A Stable-isotope Study in Healthy Normolipidemic Volunteers Comparing the Mechanisms of Action of Lifibrol and Pravastatin

Elevated lipoprotein concentrations are a major risk factor for the development of atherosclerotic cardiovascular disease. Effective reduction of low density lipoprotein (LDL)-cholesterol concentrations has been shown to greatly reduce this risk. The most widely used lipid-lowering agents are the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins), which have been shown to reduce morbidity and mortality from coronary heart disease (CHD) in large prospective clinical trials. However, despite significant LDL-C reduction approximately 70% of the events are still not avoided and the search for improved or alternative lipid-modifying drug therapies continuous. HDL-C has been addressed as a potential modifiable target for decreasing this residual risk, since a low HDL-C concentration is an acknowledged independent risk factor for CHD. However, recent studies showed that an increase in HDL-C concentrations was surprisingly not associated with a decrease in atherosclerosis, but with a possible increase. Therefore it seems that not the concentrations of HDL-C should be targeted but HDL function in reverse cholesterol transport.

Lifibrol is a lipid-modifying drug which has been shown to improve HDL particle flux via increased apoA-I production, while not having HDL-raising properties. Furthermore, it decreases dose-dependently LDL-C by up to 40%. It is of major interest to clarify the, apparently unique, mechanisms of action of a compound, whose LDL-lowering effects are comparable in magnitude to the ones of statins.

The mechanisms of lifibrol's LDL-lowering effects are not completely clarified. There is evidence suggesting that it increases hepatic LDL receptor expression by a sterol-independent mechanism, i.e. not through a reduction in cholesterol synthesis, the mechanism of action of statins. ApoB turnover studies have indicated that increased catabolism of LDL rather than a decrease in hepatic apoB production may be responsible for its cholesterol-lowering effects. Since apoB metabolism and cholesterol synthesis are closely related, we designed a study to investigate the effects of lifibrol on the metabolism of apoB-100-containing lipoproteins and on endogenous sterol synthesis in parallel, using stable isotope methods. In addition, since lifibrol may inhibit cholesterol synthesis at steps earlier than HMG-CoA reductase, we investigated [13C]acetate catabolism analyzing 13CO2 appearance in breath. The HMG-CoA-reductase inhibitor pravastatin was used as comparator, since its mode of action is well characterized. The principle questions addressed were (i) whether lifibrol exerts its cholesterol-lowering effects through decreased synthesis/enhanced catabolism of apoB-100-containing lipoproteins or through inhibition of sterol de novo synthesis and (ii) whether these effects are interrelated.