect of Fibrates on Lipid and Lipoprotein Levels in Large Outcome Studies*

 

TGs

LDL-C

HDL-C

Helsinki Heart Study- Gemfibrozil (152)

35% Decrease

11% Decrease

10% Increase

VA-HIT Study

Gemfibrozil (153)

31% Decrease

No Change

6% Increase

BIP Study

Bezafibrate (154)

21% Decrease

7% Decrease

18% Increase

Leader Study

Bezafibrate (155)

23% Decrease

8% Decrease

8% Increase

Field Study

Fenofibrate (156)

29% Decrease

12% Decrease

5% Increase

*The values are adjusted for changes in the placebo group.

 

The different fibrates in general cause similar changes in lipid and lipoprotein levels. There are only a few comparative trials of fibrates comparing their effects on lipid and lipoprotein levels and these trials have been very small. Comparisons of ciprofibrate and gemfibrozil have not shown any major differences between these two fibrates (157,158). In contrast, two very small trials have compared gemfibrozil vs. fenofibrate and reported that fenofibrate was more efficacious in lowering LDL levels than gemfibrozil (159,160).

 

In very rare instances fibrates can cause a paradoxical marked decrease in HDL-C levels (161-164). This rare paradoxical decrease in HDL-C typically occurs when fibrates are used in combination with a thiazolidinedione (rosiglitazone and pioglitazone) but can occur when fibrates are used alone or with ezetimibe (161-165). The decrease in HDL-C can be extreme with decreases of 50% to 88% reported and recovery to normal can take weeks after the fibrate is discontinued (162). The mechanism for this paradoxical effect is unknown.

 

Effect of Fibrates in Combination with Other Lipid Lowering Drugs on Lipid and Lipoprotein Levels

 

STATINS

 

Statins are the primary drugs used to treat most patients with dyslipidemia. Statins are very effective in lowering LDL-C levels but have only modest effects on TG and HDL-C levels. Therefore, it is appealing to add a fibrate to patients who on statin therapy have LDL-C levels at goal but still have elevated non-HDL-C and TG levels and decreased HDL-C levels. Therefore, there have been numerous studies examining the effect of the combination of statins and fibrates on lipid and lipoprotein levels. An example is the Safari Trial which compared the effect of simvastatin only (n=207) vs. simvastatin + fenofibrate (n=411) in patients with combined hyperlipidemia (166). The results of this trial are shown in table 15. As anticipated, adding a fibrate results in a further lowering of LDL-C, non-HDL-C, and TG levels with a further increase in HDL-C.

 

Table 15. Effect of Simvastatin Alone vs. Simvastatin + Fenofibrate on Lipid and Lipoprotein Levels

 

LDL

TG

Non-HDLC

HDL

Simvastatin

-26%

-20%

-26%

+10%

Simvastatin + Fenofibrate

-31%

-43%

-35%

+19%

 

A meta-analysis of 9 studies with over 1,200 participants compared the effect of statin alone vs. statin + fibrate on lipid and lipoprotein levels (167). The combination of statins and fibrates provided significantly greater reductions in total cholesterol, LDL-C, and TGs, and a significantly greater increase in HDL-C than treatment with statins alone. A larger meta-analysis of 13 randomized controlled trials, involving 7,712 patients, similarly demonstrated significant decreases in LDL-C (8.8mg/dL), TGs (58mg/dL), and total cholesterol (11.2mg/dL), and increases in HDL-C (4.65mg/dL) in patients receiving the combination of statins + fibrates compared with statin therapy alone (168). The combination of statins + fibrates also result in a shift of LDL particles from small dense particles to large buoyant particles whereas no change in LDL particle size was observed with statin monotherapy (169).  

 

A recent meta-analysis of 6 studies with over 400 participants compared the effect of adding a statin to fibrate therapy (fibrate alone vs. fibrate + statin) and showed similar changes (170).  The fibrate-statin combination produced significantly greater reductions in the levels of total cholesterol, LDL-C, and TGs compared to fibrate alone. In contrast there was no significant difference in HDL-C levels in the fibrate vs. fibrate + statins group.

 

EZETIMIBE

 

In patients unable to tolerate statin therapy one needs to use other drugs to treat dyslipidemia. In a study comparing the effect of ezetimibe 10mg alone, fenofibrate 145mg alone, or ezetimibe + fenofibrate the combination had a better effect on the lipid profile resulting in a greater decrease in LDL-C levels and increase in HDL-C levels than either drug alone (Table 16) (171).

 

Table 16. Effect of the Combination of Ezetimibe and Fenofibrate on Lipid and Lipoprotein Levels

 

LDL-C

HDL-C

TG

Ezetimibe

23% Decrease

2.2% Increase

10% Decrease

Fenofibrate

22% Decrease

7.5% Increase

38% Decrease

Ezetimibe + Fenofibrate

34% Decrease

11.5% Increase

38% Decrease

 

Similar results were observed in another randomized trial of ezetimibe 10mg and fenofibrate 160mg (172). Moreover, both fibrate therapy and the combination of ezetimibe and fenofibrate results in a shift of LDL particles from small dense LDL particles to large buoyant particles (172).

 

EZETIMIBE + STATIN

 

A large randomized trial has compared the effect of ezetimibe /simvastatin 10mg/20mg, fenofibrate 160mg, or ezetimibe/simvastatin + fenofibrate on lipid and lipoprotein levels. As one would expect triple drug therapy had a better effect on the lipid profile (Table 17) (173). While ezetimibe/simvastatin was very effective in lowering LDL-C levels and fenofibrate was very effective in lowering TGs and raising HDL-C levels the combination resulted in more favorable changes in TGs. In a similar study the addition of fenofibrate 135mg to atorvastatin 40 mg + ezetimibe 10 mg resulted in a greater reduction in TGs (-57% vs. -40%; p<0.001) and a greater increase in HDL (13% vs. 4.2%; p<0.001) than placebo (174).  Fibrate therapy and ezetimibe/simvastatin + fenofibrate also resulted in a shift of LDL particles from small dense LDL particles to large buoyant particles (173).

 

Table 17. Effect of the Combination of Ezetimibe/Simvastatin and Fenofibrate on Lipid and Lipoprotein Levels

 

LDL-C

HDL-C

TG

Placebo

-3.5%

+1.1

-3.1%

Ezetimibe/Simvastatin

-47%

+9.3%

-29%

Fenofibrate

-16%

+18.2

-41

Eze/Simva + Fenofibrate

-46%

+18.7

-50%

 

BILE ACID SEQUESTRANT  

 

Studies have also examined the effect of fibrates in combination with bile acid sequestrants. Participants receiving fenofibrate 160 mg/day were randomized to receive either colesevelam HCl 3.75 g/day or placebo (175). No significant differences in TG or HDL-C levels were observed between the two groups. However, LDL-C levels were decreased in the fenofibrate + colesevelam group compared to the fenofibrate + placebo group (12.4% greater decrease: p<0.001). A study of the combination of fenofibrate and colestipol also demonstrated a more marked decrease in LDL-C with that combination compared to either drug alone (colestipol -18%; fenofibrate -17%, colestipol + fenofibrate 37%) (176). The combination of both drugs did not blunt the effects of fenofibrate on VLDL and HDL. Other studies of the combination of a fibrate with a bile acid sequestrant have also demonstrated an enhanced effect in lowering LDL-C levels (177-179).

 

NIACIN

 

Surprisingly there are few large randomized trials examining the effect of combination therapy with niacin + fibrate vs. monotherapy. One very small trial did reported that while both niacin monotherapy and bezafibrate monotherapy were effective in lowering serum TGs there was no added benefit of combination therapy in reducing serum TG level although a large variance may have reduced the ability to detect statistically significant results (16). A larger trial in HIV+ patients reported that the combination of niacin and fenofibrate was better at lowering TGs and non-HDL-C and increasing HDL-C levels than monotherapy with either niacin or fenofibrate (17). It would be informative if additional trials of fibrate + niacin combination therapy were carried out in patients with marked hypertriglyceridemia that can often be difficult to control with lifestyle changes and monotherapy.

 

FISH OIL  

 

In patients with TG levels > 500mg/dL the combination of fenofibrate 130mg per day and 4 grams of Lovaza (DHA and EPA) reduced TG levels to a greater extent than monotherapy with fenofibrate alone (7% decrease; P = 0.059) (103). Not unexpectedly, LDL levels were increased to a greater extent with combination therapy (9% increase; p= 0.03). When subjects who had received 8 weeks of fenofibrate monotherapy were treated with Lovaza (DHA and EPA) during the 8-week, open-label extension study TG levels were reduced by an additional 17.5% (P = 0.003). These results indicate that the addition of omega-3-fatty acids to fenofibrate will further decrease TG levels.

 

Mechanisms Accounting for the Fibrate Induced Lipid Effects

 

Fibrates are ligands that bind and activate PPAR alpha, a member of the family of nuclear hormone receptors that are activated by lipids (180,181). PPAR alpha is highly expressed in the liver and other tissues important in fatty acid metabolism. PPAR alpha forms a heterodimer with RXR and together the PPAR alpha:RXR complex when activated binds to the PPAR response elements in a large number of genes and regulates the expression of these genes (180,181). The natural ligands of PPAR alpha are fatty acid derivatives formed during lipolysis, lipogenesis, or fatty acid catabolism (180,181).

 

TRIGLYCERIDES  

 

Fibrates lower plasma TG levels by decreasing VLDL production and by increasing the clearance of TG rich lipoproteins (182,183). The decrease in VLDL production is primarily due to PPAR alpha activation of the beta oxidation of fatty acids, which reduces the substrate available for the synthesis of TGs and the formation of VLDL (180,183). Additionally, a decrease in hepatic fatty acid synthesis may also contribute to the decrease in fatty acids (180,183). The increased clearance of TG rich lipoproteins is due to PPAR alpha stimulating the transcription of lipoprotein lipase, the key enzyme that catabolizes the TGs carried by VLDL and chylomicrons (180,183). In addition, activation of PPAR alpha also inhibits the transcription of APO C-III, which inhibits lipoprotein lipase activity (180,183). A decrease in Apo C-III enhances the clearance of TG rich lipoproteins by increasing lipoprotein lipase activity. Notably, a decrease in Apo C-III also decreases TG levels in patients deficient in lipoprotein lipase indicating that there are multiple mechanisms for its effects on TG metabolism (184). Recent studies suggest that Apo C-III inhibits the uptake of TG rich lipoproteins into the liver by the LDL receptors/ LDLR-related protein 1 axis (185). PPAR alpha activation also increases the transcription of Apo A-V, which would also facilitate the activity of lipoprotein lipase (180).

 

HIGH DENSITY LIPOPROTEINS

 

The increase in HDL induced by fibrates is due to PPAR alpha activation stimulating Apo A-I and A-II transcription (180,183). This leads to the increased production of HDL (182). In addition, a decrease in TG rich lipoproteins may result in a reduction in CETP mediated transfer of cholesterol from HDL to VLDL and of TG from VLDL to HDL (183). This would lead to less TG enrichment of HDL and a decrease in the opportunity of hepatic lipase to remove TG leading to small HDL particles that may be rapidly catabolized.

 

LOW DENSITY LIPOPROTEINS

 

As noted above the effect of fibrates on LDL-C levels is variable with increases in LDL seen in patients with high TG levels (>400mg/dL) and decreases in LDL-C levels in patients with lower TG levels. In patients with modest elevations in plasma TG levels the clearance of LDL is enhanced (182). The mechanism for this enhanced clearance could be due to a decrease in Apo C-III, as increased levels of this protein inhibits LDL receptor activity (185,186). Additionally, the shift from small dense LDL to large buoyant LDL would enhance the uptake of LDL by the LDL receptor (187). In patients with TG levels > 400mg/dL fibrate therapy decreases LDL clearance (182). Prior to treatment, patients with marked hypertriglyceridemia have hypercatabolism of LDL, which is likely due to increased uptake by the reticuloendothelial system (182). This increased clearance is LDL receptor independent. Treatment with fibrates lowers the plasma TGs leading to normalization of reticuloendothelial cell function and a decrease in LDL clearance resulting in an increase in LDL-C levels with fibrate therapy (182). In addition, the metabolism of VLDL to LDL may be enhanced by fibrates when the TG levels are markedly elevated.

 

Effect of Monotherapy with Fibrates on Cardiovascular Outcomes

 

There have been a number of studies that have examined the effect of monotherapy with a variety of different fibrates on cardiovascular disease. We will describe the major studies below.

 

  • Coronary Drug Project (CDP): CDP conducted between 1966 and 1975, was a randomized, double-blind clinical trial that determined the effect of clofibrate (1.8g/day), dextrothyroxine (6mg/day), two doses of oral estrogen (2.5 or 5mg per day), or immediate release niacin (3 grams/day) vs. placebo in 8,341 men aged 30 to 64 years of age with an electrocardiogram documented myocardial infarction on cardiovascular events and mortality (43). The mean baseline total cholesterol level was 251mg/dL and TG level was 183mg/dL. The two estrogen regimens and dextrothyroxine treatment groups were discontinued early because of increased adverse effects. Clofibrate treatment (n= 1,051) compared to placebo (n= 2,680) also did not demonstrate clinical benefit. The five-year mortality in subjects treated with clofibrate was 20.0% as compared with 20.9% in subjects on placebo therapy (P = 0.55). The results with niacin are discussed above in the section on niacin and cardiovascular outcomes.

 

  • WHO: WHO was a double-blind trial in middle-aged men, age 30-59 years of age, without evidence of heart or other major disease, who were treated with 1.6 grams/day clofibrate (n=5,000) or placebo (n=5,000) for an average of 5.3 years (188). Average serum cholesterol levels were approximately 248mg/dL and a mean reduction of approximately 9 per cent occurred in the clofibrate group. The incidence of ischemic heart disease was decreased by 20% in the clofibrate group compared to the control group (P <0.05). This decrease was confined to non-fatal myocardial infarcts which were reduced by 25% while the incidence of fatal heart attacks and angina was similar in the clofibrate and placebo groups. Importantly, the numbers of deaths and crude mortality rates from all causes were increased in the clofibrate-treated group compared to the control group (P < 0.05). The excess deaths were partially accounted for by increased deaths due to liver, biliary tract, and intestinal disease. There was also an increase in cholecystectomies in subjects treated with clofibrate. Because of increased toxicity clofibrate is no longer available.

 

  • Helsinki Heart Study (HHS): HSS was a randomized double-blind trial in middle aged men age 40-55 years of age without cardiovascular who had non-HDL-C levels greater than or equal to 200mg/dL (152). Subjects were randomized to receive 600mg gemfibrozil twice a day (n=2,051) or placebo (n=2,030) for five years. At initiation of the study total cholesterol was 289mg/dL, HDL-C 47mg/dL, non-HDL-C 242mg/dL, and TGs 176mg/dL. Gemfibrozil caused an increase in HDL-C (approximately 10%) and reductions in total (~10%), LDL-C (~11%), non-HDL-C (~14%), and TG levels (~35%). There were minimal changes in serum lipid levels in the placebo group. Fatal and non-fatal myocardial infarctions and cardiac death were the principal end points and the cumulative rate of these cardiac end points were reduced 34% in the gemfibrozil group (27.3 per 1,000 in the gemfibrozil group vs. 41.4 per 1,000 in the placebo group; P< 0.02). The decrease in cardiovascular disease in the gemfibrozil group became evident in the second year and continued throughout the remainder of the study. There was no difference in mortality between the gemfibrozil and placebo groups. The benefit of gemfibrozil therapy was greatest in participants with elevated TGs and decreased HDL-C levels (189,190). Risk reduction with gemfibrozil was 78% (P = .002) among those with BMI > 26 kg/m2 and dyslipidemia (TGs > ~200mg/dL and HDL-C < 42mg/dL) suggesting that certain types of patients are likely to derive greater benefit from fibrate treatment (191).

 

  • Veterans Affairs High-Density Lipoprotein Cholesterol Intervention Trial (VA-HIT): VA-HIT was a double-blind trial in men with coronary heart disease who had an HDL-C level <40mg/dL and LDL-C level <140mg/dL (153). Subjects were randomized to gemfibrozil 1200mg per day (n=1,264) or placebo (n=1,267) for 5.1 years. Mean lipid levels at study initiation were HDL-C 32mg/dL, LDL-C 111mg/dL, total cholesterol 175mg/dL, and TGs 160mg/dL. At one year, the mean HDL-C level was 6 percent higher, the mean TG level was 31 percent lower, and the mean total cholesterol level was 4 percent lower in the gemfibrozil group than in the placebo group. LDL-C levels did not differ significantly between the groups. The primary study outcome was nonfatal myocardial infarction or death from coronary causes. The primary outcome occurred in 21.7% of patients in the placebo group and 17.3% of patients in the gemfibrozil group (22 percent decrease; P=0.006). A 24% reduction in the combined outcome of death from coronary heart disease, nonfatal myocardial infarction, and stroke was observed in the gemfibrozil group (P< 0.001). There were no significant differences in the rates of coronary revascularization, hospitalization for unstable angina, death from any cause, and cancer. Similar to HHS the beneficial effect of gemfibrozil did not become apparent until approximately two years after treatment. A low HDL-C (<33.5mg/dL) and high TGs (>180mg/dL) at baseline predicted a beneficial response to gemfibrozil therapy (192).

 

  • Bezafibrate Infarction Prevention Study (BIP): BIP was a double-blind study in male and female patients aged 45-74 with a previous myocardial infarction or stable angina (154). Patients were randomized to receive either 400 mg of bezafibrate per day (n=1,548) or a placebo (n=1,542) and were followed for 6.2 years. At the initiation of the study total cholesterol was 212mg/dL, LDL-C was 148mg/dL, HDL-C was 34.6mg/dL, and TGs were145mg/dL. Bezafibrate increased HDL-C by 18% and reduced TGs by 21%. There was a small 7% decrease in LDL-C. The primary end point was fatal or nonfatal myocardial infarction or sudden death. The primary end point occurred in 13. 6% of the bezafibrate group vs. 15.0% of the placebo (9.4% reduction; P=0.26). Total and non-cardiac mortality rates were similar. In a post hoc analysis in the subgroup with high baseline TGs (> or =200 mg/dL), the reduction in the primary end point in the bezafibrate group was 39.5% (P=0.02). Additionally, bezafibrate reduced cardiovascular events in patients with the metabolic syndrome (193). These results again suggest that patients with high TGs are likely to derive benefit from fibrate therapy.

 

  • Leader Trial: The Leader trial was a double blind placebo controlled randomized trial in men age 35 to 92 with lower extremity arterial disease (194,195). Subjects were randomized to bezafibrate 400mg per day (n=783) or placebo (n=785). At baseline total cholesterol levels were 218mg/dL, LDL-C levels 132mg/dL, HDL-C levels 44mg/dL, and TGs 187mg/dL. Bezafibrate therapy reduced total cholesterol levels by 7.6%, LDL-C by 8.1%, and TGs by 23% and increased HDL-C levels by 8%. The primary endpoint of coronary heart disease and strokes was not reduced by bezafibrate treatment. Neither major coronary events nor strokes were significantly reduced.

 .

  • Fenofibrate Intervention and Event Lowering in Diabetes Trial (FIELD): In the FIELD Trial patients with Type 2 diabetes between the ages of 50 and 75 with or without pre-existing cardiovascular disease not taking statin therapy were randomized to fenofibrate 200 mg daily (n=4,895) or placebo (n=4,900) and followed for approximately 5 years (156). At initiation of the study total cholesterol was 196mg/dL, LDL-C was 120mg/dL, HDL-C was 43mg/dL, and TGs were 152mg/dL. Fenofibrate therapy resulted in an 11% decrease in total cholesterol, a 12% decrease in LDL-C, a 29% decrease in TGs, and a 5% increase in HDL-C levels. The primary outcome was coronary events (coronary heart disease death and non-fatal MI), which were reduced by 11% in the fenofibrate group but this difference did not reach statistical significance (p= 0.16). However, there was a 24% decrease in non-fatal MI in the fenofibrate treated group (p=0.01) and a non-significant increase in coronary heart disease mortality. Total cardiovascular disease events (coronary events plus stroke and coronary or carotid revascularization) were reduced 11% (p=0.035). These beneficial effects of fenofibrate therapy on cardiovascular disease were observed in patients without a previous history of cardiovascular disease. In patients with a previous history of cardiovascular disease no benefits were observed. Additionally, the beneficial effect of fenofibrate therapy was seen only in those subjects less than 65 years of age. The beneficial effects of fenofibrate in this study may have been blunted by the increased use of statins in the placebo group, which reduced the differences in lipid levels between the placebo and fenofibrate groups. If one adjusted for the addition of lipid-lowering therapy, fenofibrate reduced the risk of coronary heart disease events by 19% (p=0.01) and of total cardiovascular disease events by 15% (p=0.004). Additionally, many patients in the Field trial did not have elevations in TGs and decreased HDL-C levels. In a post hoc analysis, patients with high TGs 200mg/dL) and low HDL levels (<40mg for men and <50mg/dL for women) derived greater benefit from fenofibrate therapy (196).

 

  • Summary: While the above monotherapy fibrate studies suggest that fibrates reduce cardiovascular event, particularly in patients with high TG and low HDL levels, the results are not as robust or consistent as the beneficial effects of statins on cardiovascular outcomes (5).

 

Effect of Combination Therapy of Fibrates and Statins on Cardiovascular Outcomes

 

Given the marked benefits of statin therapy it is essential to know if adding fibrates to statin therapy further reduces cardiovascular events. Two large trials described below have addressed this key question.

 

  • ACCORD LIPID Trial: The ACCORD-LIPID Trial was designed to determine if the addition of fenofibrate to aggressive statin therapy would result in a further reduction in cardiovascular disease in patients with Type 2 diabetes (197). In this trial, 5,518 patients on statin therapy were randomized to placebo or fenofibrate therapy. The patients had diabetes for approximately 10 years and either had pre-existing cardiovascular disease or were at high risk for developing cardiovascular disease. During the trial, LDL-C levels were approximately 80mg/dL. There was only a small difference in HDL-C with the fenofibrate groups having a mean HDL-C of 41.2mg/dL while the control group had an HDL-C of 40.5mg/dL. Differences in TG levels were somewhat more impressive with the fenofibrate group having a mean TG level of 122mg/dL while the control group had a TG level of 144mg/dL. First occurrence of nonfatal myocardial infarction, nonfatal stroke, or death from cardiovascular causes was the primary outcome and there was no statistical difference between the fenofibrate treated group and the placebo group. Additionally, there were also no statistically significant differences between the groups with regards to any of the secondary outcome measures of cardiovascular disease. Of note, the addition of fenofibrate to statin therapy did not result in an increase in either muscle or liver side effects. On further analysis there was a suggestion of benefit with fenofibrate therapy in the patients in whom the baseline TG levels were elevated (>204mg/dL) and HDL-C levels decreased (<34mg/dL). While this was a negative study, it must be recognized that most of the patients included in this study did not have the lipid profile that would typically lead to treatment with fibrates.

 

  • PROMINENT Trial: The PROMINENT trial studied the effect of pemafibrate, a new selective PPAR-alpha activator, in reducing cardiovascular outcomes in 10,497 patients (66.9% with previous ASCVD) with diabetes (198). This was a double-blind, randomized, controlled trial, in patients with Type 2 diabetes, with mild-to-moderate hypertriglyceridemia (TG level, 200 to 499 mg/dL), LDL-C < 100mg/dL, and HDL-C levels < 40 mg/dL who received either pemafibrate (0.2-mg tablets twice daily) or placebo in addition to statin therapy (96% on statins). The primary end point was a composite of nonfatal MI, ischemic stroke, coronary revascularization, or death from cardiovascular causes. Baseline fasting TG was 271 mg/dL, HDL-C 33 mg/dL, and LDL-C 78 mg/dL. Compared with placebo, pemafibrate decreased TG by 26.2%, while HDL-C increased 5.1% and LDL-C increased 12.3%. Notably non-HDL-C levels were unchanged and Apo B levels increased 4.8%. The primary endpoint was similar in the pemafibrate and placebo group (HR 1.03; 95% CI 0.91 to 1.15). The increase in LDL-C and Apo B levels may have accounted for the failure to reduce cardiovascular events.

 

  • Summary: The results of the ACCORD and PROMINENT trials were disappointing and have greatly reduced the enthusiasm for adding fibrates to statin therapy to cardiovascular events.

 

Effect of Fibrates on Non-Cardiovascular Outcomes

 

DIABETIC RETINOPATHY

 

Small studies in the 1960’s presented suggestive evidence that treatment with clofibrate improved diabetic retinopathy (199,200). Randomized trials have confirmed these observations.

 

The Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) study was a randomized trial in patients with Type 2 diabetes mellitus. Patients were randomly assigned to receive either fenofibrate 200 mg/day (n=4,895) or placebo (n=4,900). Laser treatment for retinopathy was significantly lower in the fenofibrate group than in the placebo group (3.4% patients on fenofibrate vs 4.9% on placebo; p=0.0002) (201). Fenofibrate therapy reduced the need for laser therapy to a similar extent for maculopathy (31% decrease) and for proliferative retinopathy (30% decrease). In the ophthalmology sub-study (n=1,012), the primary endpoint of 2-step progression of retinopathy grade did not differ significantly between the fenofibrate and control groups (9.6% patients on fenofibrate vs 12.3% on placebo; p=0.19). In patients without pre-existing retinopathy there was no difference in progression (11.4% vs 11.7%; p=0.87). However, in patients with pre-existing retinopathy, significantly fewer patients on fenofibrate had a 2-step progression than did those on placebo (3.1% patients vs 14.6%; p=0.004). A composite endpoint of 2-step progression of retinopathy grade, macular edema, or laser treatments was significantly reduced in the fenofibrate group (HR 0.66, 95% CI 0.47-0.94; p=0.022).

 

In the ACCORD Lipids Study a subgroup of participants were evaluated for the progression of diabetic retinopathy by 3 or more steps on the Early Treatment Diabetic Retinopathy Study Severity Scale or the development of diabetic retinopathy necessitating laser photocoagulation or vitrectomy over a four-year period (202). At 4 years, the rates of progression of diabetic retinopathy were 6.5% with fenofibrate therapy (n=806) vs. 10.2% with placebo (n=787) (adjusted odds ratio, 0.60; 95% CI, 0.42 to 0.87; P = 0.006). Of note, this reduction in the progression of diabetic retinopathy was of a similar magnitude as intensive glycemic treatment vs. standard therapy.

 

A double-blind, randomized, placebo-controlled study in 296 patients with type 2 diabetes mellitus evaluated the effect of placebo or etofibrate on diabetic retinopathy (203). After 12 months an improvement in ocular pathology was more frequent in the etofibrate group vs the placebo group ((46% versus 32%; p< 0.001).

 

The MacuFen study was a small double-blind, randomized, placebo-controlled study in 110 subjects with diabetic macular edema who did not require immediate photocoagulation or intraocular treatment (204). Patients were randomized to fenofibric acid or placebo for 1 year. Patients treated with fenofibric acid had a modest improvement in total macular volume that was not statistically significant compared to the placebo group.

 

Taken together these results indicate that fibrates have beneficial effects on the progression of diabetic retinopathy (205). The mechanisms by which fibrates decrease diabetic retinopathy are unknown, and whether decreases in serum TG levels plays an important role is uncertain. Fibrates activate PPAR alpha, which is expressed in the retina (206). Diabetic PPARα KO mice developed more severe DR while overexpression of PPARα in the retina of diabetic rats significantly alleviated diabetes-induced retinal vascular leakage and retinal inflammation, suggesting that fibrates could have direct effects on the retina to reduce diabetic retinopathy (206).

 

DIABETIC KIDNEY DISEASE

 

The Diabetes Atherosclerosis Intervention Study (DAIS) evaluated the effect of fenofibrate therapy (n= 155) vs. placebo (n=159) on changes in urinary albumin excretion in patients with Type 2 diabetes (207). Fenofibrate significantly reduced the worsening of albumin excretion (fenofibrate 8% vs. placebo 18%; P < 0.05). This effect was primarily due to reduced progression from normal albumin excretion to microalbuminuria (fenofibrate 3% vs. 18% placebo; P < 0.001).

 

 In the Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) trial, Type 2 diabetic patients aged 50 to 75 years were randomly assigned to fenofibrate (n = 4,895) or placebo (n = 4,900) for 5 years (208). Fenofibrate reduced urine albumin/creatinine ratio by 24% vs 11% in placebo group (p < 0.001), with 14% less progression and 18% more albuminuria regression (p < 0.001) in the fenofibrate group than in participants on placebo. As expected, fenofibrate therapy acutely increased plasma creatinine levels and decreased eGFR (209). However, over the long-term, the increase in plasma creatinine was lower in the fenofibrate group compared to the placebo group (14% decrease; p=0.01). Similarly, there was a slower annual decrease in eGFR in the fenofibrate group (1.19 vs 2.03 ml/min/1.73 m2annually, p < 0.001). End-stage renal disease, dialysis, renal transplant, and renal death were similar in the fenofibrate and placebo groups, likely due to the small number of events.

 

In the ACCORD-LIPID trial, 5,518 patients on statin therapy were randomized to placebo or fenofibrate therapy (197). The patients had diabetes for approximately 10 years and either had pre-existing cardiovascular disease or were at high risk for developing cardiovascular disease. The post-randomization incidence of microalbuminuria was 38.2% in the fenofibrate group and 41.6% in the placebo group (p=0.01) and post-randomization incidence of macroalbumuria was 10.5% in the fibrate group and 12.3% in the placebo group (p=0.04) indicating a modest reduction in the development of proteinuria in patients treated with fenofibrate (197). There was no significant difference in the incidence of end-stage renal disease or need for dialysis between the fenofibrate group and the placebo group, likely due to the small number of events.

 

A small randomized study in patients with Type 2 diabetes and hypertriglyceridemia compared the effect of fenofibrate (200mg/day) (n=28) vs. no treatment (n=28) on urinary albumin excretion (210). After 180 days urinary albumin/creatine ratio was decreased in the fenofibrate group vs. controls (control -8.15 vs fenofibrate -44.05 mg/g; P<0.05).

 

These studies suggest that fibrates may have a beneficial effect on diabetic kidney disease (211). One should recognize that reducing proteinuria is a surrogate marker and may not indicate a reduction in the development of end stage renal disease. The mechanisms accounting for the decrease in proteinuria are unknown.

 

AMPUTATIONS

 

In the Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) study, patients aged 50-75 years with Type 2 diabetes were randomly assigned to receive fenofibrate 200 mg per day (n=4,895) or matching placebo (n=4,900) for 5 years' duration (212). The risk of first amputation was decreased by 36% (p=0.02) and minor amputation events without known large-vessel disease by 47% (p=0.027) in the fenofibrate treated group (212). The reduction in amputations was independent of glucose control or dyslipidemia. No difference between the risks of major amputations was seen in the placebo and fenofibrate groups. The basis for this reduction in amputations is unknown.

 

GOUT

 

In the Field trial treatment, fenofibrate reduced uric acid levels by 20% and reduced episodes of gout by approximately 50% compared to placebo (HR 0·48, 95% CI 0·37-0·60; p<0·0001) (213). Interestingly, a meta-analysis of fibrate trials found that fenofibrate but not bezafibrate reduced serum uric acid levels suggesting that the reduction in uric acid levels is not a class effect (214).

 

SUMMARY

 

The above studies provide substantial evidence that fibrates have a favorable effect on diabetic microvascular disease (155). While fibrates are not approved specifically for the prevention or treatment of diabetic microvascular disease one should consider these potential beneficial effects when deciding on treatment choices. For example, in a patient with diabetes and microvascular disease and hypertriglyceridemia needing therapy one might elect to use fibrates to lower plasma TGs given their potential beneficial effects on slowing the progression of microvascular disease. 

 

Side Effects

 

RENAL

 

Fibrate therapy leads to an increase in serum creatinine and cystatin C levels (215-217). For example, in the Field Trial serum creatinine levels increased from 0.88mg/dL to 0.99mg/dL, a 12% increase (156). This increase in creatinine has been seen with all fibrates but appears to be less profound with gemfibrozil (215). The increase in cystatin C occurs with fenofibrate but not with other fibrates (216). It must be recognized that this increase in creatinine is reversible on stopping fibrate therapy and does not reflect kidney damage (215). In fact, careful measurements of renal function have not demonstrated a decrease in glomerular filtration rate despite the increase in serum creatinine (209,218,219). As discussed above, studies of renal function in patients with diabetes actually suggests that treatment with fibrates may be protective. The precise mechanism by which fibrates increase serum creatinine levels is unknown.

 

In patients with chronic renal disease fibrates should be used with caution and at lower doses (215). Fibrates are all excreted by the kidneys and thus the excretion of fibrates is decreased in patients with renal dysfunction (215). Therefore, one needs to adjust the fibrate dose depending upon renal function. The National Kidney Foundation recommends the dose adjustments shown in Table 18 (220).

 

Table 18. Fibrate Dose Adjustments in Renal Disease

 

No Kidney Disease

GFR 30-60

GFR < 30

Kidney Transplant

Bezafibrate

400-600mg

200mg

Avoid

Avoid

Ciprofibrate

1000-2000mg

?

Avoid

Avoid

Fenofibrate

150-200mg

40-60mg

Avoid

Avoid

Gemfibrozil

1200mg

1200mg

600mg

600mg

 

GALLBLADDER DISEASE

 

It is clear that clofibrate increases the risk of gallbladder disease. In both the WHO trial and the Coronary Drug Project, cholecystectomies occurred two to three times more often in the patients treated with clofibrate compared to placebo (43,188,221). Whether gemfibrozil, fenofibrate, or other fibrates increases the risk of gallbladder disease is uncertain. In the large randomized outcome studies presented earlier (Effect of fibrates on cardiovascular outcomes section) a statistically significant increase in either gallbladder disease or cholecystectomies were not observed. However, in a sub-study of 450 Helsinki Heart Study participants a trend toward a greater prevalence of gallstones during the study in the gemfibrozil group was observed (7.5% versus 4.9% for the placebo group, a 55% excess for the gemfibrozil group) (Lopid Package Insert). A trend toward a greater incidence of gallbladder surgery was also observed in the gemfibrozil group (17 versus 11 subjects, a 54% excess) (Lopid Package Insert). In a single epidemiological trial fibrate treatment independently correlated with the presence of gallstones with a relative risk of 1.7 (p=0.04) (222).

 

All fibrates alter the composition of bile resulting in an increase in the concentration of cholesterol, which will predispose to the formation of cholesterol gallstones (215). In a comparison of clofibrate and gemfibrozil it was observed that clofibrate resulted in changes in bile composition that would be more lithogenic than gemfibrozil (223).

 

The effect of combining fibrates with statins on the risk of gallbladder disease is unknown.  An increased risk of gallbladder disease or cholecystectomies was not reported in the ACCORD-LIPID trial where fenofibrate was added to statin therapy or the PROMINENT trial where pemafibrate was added to statin therapy (197,198).

 

While it is clear that clofibrate increases the risk of gallbladder disease the effect of other fibrates either as monotherapy or in combination with other drugs is less well defined.

 

PANCREATITIS  

 

In a meta-analysis of 7 fibrate trials involving 40,162 participants conducted over 5.3 years, 144 participants developed pancreatitis (84 assigned to fibrate therapy, 60 assigned to placebo) (RR, 1.39 (95% CI, 1.00-1.95; P = .053) (224). These observations raise the possibility that fibrates may increase the risk of pancreatitis.

 

CANCER

 

A large meta-analysis of 17 randomized controlled trials, involving 44,929 participants, with an average follow-up of 5.2 years has examined if fibrates lead to an increased risk of cancer. No increase in either cancer incidence (RR = 1.02, 95% CI 0.92-1.12) or cancer death (RR = 1.06, 95% CI: 0.92-1.22) was noted with fibrate treatment (225).

 

LIVER DISEASE

 

Fenofibrate has rarely been associated with idiosyncratic hepatotoxicity manifesting as hepatocellular to cholestatic disorders (226). The hepatitis may be acute self-limited or persistent chronic hepatitis. Liver abnormalities are very rare and in large trials such as the FIELD trial described above liver function test abnormalities were similar in the fenofibrate and placebo groups (156).   

 

GLYCEMIC PARAMETERS

 

A meta-analysis of 22 randomized placebo-controlled trials involving a total of 11,402 subjects demonstrated that fibrate therapy significantly decreased fasting plasma glucose, insulin levels, and insulin resistance measured by HOMA-IR, but did not effect HbA1c levels (227).

 

MUSCLE DISORDERS

 

Fibrate monotherapy has been reported to cause myopathy (215). In a large epidemiological study the incidence of hospitalization for rhabdomyolysis per 10,000 person-years for monotherapy with a fibrate was 2.82 (95% CI, 0.58-8.24) while in patients not exposed to lipid lowering drugs the incidence was 0 (95% CI, 0-0.48) (228). The risk of rhabdomyolysis was greater with gemfibrozil therapy than with fenofibrate. Interestingly the incidence of rhabdomyolysis was greater for patients treated with fibrate monotherapy than for patients treated with statin monotherapy (incidence for atorvastatin, pravastatin, or simvastatin was only 0.44 per 10,000 person-years). In an epidemiological study focusing on myopathy similar results were observed (229). The relative risks of myopathy in current users of fibrates and statins compared with nonusers were 42.4 (95% CI = 11.6-170.5) and 7.6 (95% CI = 1.4-41.3), respectively. It should be recognized though that in large randomized clinical trials the risk of muscle symptoms was low in patients treated with fibrates and not dissimilar to that seen in the patients treated with placebo (215). For example, in the Helsinki Heart Study over 2,000 patients were treated and in the VA-HIT over 1,000 patients were treated with gemfibrozil for five years and no cases of  myopathy were reported in either trial (152,153). In the Bezafibrate Infarction Prevention Study, seven patients in the placebo group and five patients in the bezafibrate group reported muscle pain, while CPK levels greater than 2x the upper range of normal was seen in four patients in the bezafibrate group and one patient in the placebo group (154). Finally, in the Field Trial, patients with diabetes were treated with fenofibrate (n=4,895) or placebo (n=4,900) (156). Myositis was observed in one patient treated with placebo and two patients treated with fenofibrate while rhabdomyolysis was observed in one patient treated with placebo and three patients treated with fenofibrate. Elevations in CPK levels values > 10x the upper range of normal were seen in three patients on placebo and 4 patients treated with fenofibrate. Thus, while fibrates can lead to significant muscle dysfunction this is a rare event and appears to occur only slightly more often in patients treated with a fibrate than in patients treated with a placebo. The risk of serious muscle disease appears to be increased in patients with renal failure, hypothyroidism, and in the elderly (215). The mechanism by which fibrates predispose to muscle disorders is unknown.

 

The effect of fibrates in combination with statins on muscle disorders will be discussed in detail in the section on drug interactions below.

 

Drug Interactions

 

STATINS

 

The combination a fibrate and a statin may increase the risk of developing muscle symptoms (215). The degree of risk is dependent on both the specific statin and the specific fibrate that is being used in combination (215). For example, the average incidence per 10,000 person-years for hospitalization for rhabdomyolysis with monotherapy with atorvastatin, pravastatin, or simvastatin was 0.44 (95 % CI, 0.20-0.84); with fibrate alone was 2.82 (95% CI, 0.58-8.24); and with combined therapy of atorvastatin, pravastatin, or simvastatin with a fibrate was 5.98 (95% CI, 0.72-216.0) (228). Of note, the average incidence per 10,000 person-years for hospitalization for rhabdomyolysis with the combination of cerivastatin with a fibrate was 1035 (95% CI, 389-2117), clearly demonstrating an increased risk of the cerivastatin-fibrate combination compared to other statin-fibrate combinations (228). A study by Alsheikh-Ali and colleagues looking at cases of rhabdomyolysis reported to the FDA relative to the total number of prescriptions reached the conclusion that the combination of cerivastatin with a fibrate markedly increased the risk of this complication (230). Additionally, it was noted that the risk of rhabdomyolysis was greater with gemfibrozil compared to fenofibrate and that the combination of cerivastatin and gemfibrozil was particularly toxic (230). Other studies have also noted a marked risk with the combination of cerivastatin and gemfibrozil (231). Cerivastatin is no longer available.

 

Studies comparing the risk of rhabdomyolysis with gemfibrozil-statin combination therapy compared to fenofibrate-statin combination therapy have shown an increased risk with gemfibrozil (215). For example, the number of cases of rhabdomyolysis reported with fenofibrate and statins other than cerivastatin was 0.58 per million prescriptions whereas with gemfibrozil and statins other than cerivastatin was 8.6 per million prescriptions (232). Reviews of the FDA’s adverse events reporting system database have estimated that the risk of myopathy for the combination of gemfibrozil with a statin was much greater than the risk with the combination of fenofibrate with a statin (230,232).  Additionally, studies that employed the combination of gemfibrozil and statins have reported a significant occurrence of muscle related symptoms whereas studies of fenofibrate in combination with statins have not shown an increase in muscle related symptoms (215). For example, the rate of myopathy in over 4,000 patients taking lovastatin was only 0.4% but in patients on the combination of lovastatin and gemfibrozil the frequency increased to 5% (233). In contrast, in the ACCORD-LIPID Trial over 5,000 patients on statin therapy were randomized to fenofibrate or placebo for 4.7 years and no increase in the incidence of muscle related symptoms was observed with fenofibrate therapy (197). Similarly, in the Field Trial approximately 1,000 patients were taking fenofibrate and a statin and with 5 years of follow-up no cases of rhabdomyolysis were reported (156). Finally, a meta-analysis by Geng and colleagues identified 13 randomized trials with 7,712 patients receiving combination fenofibrate-statin therapy compared with statin therapy alone (168). The incidence of elevated creatine kinase levels, muscle-associated adverse events, or withdrawals attributed to muscle dysfunction did not differ significantly between the fenofibrate + statin patients vs. the statin alone patients (168). The American College of Cardiology and American Heart Association Guidelines recommend against using the combination of a statin and gemfibrozil but recognize that the use of a statin and fenofibrate is appropriate under certain circumstances (234).

 

The increased risk of combining gemfibrozil with statins is due to alterations in statin metabolism leading to increases in the serum levels of statins and hence an increased risk of myopathy (215,235). In contrast, fenofibrate does not alter statin metabolism and therefore can be safely combined with statins (Table 19) (235).   

 

Table 19. Effect of Fibrates on Statin Pharmacokinetics (215,235,236)

Statin

Gemfibrozil

Fenofibrate

Atorvastatin

Increase in C-Max by 1.5-Fold

No Change

Simvastatin

Increase in C-Max by 2-Fold

No Change

Pravastatin

Increase in C-Max by 2-Fold

No Change

Rosuvastatin

Increase in C-Max by 2-Fold

No Change

Lovastatin

Increase in C-Max by 2.8-Fold

No Change

Pitavastatin

Increase in C-Max by 41%

Unknown

Fluvastatin

No Change

No Change

  

The explanation for the difference between gemfibrozil and fenofibrate is that gemfibrozil uses the same family of glucuronidation enzymes as the statins thereby inhibiting statin metabolism (215,237). In contrast, fenofibrate uses a different family of glucuronidation enzymes and does not inhibit statin metabolism (215).

 

COUMADIN ANTI-COAGULANTS

 

Gemfibrozil and fenofibrate can potentiate the effect of coumadin anti-coagulants leading to a prolongation of prothrombin time and an increased risk of bleeding. When starting a fibrate in patients on coumadin therapy the dose of coumadin should be decreased and prothrombin times should be closely monitored (Lopid and Tricor Package Inserts).    

 

REPAGLINIDE

 

Gemfibrozil in combination with rapaglinide increases blood levels of rapaglinide and therefore this combination should not be used because of the increased risk of hypoglycemia (Lopid Package Insert).

 

Contraindications

 

Fibrates are contraindicated in patients with severe hepatic dysfunction. Additionally, patients with pre-existing gallstones should not be treated with fibrates. Fenofibrate and gemfibrozil are pregnancy category C drugs and should only be used if the potential benefit justifies the potential risk to the fetus. The combination of gemfibrozil and a statin should be avoided.

 

Conclusions

 

Fibrates are effective drugs in reducing TG levels and modestly increase HDL-C levels. Additionally, they also reduce LDL-C and non-HDL-C levels. Fibrates have a number of side effects and one should avoid using gemfibrozil in combination with statins. In contrast, fenofibrate can be used in combination with statins. Studies have not consistently demonstrated that fibrate monotherapy therapy reduces cardiovascular events and the combination of fibrates and statins in two studies has not been shown to be beneficial. Therefore enthusiasm to use fibrates to reduce cardiovascular events has markedly diminished. In patients with diabetes fibrates appear to slow the progression of microvascular disease. Finally, fibrates are effective in lowering TGs in patients with marked hypertriglyceridemia and while not proven will likely reduce the risk of the development of pancreatitis.

 

VOLANESORSEN

 

Introduction 

 

Volanesorsen (Waylivra) is an antisense oligonucleotide inhibitor of apolipoprotein C-III (apo C-III) mRNA that is approved in Europe for the treatment of familial chylomicronemia syndrome (FCS). This drug has not been approved by the FDA for use in the United States. FCS is a rare metabolic disorder involving the impaired function of lipoprotein lipase (LPL) due to mutations in LPL, Apo C-II, Apo A-V, lipase maturation factor 1, and glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) (238,239). For a detailed discussion of the diagnosis and treatment of FCS see the following references (238-240).

 

Effect of Volanesorsen on Lipid and Lipoprotein Levels

 

FAMILIAL CHYLOMICRONEMIA SYNDROME (FCS)

 

A double-blind, randomized 52-week trial (APPROACH study) evaluated the ability of volanesorsen (300 mg subcutaneously once weekly) vs. placebo to decrease TG levels in 66 patients with FCS (baseline TGs 2,209mg/dL) (241). The primary end point was the percentage change in fasting TG levels at 3 months. As expected, there was a marked reduction in Apo C-III levels (84% decrease) in the volanesorsen group and a small increase (6%) in the placebo group. Most importantly patients treated with volanesorsen had a 77% decrease at 3 months in TG levels (mean decrease of 1,712 mg/dL) whereas patients receiving placebo had an 18% increase in TG levels. The decrease in TGs in patients treated with volanesorsen persisted for 24 months (242). Significantly, 77% of the patients in the volanesorsen group vs. only 10% of patients in the placebo group had TG levels of less than 750 mg/dL, a level that would greatly reduce the risk of pancreatitis. In addition, patients who received volanesorsen had decreases in levels of chylomicron TG by 83%, apolipoprotein B-48 by 76%, non–HDL-C by 46%, and VLDL-C by 58% and increases in levels of HDL-C by 46%, apolipoprotein A1 by 14%, LDL-C by 136% (note LDL-C increased from 28 to 61 mg/dL), and total apolipoprotein B by 20%.

 

While the APPROACH study was not powered to examine the effect of volanesorsen on pancreatitis, during the study three patients in the placebo group had four episodes of acute pancreatitis, whereas one patient in the volanesorsen group had one episode. In patients with a history of recurrent pancreatitis events (≥ 2 events in the 5 years prior to study, n = 11), a reduction in pancreatitis attacks was seen in patients treated with volanesorsen compared with placebo (none of the 7 patients in the volanesorsen group and 3 of the 4 patients in the placebo group experienced a pancreatitis attack over the 52-week study period).

 

In a retrospective global web-based survey open to all patients with the FCS who received volanesorsen for ≥3 months, 22 patients responded and reported reductions in steatorrhea, pancreatic pain, and constant worry about an attack of pain/ acute pancreatitis (243). The patients also reported that volanesorsen improved overall management of symptoms and reduced interference of FCS with work/school responsibilities. Decreases in the negative impact of FCS on personal, social, and professional life were also reported.

 

HYPERTRIGLYCERIDEMIA

 

A randomized, double-blind, placebo-controlled, study evaluated volanesorsen in patients with hypertriglyceridemia (244). Patients who were not receiving TG-lowering therapy (n=57) were eligible if they had fasting TG level between 350 mg/dL and 2000 mg/dL and were assigned to volanesorsen 100, 200, or 300 mg or placebo. Patients who were receiving a fibrate (n=28) were eligible if they had a fasting TG level between 225 mg/dL and 2000 mg/dL and were randomly assigned to volanesorsen 200 or 300 mg or placebo. The study drug was administered as a single subcutaneous injection once a week for 13 weeks. Baseline TG levels were 581±291 mg/dL in patients not on fibrates and 376±188 mg/dL in patients on fibrates. In patients not on fibrates volanesorsen 300 mg decreased Apo C-III levels by 79.6% vs. an increase of 4.2% in the placebo group (P<0.001) and decreased TG levels by 70.9% compared with an increase of 20.1% in the placebo group (P<0.001). Additionally, HDL-C levels increased by 45.7% from baseline in the 300 mg group, as compared with an increase of 0.7% in the placebo group (P<0.001). LDL-C levels increased from 79.5±29.9 mg/dL to 127.8±44.9 mg/dL with 300 mg of volanesorsen and was associated with an increase in LDL particle size. However, non-HDL-C and total apo B levels remained relatively unchanged and similar to those in the placebo group. Similar changes in Apo C-III, TGs, HDL-C, non-HDL-C, VLDL-C, and total apo B levels were observed in the patients on fibrates treated with volanesorsen. Of note, LDL-C levels did not increase in the patients on fibrates treated with volanesorsen perhaps due to the lower baseline TG levels. 

 

The COMPASS study randomized 113 patients with fasting TGs ≥500 mg/dL (mean TG 1,261mg/dL) to receive either volanesorsen 300 mg or placebo subcutaneously once weekly for 26 weeks (245). Most of these patients had the multifactorial chylomicronemia syndrome but a small number had FCS. A 71% reduction in TGs from baseline after 3 months was observed in patients treated with volanesorsen vs. a 0.9% reduction in placebo-treated patients (P<0.0001). LDL-C levels increased 96% (64 to 111mg/dL), HDL-C increased 61% (25 to 39mg/dL) and non-HDL-C decreased 27% (232 to 158mg/dL) Notably pancreatitis episodes were reduced with 5 events in 3 patients occurring in the placebo group vs. none with volanesorsen treatment (P=0.036). 

 

DIABETES

 

A randomized, double-blind, placebo-controlled trial of volanesorsen 300 mg weekly or placebo was performed in 15 adult patients with type 2 diabetes (HbA1c >7.5%) and hypertriglyceridemia (TG >200 and <500 mg/dL) (246). Treatment with volanesorsen significantly reduced plasma apo C-III (-88%, P = 0.02) and TG (-69%, P = 0.02) levels and raised HDL-C (42%, P = 0.03) without altering LDL-C levels compared with placebo. These changes were accompanied by a 57% improvement in whole-body insulin sensitivity (P < 0.001) and decreases in HbA1c (-0.44%, P = 0.025) 3 months postdosing. The improvement in insulin sensitivity was strongly related to the decrease in plasma apo C-III and TGs.

 

FAMILIAL PARTIAL LIPODYSTROPY (FPL)

 

Patients with FPL were randomized to volanesorsen 300mg weekly (n=21) or placebo (n=19) (247). Median TG level was 781mg/dL in the placebo group and 749mg/dL in the volanesorsen group. Volanesorsen treatment at 3 months resulted in an 88% decrease in TG levels while in the placebo group TG levels decreased by 22% (net difference of −67%; P=0.0009). Non-HDL-HDL-C levels decreased while LDL-C and HDL-C levels increased.

 

Mechanisms Accounting for the Volanesorsen Induced Lipid Effects

 

Volanesorsen binds to apo C-III mRNA leading to increased degradation and thereby inhibits the hepatic synthesis of apo C-III protein resulting in a reduction in plasma apo C-III levels (248,249). Apo C-III has a number of important effects on the metabolism of TG rich lipoproteins (250). Apo C-III is an inhibitor of LPL and therefore decreasing apo C-III levels will enhance LPL activity. In patients with FCS this will not be important because patients with this disorder have defects in components of the LPL complex that result in the inability to increase LPL activity. However, in patients with increased TG levels not due FCS this would accelerate the clearance of TG rich lipoproteins. Studies have also shown that apo C-III stimulates the production and secretion of VLDL by the liver. This effect is also not likely to be of primary importance in patients with FCS as the very high TG levels are primarily due to chylomicrons and not VLDL. However, in other situations increased hepatic secretion of VLDL may be an important contributor to the hypertriglyceridemia. Whether apo C-III regulates chylomicron secretion by the intestine is unknown. Finally, Apo C-III inhibits the binding of TG rich lipoproteins to hepatic LDL receptors and LDL receptor–related protein 1 decreasing the clearance of TG rich lipoprotein particles. A decrease in apo C-III will accelerate the clearance of TG rich lipoproteins, which likely accounts for the ability of volanesorsen to decrease TG levels in patients with FCS.

 

Drug Administration and Pharmacokinetics

 

The recommended starting dose is 285 mg injected subcutaneously once weekly for 3 months after which the dose should be reduced to 285 mg every 2 weeks. If serum TGs decrease by less than 25% or are not below 2000 mg/dL (22.6 mmol/L) after 3 months on volanesorsen 285 mg weekly, treatment should be discontinued (package insert;https://www.ema.europa.eu/en/documents/product-information/waylivra-epar-product-information_en.pdf).

 

After 6 months of treatment one can consider increasing the dose frequency back to 285 mg weekly if the serum TG response has been inadequate and the platelet counts are in the normal range. Patients should return to 285 mg every 2 weeks if the higher 285 mg once weekly dose does not provide a significant additional TG reduction after 9 months (package insert).

 

Effect on Clinical Outcomes

 

As described above in the description of the effect of volanesorsen on lipid/lipoprotein levels in patients with FCS and marked hypertriglyceridemia there is suggestive evidence that lowering the very high TG levels with volanesorsen treatment will reduce the risk of pancreatitis and improve the quality of life.

 

Volanesorsen treatment reduced hepatic fat assessed by MRI in patients with FCS, severe hypertriglyceridemia, and familial partial lipodystrophy (251). The greater the hepatic fat the greater the decrease induced by volanesorsen.

 

The effect of volanesorsen on cardiovascular disease has not been determined. However, epidemiologic studies have demonstrated that increased Apo C-III levels are associated with an increased risk of cardiovascular events (252-254)and coronary artery calcification (255). Moreover, carriers of rare heterozygous loss-of-function mutations in Apo C-III have reduced TG levels and reduced cardiovascular disease risk (256-258). One can speculate that lowering Apo C-III and TG levels with volanesorsen will have beneficial effects on the development of cardiovascular disease.

 

Side Effects

 

Treatment with volanesorsen is very commonly associated with reductions in platelet count in patients with the FCS and may result in thrombocytopenia (package insert; https://www.ema.europa.eu/en/documents/product-information/waylivra-epar-product-information_en.pdf). Platelet counts below 140 x 109/L were observed in 75% of patients treated with volanesorsen vs. 24% of placebo patients. Reductions to below 100 x 109/L were observed in 47% of patients treated with volanesorsen compared with none of the patients in the placebo group. Bleeding secondary to low platelets may occur. Careful monitoring for thrombocytopenia is important during treatment and recommendations for adjustments to monitoring frequency and dosing are shown in table 20 (package insert). Platelet counts recover following drug discontinuation and administration of glucocorticoids where medically indicated.

 

Table 20.  Volanesorsen Monitoring and Treatment Recommendations

Platelet Count (x109/L)

Dose

Monitoring Frequency

Normal (≥140)

Starting dose: Weekly

After 3 months: Every 2 weeks

Every 2 weeks

100-139

Every 2 weeks

Weekly

75-99

Pause treatment for ≥4 weeks and resume treatment after platelet levels ≥ 100 x 109/L

Weekly

50-74

Pause treatment for ≥4 weeks and resume treatment after platelet levels ≥ 100 x 109/L

Every 2-3 days

Less than 50

Discontinue treatment

Glucocorticoids recommended

Daily

 

Renal toxicity has been observed after administration of volanesorsen. Monitoring for evidence of nephrotoxicity by routine urine dipstick is recommended on a quarterly basis. In the case of a positive assessment, one should measure serum creatinine and collect a 24-hour urine collection to quantify the proteinuria and assess creatinine clearance. Treatment should be discontinued if proteinuria ≥ 500 mg/24 hour is present, or an increase in serum creatinine ≥ 0.3 mg/dL that is >ULN occurs, or the creatinine clearance estimated by the CKD-EPI equation is ≤ 30 mL/min/1.73m2(package insert).

 

Elevations of liver enzymes have been observed after administration of volanesorsen. Serum liver enzymes and bilirubin should be monitored every 3 months. Treatment should be discontinued if there is a single increase in ALT or AST > 8 x ULN, or an increase > 5 x ULN, which persists for ≥ 2 weeks, or lesser increases in ALT or AST that are associated with total bilirubin > 2 x ULN or INR > 1.5 (package insert).

 

As expected, injection site reactions are frequently observed and were reported in 82% of patients (erythema, pain, pruritus, or local swelling) (package insert).

 

Contraindications

 

Treatment should not be initiated in patients with thrombocytopenia (platelet count <140 x 109/L). Safety and efficacy have not been established in patients with severe renal disease or patients with hepatic impairment (package insert). There are no data on the use of volanesorsen in pregnant women and it is preferable to avoid the use of volanesorsen during pregnancy (package insert).

 

Drug Interactions

 

Discontinuation of antiplatelet drugs/NSAIDs/anticoagulants should be considered for

platelet levels < 75 x 109/L. Treatment with these products must be discontinued at platelet levels < 50 x 109/L. No other drug interactions have been described (package insert)

 

Conclusions

 

Volanesorsen is a useful drug in patients with the FCS, particularly in patients who have repeated episodes of acute pancreatitis. Whether volanesorsen will be useful for the treatment of less severe hypertriglyceridemia remains to be determined, particularly given its potential side effects. Drugs similar to volanesorsen (Olezarsen) that do not adversely affect platelets are underdevelopment (259).  

 

ALIPOGENE TIPARVOVEC (GLYBERA)

 

Introduction

 

Alipogene tiparvovec is a gene therapy that was approved in Europe for adult patients with Familial Lipoprotein Lipase deficiency and a history of multiple or severe episodes of pancreatitis who have failed dietary therapy (260). The diagnosis of Familial Lipoprotein Lipase with loss of function mutations must be confirmed by genetic testing but patients need to have detectable levels of lipoprotein lipase protein (to avoid immunological reactions) (260). Alipogene tiparvovec is an adeno-associated virus gene therapy that results in the expression of the naturally occurring S447X variant of the human lipoprotein lipase gene that has increased lipoprotein lipase activity compared to “normal” lipoprotein lipase (260). Approximately 20% of Caucasians express this gene variant and these individuals have lower plasma TG levels and an increase in HDL-C levels (261,262). Because of the lack of long-term efficacy alipogene tiparvovec is no longer clinically available.

 

Effect of Alipogene Tiparvovec on Lipid and Lipoprotein Levels

 

In patients with plasma TG levels > 880mg/d, treatment with alipogene tiparvovec resulted in an approximately 40% decrease in fasting plasma TGs with half of the patients having > 40% decrease in fasting plasma TG levels at 3-12 weeks post treatment (263). By week 16-26, fasting TG levels returned to baseline values but chylomicron levels were reduced (263). While fasting TG levels returned to baseline, postprandial TG levels were reduced by approximately 60% suggesting that there are long term effects that are not reflected by fasting TG levels (264). In fact, in some patients treated with alipogene tiparvovec, lipoprotein lipase expression was demonstrated in muscle biopsies at 26 weeks (263).

 

Mechanisms Accounting for the Alipogene Tiparvovec Induced Lipid Effects

 

Gene therapy with alipogene tiparvovec results in the expression of lipoprotein lipase in muscle, which accelerates the clearance of chylomicrons (260,263). Studies have demonstrated a reduced peak level and a reduced area under the curve for postprandial chylomicrons (264).

 

Drug Administration and Pharmacokinetics

 

Alipogene tiparvovec is administered by multiple intramuscularly injections in the legs given at a single visit (260). The number of injections is > 40 and therefore the injections are given under spinal anesthesia (263). From 3 days before administration until 12 weeks after administration patients may be treated with cyclosporine (3mg/kg/day) and mycophenolate (2g/day) and on the day of administration methylprednisolone 1mg/kg) may be administered IV (260,263).

 

Effect on Clinical Outcomes

 

In patients with Familial Lipoprotein Lipase Deficiency the outcome of interest is pancreatitis. In a retrospective study of 19 patients treated with alipogene tiparvovec an approximate 50% decrease in pancreatitis was observed (265). In addition, patients treated with alipogene tiparvovec have reported benefits including discontinuing lipoprotein apheresis, increased energy, and the ability to liberalize their diet, which is difficult to comply with due to the marked limitation in dietary fat (263,266).

 

Conclusions

 

Alipogene tiparvovec may be a useful treatment for the rare patient with Familial Lipoprotein Lipase deficiency but the lack of long-term efficacy and the difficulty of giving the required injections led to this drug being removed from the market.  Because of the rarity of this disorder the information on patients treated with this drug is limited and randomized trials are impossible.

 

EVINACUMAB (EVKEEZA)

 

Introduction

 

Evinacumab is a human monoclonal antibody against angiopoietin-like protein 3 (ANGPTL3). It is approved for the treatment of Homozygous Familial Hypercholesterolemia. Evinacumab decreases LDL-C levels by mechanisms independent of LDL receptor activity. The recommended dose of evinacumab is 15 mg/kg administered by intravenous infusion over 60 minutes every 4 weeks. While it is not approved for TG lowering it is effective in lowering TG levels.

 

Effect on Evinacumab on TG Levels

 

For information on the effect of evinacumab on LDL-C levels see the Endotext chapter on “Cholesterol Lowering Drugs (5). Because of the difficulty in treating severe hypertriglyceridemia, I have focused on evinacumab in this group of patients. Phase 1 studies have shown that various doses of evinacumab lower TG levels in individuals with TG levels between 150-450mg/dL with maximal effects of approximately 80% reductions (267). As one would expect LDL-C and HDL-C levels also decreased in these individuals with modest hypertriglyceridemia.

 

A phase 2 study evaluated evinacumab in three groups of patients with severe hypertriglyceridemia; FCS patients with bi-allelic loss-of-function mutations in the lipoprotein lipase (LPL) pathway (n = 17), multifactorial chylomicronemia syndrome (MFCS) with heterozygous loss-of-function LPL pathway mutations (n = 15), and MFCS without LPL pathway mutations (n = 19) (268). Patients were randomized to evinacumab 15 mg/kg IV or placebo every 4 weeks over 12-weeks. The effect on TG and non-HDL-C levels are shown in table 21. Despite the very small number of patients the results suggest that evinacumab can lower TG levels in patients with MFCS but not in patients with FCS. This result Is not surprising based on the proposed mechanism of action of inhibiting ANGPTL3 (see below).

 

Table 21. Change in Lipid/Lipoprotein Parameters

 

FCS

MFCS/heterozygous LPL pathway mutations

MFCS/ without LPL pathway mutations

 

Placebo (n=5)

Evinacumab (n=12)

Placebo (n=8)

Evinacumab ((n=9)

Placebo (n=5)

Evinacumab (n=14)

Fasting TG

Baseline

3,918mg/dL

3,140mg/dL

1,351mg/dL

1,238mg/dL

1,030mg/dL

1,917mg/dL

% change

−22.9

−27.7

9.4

−64.8*

80.9

−81.7**

Non-HDL-C

Baseline

356mg/dL

345mg/dL

202mg/dL

220mg/dL

209mg/dL

296mg/dL

% change

−15.2

−34.2^

8.0

−31.0^^

48.4

−38.5^^^

*p= 0.0076, **p= 0.0418, ^p= 0.0074, ^^p= 0.0677, ^^^p= 0.1016.

FCS= familial chylomicronemia syndrome, MFCS= multifactorial chylomicronemia syndrome.

 

Mechanism Accounting for the Evinacumab Induced Decrease in TG

 

ANGPTL3 inhibits lipoprotein lipase (LPL) activity thereby slowing the clearance of VLDL and chylomicrons resulting in an increase in plasma triglyceride levels (269,270). Mice deficient in ANGPTL3 have lower plasma triglyceride levels while mice overexpressing ANGPTL3 have elevated plasma triglyceride levels (270). Evinacumab by inhibiting the ability of ANGPTL3 to decrease LPL activity results in an increases in LPL activity, which accelerates the clearance of TG rich lipoproteins decreasing plasma triglyceride levels (270). In patients with FCS who lack a functioning lipoprotein lipase clearance system evinacumab will not accelerate the clearance of TG rich lipoproteins. For information on the mechanism by which evinacumab lowers LDL-C and HDL-C see the Endotext chapter on “Cholesterol Lowering Drugs” (5).

 

Pharmacokinetics and Drug Interactions

 

There are no significant drug interactions.

 

Effect of Evinacumab on Clinical Outcomes

 

There are no cardiovascular outcome studies.

 

Homozygosity for loss-of-function mutations in ANGPTL3 is associated with significantly lower plasma levels of LDL-C, HDL-C, and triglycerides (familial combined hypolipidemia) (270,271). Heterozygous carriers of loss-of-function mutations in ANGPTL3, which occur at a frequency of about 1:300, have significantly lower total cholesterol, LDL-C, and triglyceride levels than noncarriers (270). Moreover, patients carrying loss-of-function variants in ANGPTL3 have a significantly lower risk of coronary artery disease (272,273). Additionally, in an animal model of atherosclerosis treatment with evinacumab decreased atherosclerotic lesion area and necrotic content (272). Taken together these observations suggest that inhibiting ANGPTL3 with evinacumab will reduce cardiovascular disease.

 

Side Effects

 

Serious hypersensitivity reactions have occurred with evinacumab. In clinical trials, 1 (1%) of evinacumab treated patients experienced anaphylaxis vs. 0% of patients who received placebo (package insert).

 

Contraindications

 

Based on animal studies, evinacumab may cause fetal harm when administered to pregnant patients (package insert). Patients should be advised of the potential risks to the fetus of pregnancy. Patients who may become pregnant should be advised to use effective contraception during treatment with evinacumab and for at least 5 months following the last dose.

 

Summary

 

Evinacumab lowers triglyceride levels in patients with severe hypertriglyceridemia due to multifactorial chylomicronemia syndrome and could be useful in selected patients with hypertriglyceridemia. Note it is not approved to treat severe hypertriglyceridemia and administration intravenously every 4 weeks will limit its use to special circumstances.

 

CLINICAL USE OF TRIGLYCERIDE LOWERING DRUGS

 

Marked Hypertriglyceridemia (>500mg/dL); Prevention of Pancreatitis

 

In patients with marked elevations in TG levels (>500-1000mg/dL) the major concern is an increased risk of pancreatitis (274,275). Because of this increased risk it is imperative to lower TG levels. The initial steps are to 1) treat any disease states that could be leading to an elevation in plasma TG levels, 2) if possible, discontinue any drugs that could be leading to an elevation in plasma TGs, and 3) initiate lifestyle changes (Table 22) (2,276).

 

Table 22. Causes of Secondary Hypertriglyceridemia

Lifestyle

Diseases

Medications

Excess calories

Poorly controlled diabetes

Corticosteroids

Excess dietary fat intake

Hypothyroidism

Oral estrogen

Excess simple sugars

Renal disease

Retinoic acid derivatives

Overweight/Obesity

HIV infection

Beta adrenergic blockers

Alcohol intake

Cushing’s syndrome

Thiazide diuretics

Pregnancy

Acromegaly

Protease inhibitors

 

Growth hormone deficiency

Bile acid sequestrants

 

Lipodystrophy

Anti-psychotic drugs

 

Paraproteinemia

Cyclosporine/tacrolimus

 

Nephrotic Syndrome

L-asparaginase

 

Inflammatory Disorders

Interferon alpha 2b

 

 

Cyclophosphamide

 

These initial steps are often sufficient to result in marked reductions in plasma TG levels eliminating the need for TG lowering medications. For example, in patients with diabetes in very poor glycemic control, treatment that results in good glycemic control can markedly lower TG levels (277). Similarly, the restoration of euthyroidism in a hypothyroid patient can also markedly lower lipid levels (278). If these initial steps do not result in a lowering of TGs into an acceptable range, then the use of drugs to lower plasma TG levels is indicated. There have been no randomized controlled trials demonstrating that treatment diminishes pancreatitis but most experienced clinicians believe that lowering TG levels to below 500-1000mg/dL reduces the risk of developing pancreatitis (274,275). The addition of either fibrates or fish oil to lifestyle changes are commonly used to lower markedly elevated TG levels. In some patients, combination therapy is required to lower plasma TGs to an acceptable range. In patients with Familial Chylomicronemia syndrome volanesorsen is a promising therapeutic tool.

 

Moderate Hypertriglyceridemia (150-500mg/dL); Prevention of Cardiovascular Disease

 

In the era of statin therapy, it is uncertain whether lowering TG levels in patients on statin therapy will further reduce cardiovascular events. As discussed in detail in the sections on individual drugs, the studies carried out so far have not shown that adding niacin or fibrates to statin therapy is beneficial with regards to cardiovascular disease. As also discussed, some of the available studies have major limitations because many of the patients in these outcome studies did not have substantial elevations in TGs. Nevertheless, at this time there is little enthusiasm for adding either fibrates or niacin to statins to lower the risk of cardiovascular event.

 

Notably, the REDUCE-IT trial, which tested the effect of high dose EPA (4 grams per day) in patients with elevated TG levels on statin therapy demonstrated a 25% reduction in cardiovascular events. However, the decrease in cardiovascular events was considerably greater than one would expect based on the reduction in TG levels suggesting that the decrease in cardiovascular events was not solely due to lowering TG levels and that other effects of EPA likely played a role. Additionally, as discussed in detail in the section discussing cardiovascular trials in the omega-3-fatty acid section there are concerns that the use of mineral oil as the placebo in the REDUCE-IT trial may have caused harmful effects leading to increased events. Thus, the role of EPA in reducing cardiovascular events is debated with some experts feeling that it is beneficial while others feeling that the evidence for benefit is very weak. Clearly additional studies are required to resolve this controversy. In the meantime, clinicians will need to use their clinical judgement on whether to treat patients with modest elevations in TG levels with EPA (icosapent ethyl; Vascepa) balancing the potential benefits of treatment vs. the potential side effects.

 

Some guidelines use non-HDL-C as a therapeutic goal and thus the use of omega-3-fatty acids and fibrates will often be required to lower TG levels to achieve these non-HDL-C goals. In contrast, other guidelines focus on LDL-C levels and the use of statins and thus de-emphasize the use of omega-3-fatty acids and fibrates. Given the absence of definitive data one needs to use clinical judgement. Consideration should also be given to the use of fenofibrate in hypertriglyceridemic patients with diabetes at high risk for microvascular disease given the studies that have shown that fibrates reduce the microvascular complications of diabetes. Because of the side effects of niacin, the use of niacin to lower TG levels has markedly diminished. In the past we used to use niacin to lower both LDL-C levels and TGs but with the availability of ezetimibe, bempedoic acid, and PCSK9 inhibitors the need to use niacin to lower LDL-C levels has markedly decreased.

 

ACKNOWLEDGEMENTS

 

This work was supported by grants from the Northern California Institute for Research and Education.

 

REFERENCES

 

  1. Geier RR, Tannock LR. Risk of Fasting and Non-Fasting Hypertriglyceridemia in Coronary Vascular Disease and Pancreatitis. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2022.
  2. Feingold KR. Approach to the Patient with Dyslipidemia. In: De Groot LJ, Chrousos G, Dungan K, Feingold KR, Grossman A, Hershman JM, Koch C, Korbonits M, McLachlan R, New M, Purnell J, Rebar R, Singer F, Vinik A, eds. Endotext. South Dartmouth (MA) 2023.
  3. Feingold KR. Obesity and Dyslipidemia. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2023.
  4. Feingold KR. The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, de Herder WW, Dhatariya K, Dungan K, Hofland J, Kalra S, Kaltsas G, Kapoor N, Koch C, Kopp P, Korbonits M, Kovacs CS, Kuohung W, Laferrere B, Levy M, McGee EA, McLachlan R, New M, Purnell J, Sahay R, Shah AS, Singer F, Sperling MA, Stratakis CA, Trence DL, Wilson DP, eds. Endotext. South Dartmouth (MA) 2021.
  5. Feingold KR. Cholesterol Lowering Drugs. In: De Groot LJ, Chrousos G, Dungan K, Feingold KR, Grossman A, Hershman JM, Koch C, Korbonits M, McLachlan R, New M, Purnell J, Rebar R, Singer F, Vinik A, eds. Endotext. South Dartmouth (MA) 2021.
  6. Altschul R, Hoffer A, Stephen JD. Influence of nicotinic acid on serum cholesterol in man. Arch Biochem Biophys1955; 54:558-559
  7. Cooper DL, Murrell DE, Roane DS, Harirforoosh S. Effects of formulation design on niacin therapeutics: mechanism of action, metabolism, and drug delivery. Int J Pharm 2015; 490:55-64
  8. Song WL, FitzGerald GA. Niacin, an old drug with a new twist. J Lipid Res 2013; 54:2586-2594
  9. Julius U. Niacin as antidyslipidemic drug. Can J Physiol Pharmacol 2015; 93:1043-1054
  10. Morgan JM, Capuzzi DM, Baksh RI, Intenzo C, Carey CM, Reese D, Walker K. Effects of extended-release niacin on lipoprotein subclass distribution. Am J Cardiol 2003; 91:1432-1436
  11. Birjmohun RS, Hutten BA, Kastelein JJ, Stroes ES. Efficacy and safety of high-density lipoprotein cholesterol-increasing compounds: a meta-analysis of randomized controlled trials. J Am Coll Cardiol 2005; 45:185-197
  12. Knopp RH, Alagona P, Davidson M, Goldberg AC, Kafonek SD, Kashyap M, Sprecher D, Superko HR, Jenkins S, Marcovina S. Equivalent efficacy of a time-release form of niacin (Niaspan) given once-a-night versus plain niacin in the management of hyperlipidemia. Metabolism 1998; 47:1097-1104
  13. Sahebkar A, Reiner Z, Simental-Mendia LE, Ferretti G, Cicero AF. Effect of extended-release niacin on plasma lipoprotein(a) levels: A systematic review and meta-analysis of randomized placebo-controlled trials. Metabolism2016; 65:1664-1678
  14. Goldberg AC. A meta-analysis of randomized controlled studies on the effects of extended-release niacin in women. Am J Cardiol 2004; 94:121-124
  15. Ballantyne CM, Davidson MH, McKenney J, Keller LH, Bajorunas DR, Karas RH. Comparison of the safety and efficacy of a combination tablet of niacin extended release and simvastatin vs simvastatin monotherapy in patients with increased non-HDL cholesterol (from the SEACOAST I study). Am J Cardiol 2008; 101:1428-1436
  16. Fazio S, Guyton JR, Polis AB, Adewale AJ, Tomassini JE, Ryan NW, Tershakovec AM. Long-term safety and efficacy of triple combination ezetimibe/simvastatin plus extended-release niacin in patients with hyperlipidemia. Am J Cardiol 2010; 105:487-494
  17. Shearer GC, Pottala JV, Hansen SN, Brandenburg V, Harris WS. Effects of prescription niacin and omega-3 fatty acids on lipids and vascular function in metabolic syndrome: a randomized controlled trial. J Lipid Res 2012; 53:2429-2435
  18. Pradhan B, Neopane A, Karki S, Karki DB. Effectiveness of nicotinic acid and bezafibrate alone and in combination for reducing serum triglyceride level. Kathmandu Univ Med J (KUMJ) 2005; 3:411-414
  19. Balasubramanyam A, Coraza I, Smith EO, Scott LW, Patel P, Iyer D, Taylor AA, Giordano TP, Sekhar RV, Clark P, Cuevas-Sanchez E, Kamble S, Ballantyne CM, Pownall HJ. Combination of niacin and fenofibrate with lifestyle changes improves dyslipidemia and hypoadiponectinemia in HIV patients on antiretroviral therapy: results of "heart positive," a randomized, controlled trial. J Clin Endocrinol Metab 2011; 96:2236-2247
  20. Carlson LA. Studies on the effect of nicotinic acid on catecholamine stimulated lipolysis in adipose tissue in vitro. Acta Med Scand 1963; 173:719-722
  21. Carlson LA, Oro L. The effect of nicotinic acid on the plasma free fatty acid; demonstration of a metabolic type of sympathicolysis. Acta Med Scand 1962; 172:641-645
  22. Kamanna VS, Ganji SH, Kashyap ML. Recent advances in niacin and lipid metabolism. Curr Opin Lipidol 2013; 24:239-245
  23. Tunaru S, Kero J, Schaub A, Wufka C, Blaukat A, Pfeffer K, Offermanns S. PUMA-G and HM74 are receptors for nicotinic acid and mediate its anti-lipolytic effect. Nat Med 2003; 9:352-355
  24. Wise A, Foord SM, Fraser NJ, Barnes AA, Elshourbagy N, Eilert M, Ignar DM, Murdock PR, Steplewski K, Green A, Brown AJ, Dowell SJ, Szekeres PG, Hassall DG, Marshall FH, Wilson S, Pike NB. Molecular identification of high and low affinity receptors for nicotinic acid. J Biol Chem 2003; 278:9869-9874
  25. Wang W, Basinger A, Neese RA, Christiansen M, Hellerstein MK. Effects of nicotinic acid on fatty acid kinetics, fuel selection, and pathways of glucose production in women. Am J Physiol Endocrinol Metab 2000; 279:E50-59
  26. Lauring B, Taggart AK, Tata JR, Dunbar R, Caro L, Cheng K, Chin J, Colletti SL, Cote J, Khalilieh S, Liu J, Luo WL, Maclean AA, Peterson LB, Polis AB, Sirah W, Wu TJ, Liu X, Jin L, Wu K, Boatman PD, Semple G, Behan DP, Connolly DT, Lai E, Wagner JA, Wright SD, Cuffie C, Mitchel YB, Rader DJ, Paolini JF, Waters MG, Plump A. Niacin lipid efficacy is independent of both the niacin receptor GPR109A and free fatty acid suppression. Sci Transl Med 2012; 4:148ra115
  27. Ganji SH, Tavintharan S, Zhu D, Xing Y, Kamanna VS, Kashyap ML. Niacin noncompetitively inhibits DGAT2 but not DGAT1 activity in HepG2 cells. J Lipid Res 2004; 45:1835-1845
  28. Grundy SM, Mok HY, Zech L, Berman M. Influence of nicotinic acid on metabolism of cholesterol and triglycerides in man. J Lipid Res 1981; 22:24-36
  29. Blond E, Rieusset J, Alligier M, Lambert-Porcheron S, Bendridi N, Gabert L, Chetiveaux M, Debard C, Chauvin MA, Normand S, Roth H, de Gouville AC, Krempf M, Vidal H, Goudable J, Laville M, Niacin" Study G. Nicotinic acid effects on insulin sensitivity and hepatic lipid metabolism: an in vivo to in vitro study. Horm Metab Res 2014; 46:390-396
  30. Hernandez C, Molusky M, Li Y, Li S, Lin JD. Regulation of hepatic ApoC3 expression by PGC-1beta mediates hypolipidemic effect of nicotinic acid. Cell Metab 2010; 12:411-419
  31. Zhang LH, Kamanna VS, Zhang MC, Kashyap ML. Niacin inhibits surface expression of ATP synthase beta chain in HepG2 cells: implications for raising HDL. J Lipid Res 2008; 49:1195-1201
  32. Jin FY, Kamanna VS, Kashyap ML. Niacin decreases removal of high-density lipoprotein apolipoprotein A-I but not cholesterol ester by Hep G2 cells. Implication for reverse cholesterol transport. Arterioscler Thromb Vasc Biol1997; 17:2020-2028
  33. Blum CB, Levy RI, Eisenberg S, Hall M, 3rd, Goebel RH, Berman M. High density lipoprotein metabolism in man. J Clin Invest 1977; 60:795-807
  34. Shepherd J, Packard CJ, Patsch JR, Gotto AM, Jr., Taunton OD. Effects of nicotinic acid therapy on plasma high density lipoprotein subfraction distribution and composition and on apolipoprotein A metabolism. J Clin Invest1979; 63:858-867
  35. Lamon-Fava S, Diffenderfer MR, Barrett PH, Buchsbaum A, Nyaku M, Horvath KV, Asztalos BF, Otokozawa S, Ai M, Matthan NR, Lichtenstein AH, Dolnikowski GG, Schaefer EJ. Extended-release niacin alters the metabolism of plasma apolipoprotein (Apo) A-I and ApoB-containing lipoproteins. Arterioscler Thromb Vasc Biol 2008; 28:1672-1678
  36. Rubic T, Trottmann M, Lorenz RL. Stimulation of CD36 and the key effector of reverse cholesterol transport ATP-binding cassette A1 in monocytoid cells by niacin. Biochem Pharmacol 2004; 67:411-419
  37. Zhang LH, Kamanna VS, Ganji SH, Xiong XM, Kashyap ML. Niacin increases HDL biogenesis by enhancing DR4-dependent transcription of ABCA1 and lipidation of apolipoprotein A-I in HepG2 cells. J Lipid Res 2012; 53:941-950
  38. van der Hoorn JW, de Haan W, Berbee JF, Havekes LM, Jukema JW, Rensen PC, Princen HM. Niacin increases HDL by reducing hepatic expression and plasma levels of cholesteryl ester transfer protein in APOE*3Leiden.CETP mice. Arterioscler Thromb Vasc Biol 2008; 28:2016-2022
  39. Seed M, O'Connor B, Perombelon N, O'Donnell M, Reaveley D, Knight BL. The effect of nicotinic acid and acipimox on lipoprotein(a) concentration and turnover. Atherosclerosis 1993; 101:61-68
  40. Croyal M, Ouguerram K, Passard M, Ferchaud-Roucher V, Chetiveaux M, Billon-Crossouard S, de Gouville AC, Lambert G, Krempf M, Nobecourt E. Effects of Extended-Release Nicotinic Acid on Apolipoprotein (a) Kinetics in Hypertriglyceridemic Patients. Arterioscler Thromb Vasc Biol 2015; 35:2042-2047
  41. Chennamsetty I, Kostner KM, Claudel T, Vinod M, Frank S, Weiss TS, Trauner M, Kostner GM. Nicotinic acid inhibits hepatic APOA gene expression: studies in humans and in transgenic mice. J Lipid Res 2012; 53:2405-2412
  42. Pieper JA. Overview of niacin formulations: differences in pharmacokinetics, efficacy, and safety. Am J Health Syst Pharm 2003; 60:S9-14; quiz S25
  43. Clofibrate and niacin in coronary heart disease. JAMA 1975; 231:360-381
  44. Canner PL, Berge KG, Wenger NK, Stamler J, Friedman L, Prineas RJ, Friedewald W. Fifteen year mortality in Coronary Drug Project patients: long-term benefit with niacin. J Am Coll Cardiol 1986; 8:1245-1255
  45. Canner PL, Furberg CD, Terrin ML, McGovern ME. Benefits of niacin by glycemic status in patients with healed myocardial infarction (from the Coronary Drug Project). Am J Cardiol 2005; 95:254-257
  46. Canner PL, Furberg CD, McGovern ME. Benefits of niacin in patients with versus without the metabolic syndrome and healed myocardial infarction (from the Coronary Drug Project). Am J Cardiol 2006; 97:477-479
  47. Carlson LA, Rosenhamer G. Reduction of mortality in the Stockholm Ischaemic Heart Disease Secondary Prevention Study by combined treatment with clofibrate and nicotinic acid. Acta Med Scand 1988; 223:405-418
  48. Carlson LA, Danielson M, Ekberg I, Klintemar B, Rosenhamer G. Reduction of myocardial reinfarction by the combined treatment with clofibrate and nicotinic acid. Atherosclerosis 1977; 28:81-86
  49. AIM-HIGH Investigators, Boden WE, Probstfield JL, Anderson T, Chaitman BR, Desvignes-Nickens P, Koprowicz K, McBride R, Teo K, Weintraub W. Niacin in patients with low HDL cholesterol levels receiving intensive statin therapy. N Engl J Med 2011; 365:2255-2267
  50. Guyton JR, Slee AE, Anderson T, Fleg JL, Goldberg RB, Kashyap ML, Marcovina SM, Nash SD, O'Brien KD, Weintraub WS, Xu P, Zhao XQ, Boden WE. Relationship of lipoproteins to cardiovascular events: the AIM-HIGH Trial (Atherothrombosis Intervention in Metabolic Syndrome With Low HDL/High Triglycerides and Impact on Global Health Outcomes). J Am Coll Cardiol 2013; 62:1580-1584
  51. Hps Thrive Collaborative Group, Landray MJ, Haynes R, Hopewell JC, Parish S, Aung T, Tomson J, Wallendszus K, Craig M, Jiang L, Collins R, Armitage J. Effects of extended-release niacin with laropiprant in high-risk patients. N Engl J Med 2014; 371:203-212
  52. Song WL, Stubbe J, Ricciotti E, Alamuddin N, Ibrahim S, Crichton I, Prempeh M, Lawson JA, Wilensky RL, Rasmussen LM, Pure E, FitzGerald GA. Niacin and biosynthesis of PGD(2)by platelet COX-1 in mice and humans. J Clin Invest 2012; 122:1459-1468
  53. Cashin-Hemphill L, Mack WJ, Pogoda JM, Sanmarco ME, Azen SP, Blankenhorn DH. Beneficial effects of colestipol-niacin on coronary atherosclerosis. A 4-year follow-up. JAMA 1990; 264:3013-3017
  54. Brown G, Albers JJ, Fisher LD, Schaefer SM, Lin JT, Kaplan C, Zhao XQ, Bisson BD, Fitzpatrick VF, Dodge HT. Regression of coronary artery disease as a result of intensive lipid-lowering therapy in men with high levels of apolipoprotein B. N Engl J Med 1990; 323:1289-1298
  55. Kane JP, Malloy MJ, Ports TA, Phillips NR, Diehl JC, Havel RJ. Regression of coronary atherosclerosis during treatment of familial hypercholesterolemia with combined drug regimens. JAMA 1990; 264:3007-3012
  56. Brown BG, Zhao XQ, Chait A, Fisher LD, Cheung MC, Morse JS, Dowdy AA, Marino EK, Bolson EL, Alaupovic P, Frohlich J, Albers JJ. Simvastatin and niacin, antioxidant vitamins, or the combination for the prevention of coronary disease. N Engl J Med 2001; 345:1583-1592
  57. Whitney EJ, Krasuski RA, Personius BE, Michalek JE, Maranian AM, Kolasa MW, Monick E, Brown BG, Gotto AM, Jr. A randomized trial of a strategy for increasing high-density lipoprotein cholesterol levels: effects on progression of coronary heart disease and clinical events. Ann Intern Med 2005; 142:95-104
  58. Sacks FM, Pasternak RC, Gibson CM, Rosner B, Stone PH. Effect on coronary atherosclerosis of decrease in plasma cholesterol concentrations in normocholesterolaemic patients. Harvard Atherosclerosis Reversibility Project (HARP) Group. Lancet 1994; 344:1182-1186
  59. Taylor AJ, Lee HJ, Sullenberger LE. The effect of 24 months of combination statin and extended-release niacin on carotid intima-media thickness: ARBITER 3. Curr Med Res Opin 2006; 22:2243-2250
  60. Taylor AJ, Sullenberger LE, Lee HJ, Lee JK, Grace KA. Arterial Biology for the Investigation of the Treatment Effects of Reducing Cholesterol (ARBITER) 2: a double-blind, placebo-controlled study of extended-release niacin on atherosclerosis progression in secondary prevention patients treated with statins. Circulation 2004; 110:3512-3517
  61. Taylor AJ, Villines TC, Stanek EJ, Devine PJ, Griffen L, Miller M, Weissman NJ, Turco M. Extended-release niacin or ezetimibe and carotid intima-media thickness. N Engl J Med 2009; 361:2113-2122
  62. Thoenes M, Oguchi A, Nagamia S, Vaccari CS, Hammoud R, Umpierrez GE, Khan BV. The effects of extended-release niacin on carotid intimal media thickness, endothelial function and inflammatory markers in patients with the metabolic syndrome. Int J Clin Pract 2007; 61:1942-1948
  63. Lee JM, Robson MD, Yu LM, Shirodaria CC, Cunnington C, Kylintireas I, Digby JE, Bannister T, Handa A, Wiesmann F, Durrington PN, Channon KM, Neubauer S, Choudhury RP. Effects of high-dose modified-release nicotinic acid on atherosclerosis and vascular function: a randomized, placebo-controlled, magnetic resonance imaging study. J Am Coll Cardiol 2009; 54:1787-1794
  64. Dunbar RL, Gelfand JM. Seeing red: flushing out instigators of niacin-associated skin toxicity. J Clin Invest 2010; 120:2651-2655
  65. Dunn RT, Ford MA, Rindone JP, Kwiecinski FA. Low-Dose Aspirin and Ibuprofen Reduce the Cutaneous Reactions Following Niacin Administration. Am J Ther 1995; 2:478-480
  66. McKenney JM, Proctor JD, Harris S, Chinchili VM. A comparison of the efficacy and toxic effects of sustained- vs immediate-release niacin in hypercholesterolemic patients. JAMA 1994; 271:672-677
  67. Henkin Y, Johnson KC, Segrest JP. Rechallenge with crystalline niacin after drug-induced hepatitis from sustained-release niacin. JAMA 1990; 264:241-243
  68. Kashyap ML, Ganji S, Nakra NK, Kamanna VS. Niacin for treatment of nonalcoholic fatty liver disease (NAFLD): novel use for an old drug? J Clin Lipidol 2019; 13:873-879
  69. Anderson TJ, Boden WE, Desvigne-Nickens P, Fleg JL, Kashyap ML, McBride R, Probstfield JL, AIM_HIGH Investigators. Safety profile of extended-release niacin in the AIM-HIGH trial. N Engl J Med 2014; 371:288-290
  70. Miettinen TA, Taskinen MR, Pelkonen R, Nikkila EA. Glucose tolerance and plasma insulin in man during acute and chronic administration of nicotinic acid. Acta Med Scand 1969; 186:247-253
  71. Poynten AM, Gan SK, Kriketos AD, O'Sullivan A, Kelly JJ, Ellis BA, Chisholm DJ, Campbell LV. Nicotinic acid-induced insulin resistance is related to increased circulating fatty acids and fat oxidation but not muscle lipid content. Metabolism 2003; 52:699-704
  72. Goldberg RB, Bittner VA, Dunbar RL, Fleg JL, Grunberger G, Guyton JR, Leiter LA, McBride R, Robinson JG, Simmons DL, Wysham C, Xu P, Boden WE. Effects of Extended-Release Niacin Added to Simvastatin/Ezetimibe on Glucose and Insulin Values in AIM-HIGH. Am J Med 2016; 129:753 e713-722
  73. Goldie C, Taylor AJ, Nguyen P, McCoy C, Zhao XQ, Preiss D. Niacin therapy and the risk of new-onset diabetes: a meta-analysis of randomised controlled trials. Heart 2016; 102:198-203
  74. Elam MB, Hunninghake DB, Davis KB, Garg R, Johnson C, Egan D, Kostis JB, Sheps DS, Brinton EA. Effect of niacin on lipid and lipoprotein levels and glycemic control in patients with diabetes and peripheral arterial disease: the ADMIT study: A randomized trial. Arterial Disease Multiple Intervention Trial. JAMA 2000; 284:1263-1270
  75. Garg A, Grundy SM. Nicotinic acid as therapy for dyslipidemia in non-insulin-dependent diabetes mellitus. JAMA1990; 264:723-726
  76. Ding Y, Li Y, Wen A. Effect of niacin on lipids and glucose in patients with type 2 diabetes: A meta-analysis of randomized, controlled clinical trials. Clin Nutr 2015; 34:838-844
  77. Gershon SL, Fox IH. Pharmacologic effects of nicotinic acid on human purine metabolism. J Lab Clin Med 1974; 84:179-186
  78. Gagne JJ, Houstoun M, Reichman ME, Hampp C, Marshall JH, Toh S. Safety assessment of niacin in the US Food and Drug Administration's mini-sentinel system. Pharmacoepidemiol Drug Saf 2018; 27:30-37
  79. Domanico D, Verboschi F, Altimari S, Zompatori L, Vingolo EM. Ocular Effects of Niacin: A Review of the Literature. Med Hypothesis Discov Innov Ophthalmol 2015; 4:64-71
  80. Zargar A, Ito MK. Long chain omega-3 dietary supplements: a review of the National Library of Medicine Herbal Supplement Database. Metab Syndr Relat Disord 2011; 9:255-271
  81. Kleiner AC, Cladis DP, Santerre CR. A comparison of actual versus stated label amounts of EPA and DHA in commercial omega-3 dietary supplements in the United States. J Sci Food Agric 2015; 95:1260-1267
  82. Wendland E, Farmer A, Glasziou P, Neil A. Effect of alpha linolenic acid on cardiovascular risk markers: a systematic review. Heart 2006; 92:166-169
  83. Eslick GD, Howe PR, Smith C, Priest R, Bensoussan A. Benefits of fish oil supplementation in hyperlipidemia: a systematic review and meta-analysis. Int J Cardiol 2009; 136:4-16
  84. Balk EM, Lichtenstein AH, Chung M, Kupelnick B, Chew P, Lau J. Effects of omega-3 fatty acids on serum markers of cardiovascular disease risk: a systematic review. Atherosclerosis 2006; 189:19-30
  85. Hartweg J, Perera R, Montori V, Dinneen S, Neil HA, Farmer A. Omega-3 polyunsaturated fatty acids (PUFA) for type 2 diabetes mellitus. Cochrane Database Syst Rev 2008:CD003205
  86. Zheng T, Zhao J, Wang Y, Liu W, Wang Z, Shang Y, Zhang W, Zhang Y, Zhong M. The limited effect of omega-3 polyunsaturated fatty acids on cardiovascular risk in patients with impaired glucose metabolism: a meta-analysis. Clin Biochem 2014; 47:369-377
  87. Chi H, Lin X, Huang H, Zheng X, Li T, Zou Y. Omega-3 fatty acid supplementation on lipid profiles in dialysis patients: meta-analysis. Arch Med Res 2014; 45:469-477
  88. Pei J, Zhao Y, Huang L, Zhang X, Wu Y. The effect of n-3 polyunsaturated fatty acids on plasma lipids and lipoproteins in patients with chronic renal failure--a meta-analysis of randomized controlled trials. J Ren Nutr 2012; 22:525-532
  89. Zhu W, Dong C, Du H, Zhang H, Chen J, Hu X, Hu F. Effects of fish oil on serum lipid profile in dialysis patients: a systematic review and meta-analysis of randomized controlled trials. Lipids Health Dis 2014; 13:127
  90. Hall AV, Parbtani A, Clark WF, Spanner E, Huff MW, Philbrick DJ, Holub BJ. Omega-3 fatty acid supplementation in primary nephrotic syndrome: effects on plasma lipids and coagulopathy. J Am Soc Nephrol 1992; 3:1321-1329
  91. Spadaro L, Magliocco O, Spampinato D, Piro S, Oliveri C, Alagona C, Papa G, Rabuazzo AM, Purrello F. Effects of n-3 polyunsaturated fatty acids in subjects with nonalcoholic fatty liver disease. Dig Liver Dis 2008; 40:194-199
  92. Oliveira JM, Rondo PH. Omega-3 fatty acids and hypertriglyceridemia in HIV-infected subjects on antiretroviral therapy: systematic review and meta-analysis. HIV Clin Trials 2011; 12:268-274
  93. De Truchis P, Kirstetter M, Perier A, Meunier C, Zucman D, Force G, Doll J, Katlama C, Rozenbaum W, Masson H, Gardette J, Melchior JC. Reduction in triglyceride level with N-3 polyunsaturated fatty acids in HIV-infected patients taking potent antiretroviral therapy: a randomized prospective study. J Acquir Immune Defic Syndr 2007; 44:278-285
  94. Harris WS, Ginsberg HN, Arunakul N, Shachter NS, Windsor SL, Adams M, Berglund L, Osmundsen K. Safety and efficacy of Omacor in severe hypertriglyceridemia. J Cardiovasc Risk 1997; 4:385-391
  95. Maki KC, Orloff DG, Nicholls SJ, Dunbar RL, Roth EM, Curcio D, Johnson J, Kling D, Davidson MH. A highly bioavailable omega-3 free fatty acid formulation improves the cardiovascular risk profile in high-risk, statin-treated patients with residual hypertriglyceridemia (the ESPRIT trial). Clin Ther 2013; 35:1400-1411 e1401-1403
  96. Pownall HJ, Brauchi D, Kilinc C, Osmundsen K, Pao Q, Payton-Ross C, Gotto AM, Jr., Ballantyne CM. Correlation of serum triglyceride and its reduction by omega-3 fatty acids with lipid transfer activity and the neutral lipid compositions of high-density and low-density lipoproteins. Atherosclerosis 1999; 143:285-297
  97. Calabresi L, Donati D, Pazzucconi F, Sirtori CR, Franceschini G. Omacor in familial combined hyperlipidemia: effects on lipids and low density lipoprotein subclasses. Atherosclerosis 2000; 148:387-396
  98. Minihane AM, Khan S, Leigh-Firbank EC, Talmud P, Wright JW, Murphy MC, Griffin BA, Williams CM. ApoE polymorphism and fish oil supplementation in subjects with an atherogenic lipoprotein phenotype. Arterioscler Thromb Vasc Biol 2000; 20:1990-1997
  99. Haglund O, Mehta JL, Saldeen T. Effects of fish oil on some parameters of fibrinolysis and lipoprotein(a) in healthy subjects. Am J Cardiol 1994; 74:189-192
  100. Beil FU, Terres W, Orgass M, Greten H. Dietary fish oil lowers lipoprotein(a) in primary hypertriglyceridemia. Atherosclerosis 1991; 90:95-97
  101. Herrmann W, Biermann J, Kostner GM. Comparison of effects of N-3 to N-6 fatty acids on serum level of lipoprotein(a) in patients with coronary artery disease. Am J Cardiol 1995; 76:459-462
  102. Shinozaki K, Kambayashi J, Kawasaki T, Uemura Y, Sakon M, Shiba E, Shibuya T, Nakamura T, Mori T. The long-term effec