Ezetimibe: cholesterol lowering and beyond

Abstract

Ezetimibe is a cholesterol absorption inhibitor that blocks the intestinal absorption of both biliary and dietary cholesterol. It appears to exert its effect by blocking intestinal sterol transporters, specifically Niemann–Pick C1-like 1 proteins, thereby inhibiting the intestinal absorption of cholesterol, phytosterols and certain oxysterols. Ezetimibe monotherapy and in combination with statin therapy is primarily indicated for lowering LDL-cholesterol levels. In addition, it may favorably affect other parameters that could potentially further reduce atherosclerotic coronary heart disease risk, such as raising HDL-cholesterol and lowering levels of triglycerides, non-HDL-cholesterol, apolipoprotein B and remnant-like particle cholesterol. Further effects of ezetimibe include a reduction in circulating phytosterols and oxysterols and, when used in combination with statins, a reduction in high-sensitivity C-reactive protein. The clinical significance of the LDL-cholesterol lowering and other effects of ezetimibe is being evaluated in clinical outcome studies.

Keywords::

• ezetimibe
• HDL-C
• non-HDL-C
• oxysterols
• phytosterols
• RLP-C

Figure 1. Overview of cholesterol metabolism.

ApoB: Apolipoprotein B; CE: Cholesterol ester; CETP: Cholesteryl ester transfer protein; chol: Cholesterol; CM: Chylomicron;

CMr: Chylomicron remnant; IDL: Intermediate-density lipoprotein; LCAT: Lecithin-cholesterol acetyltranferase; NPC1L1: Niemann–Pick C1-like 1 protein; SR-BI: Scavenger receptor type BI; TG: Triglyceride; VLDL: Very-low-density lipoprotein.

Figure 2. Hepatic cholesterol synthesis and lipoprotein packaging.

Adapted with permission from [2].

Figure 3. Analogy of free cholesterol packaging into apolipoprotein B-containing lipoproteins in peripheral tissue, liver and intestine.

The esterification and lipoprotein packaging of cholesterol by the liver (hepatocyte) and intestine (enterocyte), occur intracellularly. Peripheral cholesterol transport via HDL particles occurs in the plasma.

ACAT: Acyl-CoA:cholesterol acyl transferase; CETP: Cholesteryl ester transfer protein; LCAT: Lecithin-cholesterol acyltransferase; MTP: Microsomal triglyceride transfer protein; VLDL: Very-low-density lipoprotein.

Reproduced with permission from [2].

Figure 4. Peripheral cholesterol transport.

ABCA1: Adenosine triphosphate-binding cassette transporter; ApoA-I : Apolipoprotein A-I; B100: Apolipoprotein B-100-containing lipoproteins; B48: Apolipoprotein B-48-containing lipoproteins; CE: Cholesterol ester; CETP: Cholesteryl ester transfer protein; CM: Chylomicron; CMr: Chylomicron remnant; FC: Free cholesterol; IDL: Intermediate-density lipoprotein; LCAT: Lecithin-cholesterol acyltransferase; LDLR: LDL receptor; NPC1L1: Niemann–Pick C1-like 1 protein; SR-BI: Scavenger receptor type BI; TG: Triglyceride; VLDL: Very-low-density lipoprotein.

Figure 5. NPC1L1 controls cholesterol absorption in the intestine and is the target of ezetimibe.

ACAT: Acyl-CoA:cholesterol acyltransferase; NPC1L1: Niemann–Pick C1-like 1 protein.

Figure 6. Complementary mechanisms of action lead to broader lipid control.

ApoA-I: Apolipoprotein A-I; B100: Apolipoprotein B-100-containing lipoproteins; B48: Apolipoprotein B-48-containing lipoproteins; CE: Cholesterol ester; CETP: Cholesteryl ester transfer protein; chol: Cholesterol; CM: Chylomicron; CMr: Chylomicron remnant; IDL: Intermediate-density lipoprotein; LCAT: Lecithin-cholesterol acetyltransferase; LDLR: LDL receptor; NPC1L1: Niemann–Pick C1-like 1 protein; SR-BI: Scavenger receptor type BI; TG: Triglyceride; VLDL: Very-low-density lipoprotein.

Figure 7. Dose–response effect of ezetimibe monotherapy.

*p < 0.05 versus placebo.

EZE: Ezetimibe.

Reproduced with permission from [51].

Figure 8. Ezetimibe add-on to statin therapy.

^*p < 0.001; ^**p < 0.05; ^***p < 0.01.

HDL-C: High-density lipoprotein-cholesterol; LDL-C: Low-density lipoprotein-cholesterol; TG: Triglyceride.

Reproduced with permission from [54].

Figure 9. C-reactive protein and inflammation.

CRP: C-reactive protein.

Figure 10. Effect of ezetimibe, simvastatin and ezetimibe/simvastatin on RLP-C.

^*p < 0.001 EZE/SIMVA versus same dose SIMVA.

^**p < 0.05 EZE/SIMVA versus same dose SIMVA.

^‡p < 0.05 EZE/SIMVA versus next highest dose SIMVA.

EZE: Ezeteimibe; RLP-C: Remnant-like particle cholesterol; SIMVA: Simvastatin.

Adapted with permission from [65].

Figure 11. Cardiovascular disease risk factors and association with phytosterols and cholesterol synthesis precursors.

Adapted with permission from [122].

Figure 12. Ezetimibe and statin effect on phytosterol concentrations.

^* p < 0.001 versus placebo.

^‡p < 0.001 versus pooled SIMVA.

^§p < 0.05 versus EZE.

^¶p < 0.001 versus EZE.

^#p < 0.001 versus pooled ATORVA.

ATORVA: Atorvastatin; EZE: Ezetimibe; SIMVA: Simvastatin.

Adapted with permission from [129].

Figure 13. Ezetimibe and statin effects on cholesterol precursors.

^*p < 0.001 versus placebo.

^‡p < 0.05 versus placebo.

^§p < 0.05 versus EZE.

^¶p < 0.001 versus EZE

ATORVA: Atorvastatin; EZE: Ezetimibe; SIMVA: Simvastatin.

Adapted with permission from [129].

Reducing LDL-cholesterol (LDL-C) levels by administering statins reduces atherosclerotic coronary heart disease (CHD) risk and cardiovascular disease (CVD)-associated events by approximately 20–40% [1–3]. Further reduction of CHD risk in high-risk individuals probably requires more aggressive LDL-C lowering for longer periods of time than studied by existing CHD outcomes trials. It is possible that it is the degree of LDL-C lowering, and not the means by which it is achieved, that may be most important in reducing CHD events [4]. Therefore, combining lipid-altering drug therapies with complementary mechanisms of action for the purpose of further LDL-C lowering has been recommended as a therapeutic approach for high CHD-risk individuals [1]. Combination lipid-altering therapies may also improve other CHD-associated risk factors that go beyond LDL-C levels.

Ezetimibe is an intestinal cholesterol absorption inhibitor that, in both monotherapy and combination therapy, significantly reduces LDL-C levels [5,6]. Furthermore, ezetimibe may have additional effects. This review describes the cholesterol-lowering effects of ezetimibe, as well as other effects that may potentially reduce other CHD risk factors.

Sources of plasma cholesterol

To better describe the inter-relationship of the complementary effects that lipid-altering drugs may have on CHD risk reduction, it is important to understand clinically applicable cholesterol physiology. Simply stated, plasma cholesterol is derived from two main sources in the body: cholesterol biosynthesis (peripheral and hepatic) and intestinal cholesterol absorption (Figure 1)[2,5,7]. The relative contribution of cholesterol from these sources depends upon various other factors, including genetic predisposition, diet and drug therapies [5,8,9].

Cholesterol biosynthesis

Animal data suggest that most cholesterol synthesis occurs in peripheral tissues such as intestine, muscle and skin, with the greatest amount per gram of tissue produced in endocrine organs (e.g., ovary and adrenal glands) [5,10,11]. This is because cholesterol is an important component of cell membrane stability and serves as the backbone structure for steroidogenesis. In addition to its critical regulatory role in cholesterol homeostasis, the liver is also an important organ with regard to cholesterol synthesis and excretion into the intestine. Finally, cholesterol derived from vascular endothelial macrophages associated with arterial plaques is a clinically important, but qualitatively minor contributor to total circulating cholesterol levels [7,10,12].

In both peripheral tissues and the liver, cholesterol is synthesized through a series of enzymatic steps. The major precursor for the synthesis of cholesterol is acetyl coenzyme A (acetyl-CoA), which gives rise to hydroxyl-methylglutaryl coenzyme A (HMG-CoA) (Figure 2). The rate-limiting step in the cholesterol biosynthetic pathway is the conversion of HMG-CoA to mevalonic acid, which is catalyzed by HMG-CoA-reductase. Once synthesized from mevalonic acid, hepatic cholesterol is esterified by the action of acyl-CoA-cholesterol acyl transferase (ACAT) and incorporated into apolipoprotein B (ApoB)-100-containing very-low-density lipoprotein (VLDL) via microsomal triglyceride transfer protein (MTP). It is then secreted into the plasma for transport to peripheral tissues (Figure 3). In the blood, VLDL particles are further metabolized through hydrolysis of triglycerides (TG) by lipases, yielding intermediate-density lipoprotein (IDL) particles, some of which are cleared by the liver, with the remainder being converted to LDL particles – the major carrier of cholesterol in the plasma (Figures 1 & 2).

Cholesterol synthesis in peripheral tissues also contributes to the hepatic cholesterol pool through the transfer of cholesterol to the liver in a process mediated by HDL particles (peripheral, or sometimes referred to as ‘reverse’ cholesterol transport) (Figures 3 & 4) [13]. Free cholesterol is transferred from peripheral tissues to pre-β-HDL-C particles via the ATP-binding cassette transporter 1 (ABCA1) pathway. These nascent ApoA-I-containing lipoproteins undergo remodeling to become mature HDL-C in the plasma through esterification of the free cholesterol by lecithin-cholesterol acetyltransferase (LCAT). Circulating, mature HDL particles may undergo holoparticle uptake by the liver via a putative receptor, or the cholesterol from HDL particles may be transferred to the liver after interaction with the hepatic scavenger receptors (SR-BI). Once in the liver, the cholesterol delivered by HDL particles may be utilized for hepatic cholesterol needs, converted into bile acids, or excreted into bile and eliminated through the feces [2,14]. In addition to delivery to the liver, circulating HDL particles may transfer cholesterol to ApoB-containing atherogenic lipoproteins (such as LDL, VLDL, IDL and chylomicron remnants) through cholesteryl ester transfer protein (CETP). The cholesterol associated with LDL particles may be taken up by the liver via hepatic LDL receptors [2,14], which represents another route for peripheral tissue cholesterol and HDL-C delivery to the liver.

Although most of the cholesterol produced in the body may be derived from peripheral tissues, the major organ that regulates the amount of circulating cholesterol is the liver, through its cholesterol synthesis and packaging into lipoproteins, and through up- or downregulation of its LDL receptors [5,15]. Through the process of hepatic LDL receptor-mediated endocytosis, LDL particles and other ApoB-lipoproteins are removed from the blood and taken into hepatocytes, where they are catabolized to release cholesterol. Free cholesterol can be stored in these cells as cholesterol ester, the concentration of which plays a role in regulating cholesterol synthesis and hepatic LDL receptor activity, thus helping to regulate circulating levels of LDL-C. In humans, the cholesterol eliminated by fecal excretion is equally derived from cholesterol that is converted to acidic steroids (such as bile acids) and neutral steroids (such as cholesterol itself).

Whether from lipoprotein-mediated hepatic uptake, endogenous hepatic synthesis, or delivery from intestinal absorption, the hepatic cholesterol pool directs the degree to which cholesterol is metabolized [16]. During times of higher hepatic cholesterol availability, LDL receptors may become down-regulated and circulating cholesterol levels are increased. When hepatic cholesterol availability is reduced, LDL receptors may become upregulated and circulating cholesterol levels are decreased.

Intestinal cholesterol absorption

Intestinal sterol absorption is an important factor towards determining circulating cholesterol levels [5,9,10]. It has been suggested that dietary sterols are not a major contributor to circulating cholesterol and other sterols, except during times of very high cholesterol consumption; however, this is not to suggest that intestinal cholesterol absorption is physiologically unimportant [10]. This is because up to three-quarters of the cholesterol delivered to the intestine is derived from biliary cholesterol excretion from the liver and, thus, intestinal cholesterol absorption is a clinically relevant mechanism that significantly contributes to hepatic cholesterol.

In the intestinal lumen, both biliary and dietary cholesterol undergo micellar adaptation by bile acids, which allows for enhanced transport through the brush border membranes into intestinal epithelial cells (Figure 1). Free cholesterol within intestinal cells may then return to the intestinal lumen through adenosine-binding cassette (ABC) transporters G5 and G8 [17], or alternatively be esterified through ACAT and packaged into chylomicron particles within intestinal epithelial cells by MTP in a metabolic process somewhat analogous to that which occurs in the liver and peripheral tissue (Figure 3 & Table 1). Chylomicrons are then transported through the lymphatic system and subsequently to the blood where cholesterol may be delivered to peripheral tissue and the liver [18]. During this process, removal of TG from chylomicrons by lipases results in the formation of cholesterol-rich chylomicron remnant particles that are taken up by the liver and added to the hepatic pool.

Genetic mutations resulting in the dysfunction of ABCG5 and ABCG8 transporters are linked to sitosterolemia – an inheritable disease characterized by an accumulation of phytosterols resulting from a net increase in intestinal absorption and possibly decreased biliary removal [19,20]. Hyperabsorption of both plant sterols and cholesterol in sitosterolemic patients consequently leads to increased plasma and tissue levels of plant sterols, which can result in the development of xanthomas and premature coronary heart disease, even in the absence of significant hypercholesterolemia [21]. It appears that these transporters may also promote biliary secretion of cholesterol in mice [22], but the relation of this to cholesterol metabolism in humans is not known.

Although the liver is recognized as the major regulator of cholesterol metabolism, the intestine also appears to play an important role in cholesterol flux and maintenance of plasma cholesterol homeostasis [16]. In addition to its role in cholesterol absorption, the intestine acts as an excretion organ and may contribute directly to fecal neutral sterol (cholesterol) excretion [23]. Another way wherein the intestine contributes to cholesterol flux is through HDL particle biogenesis [16]. In rats, almost all of the total plasma ApoA-I (the principal apolipoprotein of HDL-C) is produced by the liver and intestine, up to 56% of which is synthesized and secreted by the intestine [24,25]. The intestine also expresses significant levels of the transporter, ABCA1 (critical to the formation of HDL particles), and in mice approximately 30% of total plasma HDL-C is contributed by intestinal ABCA1 [24].

Nonesterified sterols and phospholipids in enterocytes transported by ABCA1 to lipid-free ApoA-I are assembled into nascent pre-β-HDL particles. Secretion of nascent, pre-β-HDL particles by the intestine into the circulation promotes cholesterol and phospholipid efflux from peripheral cells to these particles [13,16], and thus may enhance cholesterol flux to the liver.

Relationship between cholesterol synthesis & intestinal absorption

Cholesterol homeostasis is maintained through the processes of cholesterol synthesis and metabolism and is influenced by physiologic responses to fluctuations in dietary cholesterol intake. An inter-relationship exists between cholesterol absorption and biosynthesis [5,8,15]. When the intestinal absorption of cholesterol is reduced, less cholesterol is delivered to the liver, resulting in a compensatory rise in the rate of hepatic cholesterol synthesis and an increase in the number and activity of hepatic LDL surface receptors. Conversely, when more intestinal cholesterol is absorbed, cholesterol synthesis and LDL receptor activity are downregulated. Some studies have suggested that when cholesterol synthesis is reduced with a statin, a reciprocal increase in cholesterol intestinal absorption appears to take place as indicated by increased blood levels of phytosterols; however, other studies have not shown this compensatory effect. This inconsistency is perhaps attributable to differing methodologies used to assess cholesterol intestinal absorption [8].

Mechanisms of action

Ezetimibe monotherapy

Ezetimibe is a member of a class of lipid-altering agents known as cholesterol absorption inhibitors [27]. Ezetimibe blocks the intestinal absorption of both cholesterol and phytosterols through its interaction with the sterol transporter, Niemann–Pick C1-like 1 protein (NPC1L1), located on the brush border membrane of intestinal epithelial cells (Figure 5) [26–28], although other proteins may also be involved [29]. The NPC1L1 transporter is required for the transport of intestinal cholesterol and phytosterols from the intestinal lumen to the intracellular compartments of enterocytes, where sterol esterification and incorporation into chylomicron particles takes place [26–28]. Ezetimibe may also interact with hepatic NPC1L1 proteins, which are highly expressed in human liver cells and thought to regulate biliary cholesterol concentration and, thus may potentially affect circulating cholesterol levels [30].

In animal models, inhibition of intestinal cholesterol absorption by ezetimibe results in a reduction of the cholesterol content packaged into chylomicrons, which subsequently decreases the amount of cholesterol delivered by chylomicrons to the liver [31]. This reduction in cholesterol delivered to the liver results in a compensatory upregulation of hepatic LDL receptors, thereby enhancing LDL-C clearance and lowering plasma levels of LDL-C (Figure 6) [32–34]. In monkeys fed a high-fat, high-cholesterol diet, ezetimibe treatment substantially reduced the cholesterol ester content of postprandial chylomicrons but had no effect upon the levels of ApoB-48 (the major structural protein of intestinally derived lipoprotein particles), indicating that ezetimibe depleted cholesterol from chylomicrons without reducing the number of chylomicron particles [31]. Although ApoB-48 levels remained unchanged, levels of ApoB-100 (a large hepatic glycoprotein critical to the assembly, secretion, and intravascular transport of VLDL, IDL and LDL) were reduced by ezetimibe treatment, suggesting that decreased delivery of cholesterol to the liver ultimately leads to enhanced clearance [35] and/or reduced production of atherogenic lipoproteins [31]. These effects of ezetimibe are supported by an ApoB-48 and -100 kinetic study in men with primary hypercholesterolemia [35], which has also suggested that ezetimibe monotherapy does not have a significant impact on ApoB-48 kinetics, although the catabolic rate of ApoB-100-containing VLDL, IDL and LDL may be increased.

In the LDL r^-/- (homozygous null receptor) mouse model, virtually none of the circulating cholesterol is taken up by tissues, and changes in circulating LDL-C reflect only the rate of LDL production. In this model, ezetimibe reduces the production of VLDL and LDL [34]. Ezetimibe also induces an increase in the expression of LDL receptors in LDL r^+/+ mice. In patients with homozygous familial hypercholesterolemia, an autosomal-dominant inherited disorder generally caused by mutations in LDL receptor genes [36], ezetimibe significantly reduces LDL-C levels, when added to ongoing statin therapy [37]. In totality, these data support increased clearance and reduced hepatic production as important mechanisms of ezetimibe in reducing circulating LDL-C and other atherogenic lipoprotein levels.

Ezetimibe plus statin

Statins and ezetimibe represent two complementary lipid-altering drugs that, through dual inhibition, lower LDL-C levels. Extensive evidence supports the use of statins for LDL-C lowering and CVD event reduction [38]. Statins inhibit hepatic cholesterol synthesis, which upregulates LDL receptors, resulting in increased clearance of ApoB-containing atherogenic lipoproteins, such as LDL and VLDL, from the plasma [32,39]. Enhanced receptor clearance of other lipoproteins may also mediate reductions in ApoB-48-containing chylomicrons [39,40]. Decreased hepatic cholesterol synthesis also reduces VLDL synthesis and secretion [15,39], and the amount of biliary cholesterol delivered to the intestine, which, in turn, may upregulate intestinal cholesterol absorption [39,41–43].

Ezetimibe inhibits intestinal cholesterol absorption and results in increased intestinal and liver cholesterol synthesis in mice, which could potentially lead to a partial attenuation of LDL-C lowering [26,34,44]. Thus, reducing cholesterol absorption with ezetimibe and reducing cholesterol synthesis with statins provide complementary mechanisms of action. Several studies have demonstrated that when administered with a statin, ezetimibe provides at least additive improvements in lowering LDL-C, non-HDL-C, TG and ApoB, and in raising HDL-C levels over that of statins alone (Figure 6)[6,45].

The combination of ezetimibe with statins upregulates LDL receptors and significantly reduces LDL-C and ApoB levels [46]. In a miniature pig model of cholesterol metabolism, coadministration of simvastatin resulted in significant reductions of ezetimibe-induced hepatic cholesterol synthesis and a marked increase in hepatic LDL receptor expression [46]. The combination also decreased ApoB-100 levels, attributed to reduced VLDL production and enhanced LDL r-mediated clearance of LDL-C. Ezetimibe-induced expression of LDL receptors and the subsequent increased clearance of both ApoB-100- and -48-containing lipoproteins may account for lowering of both cholesterol and tri-glyceride levels, particularly when combined with statin therapy.

LDL-C-lowering effects of ezetimibe monotherapy & combination with statins in clinical trials

In clinical studies, ezetimibe (10 mg) monotherapy significantly reduces LDL-C levels in hypercholesterolemic patients by -17.2 to -22.3% (p < 0.01 to < 0.001) compared with placebo (Table 2) [47–50]. Doses of 20 and 40 mg ezetimibe were well tolerated in early trials; however, these doses provided minimal additional lipid-altering benefit (Figure 7) [51,52]. Hence, ezetimibe 10 mg once a day is the dose indicated for the treatment of hypercholesterolemic patients [53].

In combination therapy, when added to statins, ezetimibe reduces LDL-C levels significantly beyond those attainable with statins alone (-5.9 to -21.0%; p < 0.05 to < 0.001) in hypercholesterolemic patients (Table 3 & Figure 8) [54–64]. The ezetimibe/simvastatin combination tablet (Vytorin^®) also significantly reduces LDL-C levels in an additive manner, -4.2 to -14.0% (p < 0.001) beyond that of simvastastin, atorvastatin or rosuvastatin alone [65–67]. The enhanced LDL-C lowering observed when ezetimibe is added to statins is due to the aforementioned complementary action of the two agents.

Finally, ezetimibe significantly reduces LDL-C levels when added to ongoing, stable, statin therapy in patients with metabolic syndrome (-16.7 and -21.4%) and Type 2 diabetes mellitus (T2DM) (-26.1 and -24.8%) compared with placebo (p < 0.001 for both) [68,69]. Similarly, in T2DM patients, the ezetimibe/simvastatin combination tablet significantly lowers LDL-C from baseline compared with atorvastatin at all dose comparisons (-6.6 to -15.3%; p < 0.001) [70].

Ezetimibe: lipid effects beyond LDL-C lowering

Optimal CHD risk reduction may best be achieved by modification of multiple CHD risk factors [20]. Nonattainment of optimal LDL-C levels, coupled with unfavorable HDL-C and/or TG levels, increases the adjusted risk of CHD events [20]. In addition to LDL-C lowering, ezetimibe favorably alters the levels of other parameters considered to be associated with CHD risk, such as TG, HDL-C, non-HDL-C, ApoB, high-sensitivity C-reactive protein (hsCRP) and RLP-C [1,71].

Non-HDL-C

Non-HDL-C sums cholesterol carried by atherogenic lipoproteins such as ApoB-100-containing LDL, VLDL, IDL, RLP, lipoprotein [Lp(a)] and, in certain instances, ApoB-48-containing chylomicrons [72]. Thus, non-HDL-C, a measure of total cholesterol minus HDL, is thought to be a better predictor of CHD risk than LDL-C alone [72]. As all these lipoproteins have the potential to deliver cholesterol into the arterial wall, non-HDL-C reflects a degree of atherogenic risk not quantified by measurement of LDL-C alone [73]. It is for this reason that the National Cholesterol Education Program (NCEP) ATP III has recommended non-HDL-C treatment goals, set at levels 30 mg/dl above the respective LDL-C treatment goals, as a secondary target for high-risk patients with TG of more than 200 mg/dl [1,74]. Similarly, AHA/ACC guidelines recommend that non-HDL-C should be reduced to less than 130 mg/dl in patients with TG of more than 200–499 mg/dl, and further reduction to less than 100 mg/dl in these patients is reasonable [71].

Data from animal studies have shown that reduced intestinal absorption with ezetimibe monotherapy decreases non-HDL-C levels [73]. In a pooled analysis, ezetimibe itself and in the monotherapy arms of combination studies reduced non-HDL-C significantly more than placebo (-19.7%; p < 0.001) in patients with mild-to-moderate hypercholesterolemia [75]. Ezetimibe has also shown favorable effects upon non-HDL-C when added to statins (Table 3). In add-on to statin therapy studies, ezetimibe reduced non-HDL-C from baseline significantly more (-19.9% to -25.3%; p < 0.001) than placebo [54–56]. Ezetimibe added in combination with statins also significantly reduced non-HDL-C (-4.6 to -19.0%; p < 0.05 to < 0.001) compared with statins alone at various doses and across pooled doses [57–61,64]. The ezetimibe/simvastatin combination tablet reduced non-HDL-C levels from baseline significantly more than simvastatin and ro-suvastatin (-12.7 and -4.1%, respectively; p < 0.001) [65,67].

Measurement of non-HDL-C in patients with metabolic syndrome and T2DM is particularly relevant as these patients tend to have high amounts of atherogenic-type lipoproteins often found with elevated TG, TG-rich lipoproteins (TRL), and ApoB and small, dense LDL particles [76], which may not be fully realized by measuring LDL-C levels alone. When ezetimibe was added to ongoing, stable statin therapy in patients with metabolic syndrome, non-HDL-C was significantly reduced by -15.4 and -19.5% (p < 0.001) and in T2DM patients by -23.8 and -21.8% (p < 0.001) compared with statins alone across pooled doses [68,69]. In T2DM patients, the combination ezetimibe/simvastatin 10/20 mg provided significant additional reductions in non-HDL-C levels of -13.1 and -6.7% (p < 0.001 compared with atorvastatin 10 and 20 mg, respectively) and of -5.6% with ezetimibe 10/40 mg versus atorvastatin 40 mg (p < 0.001) [70].

Reductions in LDL-C, VLDL-C, and RLP-C levels account for the ezetimibe-induced decreases in plasma non-HDL-C levels [65,76], attributed to its mechanism of action described for lowering of levels of LDL-C and TG (below), and of ApoB (below). Ezetimibe has not been shown to reduce Lp(a) levels [49,54,57,58,60,64].

HDL cholesterol

Levels of HDL-C correlate with CHD risk [1,74]. Therefore, raising HDL is recommended by some organizations in patients at higher risk for CHD [74,77].

Ezetimibe monotherapy elicits modest increases in HDL-C levels. In several monotherapy trials, ezetimibe 5 and 10 mg provided additional increases in HDL-C from baseline levels compared with placebo (2.2%; p > 0.05 and 2.3 to 3.5%; p < 0.05 to < 0.01; Table 2) [47–50]. Ezetimibe in combination with statins is also associated with modest further improvements in HDL-C levels, typically greater than those achieved with statins alone (Table 3). Addition of ezetimibe 10 mg to stable, ongoing statin therapy in patients with primary hypercholesterolemia resulted in significant additional increases from baseline in HDL-C pooled across doses in comparison to placebo (1.7 and 2.1%, p < 0.05 to < 0.001; and 2.6%, p > 0.05) [54–56]. Coadministration of ezetimibe and statins increases HDL-C levels more than statins alone at various doses and across pooled doses, with several data points achieving statistical significance (0.6 to 5.8%; p < 0.05 to < 0.001) [57–64]. Increases in HDL-C from baseline were somewhat higher for pooled doses of the combination tablet ezetimibe/simvastatin compared with simvastatin (0.4%; p > 0.05) [65]. When averaged across doses, ezetimibe/simvastatin significantly increased HDL-C levels versus atorvastatin (3.6%; p < 0.001) [66] and versus rosuvastatin, increases were comparable [67].

When added to stable, ongoing statin therapy, ezetimibe significantly increased HDL levels in patients with metabolic syndrome by 2.6% (p < 0.05), versus placebo addition, and provided comparable HDL-C increases in T2DM patients [68]. In another study, ezetimibe added to ongoing statin therapy significantly increased levels of HDL-C by 2.8 and 2.7%, compared with placebo (p = 0.002 and p < 0.001) in patients with metabolic syndrome and T2DM, respectively [69]. Treatment with ezetimibe/simvastatin 10/20 mg significantly increased HDL-C levels by 3.6 and 3.4% more than atorvastatin 10 and 20 mg (p ≤ 0.001), and with ezetimibe 10/40 mg by 4.0% v-ersus atorvastatin 40 mg (p < 0.001) in T2DM patients [70].

It is possible that the modest increases in HDL-C levels induced by ezetimibe may be attributed to its reduction in the delivery of cholesterol from the intestine to the liver, which subsequently enhances the clearance of TRL. A reduction in the levels of TRL, which are acceptors of cholesterol transferred from HDL particles by CETP, may contribute to a reduced rate of cholesterol transfer, thereby increasing levels of HDL-C [78,79].

Ezetimibe inhibition of cholesterol absorption may also enhance peripheral cholesterol transport through potential mechanisms, which may not be readily linked to HDL-C raising. The intestine and liver both contribute to plasma levels of ApoA-I [24] and synthesize significant levels of ABCA1, which mediates the flux of free cholesterol and phospholipids toward nascent ApoA-I in the formation of HDL particles. In mice deficient in intestinal-specific ABCA1, plasma HDL-C levels were reduced and ABCA1 was shown to be responsible for the direct lipidation of lipid-poor ApoA-1-containing particles [24,80]. Moreover, the absence of intestinal ABCA1 resulted in a decreased transport of dietary cholesterol into HDL particles. Since nascent HDL particles secreted by the intestine contain nonesterified sterols and free cholesterol transported by ABCA1 from the enterocyte, ezetimibe inhibition of cholesterol absorption may increase the proportion of lipid-free intestinal ApoA-I available, creating HDL particles with greater capacity to take up more cholesterol, thereby promoting cholesterol and phospholipid efflux from peripheral cells [13,16].

In a similar manner, chylomicrons may also play a role in promoting HDL-mediated flux of cholesterol from peripheral tissues [81]. These large, TG-rich particles deliver cholesterol to the liver via ApoE receptors. It is possible that ezetimibe-induced reduction of the cholesterol content of the chylomicrons may enhance the potency of chylomicrons as acceptors of cholesterol ester in the HDL-mediated flux of cholesterol. Potentially, enhanced HDL flux resulting in greater delivery of cholesterol to the liver is supported by findings that ezetimibe and ezetimibe/simvastatin treatment resulted in increased fecal sterol excretion in humans [82]. Ezetimibe also promoted the excretion of macrophage and HDL-derived cholesterol in the feces of mice, but had no effect on HDL metabolism [83]. This may be attributed not only to its blockade of intestinal cholesterol and sterol transport, but also to enhanced HDL-mediated cholesterol flux.

Triglycerides

Hypertriglyceridemia is considered to be an important risk factor, although it is not clear if elevated triglyceride levels are always an independent CHD risk factor and whether the lowering of TG levels is beneficial in reducing CVD risk [38,84,85]. If after LDL-C goals are reached, and TG levels are still elevated (>200 mg/dl), then it is recommended that non-HDL-C levels be assessed and specific non-HDL-C treatment goals be achieved [1,74].

In monotherapy studies, ezetimibe (10 mg) generally decreased TG levels compared with placebo (-2.7 and -4.1%, p > 0.05; and -11.4%, p < 0.01; Table 2) [47,49,50]. When coadministered with various doses of statins, ezetimibe resulted in greater additional reductions in TGs levels compared with placebo (-11.4 to -13.8%; p < 0.001) and statins alone across pooled doses (-7.4% to -12.8%; p < 0.01 to < 0.001) and at milligram-equivalent doses (0.0 to -11.0%; p > 0.05 to < 0.001) (Table 3) [57–64]. The combination ezetimibe/simvastatin tablet al.o reduces TG levels versus simvastatin (-3.5%, p < 0.001), atorvastatin (-1.4%; p > 0.05), and rosuvastatin (-2.7%; p < 0.001) when compared across pooled doses [65–67].

TG levels are often elevated in patients with metabolic syndrome and T2DM. The coadministration of ezetimibe and statins provides additional significant reductions in TG levels from baseline in these patients. When ezetimibe was added to stable, ongoing statin therapy, TG levels were significantly reduced compared with statins alone in patients with metabolic syndrome by an additional -6.5 to -10.7% (p < 0.001) and in T2DM patients by -10.9 to -12.3% (p < 0.001) [68,69]. The combination tablet, ezetimibe/simvastatin 10/20 mg, reduced TG levels significantly more than atorvastatin at 10 mg (-4.4%; p = 0.02), with comparable reductions at 10/20 mg ezetimibe/simvastatin versus atorvastatin 20 mg and 10/40 mg ezetimibe/simvastatin versus atorvastatin 40 mg in T2DM patients [70].

The mechanism of ezetimibe-induced reduction of TG levels is unclear. Reductions in plasma and hepatic TG are observed in mice and hamsters administered ezetimibe [34]. Given that ezetimibe does not block fat absorption, decreased TG levels are likely attributed to a secondary effect resulting from the reduced delivery of cholesterol from the intestine to the liver, which in turn decreases VLDL production. As noted before, the clearance of TRLs is potentially increased through up-regulation of ApoB-100 and ApoB-48 receptors, resulting in an increased clearance of VLDL, chylomicrons and their respective remnants.

ApoB-100

As the majority of the total ApoB-containing lipoproteins in the plasma are associated with LDL particles, measurement of ApoB primarily reflects ApoB-100 levels [86]. Similar to non-HDL-C, it has been suggested that measurement of plasma ApoB levels is a better predictor of CHD risk than LDL-C [86–89] and concentrations less than 90 mg/dl may be considered an alternative secondary target for high-risk patients by the ATP III and CMA/Canadian guidelines [74,90]. Non-HDL-C atherogenic lipoproteins contain ApoB, and both measurements correlate to increased CHD risk [72,86], although the concordance of this in various population distributions remains unclear [87,91]. Measurement of ApoB is thought by some to provide a more accurate assessment of lipoprotein atherogenicity than non-HDL-C [86–89] because unlike non-HDL-C, which measures the cholesterol content of particles, ApoB provides a direct assessment of atherogenic particle number. Particle number is thought to be an important determinant of atherogenic burden and CHD risk, although the association of particle size and composition with CHD risk remains controversial [72,86,88,89]. Chylomicron particles contain 1 molecule of ApoB-48, and VLDL, IDL and LDL-C particles contain one molecule each of ApoB-100. Thus, plasma ApoB levels represent the best estimate of the total number of these atherogenic lipoproteins, in particular, ApoB-100-containing lipoproteins [86].

In monotherapy studies, ezetimibe was shown to reduce ApoB from baseline significantly more than placebo (-14.1 and 14.3%; p < 0.01; Table 2) [49,50]. In the monotherapy arm of a pooled analysis, ezetimibe (10 mg) decreased ApoB significantly more than placebo (-15.7%; p < 0.05) [62]. When added to ongoing statin therapy, ezetimibe significantly reduced ApoB compared with placebo (-15.5 and -16.3%; p < 0.001; Table 3) [54,55]. Long-term ezetimibe treatment added to ongoing simvastatin therapy (48 weeks) was also associated with an improvement in ApoB levels compared with placebo (-13.9%, significance not assessed) [56]. Similarly, ezetimibe coadministered with statins was significantly more effective in reducing ApoB levels from baseline at various dose comparisons and when pooled across doses compared with statins alone (-2.9 to -17.0%; p < 0.05 to < 0.001; Table 3) [57–64]. The ezetimibe/simvastatin combination tablet al.o reduced ApoB significantly more (p < 0.001) from baseline compared with simvastatin (-10.7%) and rosuvastatin (-2.9%) [65,67].

In patients with metabolic syndrome and in patients with T2DM, measurement of ApoB is considered an important indicator of CVD risk for atherogenic lipoprotein particles [1,72]. Adding ezetimibe to ongoing statin therapy significantly reduces ApoB levels compared with placebo in metabolic syndrome and T2DM patients, respectively (-13.2 and -18.1%; p < 0.001) [68]. The ratio of ApoB/ApoAI, thought to be one of the most correlative predictors of CVD risk [88], was significantly reduced by the addition of ezetimibe compared with placebo to ongoing statin therapy in both metabolic syndrome and T2DM patients (-16.6 and -17.7%; p < 0.001) [69].

Ezetimibe may lower plasma levels of ApoB lipoproteins through its reduction of hepatic cholesterol concentrations, leading to decreased atherogenic ApoB lipoprotein production [92]. Additionally, kinetic studies have shown that ezetimibe reduction of cholesterol delivery to the liver increases the hepatic uptake and catabolism of ApoB-100-containing lipoproteins VLDL, IDL and LDL [46], which would further reduce ApoB levels.

High-sensitivity C-reactive protein

It is recognized that CHD is largely an inflammatory process, and inflammation is often assessed by measurements of the inflammatory marker hsCRP. Elevated hsCRP is indicative of increased CHD risk [93]. CHD risk factors may contribute to inflammation and subsequent vascular injury, which eventually lead to atherosclerotic disease (Figure 9). CRP is an acute phase reactant protein produced primarily in the liver, which is released in response to acute injury, infection or other inflammatory stimuli, and is considered to be an emerging marker for the diagnosis and management of CHD, according to ATP III guidelines and a scientific statement from the AHA/CDC [74,94].

The reduced progression of CHD associated with intensive statin treatment correlates with reductions in hsCRP levels [95,96]. While ezetimibe monotherapy reduces hsCRP compared with placebo, these differences (-6.1 to 10.3%) have generally not been found to be statistically significant [62,63,65,97,98]. By contrast, ezetimibe added to ongoing statin therapy for 6–8 weeks significantly reduces hsCRP (-9.7 and -10.0%; p < 0.05) compared with added placebo when pooled across doses (Table 3) [54,55] and when assessed long-term after 48 weeks of treatment (-14.3%; significance not assessed) [56]. Similarly, in a pooled analysis, ezetimibe added to a baseline of statin therapy significantly reduced hsCRP (-10.4%; p < 0.001) compared with statins alone [98].

When coadministered in combination with simvastatin or atorvastatin, ezetimibe provided significant additional reductions in hsCRP compared with either statin alone (-10.0 to -24.6%; p < 0.01 to < 0.001) (Table 3) [59,62–64], which was similar to the findings when the ezetimibe/simvastatin combination tablet was compared with simvastatin pooled across doses (-14.3%; p < 0.001) [65]. The combination tablet ezetimibe/simvastatin produced additional hsCRP reductions (-0.3 and -2.9%), which were comparable to atorvastatin or rosuvastatin, respectively [66,67]. In a pooled analysis of several studies, combination therapy with ezetimibe/simvastatin produced significantly greater reductions in hsCRP when compared with simvastatin (-16.7%; p < 0.001) across doses and at individual dose comparisons (-14.1 to -19.1%; p < 0.05 to < 0.01) [97]. Reductions of similar magnitude were observed when compared with atorvastatin at individual doses (-4.2 to 3.2%) and across doses (-0.3%) [97]. Overall, these results suggest an additional anti-inflammatory action of combining ezetimibe plus statins [63].

Elevated levels of hsCRP are also associated with an increased risk of CHD in T2DM patients [99]. In patients with metabolic syndrome and T2DM, ezetimibe added to a baseline of statin therapy reduced hsCRP significantly more (-6.5 and -15.4%) compared with addition of placebo (p < 0.05) [68]. In another study, ezetimibe added to ongoing statin therapy reduced hsCRP significantly more (-14.8%; p < 0.001) than placebo in T2DM patients (p < 0.001), and in metabolic syndrome patients, hsCRP was further reduced by -3.2%; however, this was nonsignificant [69]. In T2DM patients, ezetimibe/simvastatin 10/20 mg significantly reduced hsCRP versus atorvastatin 10 mg (-10.2%; p < 0.05) and was comparable at the other doses [70].

The mechanism by which ezetimibe potentiates statin reduction of hsCRP is not known. Indirectly, this could be due to improvements in various lipid parameters, which attenuate the inflammatory process, as reflected by a reduction in markers of inflammation such as hsCRP. In single studies, statin-induced lowering of LDL-C and reductions in hsCRP correlate weakly [100], though a recent meta-analysis of LDL-C lowering intervention trials showed a strong correlation between LDL-C and hsCRP [101]. This suggests that different individuals may vary in achieving the amount of LDL-C lowering required to reduce inflammatory responses reflected by hsCRP [102].

Although the clinical trial evidence strongly supports that the vast majority of benefit with the use of lipid-altering drugs relates to the alteration of lipids, potential nonlipid anti-inflammatory effects on hsCRP levels cannot be ruled out [102]. It also cannot be discounted that hsCRP may potentially have some direct role in the pathogenesis of atherosclerosis, such as enhancing the uptake of oxidized LDL by endothelial cells and macrophages, decreasing nitric oxide production and inducing the expression of vascular cell adhesion molecules [95]. Studies have indicated that ezetimibe and statins interact directly with monocytes and with macrophages, which may lead to further reductions in the pathological effects of inflammation on CHD [103,104].

Remnant-like lipoproteins & lipoprotein subclass distribution

Remnant lipoproteins

Elevated levels of TRL are associated with an increased risk of CHD [72,73,77]. In particular, remnant-like lipoproteins (RLPs), a subspecies of smaller, partially catabolized TRL, are thought to be atherogenic and have consequently been identified as a potential risk factor for CHD [105–108]. Elevated plasma levels of RLP-cholesterol (RLP-C) are characteristic of many patient groups at increased risk for CVD [107,108]. RLP-C is included in the measurement of non-HDL-C levels, a secondary treatment target after LDL-C lowering in hypertriglyceridemic patients (>200 mg/dl), as mentioned previously [74]. Additionally, following isolation from plasma, RLPs can be further characterized by various methods based on density, size, charge, apolipoprotein composition and immunoabsorption [108,109].

RLPs are the result of intravascular remodeling of TG-rich chylomicron and VLDL metabolic by-products into smaller and denser particles, which have higher atherogenic potential than larger, more buoyant, particles (Figures 1, 3 & 4) [107,108]. In the bloodstream, both chylomicrons and VLDL undergo lipolysis, primarily through the action of lipoprotein lipases, in a process that removes most of the associated TG and ApoC proteins. Cholesterol ester may then be added by CETP. Remnant chylomicrons return to the liver via LDL receptor-mediated endocytosis and VLDL can be further metabolized to form IDL, which can also be taken up by the liver. The remaining IDL particles in the bloodstream are further degraded to LDL particles. Studies have indicated that the increased atherogenicity of these partially lipolyzed TRLs is attributed to an increase in arterial intima diffusion and retention, oxidative modification, enhanced endothelial cell layer permeability and longer residence time in the plasma due to reduced affinity for LDL receptors [107,108].

In patients with hypercholesterolemia, ezetimibe treatment had a more pronounced effect on reducing RLP-C than placebo (-21%), although the statistical significance of this was not assessed [65]. Treatment with the ezetimibe/simvastatin combination resulted in significantly greater reductions in RLP-C levels (-11.3%; p < 0.001) across pooled doses compared with the corresponding pooled doses of simvastatin (Figure 10) [65]. The ezetimibe-mediated decrease in RLP-C levels may also be related to increased catabolism of TRL, as previously noted for non-HDL-C and ApoB.

Lipoprotein subclass

Each of the five major classes of plasma lipoproteins (chylomicrons, VLDL, IDL, LDL and HDL particles) may be defined on the basis of density and/or size, apolipoprotein composition, and differing proportions of associated cholesterol and TG. These lipoprotein subclasses are further comprised of distinct, particle subclass distributions that differ in physicochemical properties, certain of which are characteristic of the atherogenic phenotype [72,77,108]. Several studies have demonstrated that patients with mixed dyslipidemia often have high levels of small, dense, LDL particles, large TG-rich VLDL, small, dense, HDL particles and low levels of HDL-C [110,111]. There is considerable heterogeneity in the size and density of LDL particles that range from large, buoyant to small, dense particles (the latter considered to have the most atherogenic potential) [111–113]. On this basis, the LDL profile of an individual can be classified as either pattern A phenotype, characterized by a predominance of large, buoyant particles or pattern B, characterized by a predominance of small, dense LDL particles [114].

Limited studies have assessed the effect of ezetimibe on lipoprotein subclass distribution. In the monotherapy arms of coadministration studies, ezetimibe itself reduced the cholesterol content of lipid subfractions (VLDL-C, IDL-C, LDL-CR, VLDL-C1+2, VLDL-C3, LDL-C subclasses) when evaluated by vertical autoprofile (VAP) analysis in patients with mixed hyperlipidemia and primary hypercholesterolemia [115–117]. When combined with simvastatin, these effects were significant [117]. However, ezetimibe and ezetimibe/simvastatin had minimal effect on the size and distribution of LDL particles, with modest shifts in size from small, dense to larger, more buoyant particles, assessed by segmented gradient gel electrophoresis (S₃GGE) in these patients.

Because the size of TRL pools are related to lipid subfraction patterns [118,119], similar to statins, the ezetimibe effect on particle size will be most pronounced in hypertriglyceridemic patients. In a recent study of patients with primary dyslipidemias, ezetimibe reduced the concentration of all LDL subfractions (tube gel, Lipoprint), with significant increases in the reduction of small, dense LDL particles and a shift toward larger and less atherogenic LDL particles, especially in patients with hypertriglyceridemia (TG > 200 mg/dl) [118]. Similarly, ezetimibe coadministered with fenofibrate produced greater reductions in lipoprotein subfractions (VAP), in addition to a preferential loss in small, dense LDL particles (S₃GGE), compared with fenofibrate alone [115], although this was largely attributed to the TG-lowering effect of fenofibrate.

Taken together, these studies indicate that ezetimibe and the combination of ezetimibe and simvastatin reduce the cholesterol content of lipid subfractions, but have little effect on subfraction distribution, aside from a pronounced reduction of small, dense LDL particles in patients with primary dyslipidemias and elevated TG. As mentioned previously, the mechanism by which ezetimibe reduces levels of atherogenic lipoproteins is attributed to its reduction of the intestinal delivery of cholesterol to the liver, leading to increased LDL receptor activity, which correlates with diminished VLDL synthesis and enhanced clearance and catabolism of TRL [35,39]. Correspondingly, if the amount of cholesterol in chylomicrons is decreased, then the amount of cholesterol in the respective remnants of chylomicrons following ezetimibe treatment may also be reduced. Ezetimibe, combined with statin inhibition of HMG CoA reductase, which also results in diminished VLDL production, and increased catabolism and clearance of TRL, can lead to an overall enhanced reduction of atherogenic lipoproteins and RLP.

Ezetimibe: phytosterol effects

Phytosterols and phytostanols are minor components of vegetable oils, cereals, fruits, vegetables, seeds and nuts that are effectively absorbed into the enterocyte, and then very efficiently pumped back into the intestinal lumen. Thus, dietary phytosterols and stanols undergo minimal systemic absorption from the GI tract; however, significant variability in intestinal absorption of phytosterols can occur based on genetic background and other factors. Elevated blood levels of plant sterols may increase CHD risk, although the evidence regarding this association is mixed [120–127]. In particular, the excessive intestinal systemic absorption and impaired biliary removal that occurs in hypersitosterolemia patients promotes abnormally high blood levels of these phytosterols and a tendency to develop CHD as early as the teen years [21]. The net hyperabsorption of both plant sterols and cholesterol in sitosterolemia patients usually (but not always) leads to hypercholesterolemia.

In these patients, high blood sterol levels cause characteristic development of tendon xanthomas (cholesterol deposits in skin and tendons), and red blood cell abnormalities that lead to chronic anemia. Sitosterolemia patients have normal or only moderately elevated levels of total sterols, but have a very high ratio of plant sterol to total cholesterol. Historically, sitosterolemia has been treated by strict limitation of dietary cholesterol and plant sterols and with cholestyramine resin administration to promote sterol excretion from the body. Due to its proven efficacy to reduce plant sterol intestinal absorption, ezetimibe is the only lipid-altering agent approved as adjunctive therapy for the treatment of this disorder [53].

A variety of techniques can be used to estimate cholesterol absorption and synthesis, including measurement of mass absorption of cholesterol, plasma isotope ratio, and circulating levels of phytosterols (e.g., sitosterol and campesterol) and cholesterol synthesis precursors (e.g., desmosterol and lathosterol) (Figures 2 & 11) [8,43]. In addition to substantially inhibiting fractional cholesterol absorption, ezetimibe monotherapy produces significant, progressive reductions in plasma concentrations of phytosterols in patients with hypersitosterolemia, consistent with the hypothesis that ezetimibe inhibits intestinal absorption of both plant sterols and cholesterol [21]. In patients with hypercholesterolemia, ezetimibe reduced circulating levels of campesterol by -48% and sitosterol by -41%, compared with placebo (p < 0.001 for both) [128], again supporting a marked reduction in plant sterol absorption. In both studies, the reduction of cholesterol absorption by ezetimibe was accompanied by an increased ratio of lathosterol to cholesterol, suggesting enhanced hepatic cholesterol synthesis [43].

In a study that compared the effect of ezetimibe, statins, and ezetimibe/statins on noncholesterol sterols, ezetimibe monotherapy significantly lowered plasma concentrations of sitosterol and campesterol from baseline compared with placebo (p < 0.001) (Figure 12) [129]. By contrast, these phytosterols were modestly increased by atorvastatin and mildly decreased by simvastatin (Figure 12). These changes were significantly different compared with the decreases in phytosterols induced by ezetimibe alone (p < 0.05 to p < 0.001). As anticipated, statins also reduced cholesterol synthesis precursors, lathosterol and desmosterol (Fi-gure 13), whereas ezetimibe increased the levels of these cholesterol precursors, suggesting an enhancement of cholesterol synthesis by ezetimibe. However, combination therapy with ezetimibe and simvastatin or atorvastatin reduced both the concentrations of phytosterols (campesterol and sitosterol) and cholesterol precursors (lathosterol and desmosterol) (Figures 12 & 13). These data reflect a complementary benefit of simultaneous inhibition of both sterol absorption and synthesis.

Ezetimibe: oxysterol effects

Oxysterols include oxygenated derivatives of cholesterol and plant sterols. Enzymatic and nonenzymatic oxygenation of cholesterol results in a number of circulating and intracellular derivatives, which may have a variety of biologic activities, such as roles in sphingolipid metabolism, platelet aggregation, apoptosis and protein prenylation [130]. Cholesterol is also converted to a variety of oxysterols as a precursor to bile acid synthesis. Due to processing, heating or prolonged storage of animal-derived foods, the typical Western diet contains substantial quantities of oxidized cholesterol [131,132]. Studies in feeding animal models and epidemiological studies in humans have indicated that the addition of oxidized cholesterol to the diet increases the development of atherosclerosis [132]. In addition, plant sterols can also be nonenzymatically auto-oxidized, especially during food storage.

Oxysterols are absorbed by the small intestine and incorporated into chylomicrons and LDL-C. Identified in the arterial wall, oxysterols may contribute to increased levels of oxidized lipoproteins in the intima [132]. Ezetimibe (10 mg) reduces the serum levels of oxysterols, and the incorporation of oxidized cholesterol into postprandial chylomicrons and LDL particles [133]. The absence of changes in postprandial TGs indicated that the decrease in oxysterol levels was attributed to the inhibition of oxidized sterol absorption rather than enhanced clearance.

Conclusion & expert commentary

Ezetimibe 10 mg/day significantly reduces LDL-C from baseline levels compared with placebo, and when added to statin therapy, provides significant further reductions in LDL-C levels. Comparable results are obtained with the combination tablet, ezetimibe/simvastatin (Vytorin). The LDL-lowering effects of ezetimibe and ezetimibe/simvastatin have been consistently demonstrated in different populations including patients with metabolic syndrome and T2DM.

Existing data support the hypothesis that ezetimibe has complementary and additive therapeutic lipid effects when combined with statins in the treatment of patients with dyslipidemia. In addition to LDL-C lowering, ezetimibe significantly improves other aspects of the lipid profile. Ezetimibe monotherapy reduces levels of TG and ApoB and increases HDL-C in comparison with placebo. When added to statins, ezetimibe provides significantly greater improvements in TG, HDL-C, non-HDL-C, ApoB and hsCRP. Similar effects on these parameters are consistently observed in different patient populations, including those with metabolic syndrome and T2DM. Plasma levels of RLP-C and lipoprotein subfractions are also favorably altered. These effects (and others) may impact cholesterol and lipid metabolism, as well as help to stimulate peripheral cholesterol transport. Ezetimibe itself lowers plasma phytosterol levels. It has also been shown to reduce serum levels of oxysterols and the incorporation of oxidized oxysterols into postprandial chylomicrons and LDL particles.

The potential clinical benefits of ezetimibe’s LDL-C lowering, and its other effects, await further assessment in clinical trials. The Ezetimibe and Simvastatin in Hypercholesterolemia Enhances Atherosclerosis Regression (ENHANCE) trial was a surrogate outcome trial that assessed changes in intima media thickness (IMT) in a familial hypercholesterolemic patient population. In this unique study, treatment with ezetimibe/simvastatin (10/80 mg) for 2 years did not demonstrate significant differences in IMT when compared with simvastatin 80 mg alone. [134,201]. However, ENHANCE was not a CHD outcomes study. Thus, the potential clinical benefits of ezetimibe, as assessed by traditional determinations of actual CHD events, and in a different patient population, await the results of specifically designed CHD outcomes studies (Table 4).

From a safety perspective, ezetimibe administered as monotherapy or as combination therapy is generally well tolerated. Ezetimibe itself has an adverse event profile similar to that of placebo for up to 12 weeks of treatment [47–51]. Similarly, coadministration of ezetimibe (10 mg) with statins (10–80 mg doses) results in no significant differences in the incidences of laboratory and clinical adverse events, including gastrointestinal, liver or muscle effects during 6–24 weeks of treatment [53–61,64]. The safety profile of the combination tablet ezetimbe/simvastatin at doses up to 10/80 mg also was found to be well tolerated in clinical studies when compared with atorvastatin (20 to 80 mg) and rosuvastatin (10 to 40 mg) [65–67]. In patients with metabolic syndrome and T2DM, safety profiles were also similar to those demonstrated in other patient populations [68–70].

Having said this, while ezetimibe monotherapy does not appear to increase liver enzymes, pooled data suggest that the frequency of liver enzyme elevation may be slightly (∼1%) higher when ezetimibe is added to statins. Thus, the prescribing information recommends that when ZETIA^® is coadministered with a statin, that liver enzyme tests should be performed at initiation of therapy and according to the recommendations of the statin [53]. From a practical standpoint, an interpretation of this guidance could be that adding ezetimibe to a statin may be considered as being analogous to increasing the dose of a statin. In other words, if it is recommended that increasing a statin dose from 40 to 80 mg should be accompanied by subsequent liver enzyme monitoring, then it would seem to be prudent to do the same if ezetimibe is added to a statin. Finally, while it is true that muscle adverse experiences have not been reported in controlled clinical trials of ezetimibe, isolated case reports of myopathy have been described with ezetimibe. Thus, the prescribing information recommends that all patients starting therapy with ezetimibe should be advised of the risk of myopathy and told to report promptly any unexplained muscle pain, tenderness or weakness. If myopathy is diagnosed or suspected, ezetimibe and any statin and/or fibrate should be discontinued immediately.

Five-year view

Whether LDL-C lowering through dual inhibition of reduced cholesterol absorption and synthesis translates to enhanced clinical benefit for reducing CHD events awaits further assessment in longer term, outcome-based clinical trials (Table 4)[135–137]. During the next several years, the results of these CHD outcome studies will help to better clarify the clinical importance of the metabolic effects of ezetimibe, with respect to LDL-C lowering and beyond.

Table 1. ABC transporters and disease.

Transporter	Location	Function	Disease/disorder caused by dysfunction of transporter
ABC A1	Plasma membranes of multiple body tissues including macrophages, liver, placenta, adipose tissue and spleen	Transports phospholipids and cholesterol, and is believed to be a rate-limiting step in cholesterol transport from the peripheral tissue to the liver	Tangier disease Familial HDL deficiency
ABC G5	Liver and intestine	Transports intestinally absorbed cholesterol and plant sterols/stanols	Sitosterolemia
ABC G8	Liver and intestine	Transports intestinally absorbed cholesterol and plant sterols/stanols	Sitosterolemia

ABC: Adenosine-binding cassette.

Adapted with permission from [2].

Table 2. Ezetimibe monotherapy studies.

Study (year)	Treatment			% difference compared with placebo^* (mg/dl)				Ref.
	n	Weeks	mg	LDL-C	TG	HDL-C	ApoB
Sudhop et al. (2002)	18	2	E 10	-22.3^‡	-2.7	2.2	ND	[47]
Bays et al. (2001)	432	6	E 5 E 10	-15.7^§ -18.5^§#	ND	2.9^¶ 3.5^¶	ND	[48]
Dujovne et al. (2002)	816	12	E 10	-17.2^§	-11.4^§	2.9^§	-14.1^§	[49]
Knopp et al. (2003)	827	12	E 10	-18.5^§	-4.1	2.3^§	-14.3^§	[50]

^*Based on difference of individual LS means for LDL-C, HDL-C and Apo B, and on difference of individual medians for TG.

^‡p < 0.001 versus placebo.

^§p < 0.01.

^¶p < 0.05.

^#p < 0.05 versus 5 mg ezetimibe.

Apo B: Apolipoprotein B; E: Ezetimibe; HDL-C: HDL-cholesterol; LDL-C: LDL-cholesterol; ND: Not determined; TG: Triglyceride.

Table 3. Ezetimibe combination studies.

Study (year)	Treatment			% difference compared with placebo or indicated statin^### (mg/dl)						Ref.
	n	Weeks	mg	LDL-C	TG	HDL-C	Non-HDL-C	ApoB	hsCRP (mg/l)
Add-on studies^*
Gagne et al. (2002)	769	8	All E/Stat^** vs placebo	-21.3^#	-11.4^#	1.7^§	-19.9^#	-15.6^#	-9.7^§	[54]
Pearson et al. (2005)	3030	6	All E/Stat^‡‡ vs placebo	-23.1^#	-11.5^#	2.1^#	-20.6^#	-16.3^#	-10.0^#	[55]
Masana et al. (2005) (extension of [54])	433	12	All E/S^§§ vs placebo	-27.0^#	-13.8^#	2.6	-25.3^#	-13.9^¶¶¶	-14.3^¶¶¶	[56]
Combination studies^‡
Davidson et al. (2004)	2382	12	All E/Stat^¶¶ vs All Stat^¶¶	-13.3^¶	-9.0^¶	2.9^¶	-13.0^¶	-10.6^¶	ND	[57]
Davidson et al. (2002)	668	12	All E/S^## vs All S^##	-13.8^¶	-7.4^¶	2.4^§	-13.5^¶	-10.9^¶	ND	[58]
Goldberg et al. (2004)	887	12	All E/S^## vs All S^##	-14.8^#	-12.8^#	0.6	-14.4^#	-12.9^#	-24.6^#	[59]
Feldman et al. (2004)	710	5	E/S10 vs S 20	-9.0^#	-0.0	1.1	-8.0^#	-6.0^#	ND	[60]
		5	E/S 20 vs S 20	-15.0^#	-6.0^§	2.9^§	-14.0^#	-11.0^#	ND
		5	E/S 40 vs S 20	-21.0^#	-11.0^#	2.3	-19.0^#	-17.0^#	ND
Ballantyne et al. (2004)	788	6	E/S 10 vs A 10	-8.9^§	-3.3	2.9^§	-7.6^§	-6.0^§	ND	[61]
		6	E/S 20 vs A 10	-13.1^§	-2.6	4.4^§	-11.1^§	-9.5^§	ND
		12	E/S 20 vs A20	-5.9^§	0.9	2.1	-4.6^§	-2.9^§	ND
		12	E/S 40 vs A20	-10.0^§	-3.1	5.5^§	-8.7^§	-7.2^§	ND
		18	E/S 40 vs A 40	-6.5^§	0.7	3.6^§	-5.1^§	-3.0^§	ND
		24	E/S 80 vs A 80	-6.9^§	-0.9	5.8^§	-5.0^§	-3.4^§	ND
Sager et al. (2003) (Post hoc of [58])	575	12	All E 10 vs placebo	-17.8^§	-12.4^¶	4.5^¶	ND	-15.7^¶	-10.3	[62]
			All E/S^## vs All S^##	-14.3^¶	-9.8^¶	2.5^¶	ND	-11.5^¶	-16.6^¶
Sager et al. (2005) (Post hoc of [58] and [59])	575	12	All E/S^## vs All S^##	-14.8^¶	-10.5^¶	1.5	ND	-12.0^¶	-19.0^¶	[63]
Ballantyne et al. (2003)	1703	12	All E/A^*** vs All A^***	-12.1^¶	8.0^¶	3.0^¶	-11.2^¶	-9.3^¶	-10.0^¶	[64]
Combination tablet^‡
Bays et al. (2004)	1528	12	All E/S^## vs All S^##	-14.0^#	-3.5^#	0.4	-12.7^#	-10.7^#	-14.3^#	[65]
Ballantyne et al. (2005)	1902	6	All E/S^## vs All A^***	-8.1^#	-1.4	3.6^#	ND	ND	-0.3	[66]
Catapano et al. (2006)	2959	6	All E/S^‡‡‡ vs All R^§§§	-4.2^#	-2.7^#	0.0	-4.1^#	-2.9^#	-2.9	[67]

^*Baseline measured at time of ezetimibe or placebo add-on to ongoing stable statin therapy.

^‡Baseline measured following placebo run-in prior to active treatment.

^§p < 0.05.

^¶p < 0.01.

^#p < 0.001 for ezetimibe or ezetimibe/statin vs placebo or indicated statin.

^**Simvastatin (10, 20, 40, 80 mg), atorvastatin (10, 20, 40, 80 mg), lovastatin (10, 20, 40 mg), pravastatin (10, 20, 40 mg), fluvastatin (20, 40, 80 mg), cerivastatin (0.2, 0.3, 0.4, 0.8 mg).

^‡‡Atorvastatin (10, 20, 40, 80 mg), simvastatin (10, 20, 40, 80 mg), pravastatin (20, 40 mg), other statins.

^§§Dose of simvastatin equipotent to patient’s current statin therapy.

^¶¶10, 20, 40 mg of atorvastatin, simvastatin, pravastatin or lovastatin.

^##Simvastatin (10, 20, 40, 80 mg).

^***Atorvastatin (10, 20, 40, 80 mg).

^‡‡‡Simvastatin (20, 40, 80 mg).

^§§§Rosuvastatin (10, 20, 40 mg).

^¶¶¶48 weeks.

^###Based on difference of individual LS means for LDL-C, HDL-C, non-HDL-C and Apo B, and on difference of individual medians or nonparametric analysis for TG and hsCRP.

A: Atorvastatin; Apo B: Apolipoprotein B; E: Ezetimibe10 mg; HDL-C: HDL-cholesterol; hsCRP: High-sensitivity C-reactive protein; LDL-C: LDL-cholesterol; ND: Not determined; R: Rosuvastatin; S: Simvastatin; Stat: Statins; TG: Triglyceride.

Table 4. Clinical outcome studies.

Study	Treatment	Clinical end points	Population	Ref.
SEAS	Combination ezetimibe 10 mg plus simvastatin 40 mg vs placebo	Composite of major CVD events and progression of existing aortic stenosis	n = 1873 pateints with asymptomatic aortic stenosis	[135]
SHARP	Combination ezetimibe 10 mg plus simvastatin 20 mg vs placebo	Incidence of stroke, myocardial infarction and revascularization	n = 9000 patients with chronic kidney disease	[136]
IMPROVE-IT	Ezetmibe/simvastatin 10/40 mg combination tablet vs simvastatin 40 mg	CVD outcomes	n = 10,000 patients with acute coronary syndrome	[137]

CVD: Cardiovascular disease; IMPROVE-IT: Improved Reduction of Outcomes: Vytorin Efficacy International Trial; SEAS: Simvastatin and Ezetimibe in Patients with Aortic Stenosis; SHARP: Study of Heart and Renal Protection.

Key issues

•	Ezetimibe is an inhibitor of cholesterol absorption indicated to improve lipid levels in hypercholesterolemic patients. At a 10 mg dose, ezetimibe effectively reduces LDL-C levels compared with placebo. Ezetimibe also reduces levels of triglyceride and apolipoprotein (Apo)B and modestly increases HDL-C.
•	When combined with statins, ezetimibe’s complementary mechanism of action provides further improvements in the overall lipid profile. When ezetimibe is concomitantly administered with statins, LDL-C levels may be reduced up to approximately 60%. Ezetimibe added to statins significantly reduces TG, HDL-C, non-HDL-C, ApoB and high-sensitivity C-reactive protein compared with statins alone. Ezetimibe reduces RLP-cholesterol levels and, in hypertriglyceridemic patients, may improve lipoprotein subfractions. These effects (and others) may impact cholesterol and lipid metabolism, as well as help to stimulate peripheral cholesterol transport.
•	Ezetimibe administered as monotherapy or as combination therapy is generally well tolerated. Ezetimibe administration results in no significant increases in clinical and laboratory adverse experiences, including gastrointestinal, hepatic and muscle effects.
•	Assessment of the clinical benefit of ezetimibe in reducing cardiovascular disease in patients awaits further evaluation in coronary heart disease outcome-based clinical trials. Several clinical trials are underway to evaluate the effect of ezetimibe coadministered with simvastatin on cardiovascular events and outcomes.

Financial & competing interests disclosure

Harold Bays has served as a Clinical Investigator for (and has received research grants from) pharmaceutical companies such as Abbott, Amylin, Alteon, Arena, AstraZeneca, Aventis, Bayer, Boehringer Ingelheim, Boehringer Mannheim, Bristol Myers Squibb, Ciba Geigy, Daiichi Sankyo, Eli Lilly, Esperion, Fujisawa, GelTex, Genetech, GlaxoSmithKline, Hoechst Roussel, Hoffman LaRoche, InterMune, KOS, Kowa, Lederle, Marion Merrell Dow, Merck, Merck Schering Plough, Miles, Microbia, Novartis, Obecure, Orexigen, Parke Davis, Pfizer, Pliva, Purdue, Reliant, Roche, Rorer, Regeneron, Sandoz, Sanofi, Sciele, Searle, Shionogi, Schering Plough, SmithKline Beacham, Takeda, TAP, UpJohn, Upsher Smith, Warner Lambert, and Wyeth-Ayerst. He has also served as a consultant, speaker and/or advisor to and for pharmaceutical companies such as Abbott, Arena, AstraZeneca, Aventis, Bayer, Bristol Myers Squibb, Daiichi Sankyo, KOS, Merck, Merck Schering Plough, Metabasis Therapeutics, Microbia, Novartis, Nicox, Ortho-McNeil, Parke Davis, Pfizer, Reliant, Roche, Sandoz, Sanofi Aventis, Schering Plough, SmithKline Beacham, Takeda, UpJohn, and Warner Lambert. Drs Neff, Tershakovec and Tomassini are employees of Merck and own stock and stock options.

No writing assistance was utilized in the production of this manuscript.