1. Pathway of Exogenous Cholesterol Metabolism
INTESTINAL
LUMEN
BILIARY TRACT
Plaque
formation CE
Bile acids
Synthesis
Free cholesterol
Remnants
DIET
Unstirred
water
layer
Summary Point: Exogenous cholesterol is carried in the plasma from the intestine to the liver via the lymphatic system. Endogenous cholesterol is synthesized in the liver and delivered to different organs by plasma.
Exogenous cholesterol is hydrolyzed to free cholesterol (FC) by pancreatic lipases secreted into the intestine. It is then transferred by micelles to the brush border of enterocytes, where it diffuses through the epithelial membrane to the interior of the cell. In the smooth endoplasmic reticulum, exogenous cholesterol is converted to cholesteryl ester (CE) by acyl CoA:cholesterol acyltransferase (ACAT), then packaged into chylomicrons. These chylomicrons are pumped through the thoracic lymph duct into the blood, where lipoprotein lipase (LPL) hydrolyzes them into chylomicron remnants.1-3 These particles are then transported to the liver for further processing, or if they are small enough (≤ 45 nm), they may penetrate arterial cells via transcytotic vesicles.4
Hepatic LDL receptors rapidly clear chylomicrons from the plasma. Liver cells metabolize the CE into bile acids or directly secrete it into the bile. Bile acids aid in the formation of micelles, and the secreted cholesterol is excreted via the feces.5
Hepatic cells also synthesize endogenous cholesterol and release VLDL and LDL-C into the circulation. These molecules are delivered to peripheral organs for either oxidation or storage.5
Cholesterol
FC biosynthesis
ACAT
Cholesteryl Ester (CE) FC
ENTEROCYTE
Brush
Border
BLOOD CE
Micelles
Chylomicrons
LYMPH
FUTURE RESEARCH

2. Complementary Mechanisms of Action Lead to Broader Lipid Control
VLDL
IDL
LDL
Statins
synthesis
Summary Point: Inhibition of cholesterol absorption combined with inhibition of cholesterol synthesis with a statin may provide greater LDL-C reductions given the complementary targets of exogenous and endogenous cholesterol with these approaches.
Inhibition of endogenous cholesterol synthesis with statins is a common therapeutic approach to lowering LDL-C. Statins competitively inhibit HMG-CoA reductase, an enzyme that catalyzes the conversion of HMG-CoA to mevalonate, a precursor of cholesterol. Given the dose-response limitations of statins1,2, known as the “Rule of Six”, and potential for increased liver transaminases with statin dose escalation, new approaches to lowering LDL-C are needed.
New concepts in lipid management include the selective inhibition of cholesterol absorption via the exogenous pathway. Inhibition of dietary and biliary cholesterol absorption may provide an additional approach to reducing plasma cholesterol.
BILIARY SECRETION
Absorption
Cholesterol
Absorption
Inhibition
INTESTINE
DIETARY
CHOLESTEROL
Excretion
FUTURE RESEARCH

3. Inhibition of Cholesterol Absorption
Summary Point: The process of cholesterol absorption is complex and currently only partly understood, but it can be blocked in several different ways.
Most of the cholesterol entering the intestinal lumen is biliary cholesterol; the remainder is obtained from the diet. The luminal cholesterol is initially packaged into micelles, then taken up by microvilli in the brush border membrane (BBM) of enterocytes. Bile acid sequestrants (e.g. cholestyramine), reduce the efficiency of cholesterol absorption by reducing the size of the bile acid pool and by reducing micelle formation.1 Plant sterols limit cholesterol absorption by either competing with cholesterol for solubilization into micelles2,3 or competing with cholesterol for proteins that facilitate sterol uptake into the small intestine.4
Uptake of micellar cholesterol is driven by the concentration of cholesterol in the micelle and the permeability coefficient. Once in the epithelial cell, cholesterol is either esterified by ACAT-2,5 incorporated into chylomicrons and absorbed, or returned to the lumen by the ABCA-1 transporter.6 If ACAT-2 activity is inhibited or if ABCA-1 gene activity is upregulated, cholesterol absorption will be inhibited.5,6
New approaches to lipid management include the selective inhibition of intestinal cholesterol absorption via the exogenous cholesterol pathway.
FUTURE RESEARCH

4. Ezetimibe Phase III Pooled Monotherapy: Efficacy Results
LDL-C
HDL-C TG
Placebo
Placebo (n=409)
Ezetimibe
10 mg
(n=1234)
Placebo (n=409)
Ezetimibe
10 mg
(n=1234)
Placebo (n=409)
Ezetimibe 10 mg
(n=1234)
EZ 10 mg
3.5
0.3
1.0* -5
-10
-15
-20
-1.6
Mean % Change in LDL from Baseline at Week 12
-4.2*
-17.4*
* p <0.01 vs. placebo
FUTURE RESEARCH

5. Ezetimibe Phase III Monotherapy: Pooled Safety Results
No. of Patients/Total (%)
Placebo Ezetimibe
(n=431) (n=1288)
Adverse events 285 (66) 802 (62)
Gastrointestinal 93 (22) 230 (18)
DC 2° AE (2.6) 51 (4)
Liver function tests (3 x ULN)
ALT (<1) (<1)
AST (<1) (<1)
GGT 10 (2) 20 (2)
Total bilirubin
ALP
Creatine kinase (CK) elevations
5-10 x ULN (<1)
10 x ULN (<1) (<1)
FUTURE RESEARCH

6. Results: Added Efficacy Across the Lipid Profile Regardless of Statin Used* ‡ * * * * * * * *
Lovastatin
Pravastatin
Simvastatin *
Atorvastatin *
*p<0.01 for EZE + statin vs statin alone; ‡p=0.22 for EZE + statin vs statin alone
FUTURE RESEARCH

7. Ezetimibe Co-administered with Statins
Ezetimibe co-administered with low dose statins, offers broader lipid control than that achieved by increasing the dose of the statin alone
Additional 5% 6%
HDL–C TG
40%
Additional -10%
LDL–C
20% TG
Additional -14 to 18 %
FUTURE RESEARCH

8. Homozygous FH Study: Efficacy on LDL-C When Added to Ongoing 40 mg of Statin
% Change From Baseline
(Statin 40 mg)
14%
21% * *
n=50
*P<0.01 vs statin 80 mg
FUTURE RESEARCH

9. Structure of Ezetimibe (SCH 58235)OH N O