Current Topics in Medicinal Chemistry - Volume 7, Issue 11, 2007

Melanocortin receptors (MCRs) belong to the superfamily of 7-transmembrane G-protein coupled receptors (GPCRs). Five MCR subtypes, named MC1-5 receptors, have been identified and cloned to date. The identification of hypothalamic melanocortin receptors, MC3R and MC4R, as central components of feeding behavior and metabolism have helped build a picture, albeit incomplete, of the neuronal pathways involved in energy homeostasis. Thus, the intervention of neural MCR pathway may be a viable way to treat metabolic disorders such as obesity. Neural MCRs also participate in the regulation of sexual function, thus providing an alternative, centrally mediated therapeutic approach for erectile and other sexual dysfunctions. To this end, MC3/4R agonists and antagonists based on peptide, peptidomimetic, and small molecule structures have been prepared and evaluated for their biological activities. This, in turn, has enhanced our understandings on the physiological functions of the central melanocortin system. This special issue of “Current Topic in Medicinal Chemistry” is organized to highlight recent advances in the field of melanocortins. Six articles from researchers involved in the investigation of MCR ligands and their potential therapeutic uses are included. Hongmao Sun and David Fry start the issue with an in-depth review on the molecular modeling of melanocortin receptors. The understanding of how a MCR ligand interacts with each MCR subtype is essential to design potent, selective MCR ligands. In their article, Sun and Fry describe three molecular modeling approaches for MCRs, i.e., structure-based, ligand-based methods and proteochemometrics. The strengths and limitations of these approaches are evaluated. The authors conclude that the determination of the high resolution X-ray crystal structure of bovine rhodopsin (a GPCR with structure closely related to MCR), the availability of many selective MC4R agonists and the mature proteochemometrics all provide better than ever useful information to assist the design of potent and selective MCR ligands. The next three articles are focused on the design and synthesis of potent, selective MC3R and MC4R ligands. First, Liang Zeng Yan, Hansen M. Hsiung, Mark L. Heiman, Robert A. Gadski, Paul J. Emmerson, JeAnne Hertel, David Flora, Patrick Edwards, Dave Smiley, Lianshan Zhang, Saba Husain, Steven D. Kahl, Richard D. DiMarchi, and John P. Mayer from Eli Lily present their work on MC4R agonists based on the native β-MSH(5-22) sequence. Optimization efforts led them to identify a novel series of disulfide constrained hexapeptides that showed improved potency in binding MC4R and enhanced selectivity over MC1R. These cyclic peptides also demonstrated in vivo efficacy in reducing food intake in animal feeding models. The next article describes the recent development of small molecule-based MC4R agonists and antagonists. During the discussion of small molecule MC4R agonists, Feroze Ujjainwalla and Iyassu K. Sebhat divide the subject into two portions: rational peptidomimetic design which includes privileged structure approach, as well as cyclization and molecular modeling approaches; and hit to lead discovery of MC4R agonists. Their insightful structural analysis of MC4R ligands will benefit anyone who is interested in this field. In the following review, Professor Victor J. Hruby, Minying Cai, James P. Cain, Alexander V. Mayorov, Matthew M. Dedek and Devendra Trivedi discuss the biological functions attributed to the MC3R, and summarize the efforts to design and synthesize ligands that are potent and selective for the MC3R. Various linear and cyclic peptide templates based on α-, β-, and γ-MSH, as well as several peptidomimetic and small molecule design approaches leading to potent and selective hMC3R agonists and antagonists are provided.........

Molecular modeling of G-protein coupled receptors (GPCRs) remains a challenge due to the limited availability of structural information for the receptors. Molecular modeling approaches for melanocortin receptors (MCRs) fall into three categories: structurebased, ligand-based, and proteochemometric. Homology modeling combined with the information obtained from site-directed mutagenesis of receptors, recombined chimeric mutations of receptors and the structures of melanocortin type 4 receptor (MC4R) peptide ligands, has provided insights on detailed ligand-receptor interactions. Still, homology models based on the structures of bacteriorhodopsin (bR) or bovine rhodopsin as templates have not reached atomic level accuracy, making them unsuitable for rational drug design. On the other hand, availability of a large number of potent ligands of MCRs, especially those for the therapeutically important MC4R, has fueled ligand-based approaches, including automated pharmacophore query optimization and pharmacophorebased virtual screening. Proteochemometrics, a novel technology for the analysis of intermolecular interactions between ligand and receptor, has also shown great value in obtaining detailed informaton on molecular recognition and providing guidance to ligand design. In this review, the strengths and limitations of homology modeling, pharmacophore modeling and proteochemometrics modeling of MCRs are evaluated.

The recent emergence of obesity as a major health threat in the industrialized world has intensified the search for novel and effective pharmacologic treatment. The proopiomelanocortin (POMC)-melanocortin 4 receptor (MC4R) axis has been shown to regulate food intake and energy homeostasis and is considered among the most promising antiobesity targets. Our initial efforts in this area have focused on affinity and selectivity directed optimization of the native β-MSH(5-22) sequence and resulted in the discovery of a potent MC4R agonist: Ac-Tyr-Arg-[Cys-Glu-His-D-Phe-Arg-Trp-Cys]-NH2 (10). Subcutaneous administration of this peptide produced an excellent in vivo efficacy in reducing food intake and increasing fat metabolism. Additionally, suppression of food intake was observed in wild type but not in MC4R deficient mice, suggesting that the effects observed in the wild type mice were mediated through MC4R signaling. Subsequent optimization efforts led to the identification of a novel series of disulfide constrained hexapeptides as exemplified by Ac-[hCys-His-D-Phe-Arg-Trp-Cys]-NH2 (100). These cyclic hexapeptides showed a further improved potency in binding MC4R and an enhanced selectivity over MC1R. At a dose of 0.07 mg/kg analog 102 reduced food intake by 38% and increased fat utilization by 58% in rats. These cyclic peptides provide novel and enhanced reagents for the elucidation of melanocortin receptors biology and may find applications in the treatment of obesity and related metabolic disorders.

Genetic and pharmacological studies, involving both animals and humans, suggest that the central MC4 receptor plays a key role in homeostatic control, most probably via regulation of appetite and energy expenditure. This has stimulated intense research efforts in the field of drug discovery to identify MC4 receptor agonists and antagonists for the therapeutic treatment of obesity and diseases associated with loss of body weight. This article constitutes a near comprehensive review of the published scientific literature on small molecule ligands of the hMC4 receptor since 2002.

The processed products of the proopiomelanocortin gene (ACTH, α-MSH, β-MSH, γ-MSH, etc.) interact with five melanocortin receptors, the MC1R, MC2R, MC3R, MC4R, and MC5R to modulate and control many important biological functions crucial for good health both peripherally (as hormones) and centrally (as neurotransmitters). Pivotal biological functions include pigmentation, adrenal function, response to stress, fear/flight, energy homeostasis, feeding behavior, sexual function and motivation, pain, immune response, and many others, and are believed to be involved in many disease states including pigmentary disorders, adrenal disorders, obesity, anorexia, prolonged and neuropathic pain, inflammatory response, etc. The melanocortin-3 receptor (MC3R) is found primarily in the brain and spinal cord and also in the periphery, and its biological functions are still not well understood. Here we review some of the biological functions attributed to the MC3R, and then examine in more detail efforts to design and synthesize ligands that are potent and selective for the MC3R, which might help resolve the many questions still remaining about its function. Though some progress has been made, there is still much to be done in this critical area.

The central melanocortin system plays a critical role in energy homeostasis. It is well established that melanocortin-containing neurons are nutritionally regulated and that genetic alterations in the melanocortin system produce profound effects on food intake, energy expenditure, and body weight. Within the brain, melanocortin-producing neurons originate in the arcuate nucleus of the hypothalamus (ARC) and the nucleus of the solitary tract (NTS) in the brainstem and project to various nuclei modulating energy balance. A large body of pharmacological and genetic evidence implicates the central melanocortin 4 receptors (MC4Rs) in the effects of melanocortin peptides on ingestive behaviour, energy expenditure, and body weight. Preclinical studies with endogenous and synthetic melanocortin ligands demonstrate that they produce potent effects on food intake and energy expenditure. Clinical studies thus far have been somewhat less successful and have been hampered by the induction of side effects, which present obstacles to the development of successful therapeutic agents. However, various promising strategies are being pursued to overcome these limitations, including the synthesis of more selective and potent melanocortin analogs.

Penile erection is a complex physiologic event resulting from the interactions of the nervous system on a highly specialized vascular organ. Activation of central nervous system melanocortinergic (MC) receptors with either endogenous or synthetic melanotropic ligands may initiate and/or facilitate spontaneous penile erection. While the CNS contains principally the MC3 and MC4 receptor subtypes, there is conflicting data as to which receptor mediates erection. Although the MC4R is emerging as the principle effector of MC induced erection, the role of the MC3R is poorly understood. Manipulation of each receptor subtype with newly synthesized receptor specific agonists and antagonists, as well as knockout mice, has elucidated their individual contributions. Novel data from our laboratories suggests that antagonism of forebrain MC3R may enhance melanocortin-induced erections. Furthermore, melanocortin agents may interact with better-studied systems such as oxytocinergic pathways at the hypothalamic, brainstem or spinal level. Current therapies for erectile dysfunction target end organ vascular tissue. Manipulation of MC receptors may provide an alternative, centrally mediated therapeutic approach for erectile and other sexual dysfunctions. The non-specific “superpotent” MC agonist, PT-141, which is the carboxylate derivative of MT-II, has reached phase II human trials. Through their centrally mediated activity, melanocortin agonists have potential to treat erectile dysfunction as well as possible applications to the unmet medical needs of decreased sexual motivation and loss of libido.

From the initial discovery of its role in amphibian skin pigmentation to the more recently established involvement in multiple physiological processes such as food intake, energy balance, inflammation, sexual function, depression and anxiety, the history of the melanocortin system represents one of the more important chapters in pituitary endocrinology. Although certain therapeutic applications were foreseeable decades ago, it was only following the cloning, expression and biological characterization of the MC-1 through MC-5 receptor subtypes in the early 1990's that the scientific community began to contemplate the full therapeutic utility of the melanocortin pathway. While each of the five receptor subtypes represents a potential drug target, the MC-4 receptor (MC-4R) subtype has received by far the greatest share of interest and visibility. In this issue of Current Topics in Medicinal Chemistry we have focused on four MC-4R related therapeutic indications which have received considerable recognition. 1) Extensive preclinical and clinical evidence suggests that the MC-4R subtype is intimately involved in the control of food intake and energy expenditure. Numerous animal studies with either melanocortin ligands or transgenic rodents have implicated the involvement of this receptor in food intake and body weight regulation. The correlation of mutations in the human MC-4R with obesity has reinforced these preclinical findings and suggested that stand alone or adjunctive administration of a MC-4R agonist may be beneficial in antiobesity therapy. Progress in this area is summarized by Paul Emmerson and co-workers from Eli Lilly and Co. 2) Based on the above preclinical and clinical validation, an equally convincing argument can also be made for the use of MC-4R antagonists for the treatment of conditions characterized by caloric deficit such as cachexia and anorexia resulting from cancer or AIDS. In these scenarios, the down regulation of MC-4R signaling could offer an important treatment option. This therapeutic approach has been validated in a number of animal models of tumor-induced wasting. The rationale and progress in this area is reviewed by Alan Foster and Chen Chen from Neurocrine Biosciences. 3) Treatment of male and female sexual dysfunction represents another potential therapeutic indication for MC-4R agonists. Stimulation of penile erection has been demonstrated with selective MC-4R agonists in a number of rodent models. More importantly, a statistically significant erectile response has been shown clinically upon intranasal or subcutaneous administration of bremelanotide, PT-141, a nonselective melanocortin agonist. Additionally, these agents may also be useful in the treatment of female sexual arousal disorder. These indications are updated by Annette Shadiack and co-workers from Palatin Technologies. 4) The potential utility of MC-4R antagonists in the treatment of CNS disorders is highlighted through application to anxiety and depression. The key observation that melanocortin agonists produce stress-induced grooming behavior in rodents which is reversible through administration of a selective antagonist prompted exploration of this pathway for the treatment of anxiety and depression. This novel therapeutic concept is reviewed by Shigeyuki Chaki and Taketoshi Okubo from the Taisho Pharmaceutical Company.

The melanocortin family of receptors (MC 1 - 5R) and their endogenous peptide ligands (α, β, γ - MSH and ACTH) have been implicated in the control of a wide variety of behavioral and physiological functions including the homeostatic control of food intake and body weight. In rodent models, melanocortin agonists including the nonselective peptide MTII have been shown to decrease food intake and body weight while antagonists such as SHU9119 and AGRP have been shown to stimulate food intake and increase body weight. Deletion of either the MC3R or MC4R in mice was found to be associated with obesity although hyperphagia was only observed in the MC4R deficient mice. Similarly in humans, inactivating mutations of the MC4R have been found in as many as six percent of obese individuals. The suggestion from these findings that activation of MC4Rs would have an anorectic effect in humans has resulted in efforts to produce selective agonists for the treatment of obesity. Over the past decade, efforts to develop MC4R selective small molecule and peptide agonists have been met with fractional success. Many small molecule agonists have been identified; however, few have been shown to have activity in vivo. While their use as therapeutics may have limitations, selective and potent peptide agonists have been shown by several investigators to decrease food intake and body weight in rodent models. The subject of the current review is to examine the progress made to date on producing both small molecule and peptide MC4R agonists as potential therapeutics for obesity.

The melanocortin-4 (MC4) receptor subtype plays a pivotal role in body weight regulation. Knock-out or mutation of MC4 receptors in animals or humans leads to severe obesity and acute or sub-acute antagonism of central MC4 receptors produces an increase in food intake and a decrease in metabolism. Knock-out or antagonism of MC4 receptors in animal models of cachexia leads to a protection from anorexia and the loss of both lean and fat body mass, suggesting that an MC4 antagonist may be beneficial in wasting diseases, which are poorly treated by available therapies. Considerable progress has been made in the discovery of non-peptide antagonists with high affinity and selectivity for MC4 receptors. Optimization of these compounds has produced molecules that are active upon systemic administration and are effective in protecting against cachectic symptoms in animal models of tumor-induced wasting. Further development of such compounds is greatly anticipated as a potential means to combat the cachexia that results from chronic diseases such as cancer, AIDS, renal failure, liver failure, congestive heart failure and lung disease.

Melanocortinergic agents are currently being investigated for a possible therapeutic role in male and female sexual dysfunction. These investigations were sparked by findings that systemic administration of a synthetic analog of alpha-MSH, MT-II, causes penile erections in a variety of species, including humans. Several other melanocortinergic agents including HP-228, THIQ, and bremelanotide (PT-141) have since been shown to have erectogenic properties thought to be due to binding to melanocortin receptors in the central nervous system, particularly the hypothalamus. Bremelanotide, a nasally administered synthetic peptide, is the only melanocortinergic agent that has been clinically studied in both males and females. Data from Phase II clinical trials of bremelanotide support the use of melanocortin-based therapy for erectile dysfunction. Studies using animal models have demonstrated that precopulatory behaviors in female rats analogous to sexual arousal are evoked, and preliminary clinical data also suggest a role in promoting sexual desire and arousal in women. Based on bremelanotide clinical experience, administration of a melanocortin agonist is well tolerated and not associated the hypotension observed with phosphodiesterase-5 inhibitors currently used to treat erectile dysfunction. This review discusses investigations of melanocortin agonists for the treatment of sexual dysfunction with emphasis on proposed sites and mechanisms of action in the central nervous system that appear to be involved in melanocortinergic modulation of sexual function. Current research validates use of melanocortinergic agents for the treatment of both male and female sexual dysfunction.

Derived from proopiomelanocortin by proteolytic processing, melanocortins have been implicated in a wide range of physiological processes. Melanocortins exert their physiological effects by binding to specific receptors on the surface of cell membranes. To date, five subtypes of melanocortin receptors (MC1 - MC5) have been identified, all of which are G-protein coupled receptor whose activation leads to increase in intracellular cyclic 3',5'-adenosine monophosphate formation. Of these, the MC4 receptor is expressed predominantly throughout the central nervous system, particularly, in areas related to stress responses and emotional states. Expression of the MC4 receptor is regulated by stress exposure. Reports also indicate that stimulation of the MC4 receptor activates the hypothalamus-pituitary-adrenal axis, and that the MC4 receptor mediates stress-related behaviors and anxiety in rodents. Recently developed selective MC4 receptor antagonists have demonstrated antidepressant and anxiolytic effects in several animal models of depression and anxiety. MC4 receptor antagonists are effective, particularly under conditions of high stress, which may be consistent with the etiology of depression and anxiety. This review describes the involvement of the MC4 receptor in stress response and discusses the potential value of MC4 receptor antagonists for the treatment of stress-related disorders such as depression and anxiety.

Pharmaceutical aphrodisia. Almost a decade ago, 43% of women and 31% of men in the United States were believed to suffer from one form or another of sexual dysfunction [1]. Viagra™ and other phosphodiesterase type 5 (PDE5) inhibitors have provided viable pharmacological treatment options for men over the past decade. However, these drugs lack efficacy in women, and similarly successful drug treatments for female sexual dysfunction have yet to emerge [2]. Furthermore, despite the high efficacy and widespread tolerability of PDE inhibitors in men, these drugs are not without drawbacks. Headache and vision-related side effects are present in some individuals, and these drugs are contraindicated in patients concurrently taking nitrates or related drugs that may carry hypotension risks [3]. Additionally, it is estimated that half of men prescribed sildenafil (Viagra™) subsequently cease use over time [4]. Bremelanotide (formerly PT-141), a drug currently under development by Palatin Technologies [5], could prove a novel and effective treatment for female and male sexual dysfunction. The main reason underlying the drug’s potential efficacy in both sexes lies in its CNS-acting mechanism. Bremelanotide is a cyclic heptapeptide melanocortin analog, which acts as an agonist at the primarily CNS-expressed melanocortin receptors, MC3-R and MC4-R [6, 7]. The mechanism by which melanocortin receptor activation mediates sexual arousal and behavior is not well established; however, it likely involves an increase in nitric oxide signaling in the CNS [7]. Central activation of the melanocortin receptors by known agonist such as alpha-melanocyte-stimulating hormone (alpha-MSH) causes increased sexual arousal and mating behavior in animals [8]. Likewise, the synthetic melanocortin analog Melanotan-II, which is structurally similar to Bremelanotide (and is the parent compound from which the latter was derived), induces penile erection in men suffering from erectile dysfunction (ED) [9]. Rodent and non-human primate studies demonstrate a potent erection-promoting effect of Bremelanotide [10]. Women with female sexual arousal disorder given Bremelanotide (by intranasal delivery) report significantly increased sexual desire after treatment, as well as more satisfaction with their level of arousal when attempting intercourse within 24 hours [11]. In both normal and ED men, treatment with Bremelanotide causes significant increases in erectile activity, in a dose-dependant manner [10]. Clinically and statistically effective doses of the drug (up to 20mg) are safe and well tolerated by both normal and ED patients [12]. Having completed all Phase II trials in men with promising results, Bremelanotide is set to enter Phase III for men in the first half of this year. In women, positive data from a Phase IIa trial and the recent commencement of a Phase IIb trial suggest that entry to Phase III for women may also be on the horizon. If approved, Bremelanotide would be the first FDA-approved synthetic aphrodisiac. This novel therapy could potentially address the substantial need for an effective female sexual dysfunction treatment as well as offer an alternative to traditional PDE inhibition in ED men. REFERENCES [1] Laumann, E.O; Paik, A; Rosen, R.C. ‘Sexual Dysfunction in the United States. JAMA, 1999, 281, 537-544. [2] Shields, K.M.; Hrometz, S.L. Use of Sildenafil for Female Sexual Dysfunction. Ann. Pharmacother., 2006, 40(5), 931-4. [3] Cheitlin, M.D.; Hutter, A.M.; Brindis, R.G. et al. Use of sildenafil (Viagra) in patients with cardiovascular disease. Circulation, 2000, 101(8), E90. [4] Souverein, P.C.; Egberts, A.C.; Meuleman, E.J. et al. Incidence and determinants of sildenafil (dis) continuation the Dutch cohort of sildenafil users. Int. J. Impot. Res., 2002, 14, 259-265. [5] Palatin Technologies Inc., Cranbury, New Jersey, USA. http:// www.palatin.com. [6] Wessells, H.; Gralnek, D.; Dorr, R. et al. Effect of an alphamelanocyte stimulating hormone analog on penile erection and sexual desire in men with organic erectile dysfunction. Urology, 2000, 56, 641-646. [7] Giuliano, F. Control of Penile Erection by the Melanocortinergic System: Experimental Evidences and Therapeutic Perspectives. J. Andrology, 2004, 25: 5, 683-91........