Current Topics in Medicinal Chemistry - Volume 5, Issue 3, 2005

In the closing days of 2004, the American Heart Association released its annual statistics on heart disease and stroke (http: / / www.americanheart.org). To the surprise of no one, these data showed that cardiovascular disease remains the leading cause of death in the US, claiming over 900,000 lives in 2002. The direct and indirect costs of cardiovascular disease are estimated to reach $393 billion in 2005. Almost as disturbing as the size of the problem is the realization that the situation persists in the face of all that has been done to understand and treat this disease. Despite decades of research, tremendous advances in treatment and prevention, and a broad-based public awareness campaign on the risk factors for cardiovascular disease, Americans continue to die from coronary events at the rate of about one every 34 seconds. The epidemic of cardiovascular disease is not limited to the United States. The WHO estimates that worldwide 17 million people die every year from cardiovascular disease, especially heart attack and stroke (http: / / www.who.int). Of the many risk factors for cardiovascular disease, dyslipidemia is among the best understood and one that has clearly lent itself to both life-style and pharmacological intervention. Several large clinical studies conducted over the last few decades have indicated that overall mortality rates from cardiovascular disease can be significantly reduced with aggressive pharmacological intervention and risk factor management. If the prevalence of cardiovascular disease is no surprise, then it should be equally understandable that drugs that lower cholesterol and triglyceride levels are consistently at the top of global pharmaceutical sales. That death from this disease remains at such a staggering level attests to the need for more to be done. This issue of Current Topics in Medicinal Chemistry highlights several areas of current interest in the area of lipid modulation for the possible treatment and prevention of cardiovascular disease. Clearly a problem of this magnitude demands an intensive and multi-pronged attack, and a single volume can hardly do justice to the area, even if restricted to drugs affecting lipids. The topics for this volume were chosen to at least sample the range of approaches from the latest in established lipid lowering therapies, through the newest approaches that have been approved recently or may be available in the next few years, and ending with a brief glimpse of things that may be on the more distant horizon.

The treatment of dyslipoproteinemia has proven a successful strategy in the prevention of cardiovascular diseases. The major target of hypolipidemic drugs is the reduction of low density lipoprotein cholesterol (LDL-C). HMGCoA reductase inhibitors (HMGRI) effectively lower LDL-C by inhibiting the mevalonate pathway and enhancing the activity of the LDL receptor (LDL-R). Numerous clinical studies demonstrated convincingly, that the reduction of LDL-C lowers the incidence of cardiovascular events in primary and secondary prevention. Two new HMGRI, rosuvastatin and pitavastatin, have been evaluated in clinical trials. Both drugs demonstrated efficacy in lowering atherogenic lipoproteins. In addition to the reduction of LDL-C, they may have a higher potency to lower triacylglycerides (TG) and to increase HDL cholesterol (HDL-C) compared to currently available HMGRI. Other therapeutic strategies examined in experimental animals are the inhibition of squalene synthase, the first enzyme of the mevalonate pathway, which is specifically committed to cholesterol biosynthesis, and the direct up-regulation of LDL receptor activity. The latter compounds, the SCAP ligands, are the first members of a new class of hypolipidemic agents affecting the transcriptional regulation of genes involved in lipid metabolism. Recent treatment guidelines emphasise the importance of modifying lipid metabolism beyond lowering LDL-C, mainly by lowering TG and raising HDL-C. Although these actions are not primary targets of the compounds discussed here, it is interesting that drugs inducing the LDL-R usually also lower TG and, in the case of HMGRI, increase HDL-C.

Atherosclerotic coronary artery disease remains a major healthcare concern especially in developed countries. While lowering plasma cholesterol levels via diet, exercise, and pharmacoptherapy can reduce this risk of developing coronary artery disease, there remains a need for more effective drug therapies. The azetidinone cholesterol absorption inhibitors typified by ezetimibe represent the first new approach to lipid lowering therapy in more than a decade. This review summarizes the medicinal chemistry of the azetidinone cholesterol absorption inhibitors as a class, with emphasis on factors that contributed both to the discovery of ezetimibe as well as to our overall understanding of S.A.R. trends in this area.

Cholesteryl ester transfer protein (CETP) facilitates the exchange of neutral lipids (such as cholesteryl esters and triglycerides) between anti-atherogenic HDL particles and pro-atherogenic VLDL and LDL particles in human plasma. Individuals possessing a genetic deficiency for CETP have higher HDL cholesterol and lower LDL cholesterol and may have a reduced risk for developing cardiovascular disease. Small molecule inhibitors of CETP are being developed that would appear to provide a beneficial change in lipoprotein profile. However, randomized clinical trials are ultimately required to determine whether CETP inhibition will afford a reduction in cardiovascular events.

This review describes the role of nuclear receptors in the regulation of genes involved in cholesterol transport and synthetic modulators of these receptors. Increasing the efflux of cholesterol from peripheral cells, such as lipid-laden macrophages, through a process called reverse cholesterol transport (RCT) requires HDL. Increasing the circulating levels of HDL, as well as the efficiency of the RCT process, could result in a reduction in the development of coronary artery disease and atherosclerosis. Nuclear receptors of the RXR heterodimer family have recently been shown to regulate key genes involved in HDL metabolism and reverse cholesterol transport. These include the PPARs (peroxisome proliferator activated receptors), the LXR (liver X receptor) and the farnesoid X receptor (FXR). The synthesis of specific and potent ligands for these receptors has aided in ascertaining the physiological role of these receptors as lipid sensors and the potential therapeutic utility of modulators of these receptors in dyslipidemias and cardiovascular disease.

The microsomal triglyceride transfer protein (MTP), along with its partner, protein disulphide isomerase, performs a wide range of lipid transport functions necessary for maintenance of whole-body lipid homeostasis. In this review, we summarize the recent deluge of comparative and functional genomic data that have forced a radical reappraisal of the evolutionary processes that established the major lipid transport pathway in man, and the different structural and lipid transfer roles MTP plays within it. This is followed by an overview of MTP structure-function relationships, highlighting two newly identified functional roles: first, the production of small, apolipoprotein (apo)Bcontaining lipoprotein particles in cardiac myocytes and, second, the lipidation of a major histocompatibility complex class-I related molecule (CD1d) that presents glycolipid antigens to distinct subsets of natural killer T cells. We also discuss the interactions of MTP with proteins such as apoB and CD1d, and the complex mechanisms regulating MTP transcription in different cell types and nutritional states. The past five years has witnessed remarkable progress in teasing out the different functionalities of MTP, and the properties of the different molecules that inhibit MTP activity, data that are likely to underpin the design of the next generation of MTP/apoB inhibitors for preventing cardiovascular disease attributable to the increased production of atherogenic lipoproteins.

Arylacetamide analgesics that stimulate k opioid receptors in the central nervous system mediate dysphoria and psychosis as well as analgesia. However, the naturally occurring peptide agonist, dynorphin A, is analgesic in the absence of dysphoria and psychosis, indicating that the therapeutic effects of k opioid agonists may be separated from their side effects. As part of our effort to discover κ opioid receptor analgesics lacking side effects, we designed and constructed two μ/κ chimeric receptors, composed primarily of amino acid residues derived from the μ opioid receptor, that were expected to bind dynorphin A with high affinity. In one, extracellular loop 2 and transmembrane domain 4 were derived from the κ opioid receptor and in the other, only extracellular loop 2 was derived from the κ opioid receptor. Most competitors of [3H]diprenorphine binding from a variety of structural classes bound to the chimeras with affinities similar to those with which they bound to the m opioid receptor. In contrast, dynorphin A analogs bound to the chimeras with affinities similar to those with which they bound to the κ opioid receptor. Pharmacological characterization of [35S]GTPgS binding mediated by the chimera with extracellular loop 2 derived from the κ opioid receptor showed that it behaved as if it were m opioid receptor with high affinity for dynorphin A analogs. These chimeras may be useful in identifying novel κ receptor agonists that bind to the second extracellular loop of the receptor and share the desirable therapeutic profile of dynorphin A.

As a whole, the G protein-coupled receptor (GPCR) superfamily displays no overall sequence homology. Nevertheless, enough short sequences and even individual amino acid residues are shared by these receptors to afford a common three-dimensional structure and a similar signal transduction mechanism. Some of these sequence commonalities, or structural motifs, are dedicated to preserving receptor infrastructure, while others are critical to agonistmediated signaling. Certain structural motifs common to GPCRs and other signal transducing integral membrane proteins are present in the conventional opioid receptors, although several of the motifs are not well characterized in this receptor family. Here we focus on six structural motifs found in the mu, delta and kappa opioid receptors as well as the opioid like receptor ORL-1. The motifs are discussed in terms of their dynamic roles in the signaling mechanism documented for several Class A GPCRs including the opioid receptors. Clarification of the roles of GPCR structural motifs provides a blueprint for structure-function studies on newly discovered or recently cloned receptors in the superfamily. Characterization of these motifs in the opioid receptors should enhance understanding of what makes an opioid ligand a full, partial or inverse agonist or antagonist at a given receptor, possibly leading to rational design of therapeutics useful for combating opiate dependence or for pain management.

The opioid receptor-like 1 (ORL1) system has attracted a lot of attention owing to its diverse physiological role and by its close structural proximity toward the classical opioid receptors. Even though they share a close sequence similarity, the ligand recognition pattern for the ORL1 receptor and the classical opioid receptors remains highly distinct. In addition, functional diversification observed between the ORL1 receptor system and classical opioid receptors clearly indicates that subtle changes in the structural makeup of a receptor are enough to delineate them. A clear understanding of the structural requirements for ligand selectivity by classical opioid receptors and identification of a common “opioid binding pocket” has not been achieved yet. At this juncture, the ORL1 receptor system presents itself as a potential tool in the quest for elucidating critical elements directing ligand selectivity. The current paper is a compilation of several sitedirected mutagenesis studies conducted on the ORL1 receptor system. The mutagenesis studies concentrated on the transmembrane domain residues are reported with the changes observed in terms of both binding and functional activation of the receptor. Given the critical role played by this G-protein coupled receptor, molecular level understanding of this ORL1 receptor system would aid in rational design and development of agonists and antagonists with multiple therapeutic applications.

Opioids are involved in the physiological control of numerous functions of the central nervous system, particularly nociception. It appears that some endogenous neuropeptides, called “anti-opioids“, participate in an homeostatic system tending to reduce the effects of opioids. Neuropeptide FF (NPFF) and cholecystokinin (CCK) possess these properties and, paradoxically, the opioid peptides nociceptin and dynorphin display some anti-opioid activity. All these peptides exhibit complex properties as they are able to both counteract and potentiate opioid activity, acting rather as modulators of opioid functions. The purpose of this review is to highlight that two different mechanisms are clearly involved in the control of opioid functions by opioid-modulating peptides: a “circuitry-induced” mechanism for nociceptin and dynorphin, and a “cellular anti-opioid” mechanism for NPFF and CCK. The knowledge of these mechanisms has potential therapeutic interest in the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

Pain management using opioid analgesics strives to achieve three goals: maximum efficacy, minimal risk of tolerance and physical dependence, and negligible side effects. Following the cloning of opioid and nociceptin receptors, novel ligands can be designed to target specific residues of these membrane proteins with the goal of improving efficacy and reducing side effects through selectivity. For the most part, ligand design has focused on binding sites located in the transmembrane region of the receptors, and has ignored the extracellular domains. In this review, we discuss the evidence for the interaction of the extracellular regions with opioids and show how computational biology tools can be used to model these domains for use in drug discovery. A computational model of the κ-opioid receptor which includes the loop regions is presented. The model combines knowledge-based information, bioinformatics and computational tools to identify regions of the extracellular loop domains that can be targeted by drug design.