High-density lipoprotein cholesterol targeting for novel drug discovery: where have we gone wrong?

Lowering low-density lipoprotein-cholesterol (LDL-C) is an effective strategy to reduce cardiovascular risk. However, a significant residual risk remains in statin-treated patients. High-density lipoprotein cholesterol (HDL-C) is a strong, independent and inverse predictor of cardiovascular risk in many epidemiologic studies and has, therefore, emerged as a potential novel therapeutic target for addressing this substantial residual risk. Nevertheless, the failure of cholesteryl ester transfer protein (CETP) inhibitors and niacin in clinical trials has generated considerable speculation about the beneficial effects of HDL. Experimental studies have identified several HDL cardioprotective functions, including enhancement of macrophage reverse cholesterol transport and endothelial function and its antioxidant, anti-inflammatory and anti-thrombotic properties. Furthermore, HDL is highly heterogeneous and the atheroprotective functions of the different HDL subpopulations are not completely understood. Current available data indicate that increased HDL-C levels do not always correlate with enhanced HDL functions and, therefore, should not be considered a biomarker of HDL functionality. The clinical application of the novel HDL-based therapeutic approaches requires the development of validated and reproducible measures of key HDL functions.