Effects of anacetrapib on plasma lipids, apolipoproteins and PCSK9 in healthy, lean rhesus macaques

Abstract

Inhibition of cholesteryl ester transfer protein (CETP) has been vigorously pursued as a potential therapy to treat patients who are at an elevated risk for coronary artery disease. Anacetrapib, a novel CETP inhibitor, has been shown clinically to raise HDL cholesterol and reduce LDL cholesterol when provided as monotherapy or when co-administered with a statin. Preclinically, the effects of anacetrapib on the functionality and composition of HDL have been extensively studied. In contrast, the effects of anacetrapib on other parameters related to lipoprotein metabolism and cardiovascular risk have been difficult to explore. The aim of the present investigation was to evaluate the effects of anacetrapib in rhesus macaques and to compare these to effects reported in dyslipidemic humans. Our results from two separate studies show that administration of anacetrapib (150 mg/kg q.d. for 10 days) to rhesus macaques results in alterations in CETP activity (reduced by more than 70%) and HDL cholesterol (increased by more than 110%) which are similar to those reported in dyslipidemic humans. Levels of LDL cholesterol were reduced by more than 60%, an effect slightly greater than what has been observed clinically. Treatment with anacetrapib in this model was also found to lead to statistically significant reductions in plasma PCSK9 and to reduce cholesterol excursion in the combined chylomicron and remnant lipoprotein fraction isolated from plasma by fast protein liquid chromatography. Collectively, these data suggest that rhesus macaques may be a useful translational model to study the mechanistic effects of CETP inhibition.

Introduction

Cardiovascular disease remains the leading cause of death worldwide and morbidity and mortality continues to rise (Dahlof, 2010). Risk for cardiovascular disease can be significantly mitigated by reducing the level of low density lipoprotein (LDL) cholesterol in the blood. For patients who require lipid-lowering therapy to control levels of LDL cholesterol (LDL-c), treatment with statins is the standard of care. Statins work by inhibiting HMG-CoA reductase, the rate-limiting enzyme for cholesterol synthesis, and typically reduce LDL-c by 18–55% in patients (Istvan and Deisenhofer, 2001). Despite the significant improvements many patients obtain with this treatment; residual risk remains for those who cannot achieve levels of LDL-c below ~100 mg/dL and for those that are statin-intolerant, underscoring the need for additional therapies to further manage this risk.

Genetic and epidemiological studies have shown that high density lipoprotein cholesterol (HDL-c) levels are inversely correlated with cardiovascular risk (1–6) and several strategies have been pursued to raise levels of HDL-c pharmacologically (Barylski et al., 2013). Cholesteryl ester transfer protein (CETP) plays a well-established role in cholesterol homeostasis by exchanging neutral lipids (cholesteryl esters (CE) and triglycerides (TG)) between HDL and apoB-containing lipoproteins such as low density lipoprotein, very low density lipoprotein (VLDL) and chylomicrons (CM). Pharmacologically, CETP is often classified as an HDL-raising target due to the robust elevations in HDL-c that have been reported with CETP inhibitors in the clinic. Certain CETP inhibitors have also been shown to significantly reduce LDL-c and trends towards reductions in plasma triglycerides are also occasionally reported (Barter et al., 2007, Cannon et al., 2010, Nicholls et al., 2011, Schwartz et al., 2012). Anacetrapib, a selective, potent small molecule inhibitor of CETP has been clinically shown both to increase HDL cholesterol by up to 139% and to reduce LDL cholesterol by up to 40% in dyslipidemic patients (Bloomfield et al., 2009). When co-administered with atorvastatin further reductions in LDL cholesterol of up to 70% were achieved with comparable increases in HDL cholesterol. Presently, the specific mechanisms by which anacetrapib acts to lower LDL cholesterol are still under investigation.

Experiments in preclinical species can be of considerable value in mechanistic evaluations, provided that the animal model investigated bears sufficient translatability to the clinic. The effects of anacetrapib on reverse cholesterol transport and HDL functionality have been thoroughly investigated in a hamster model of dyslipidemia (Castro-Perez et al., 2011, Wang et al., 2013). In the more recent report a statistically significant reduction in LDL-c was also observed. Recently, Yin et al. evaluated 24 preclinical models commonly used in cardiometabolic research and compared these to dyslipidemic humans across several important metrics including lipid and lipoprotein profiles, CETP activity and responsiveness to statin therapy (Yin et al., 2012). The authors concluded that among all the models evaluated non-human primates generally presented the lipid and lipoprotein profiles most similar to humans. We subsequently undertook a series of studies aimed at comparing the effects of anacetrapib in a non-human primate model (male rhesus macaques) to those reported in dyslipidemic patients. The ultimate goal of these primate studies was twofold: (1) to assess the value of rhesus macaques as a translational model for evaluating the effects of CETP inhibition in vivo and (2) to develop a greater understanding of the effects of anacetrapib on lipid metabolism beyond HDL-c and which could potentially contribute to cardiovascular protection. In this initial report we present data on the effects of anacetrapib in this rhesus model on plasma lipids, apolipoproteins and PCSK9. Measurements were made both at fasting (to facilitate comparison with reported clinical effects) and following oral and intravenous lipid challenges to explore the effects of anacetrapib on dynamic metabolism in a simulated, transient hyperlipidemic state.