Print Version

Reverse Cholesterol Transport Made Simple

H. Bryan Brewer, Jr, MD, Director, Lipoprotein and Atherosclerosis Research, Cardiovascular Research Institute, MedStar Research Institute, Washington Hospital Center, Washington, DC

The ATP-binding cassette, sub-family A, member 1 (ABCA1) transporter protein controls the rate-limiting step in high-density lipoprotein (HDL) particle assembly by mediating cholesterol efflux from cells. Oxysterol-induced RXR/LXR transcription factors induce the expression of the ABCA1 gene. Approximately 5% of plasma HDL is pre- b-HDL, which initiates cholesterol efflux from cells via ABCA1. About 95% of plasma HDL is mature α-HDL, which promotes cholesterol efflux via SR-B1 and ABCG1 transporters.

HDL is synthesized in the liver and intestine. ABCA1 transporter is also present in the liver and intestine. Approximately 70% of cholesterol is from the liver, 25%-30% from intestine through the regulation of the ABCA1 transporter, and 5%-6% from the periphery (macrophages). Hepatic, but not extrahepatic, ABAC1 is critical in maintaining the circulation of mature HDL particles.

Excess accumulation of cholesterol in macrophages results in foam cell production and lesion development. Reverse cholesterol transport is a key component of cholesterol efflux from macrophages. Although macrophage ABCA1 contributes minimally to the overall plasma level of HDL, it does contribute to HDL formation that is critical to the prevention of atherosclerosis.

Potential New Targets for Therapy

Dr. Davidson summarized potential new targets for high-density lipoprotein.

1) Prolong half-life of HDL. Potential target could be to reduce HDL catabolism by reducing hepatic uptake.

2) Improve tolerance to niacin. Understanding the molecular effects of this receptor may lead to improved tolerability of nicotinic acid preparations. HM74A was recently described as a nicotinic acid receptor; it is the molecular target for the actions of nicotinic acid on adipose tissue and is involved in the nicotinic acid–induced flushing response.

3) CETP inhibition. Cholesterol ester transfer protein (CETP) catalyzes the transfer of cholesterol ester from HDL to apolipoprotein B–containing lipoproteins in exchange for triglyceride, and thereby plays a major role in lipoprotein metabolism. In subjects with low HDL-C, CETP inhibition with torcetrapib markedly increased HDL-C levels and also decreased LDL-C levels, both when administered as monotherapy and when administered in combination with a statin. CETP inhibition may also produce a shift from small, dense LDL to larger, more buoyant particles.

4) Use of LXR agonists. Liver X receptors (LXRs) are ligand-activated transcription factors involved in the control of lipid metabolism and inflammation. Synthetic LXR agonists inhibit the progression of atherosclerosis in mice. Preclinical data demonstrate that administration of the synthetic LXR agonist GW3965 increases the rate of reverse cholesterol transport from macrophages to feces in vivo. LXR agonists upregulate both ABCA1 and ABCG1 transporters in macrophages, thus promoting efflux of cholesterol. However, nonselective LXR agonists also upregulate lipogenic enzymes; thus, selective LXR agonists are needed.

5) Vaccine-based therapy. CETi-1, a vaccine that stimulates antibodies specific for CETP, has been demonstrated to significantly increase HDL-C and reduce the development of atherosclerosis in rabbits. In a Phase I human trial with CETi-1, 53% (8/15) of patients who received a second injection of the active vaccine developed anti-CETP antibodies compared with 0% (0/8) in the placebo group.

Clinical Trials in Reverse Cholesterol Transport

Margaret E. Brousseau, PhD, Lipid Metabolism Laboratory, New England Medical Center and Tufts University, School of Medicine, Boston, MA

Cholesterol ester transfer protein (CETP) mediates the exchange of cholesterol ester in HDL for triglyceride in very-low-density lipoprotein (VLDL).The reciprocal increase in HDL-C and decrease in low-density lipoprotein cholesterol (LDL-C) associated with CETP deficiency led to the search for synthetic CETP inhibitors. Torcetrapib and JTT-705 are potent CETP inhibitors. JTT-705 inhibits CETP activity by forming a disulphide bond with CETP.

Kuivenhoven et al 2005 demonstrated that combination therapy using JTT-705 and pravastatin increases HDL-C levels and is safe and well tolerated for up to 4 weeks of administration. The study was a randomized, double-blind, placebo-controlled trial, in which 155 patients with type II dyslipidemia using pravastatin 40 mg were treated with placebo or JTT-705 300 or 600 mg. Four weeks of treatment with JTT-705 600 mg led to a 30% decrease in CETP activity ( P <0.001), a 28% increase in HDL cholesterol ( P<0.001), and a 5% decrease in low-density lipoprotein cholesterol ( P<0.03).

The mechanism of action of torcetrapib differs from that of JTT-705. Torcetrapib binds specifically to CETP with 1:1 stoichiometry and blocks both neutral lipid and phospholipid (PL) transfer activities. Data are consistent with a mechanism whereby torcetrapib blocks all of the major lipid-transfer functions of plasma CETP by inducing a nonproductive complex between the transfer protein and HDL.

The ability of torcetrapib to raise HDL-C levels in healthy young subjects was first tested in an initial phase 1 multidose study. Five groups of 8 subjects each were randomized to placebo (n=2) or torcetrapib (n=6) at 10, 30, 60, and 120 mg daily and 120 mg twice daily for 14 days. The correlation of plasma drug levels with inhibition was as expected based on in vitro potency, and increases in CETP mass were consistent with the proposed mechanism of inhibition. CETP inhibition increased with escalating dose, leading to elevations of HDL-C of 16% to 91%. Total plasma cholesterol did not change significantly because of a reduction in non-HDL-C, including a 21% to 42% lowering of LDL-C at the higher doses. Apolipoprotein A-I and E were elevated 27% and 66%, respectively, and apolipoprotein B was reduced 26% with 120 mg twice daily. Torcetrapib was well tolerated, with all treated subjects completing the study.

The two Phase 2 studies involved torcetrapib and atorvastatin. In a study of 493 patients, those who received torcetrapib (60 mg) and atorvastatin (10, 20, 40, 80 mg) had increases in HDL-C of 44% to 66%. At the same time, their LDL-C dropped between 41% and 60%. A separate study of 40 patients, raising HDL-C with torcetrapib increased cholesterol efflux from cells, an effect seen to be functionally similar to patients with naturally occurring high levels of HDL-C.

Brousseau et al showed that in subjects with low HDL-C levels, CETP inhibition with torcetrapib markedly increased HDL-C levels and also decreased LDL cholesterol levels, both when administered as monotherapy and when administered in combination with a statin. These data are from a single-blind, placebo-controlled study with 19 subjects with low levels of HDL-C (<40 mg per dL [1.0 mmol per liter]), 9 of whom were also treated with 20 mg of atorvastatin daily. All subjects received placebo for 4 weeks and then received 120 mg of torcetrapib daily for the following 4 weeks. Six of the subjects who did not receive atorvastatin also participated in a third phase, in which they received 120 mg of torcetrapib twice daily for 4 weeks. Treatment with 120 mg/d of torcetrapib increased HDL-C by 61% ( P<0.001) and 46% ( P=0.001) in the atorvastatin and nonatorvastatin cohorts, respectively. Treatment with 120 mg twice daily increased HDL-C by 106% ( P<0.001). Torcetrapib also reduced low-density lipoprotein (LDL) cholesterol levels by 17% in the atorvastatin cohort ( P=0.02).

In summary, results form clinical trials show that torcetrapib significantly increases HDL-C levels, either as monotherapy or in combination with atorvastatin; it significantly reduces LDL-C levels, particularly when given in combination with atorvastatin, and is well tolerated in doses up to 120 mg/d. However, in some of the larger trials, torcetrapib therapy was associated with increases in systolic blood pressure.