On- and Off-Target Pharmacology of Torcetrapib Current Understanding and Implications for the Structure Activity Relationships (SAR), Discovery and Development of Cholesteryl Ester-Transfer Protein (CETP) Inhibitors

Lowering of serum low-density lipoprotein cholesterol (LDL-C) levels remains the primary aim of lipid management. Much progress has been made in reducing rates of cardiovascular disease morbidity and mortality, largely through increased awareness of lipid-lowering therapies and particularly through the use of high-efficacy LDL-C-lowering HMG-CoA reductase inhibitors (statins). While statins have been effective in reducing cardiovascular disease risk, many patients do not adequately achieve guideline-recommended LDL-C goals and may benefit from additional cholesterol management therapies. Low serum levels of high-density lipoprotein cholesterol (HDL-C) are considered another important determinant of cardiovascular disease risk, and increased serum HDL-C levels have been shown to be associated with reductions in the incidence of cardiovascular disease. One approach toward raising serum HDL-C levels is the inhibition of cholesteryl ester-transfer protein (CETP), a plasma protein that promotes the transfer of cholesteryl ester from HDL particles and other lipoprotein fractions to pro-atherogenic apolipoprotein B-containing lipoproteins. The inhibition of this protein raises HDL-C levels and also reduces LDL-C levels. The concept of raising HDL-C levels through pharmacological intervention of this target was validated in preclinical and clinical studies with torcetrapib, the first CETP inhibitor to be assessed in late-stage clinical trials. The large clinical outcomes trial, ILLUMINATE, was prematurely terminated due to other unexpected pharmacological effects of torcetrapib that led to an increased risk of cardiovascular events and deaths. Thus, the ultimate effect of CETP inhibition on cardiovascular disease outcomes remains to be determined. Other CETP inhibitors currently in development do not have the adverse effects of increased blood pressure and circulating levels of aldosterone shown to be structurally related to torcetrapib. Preclinical and pharmacology studies have shown that these CETP inhibitors are distinct compared with torcetrapib and lack the features related to its off-target pharmacology. These findings indicate that the off-target activities of torcetrapib are not necessarily class effects of CETP inhibitors. Recent clinical trials have shown that dalcetrapib, anacetrapib and evacetrapib, the most advanced of these compounds in development, effectively raise HDL-C levels and lower LDL-C in the absence of off-target activities. The results of these trials are encouraging within the limits of study size and duration and provide a rationale for conducting further studies, including large clinical outcomes trials to assess whether CETP inhibition can lead to cardioprotective effects. This review summarizes the data supporting the development of CETP inhibitors as HDL-C-raising therapy, including structure-activity relationships and preclinical and clinical pharmacology studies of known CETP inhibitors.