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Journal of Lipid Research, Vol. 46, 2692-2698, December 2005
Copyright © 2005 by American Society for Biochemistry and Molecular Biology

Inhibition of cholesterol absorption by the combination of dietary plant sterols and ezetimibe

Lily Jakulj^*, Mieke D. Trip, Thomas Sudhop, Klaus von Bergmann, John J. P. Kastelein^* and Maud N. Vissers^1,*

^* Departments of Vascular Medicine, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands
Cardiology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands
Department of Clinical Pharmacology, University of Bonn, Bonn, Germany

Published, JLR Papers in Press, September 14, 2005. DOI 10.1194/jlr.M500260-JLR200

¹ To whom correspondence should be addressed. e-mail: m.n.vissers{at}amc.uva.nl

	ABSTRACT

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Consumption of plant sterols and treatment with ezetimibe both reduce cholesterol absorption in the intestine. However, the mechanism of action differs between the two treatments, and the consequences of combination treatment are unknown. Therefore, we performed a double-blind, placebo-controlled, crossover study for the plant sterol component with open-label ezetimibe treatment. Forty mildly hypercholesterolemic subjects were randomized to the following treatments for 4 weeks each: 10 mg/day ezetimibe combined with 25 g/day control spread; 10 mg/day ezetimibe combined with 25 g/day spread containing 2.0 g of plant sterols; 25 g/day spread containing 2.0 g of plant sterols; and placebo treatment consisting of 25 g/day control spread. Combination treatment of plant sterols and ezetimibe reduced low density lipoprotein cholesterol (LDL-C) by 1.06 mmol/l (25.2%; P < 0.001) compared with 0.23 mmol/l (4.7%; P = 0.006) with plant sterols and 0.94 mmol/l (22.2%; P < 0.001) with ezetimibe monotherapy. LDL-C reduction conferred by the combination treatment did not differ significantly from ezetimibe monotherapy (–0.12 mmol/l or –3.5%; P = 0.13). Additionally, the plasma lathosterol-to-cholesterol ratio increased with all treatments. Sitosterol and campesterol ratios increased after plant sterol treatment and decreased upon ezetimibe and combination therapy.

Our results indicate that the combination of plant sterols and ezetimibe has no therapeutic benefit over ezetimibe monotherapy in subjects with mild hypercholesterolemia.

Supplementary key words low density lipoprotein • humans • sitosterol • hypercholesterolemia

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Plasma levels of total cholesterol (TC) and low density lipoprotein cholesterol (LDL-C) can be substantially reduced by inhibition of cholesterol absorption within the intestine. This reduction in cholesterol absorption can be achieved either by daily consumption of plant sterols or stanols (1) or by treatment with ezetimibe (2). In fact, consumption of 2 g of plant sterols per day decreases plasma LDL-C by 10% (reviewed in 1, 3, 4), and treatment with 10 mg of ezetimibe once daily reduces plasma LDL-C by 15–20% (5, 6).

The precise mechanism by which free cholesterol is absorbed in the small intestine is not fully understood. Recently, two novel ATP binding cassette transporters, ABCG5 and ABCG8, were identified in this pathway. These proteins are expressed in the intestine as well as in the liver, where they function as heterodimer efflux transporters. Positioned at the apical surface of the intestinal and hepatic cells, they promote intestinal and biliary sterol excretion (7–9). Even more recently, other transporters that may be involved, the Niemann-Pick C1-Like 1 (NPC1L1) and aminopeptidase N proteins, have been identified (10, 11). Both proteins reside on the brush border membrane of enterocytes in the small intestine. They may play a role in cholesterol and plant sterol absorption and may be molecular targets for ezetimibe (11, 12).

Plant sterols and stanols are thought to compete with dietary and biliary cholesterol for incorporation into mixed micelles, thereby reducing the amount of cholesterol available for uptake by the enterocyte (13, 14). Plant sterols themselves are absorbed in exceedingly small amounts, because of the active secretion of those sterols back into the enteric lumen by ABCG5 and ABCG8 (7, 8). Net absorption of plant sterols ranges from 5% to 18% of total sterol mass, depending on the type of sterol (15, 16). In contrast to plant sterols, which increase plasma plant sterol concentrations, plant stanols, the saturated counterparts of plant sterols, reduce the absorption and, consequently, the plasma concentration of plant sterols (16). Ezetimibe, on the other hand, exerts its action in the brush border of the small intestine and reduces cholesterol absorption by inhibiting the uptake of dietary and biliary sterols, most likely through interaction with the NPC1L1 (10, 12) and/or aminopeptidase N (11) protein. Like plant stanols, ezetimibe inhibits both cholesterol and plant sterol absorption (12, 17, 18).

Prescription of ezetimibe to hypercholesterolemic patients is rapidly increasing, but these patients also often use food products that are enriched with plant sterols or stanols. Hence, it seems clinically relevant to investigate whether these two different modalities exhibit any interaction in terms of the modification of plasma lipid levels. Also, the safety of such a combination should be determined. Therefore, we designed a study to assess the efficacy of ezetimibe and plant sterols alone as well as their combination toward cholesterol and noncholesterol sterol plasma levels.

	SUBJECTS AND METHODS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Subjects
Subjects were recruited via advertisements in local newspapers. Each subject gave written informed consent. The study was approved by the institutional review board.

Subjects were included in the study if they were 18 years or older and had a plasma LDL-C concentration between 3.5 and 5.0 mmol/l. Exclusion criteria were a history of arterial disease, including unstable angina, myocardial infarction, transient ischemic attack, or a cerebrovascular accident; diabetes mellitus; uncontrolled hypertension; familial hypercholesterolemia; plasma triglyceride (TG) concentration > 4.0 mmol/l at baseline; or excessive alcohol consumption (>3 units per day). During the study, subjects were not allowed to use any other lipid-lowering medication or food products.

Study design
The study was a double-blind, placebo-controlled, crossover study for the plant sterol component with open-label ezetimibe treatment. Subjects started with a 2-week run-in period, in which they were not allowed to consume any plant sterol- or stanol-enriched food products or dietary vitamin supplements. Subjects who regularly consumed plant sterol or stanol products or who used cholesterol-lowering medication started with a 6-week run-in period. After the run-in period, subjects were randomly assigned to one of the following four treatment arms: a) 10 mg/day ezetimibe in combination with 25 g/day control spread; b) 10 mg/day ezetimibe in combination with 25 g/day spread containing 2.0 g of plant sterols; c) 25 g/day spread containing 2.0 g of plant sterols; or d) placebo treatment that consisted of 25 g/day control spread. After 4 weeks of treatment, subjects crossed over to the next study treatment, until they had completed the four treatments. Subjects were requested to maintain their usual pattern of food, drink, and smoking habits as well as physical activity during the whole study.

Plasma levels of TC, LDL-C, high density lipoprotein cholesterol (HDL-C), and TG were measured after an overnight fast of at least 12 h at baseline and at the end of each treatment period, as were liver and muscle enzymes. Plasma concentrations of lathosterol as well as those of cholestanol, sitosterol, and campesterol were measured at the end of each treatment period, because these noncholesterol sterols reflect cholesterol synthesis and absorption, respectively (19). Blood pressure and weight were assessed at each study visit. Physical examination was performed at baseline and at the end of the study.

Spread composition and administration
The placebo and the plant sterol-enriched spreads were produced and blinded by Unilever Research (Vlaardingen, The Netherlands). The placebo spread contained 8.8 g of fat, composed of 25% saturated fatty acids, 30% MUFAs, and 45% PUFAs per 25 g of spread. The plant sterol-enriched spread contained 8.8 g of fat, with a fatty acid composition of 23.5% saturated fatty acids, 25.6% MUFAs, and 49.9% PUFAs, and 2.0 g of plant sterols, composed of 46.5% sitosterol, 29.0% campesterol, 14.5% stigmasterol, and 10% other sterols, per 25 g of spread. The spreads were distributed in identical-looking 25 g tubs, labeled with different colors for each treatment arm. Subjects were instructed to keep the spreads in the refrigerator and to use one tub per day as a spread on sandwiches or as part of a hot meal, by mixing the spread with the food on the plate. The tubs were distributed at the start of each treatment period. Compliance was measured at each study visit by collection and calculation of the empty and full tubs of spread.

Ezetimibe
During two treatment periods, subjects were treated with 10 mg of ezetimibe (Ezetrol^®) per day, in addition to their daily consumption of spread. The ezetimibe treatment was not placebo-controlled; therefore, subjects only received tablets during the two ezetimibe periods, one with plant sterol spread, and one with control spread. Subjects were instructed to take one tablet of ezetimibe per day and to return the empty and full blister packs to evaluate compliance.

Plasma analyses
Plasma TC, HDL-C, and TG levels as well as the hepatic transaminases, aspartate aminotransferase (ASAT) and alanine aminotransferase (ALAT), and creatine phosphokinase (CPK) were measured with standard (automated) methods. Plasma LDL-C levels were calculated using the Friedewald equation (20).

Plasma concentrations of lathosterol, cholestanol, sitosterol, and campesterol were analyzed as trimethylsilylethers by gas-liquid chromatography (Hewlett-Packard 5890) using an automatic injection system (Automatic Sampler; Hewlett-Packard 7673A) with 5-cholestane as the internal standard (15). Because noncholesterol sterols are transported in serum by lipoproteins, changes in lipoprotein concentrations also affect concentrations of noncholesterol sterols (21). Therefore, noncholesterol sterols are expressed in concentrations (µg/dl) as well as in ratios to cholesterol (µg/mg).

Statistical analysis
Based on 40 subjects, our study had a statistical power of 80% to detect a difference of 0.15 mmol/l in LDL-C. In other words, we were able to detect an effect of 3.8% with a baseline level of 4 mmol/l and of 5% with a baseline level of 3 mmol/l.

Data were analyzed by ANOVA using the general linear model (GLM) of SAS (SAS Institute, Inc., Cary, NC). Tukey's procedure was used for pairwise comparisons and for the calculation of 95% confidence intervals between active treatments and placebo. Data for TG and liver and muscle enzymes were skewed and, therefore, log-transformed before statistical testing. Carry-over effects were checked by introducing a treatment-by-period interaction term in the model. P < 0.05 was considered statistically significant.

	RESULTS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
General
All 40 subjects completed the trial. The clinical characteristics of these subjects are shown in Table 1. Five of the subjects were females, aged 53–63 years, and none of them was on hormone replacement therapy. Based on the number of returned empty tubs of spread, the mean compliance for both types of spread was >94%. Based on the number of empty packs of ezetimibe, the mean compliance for ezetimibe treatment was >91%. One subject was excluded from statistical analysis because of TG concentrations of >4.0 mmol/l after treatment and probable noncompliance to the study protocol.

View larger version (14K): [in this window] [in a new window]   Fig. 1. Percentage changes (±SD) of total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), high density lipoprotein cholesterol (HDL-C), and triglyceride (TG) after treatment with plant sterols, ezetimibe, and the combination of plant sterols and ezetimibe. * Significant difference between treatment and placebo (P < 0.05).

View larger version (25K): [in this window] [in a new window]   Fig. 2. Individual differences in plasma low density lipoprotein cholesterol (LDL-C) concentration between the end of the 4-week treatment with plant sterols, ezetimibe, or their combination and the end of the 4-week placebo treatment.

View larger version (16K): [in this window] [in a new window]   Fig. 3. Plasma lathosterol- (A), cholestanol- (B), and campesterol- (C) to-cholesterol ratios at the end of treatment with placebo, plant sterols, ezetimibe, and the combination of plant sterols and ezetimibe. Values with different letters are significantly different from each other (P < 0.05).

	DISCUSSION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

The LDL-C lowering effects of plant sterols and ezetimibe monotherapy per se are in agreement with previous results. Although the LDL-C lowering effect of plant sterols in our study was relatively small, it was within the range of previous findings (1, 3, 4). Conversely, the LDL-C lowering effect of 22% with ezetimibe was relatively pronounced, because a mean reduction of 18% is more frequently reported (5, 6). There were no carry-over effects that could explain these results, and previous findings by others also do not suggest the existence of a carry-over effect in the current study. Mensink et al. (22) showed that the effects of 4-week treatment with 3 g/day plant stanols on TC and LDL-C concentrations were already maximal after 1 week, and values between the active treatment and placebo groups were comparable already 2 weeks after discontinuation of the treatments. Knopp et al. (23) showed that the maximal LDL-C reduction by ezetimibe was evident at 2 weeks. Moreover, Sudhop et al. (17) demonstrated that there was no carry-over effect in a crossover study with ezetimibe if subjects were treated with ezetimibe and placebo for 2 weeks each. Although they included a 2-week washout period between the two treatments, they also measured the effect of the one treatment 4 weeks after discontinuing the other. Therefore, the existence of carry-over effects is not very likely.

To our knowledge, this is the first study to investigate the combined effects of plant sterols and ezetimibe in humans. The combination has previously been studied in wild-type Kyoto rats (24), but their cholesterol levels increased after ezetimibe in combination with or without plant sterol treatment, probably because they possess a homozygous guanidine-to-thymine transversion in exon 12 of the Abcg5 gene (24). In our study, ezetimibe decreased cholesterol levels in correspondence with other human studies (5, 6). However, the combined effect of ezetimibe and plant sterols on lipid levels was unknown, although their mechanism of action is probably different. Ezetimibe reduces the absorption of cholesterol as well as that of plant sterols (17, 18, 25), probably through an interaction with the pathway that also involves NPC1L1 (10, 12). Plant sterols, on the other hand, are hypothesized to compete with cholesterol for incorporation into mixed micelles (26–29). Combining plant sterols and ezetimibe did not result in a significant decrease in LDL-C compared with ezetimibe alone, which may be attributable to a reduced amount of cholesterol that is available for the cholesterol transporter in the case of the combination treatment. As a consequence of the competition between plant sterols and cholesterol, the proportion of plant sterols in the micelles will increase and that of cholesterol will decrease (28). Because the rate of cholesterol and sitosterol absorption is directly proportional to their contents in micelles (30), an increased amount of plant sterols and a reduced amount of cholesterol will be transported through the brush border membrane and consequently be blocked by ezetimibe. In other words, the amount of cholesterol that can be blocked in the brush border may decrease when ezetimibe is combined with plant sterols because of an altered proportion of sterols in the micelles. However, more studies are needed to confirm this hypothesis.

The results of the cholesterol absorption markers support the hypothesis that ezetimibe strongly inhibits the absorption of plant sterols (12, 17, 18). When we combined treatment with plant sterols and ezetimibe, the sitosterol and campesterol levels and their ratios to cholesterol were increased compared with those after ezetimibe monotherapy (Fig. 3, Table 3). However, the increase of plasma sitosterol and campesterol levels and ratios by the addition of plant sterols to ezetimibe treatment was only approximately one-third of the increase after plant sterol consumption alone, which confirms that the absorption of plant sterols was reduced by ezetimibe treatment. Surprisingly, plasma sitosterol and campesterol concentrations and ratios were lower after the combination than after placebo treatment. On the one hand, this finding suggests that ezetimibe decreases the absorption of a larger amount of plant sterols than is additionally available from micelles by the consumption of 2 g/day plant sterols, but it also confirms that ezetimibe strongly inhibits plant sterol absorption.

It has also been suggested that plant sterols may exert a cholesterol-lowering effect within the enterocyte besides the competition with cholesterol for incorporation into the micelles (29, 31). For instance, an increased amount of plant sterols within the enterocyte may upregulate liver X receptor and thereby ABCA1 or ABC transporters involved in yet elusive mechanisms of cholesterol absorption (25, 29, 32). If plant sterols also affect cholesterol metabolism within the enterocyte, ezetimibe may in fact also counteract this effect by decreasing the amount of plant sterols within the cell. However, a recent study showed that the cholesterol-lowering effect of plant stanol esters was unrelated to changes in mRNA levels of intestinal ABC transporters or NPC1L1 (33); thus, data with respect to the upregulation of transport proteins by plant sterols or stanols are as yet inconsistent.

Our study had 80% power to detect a difference in LDL-C of 0.15 mmol/l, corresponding with 3.8% at a baseline level of 4 mmol/l or 5% at a baseline level of 3 mmol/l. Consequently, our study may have been underpowered to detect an effect of 0.12 mmol/l or 3.5% by adding plant sterols to ezetimibe. By increasing the number of subjects, the study would have gained more power and a difference of 0.12 mmol/l would have been statistically significant. However, an additional effect of only 0.12 mmol/l does not have much therapeutic advantage. We chose an LDL-lowering effect of >0.15 mmol/l to be clinically relevant, which was realized by plant sterols alone as well as by ezetimibe monotherapy (Table 2), but not by adding plant sterols to ezetimibe treatment. On the other hand, our subjects were mildly hypercholesterolemic, and including subjects with severe hypercholesterolemia may have resulted in a more pronounced difference between the combination therapy and ezetimibe monotherapy. Therefore, our results indicate that there is no therapeutic benefit of the combination of plant sterols and ezetimibe over ezetimibe monotherapy, at least in subjects with mildly increased cholesterol levels.

The lack of a placebo treatment for the ezetimibe arm is a limitation of this study. However, we included a placebo period consisting of a placebo spread to compare the effects of the other treatments. Usually, when a study lacks a placebo treatment, the effect of the intervention is calculated by subtracting baseline levels from levels after treatment, and consequently, a time effect or regression to the mean cannot be excluded. Because we included a placebo period, the effect of ezetimibe is not likely to be attributable to a time effect. Thus, even though the study design would have been more appropriate if a placebo treatment for the ezetimibe arm had been included, we think our data reflect real treatment consequences.

In conclusion, our results demonstrate that combination therapy of plant sterols and ezetimibe is safe. Nevertheless, the LDL-C reducing effect of combination therapy was not significantly greater than that of ezetimibe monotherapy, which indicates that combining the two cholesterol absorption inhibitors has hardly any therapeutic benefit compared with ezetimibe monotherapy in subjects with mild hypercholesterolemia.

	ACKNOWLEDGMENTS

 
The authors gratefully thank Elke Trautwein and Yvonne Veldhuizen of Unilever Research for their cooperation in this study and for providing the sterol-enriched and control spreads. The authors also thank Silvia Friedrichs for measurement of the noncholesterol sterols, Barbara Hutten for her assistance with statistical analyses, Bert Groen for valuable discussions during the preparation of the manuscript, and the volunteers for their enthusiastic participation.

Manuscript received June 20, 2005 and in revised form September 1, 2005.

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