Current Medicinal Chemistry - Immunology, Endocrine & Metabolic Agents - Volume 5, Issue 4, 2005

Reverse cholesterol transport (RCT) is one pathway for removing excessive cholesterol from extrahepatic cells and tissues and eventual transport to the liver for excretion thus reducing the accumulation of cholesterol in arteries. Activity of RCT is believed to be affected at least partially by the high density lipoprotein (HDL) concentration in the blood, since HDL is the major carrier of cellular cholesterol through RCT. This presumption lead to an assertion that raising HDL-C levels alone would improve RCT and provide enhanced protection against development of atherosclerosis. However, studies on RCT show that the concentration of HDL required for maximum recruitment of cellular cholesterol is far below the concentration of HDL in plasma. A more likely explanation is that the rate of RCT and the HDL concentration, which is partially determined by RCT, protect against atherosclerosis independently of each other. RCT may result in formation of dysfunctional HDL and a high level of HDL-C is not always synonymous with an efficient RCT. RCT consists of three major stages: cholesterol efflux, transport of cholesterol through the plasma compartment and uptake and excretion of cholesterol by liver. Each stage is a muti-step pathway or a combination of parallel pathways. The contribution and overall efficiency of these pathways often depends on specific metabolic circumstances. Finding the determinants of RCT would be valuable for choosing targets and evaluating the efficiency of possible therapy aimed at boosting RCT, raising HDL and enhancing protection against atherosclerosis.

The concentration of HDL cholesterol is a powerful predictor of the occurrence of cardiovascular events, with a possible causal relationship between these variables. As such, the ability of pharmacological agents to influence HDL levels, and to affect the related processes such as reverse cholesterol transport, is of considerable relevance to the prevention and treatment of cardiovascular disease. Whilst currently available lipid modifying therapy does influence HDL cholesterol levels, and those of its sub-fractions, the effects are generally modest in comparison with the effects which can be achieved in reducing levels of LDL cholesterol. Thus, HMG CoA reductase inhibitors at the prevailing doses generally increase HDL cholesterol by no more than 10%. Fibrates induce a somewhat similar response. Somewhat larger effects are seen with niacin, and smaller effects with ezetemibe. CETP inhibition by agents such as torcetrapib are able to produce more marked effects. Knowledge of the effects of these agents on functional processes in man, such as reverse cholesterol transport is limited. Evidence that elevation of plasma HDL levels can causally influence cardiovascular outcomes is largely dependent on studies with agents which principally influence HDL and triglyceride levels, rather than LDL, and in subjects with low HDL cholesterol levels at baseline. Trial data does not suggest that HDL modification is a major contributor to the clear beneficial effects of HMG CoA reductase inhibitions. Outcome trials with agents, such as torcetrapib, which effect larger changes in HDL cholesterol should give clearer insight into the potential therapeutic benefits of such strategies. However, newer approaches will be required to evaluate the efficacy of agents designed to improve functional aspects of HDL cholesterol such as cellular cholesterol efflux, as distinct from functions directly dependent on HDL cholesterol levels.

High-density lipoproteins (HDLs) have several metabolic actions in vitro that are potentially anti-atherogenic. In addition to their role in reverse cholesterol transport, native HDLs have been shown to protect low-density lipoproteins (LDLs) against oxidative modification, to have anti-inflammatory properties, and to inhibit platelet aggregation. These actions have been shown to occur also in vivo in both experimental animals and humans, when plasma HDL concentration is raised by intravenous infusion of native HDLs or reconstituted discoidal particles composed of the major HDL protein, apolipoprotein (apo) A-I, in association with phosphatidylcholine (PC) and, in the case of protection of LDLs against oxidative change, by lipid-free apo A-I and apo A-I mimetic polypeptides. Intravenous infusion of native HDLs, lipid-free apo A-I and apo A-I/PC discs, and oral administration of apo A-I peptides, have been found to prevent or reverse experimentally induced atherosclerosis in animals. A mutant form of apo A-I discovered in Italy (apo A-IMilano), the biological properties of which differ somewhat from those of normal apo A-I appears to be even more potent in this regard. The hope that this approach will provide a new effective therapy for atherosclerosis has been supported by a multi-center clinical trial, in which five weekly infusions of apo A-IMilano/PC discs induced significant regression of coronary lesions, as quantified by intravascular ultrasound, in men with clinical coronary heart disease. This article reviews the data from animal and human studies in this rapidly developing area.

Apolipoprotein A-I (apoA-I) is a 243 amino-acid cysteine deficient protein, containing 10 amphipathic domains, that allow its binding to a lipid-rich surface. It is the major protein component of high-density lipoproteins (HDL). Each HDL contains 2 or 3 copies of apoA-I plus various combinations of additional apolipoproteins (e.g. apoE, apoA-IV, apoA-II), lipid transport proteins (e.g. CETP and PLTP), and surface acting enzymes (e.g. paraoxonase, LCAT, PAF). This heterogenous and dynamic class of lipoproteins mainly functions in reverse cholesterol transport, a process resulting in the net removal of excess cholesterol from tissues, to be transported to the liver for excretion. The apolipoprotein AIMilano (A-IMilano) is a rare and naturally occurring variant form of the native apoA-I. The A-IMilano variant was a chance discovery in an individual with very low HDL-cholesterol (7-10 mg/dL) compared to the normal human levels of 40-50 mg/dL. This clinical observation dates back 30 years, and was accompanied by similar findings in two of the proband's children as well as in his father. Surprisingly, these carriers appeared to be remarkably free from any vascular abnormalities. Extensive studies led to the identification of the mutation, a Cysteine for Arginine substitution at position 173, i.e. the first description of a human apolipoprotein mutation associated to altered lipoprotein composition and metabolism. This led to an extensive evaluation of the population of Limone sul Garda for the presence of the mutation and the cardiovascular disease status of the population, confirming the earlier suggestion that this might be a protective variant. In the past 25 years some important additional insights were provided. Particularly, it could be shown that the HDL containing AIMilano is endowed with a remarkably enhanced capacity to remove cholesterol from tissues. "HDL therapy" is a novel and emerging area of therapeutic development in the cardiovascular field. It attempts to supplement and improve the vascular benefit exerted by other agents active on lipid metabolism, ie hypolipidemic drugs. Further, it takes advantage on novel techniques of coronary evaluation. A number or reports have examined the potential therapeutic properties of synthetic HDL prepared by complexing recombinant apo A-IMilano with phospholipids. The availability of synthetic HDL complexes containing recombinant apoA-IMilano has opened a new era of therapeutic management of coronary disease. Prototype HDL formulations of recombinant apoA-IMilano/phospholipid complexes have used dipalmitoylphosphatidylcholine (DPPC) or 1-palmitoyl-2-oleyl phosphatidylcholine (POPC) as phospholipid component. HDL containing apoAIMilano/ DPPC have clearly shown rapid regression of a focal carotid atheroma in a rabbit model. The mechanism of this very rapid effect appears to be associated with the cholesterol removing capacity and extended serum half-life of HDL containing A-IMilano. In a pilot study, ETC-216 (apoA-IMilano/POPC) similarly showed significant reduction of coronary plaque burden after five weekly treatments as assessed by intravascular ultrasound (IVUS) in patients with acute coronary syndromes. It is likely that additional safety, efficacy and dose regimen studies in a larger patient population will need to be performed to fully evaluate the potential of this agent as an "HDL therapy" product candidate. "HDL therapy" such as ETC- 216 is being evaluated for use with the current standard of care for patients with acute coronary syndromes, including the use of statins.

Apolipoprotein A-I (apoA-I) is the major protein in high density lipoprotein (HDL) and a major component of the reverse cholesterol transport pathway. ApoA-I also has been shown to have multiple anti-inflammatory properties including the ability to remove pro-inflammatory lipids, reduce monocyte chemotaxis and spreading, and alter interactions between T-cells and monocytes. A variant of apoA-I, apoA-IMilano, has been shown to reduce plaque volume in human coronary arteries after intravenous infusion. ApoA-IMilano and other forms of human apoA-I contain 243 amino acids. An apoA-I mimetic peptide, D-4F, synthesized from just eighteen D-amino acids has been shown to dramatically reduce atherosclerosis in mouse models after oral administration. After oral administration this peptide caused the formation of pre-β HDL, reduced lipoprotein lipid hydroperoxides, increased paraoxonase activity, rendered HDL anti-inflammatory and significantly increased reverse cholesterol transport from macrophages in apoE null mice. When D-4F was given by injection to LDL receptor null mice infected with influenza A virus, HDL was rendered anti-inflammatory and macrophage trafficking into the arteries of LDL receptor null mice was dramatically reduced. In vitro D-4F caused a shift in cytokine production by human monocytes exposed to human T cells from a Th1 to a Th2 profile and also protected human Type II pneumocytes against influenza A infection. We conclude that apoA-I mimetic peptides have therapeutic potential as antiinflammatory agents.

The atheroprotective effects of high density lipoprotein (HDL) have been established over the past several decades and this in turn has led to efforts to develop more effective and well-tolerated means for elevating this lipoprotein. The high levels of HDL associated with human deficiency of cholesteryl ester transfer protein (CETP) motivated the pharmaceutical industry to initiate programs throughout the 1990s aimed towards the inhibition of this transfer protein. Because the CETP mediated transfer of lipid from donor to acceptor lipoproteins appears to occur by a carrier mechanism there exists several means for achieving inhibition. These include interference with lipid binding to CETP and causing insufficient or excessive binding of CETP to its lipoprotein substrates. Many CETP inhibitors have now been identified and several mechanisms of inhibition are represented. Two of these synthetic inhibitors, JTT-705 and torcetrapib, have progressed into clinical trials. The results from these initial trials are consistent with prior reports regarding partial CETP deficiency in humans. For both inhibitors, whether administered as a monotherapy or in combination with a statin, a reduction in plasma CETP activity has resulted in an elevation of HDL, as well as a lowering of low density lipoprotein (LDL). Whether this class of agents will prove beneficial in the treatment and prevention of atherosclerosis awaits the results of longer trials. However, emerging data, including the role of the ABCG1/ABCG4 transporters in the efflux of cholesterol to mature HDL and the importance to reverse cholesterol transport of free cholesterol transport via HDL, reinforce the case for optimism.

Because HDL is the best lipid biomarker of cardiovascular risk, much attention has been given to the effects of nutrients on the concentration and composition of HDL. Interpretation has sometimes been inappropriate since increments in HDL do not necessarily imply benefit and vice versa. In particular the strong genetic determinant of the HDL concentration and the heterogeneity of HDL composition need to be considered. Certain disorders in which nutrition plays a key role such as obesity and the metabolic syndrome are characteristically associated with low HDL cholesterol (HDL-C) probably because HDL removal is accelerated. However weight loss that improves the overall metabolic profile does not necessarily lead to improved HDL-C implying that low HDL is part of the genetic disturbance of the disorder. Possibly undue emphasis has been placed on the reduction in HDL-C with carbohydrate rich diets that reflects wider changes in lipoprotein metabolism and is not necessarily detrimental. Similarly the rise in HDL-C with dietary saturated fat and the occasional fall with dietary polyunsaturated fat need not imply major changes in cardiovascular risk and may be influenced by polymorphic variations in regulatory genes and by gender. The type of carbohydrate also influences the HDL response to high carbohydrate diets. Alcohol is the most consistent nutrient to affect HDL-C but it is doubtful that this mechanism alone explains the cardiovascular risk reduction from drinking alcohol.