MC4R Agonists: Structural Overview on Antiobesity Therapeutics

Melanocortin-4 receptor appears to be a key monogenic cause for obesity in humans, and thus is one of the most viable targets for antiobesity therapeutics. Setmelanotide and PL-8905 are currently in clinical trials for obesity treatment. Bivalent structures show a great promise for the development of novel MC4R agonists. The melanocortin-4 receptor (MC4R) regulates adipose tissue formation and energy homeostasis, and is believed to be a monogenic target for novel antiobesity therapeutics. Several research efforts targeting this receptor have identified potent and selective agonists. While viable agonists have been characterized in vitro, undesirable side effects frequently appeared during clinical trials. The most promising candidates have diverse structures, including linear peptides, cyclic peptides, and small molecules. Herein, we present a compilation of potent MC4R agonists and discuss the pivotal structural differences within those molecules that resulted in good selectivity for MC4R over other melanocortins. We provide insight on recent progress in the field and reflect on directions for development of new agonists. The melanocortin-4 receptor (MC4R) regulates adipose tissue formation and energy homeostasis, and is believed to be a monogenic target for novel antiobesity therapeutics. Several research efforts targeting this receptor have identified potent and selective agonists. While viable agonists have been characterized in vitro, undesirable side effects frequently appeared during clinical trials. The most promising candidates have diverse structures, including linear peptides, cyclic peptides, and small molecules. Herein, we present a compilation of potent MC4R agonists and discuss the pivotal structural differences within those molecules that resulted in good selectivity for MC4R over other melanocortins. We provide insight on recent progress in the field and reflect on directions for development of new agonists. Over the last thirty years, the incidence of obesity has increased approximately 9% worldwide, contributing to 3.4 million deaths in 2010 [1Ng M. et al.Global, regional, and national prevalence of overweight and obesity in children and adults during 1980–2013: a systematic analysis for the Global Burden of Disease Study 2013.Lancet. 2014; 384: 766-781Abstract Full Text Full Text PDF PubMed Scopus (2800) Google Scholar]. With the number of obesity cases rising yearly, it is urgent to find a therapy that can regulate body weight and appetite. The melanocortin-4 receptor (MC4R; see Glossary) is one attractive potential target for novel therapy. Upon activation, the MC4R regulates energy homeostasis and reduces adipose tissue. MC4R is a G protein-coupled receptor (GPCR), which is collectively the largest family of transmembrane (TM) receptors in humans, consisting of nearly 800 distinct genes and their corresponding gene products. The general structure of GPCRs has been largely conserved through evolution and manifests as glycoproteins possessing seven TM spanning segments that organize themselves in the cell’s plasma membrane. As they possess both extracellular and intracellular elements, they serve as ideal transducers of stimuli across the cellular membrane. GPCRs can recognize a wide variety of extracellular stimuli, including small molecules, ions, photons, peptides, and proteins, and communicate the stimuli across an impermeable membrane barrier to the intracellular domain to effect changes in cell function. Signaling is transmitted through a cascade of protein interactions and activation events, thereby evoking changes in the levels of intracellular biochemical mediators of cell function (e.g., second messengers), resulting in the regulation of several physiological functions. Of the GPCR superfamily, one subfamily that has significant therapeutic promise is the melanocortin receptor (MCR) subfamily (Figure 1). MCRs are derived from five distinct genes encoding receptors (MC1R–MC5R). MC1R governs the mammalian skin and hair color by regulating the production of melanin [2Gantz I. et al.Molecular cloning of a novel melanocortin receptor.J. Biol. Chem. 1993; 268: 8246-8250Abstract Full Text PDF PubMed Google Scholar]. Data from in vivo studies showed that rats with an ortholog of the receptor exhibited different fur color than the wild type, thus providing genetic evidence that this receptor plays an important role in the pigmentation [3Schaffer J.V. Bolognia J.L. The melanocortin-1 receptor: red hair and beyond.Arch. Dermatol. 2001; 137: 1477-1485Crossref PubMed Google Scholar]. Unlike the other melanocortins, MC2R is mainly located in the adrenal cortex and controls the glucocorticoids – so-called stress hormones that are responsible for modulating the immune system, regulating the blood sugar level, and helping trigger nerve cell signaling in the brain [4Webb T.R. Clark A.J.L. Minireview: the melanocortin 2 receptor accessory proteins.Mol. Endocrinol. 2010; 24: 475-484Crossref PubMed Scopus (0) Google Scholar, 5Chhajlani V. et al.Molecular cloning of a novel human melanocortin receptor.Biochem. Biophys. Res. Commun. 1993; 195: 866-873Crossref PubMed Scopus (0) Google Scholar]. Moreover, a fundamental regulatory component of the hypothalamus–pituitary–adrenal axis in mammals is the ligand selectivity of MC2R for adrenocorticotropic hormone (ACTH; also known as adrenocorticotropin) [6Dores R.M. Garcia Y. Views on the co-evolution of the melanocortin-2 receptor, MRAPs, and the hypothalamus/pituitary/adrenal-interrenal axis.Mol. Cell. Endocrinol. 2015; 408: 12-22Crossref PubMed Scopus (9) Google Scholar]. The control of energy homeostasis is also achieved by MCR agonists, predominantly through MC3R and MC4R [7Huszar D. et al.Targeted disruption of the melanocortin-4 receptor results in obesity in mice.Cell. 1997; 88: 131-141Abstract Full Text Full Text PDF PubMed Google Scholar, 8Tao Y.-X. Mutations in the melanocortin-3 receptor (MC3R) gene: impact on human obesity or adiposity.Curr. Opin. Investig. Drugs. 2010; 11: 1092-1096PubMed Google Scholar]. In mammals, the equilibrium between the energy intake (as caloric consumption) and energy expenditure (physical activity, thermoregulation, and basal metabolism) is regulated by a sensory feedback system located in the central nervous system. While some acute fluctuations can be tolerated and mitigated, continued imbalance of this equilibrium can lead to severe health implications, such as cancer, cardiovascular diseases, and type II diabetes [9Krashes M.J. et al.Melanocortin-4 receptor-regulated energy homeostasis.Nat. Neurosci. 2016; 19: 206-219Crossref PubMed Google Scholar, 10Guh D.P. et al.The incidence of co-morbidities related to obesity and overweight: a systematic review and meta-analysis.BMC Public Health. 2009; 9: 88Crossref PubMed Scopus (1166) Google Scholar]. Of the MCRs, MC3R remains the least characterized in terms of its physiological role(s). Studies on knockout MC3R mice showed increasing accumulation of adipose tissue, but it is believed that its obesity phenotype is different than that for MC4R, since investigations on the deletions of both MC3R and MC4R revealed an accumulative effect of adipose tissue. In contrast to the deletion of MC4R, MC3R knockout mice showed a susceptibility to weight loss, indicating a loss in the lean mass [7Huszar D. et al.Targeted disruption of the melanocortin-4 receptor results in obesity in mice.Cell. 1997; 88: 131-141Abstract Full Text Full Text PDF PubMed Google Scholar, 11Butler A.A. et al.A unique metabolic syndrome causes obesity in the melanocortin-3 receptor-deficient mouse.Endocrinology. 2000; 141: 3518-3521Crossref PubMed Scopus (0) Google Scholar, 12Marks D.L. et al.The regulation of food intake by selective stimulation of the type 3 melanocortin receptor (MC3R).Peptides. 2006; 27: 259-264Crossref PubMed Scopus (66) Google Scholar, 13Renquist B.J. et al.Physiological roles of the melanocortin MC3 receptor.Eur. J. Pharmacol. 2011; 660: 13-20Crossref PubMed Scopus (28) Google Scholar, 14Ellacott K.L.J. et al.Obesity-induced inflammation in white adipose tissue is attenuated by loss of melanocortin-3 receptor signaling.Endocrinology. 2007; 148: 6186-6194Crossref PubMed Scopus (0) Google Scholar]. Mice with double knockout (MC3R and MC4R) genes were significantly more obese than mice with MC4R knockout, whereas mice with suppressed MC3R weighed the same as the wild type [15Atalayer D. et al.Food demand and meal size in mice with single or combined disruption of melanocortin type 3 and 4 receptors.Am. J. Physiol. Integr. Comp. Physiol. 2010; 298: R1667-R1674Crossref PubMed Scopus (0) Google Scholar]. Separate studies on MC3R physiological activity suggest that this receptor also plays a significant role in the energy homeostasis, immune response, natriuresis, and the circadian rhythm [13Renquist B.J. et al.Physiological roles of the melanocortin MC3 receptor.Eur. J. Pharmacol. 2011; 660: 13-20Crossref PubMed Scopus (28) Google Scholar, 16Leoni G. et al.Inflamed phenotype of the mesenteric microcirculation of melanocortin type 3 receptor-null mice after ischemia-reperfusion.FASEB J. 2008; 22: 4228-4238Crossref PubMed Scopus (0) Google Scholar, 17Getting S.J. et al.A role for MC3R in modulating lung inflammation.Pulm. Pharmacol. Ther. 2008; 21: 866-873Crossref PubMed Scopus (25) Google Scholar, 18Begriche K. et al.The role of melanocortin neuronal pathways in circadian biology: a new homeostatic output involving melanocortin-3 receptors?.Obes. Rev. 2009; 10: 14-24Crossref PubMed Scopus (0) Google Scholar, 19Callahan M.F. et al.Sympathetic nervous system mediation of acute cardiovascular actions of gamma 2-melanocyte-stimulating hormone.Hypertension. 1985; 7: I145-I150Crossref PubMed Google Scholar]. In contrast to MC3R, the role of MC4R in governing appetite and metabolic homeostasis has been well characterized. Huszar et al. [7Huszar D. et al.Targeted disruption of the melanocortin-4 receptor results in obesity in mice.Cell. 1997; 88: 131-141Abstract Full Text Full Text PDF PubMed Google Scholar] found that mice with MC4R deficiency show increased body fat tissue, hyperphagia, and hyperinsulinemia. The same study also showed that the ectopic expression of the natural antagonists of the MC4R ligand resulted in obesity in these mice. These findings were simultaneously corroborated by an independent study from Vaisse et al. [20Vaisse C. et al.A frameshift mutation in human MC4R is associated with a dominant form of obesity.Nat. Genet. 1998; 20: 113-114Crossref PubMed Scopus (703) Google Scholar] and Yeo et al. [21Yeo G.S.H. et al.A frameshift mutation in MC4R associated with dominantly inherited human obesity.Nat. Genet. 1998; 20: 111-112Crossref PubMed Scopus (718) Google Scholar] where mutations in the MC4R resulted in a nonsyndromic form of obesity in humans. With increased content of fat in their diet, wild-type mice tend to increase diet-induced thermogenesis and their physical activity. At the same time they decrease food intake within a few days, opposed to knockout mice that increase food intake [9Krashes M.J. et al.Melanocortin-4 receptor-regulated energy homeostasis.Nat. Neurosci. 2016; 19: 206-219Crossref PubMed Google Scholar, 22Ste Marie L. et al.A metabolic defect promotes obesity in mice lacking melanocortin-4 receptors.Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 12339-12344Crossref PubMed Scopus (247) Google Scholar, 23Butler A.A. et al.Melanocortin-4 receptor is required for acute homeostatic responses to increased dietary fat.Nat. Neurosci. 2001; 4: 605-611Crossref PubMed Scopus (0) Google Scholar]. A study from Fan et al. [24Fan W. et al.Role of melanocortinergic neurons in feeding and the agouti obesity syndrome.Nature. 1997; 385: 165-168Crossref PubMed Scopus (1466) Google Scholar] demonstrated that a melanocortin agonist is capable of inhibiting feeding in four mice models with hyperphagia and also inhibits the normal nocturnal food intake. In the same study, the authors also indicated that an MC3R and MC4R antagonist specifically induced hyperphagia, establishing that melanocortinergic neurons exert regulatory effects on feeding behavior [24Fan W. et al.Role of melanocortinergic neurons in feeding and the agouti obesity syndrome.Nature. 1997; 385: 165-168Crossref PubMed Scopus (1466) Google Scholar]. Other phenotypical changes have also been registered on MC4R-deficient mice, such as blood pressure and cardiovascular regulation [7Huszar D. et al.Targeted disruption of the melanocortin-4 receptor results in obesity in mice.Cell. 1997; 88: 131-141Abstract Full Text Full Text PDF PubMed Google Scholar, 25Wannamethee S.G. et al.Hypertension, serum insulin, obesity and the metabolic syndrome.J. Hum. Hypertens. 1998; 12: 735-741Crossref PubMed Google Scholar, 26Li S.J. et al.Melanocortin antagonists define two distinct pathways of cardiovascular control by alpha- and gamma-melanocyte-stimulating hormones.J. Neurosci. 1996; 16: 5182-5188PubMed Google Scholar, 27Humphreys M.H. et al.Cardiovascular effects of melanocortins.Eur. J. Pharmacol. 2011; 660: 43-52Crossref PubMed Scopus (15) Google Scholar]. Hereditary studies on eating disorder show that heritability of fat mass in monozygotic twins ranges between 70–90% and 35–45% in dizygotic twins [28Berrettini W. The genetics of eating disorders.Psychiatry (Edgmont). 2004; 1: 18-25PubMed Google Scholar, 29Maes H.H. et al.Genetic and environmental factors in relative body weight and human adiposity.Behav. Genet. 1997; 27: 325-351Crossref PubMed Scopus (919) Google Scholar]. Analysis of human populations with varying ethnic backgrounds indicates that 6–8% of the obesity cases are derived from MC4R gene mutations [30Yeo G.S.H.H. et al.Mutations in the human melanocortin-4 receptor gene associated with severe familial obesity disrupts receptor function through multiple molecular mechanisms.Hum. Mol. Genet. 2003; 12: 561-574Crossref PubMed Scopus (0) Google Scholar, 31O’Rahilly S. et al.Minireview: human obesity-lessons from monogenic disorders.Endocrinology. 2003; 144: 3757-3764Crossref PubMed Scopus (0) Google Scholar, 32van den Berg L. et al.Melanocortin-4 receptor gene mutations in a Dutch cohort of obese children.Obesity. 2011; 19: 604-611Crossref PubMed Scopus (18) Google Scholar, 33Farooqi I.S. et al.Clinical spectrum of obesity and mutations in the melanocortin 4 receptor gene.N. Engl. J. Med. 2003; 348: 1085-1095Crossref PubMed Scopus (905) Google Scholar]. These studies emphasize the central role of MC4R in energy homeostasis and obesity [9Krashes M.J. et al.Melanocortin-4 receptor-regulated energy homeostasis.Nat. Neurosci. 2016; 19: 206-219Crossref PubMed Google Scholar, 21Yeo G.S.H. et al.A frameshift mutation in MC4R associated with dominantly inherited human obesity.Nat. Genet. 1998; 20: 111-112Crossref PubMed Scopus (718) Google Scholar, 34Nogueiras R. et al.The central melanocortin system directly controls peripheral lipid metabolism.J. Clin. Invest. 2007; 117: 3475-3488Crossref PubMed Scopus (262) Google Scholar, 35Ersoy B.A. et al.Mechanism of N-terminal modulation of activity at the melanocortin-4 receptor GPCR.Nat. Chem. Biol. 2012; 8: 725-730Crossref PubMed Scopus (0) Google Scholar]. Parallel studies, exploring the physiological role of MC4R, found that activation of MC4R increases erectile function and modulates sexual behavior [36Van der Ploeg L.H.T. et al.A role for the melanocortin 4 receptor in sexual function.Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 11381-11386Crossref PubMed Scopus (217) Google Scholar, 37Wessells H. et al.Effect of an alpha-melanocyte stimulating hormone analog on penile erection and sexual desire in men with organic erectile dysfunction.Urology. 2000; 56: 641-646Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 38Safarinejad M.R. Hosseini S.Y. Salvage of sildenafil failures with bremelanotide: a randomized, double-blind, placebo controlled study.J. Urol. 2008; 179: 1066-1071Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. Initial reports on the physiological role of MC5R stated that this receptor was involved in exocrine gland dysfunction, sebum regulation, and the waxy skin fatty protection, as well as in pathogenesis of acne [39Zhang L. et al.Melanocortin-5 receptor and sebogenesis.Eur. J. Pharmacol. 2011; 660: 202-206Crossref PubMed Scopus (18) Google Scholar, 40Morgan C. Cone R.D. Melanocortin-5 receptor deficiency in mice blocks a novel pathway influencing pheromone-induced aggression.Behav. Genet. 2006; 36: 291-300Crossref PubMed Scopus (0) Google Scholar, 41Chen W. et al.Exocrine gland dysfunction in MC5-R-deficient mice: evidence for coordinated regulation of exocrine gland function by melanocortin peptides.Cell. 1997; 91: 789-798Abstract Full Text Full Text PDF PubMed Scopus (340) Google Scholar]. Zhang et al. [39Zhang L. et al.Melanocortin-5 receptor and sebogenesis.Eur. J. Pharmacol. 2011; 660: 202-206Crossref PubMed Scopus (18) Google Scholar, 42Eisinger M. et al.A melanocortin receptor 1 and 5 antagonist inhibits sebaceous gland differentiation and the production of sebum-specific lipids.J. Dermatol. Sci. 2011; 63: 23-32Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar] observed that when MC5R is inactive, the levels of sebocyte differentiation in vitro decreased due to inefficient water repulsion. In human skin, MC5R can be found in differentiated cells, including Harderian, lacrimal, preputial, and sebaceous glands, and it is shown to be essential for the production of porphyrins by stress-regulated synthesis. The aforementioned physiologic effects are mediated by a series of peptide ligands derived from the precursor proopiomelanocortin (POMC) peptide (Figure 2). Through proteolytic processing, POMC is cleaved to generate several possible ligands including ACTH and various melanocyte-stimulating hormone (MSH) variants such as α-MSH, β-MSH, and γ-MSH [43Millington G.W. The role of proopiomelanocortin (POMC) neurones in feeding behaviour.Nutr. Metab. (Lond.). 2007; 4: 18-34Crossref PubMed Scopus (116) Google Scholar, 44Poggioli R. et al.ACTH-(1-24) and alpha-MSH antagonize feeding behavior stimulated by kappa opiate agonists.Peptides. 1986; 7: 843-848Crossref PubMed Scopus (188) Google Scholar]. ACTH, represented by the green box in Figure 2, can be further processed to α-MSH (green), a promiscuous ligand that can activate all MCRs, except MC2R. MC1R and MC3–5R are also activated by β-MSH (blue) and γ-MSH (orange). In particular, with respect to MC4R, the rank-order potency relationship for the POMC-derived agonists has been observed as follows: α-MSH = ACTH > β-MSH >> γ-MSH [45Abdel-Malek Z.A. Melanocortin receptors: their functions and regulation by physiological agonists and antagonists.Cell. Mol. Life Sci. 2001; 58: 434-441Crossref PubMed Google Scholar]. It has been hypothesized that endogenous agonists adopt a β-turn conformation, which presents the His-Phe-Arg-Trp motif for optimal binding with the receptor [46Hruby V.J. et al.Alpha-melanotropin: the minimal active sequence in the frog skin bioassay.J. Med. Chem. 1987; 30: 2126-2130Crossref PubMed Google Scholar, 47Castrucci A.M. et al.Alpha-melanotropin: the minimal active sequence in the lizard skin bioassay.Gen. Comp. Endocrinol. 1989; 73: 157-163Crossref PubMed Google Scholar]. This core motif interacts with the ionic and aromatic residues in the upper regions of the TM domains of MC4R between TM2 and TM3 [48Holder J.R. et al.Structure–activity relationships of the melanocortin tetrapeptide Ac-His-d-Phe-Arg-Trp-NH2 at the mouse melanocortin receptors. 4. Modifications at the Trp Position.J. Med. Chem. 2002; 45: 5736-5744Crossref PubMed Scopus (49) Google Scholar, 49Holder J.R. et al.Structure–activity relationships of the melanocortin tetrapeptide Ac-His-d-Phe-Arg-Trp-NH2 at the mouse melanocortin receptors. 1. Modifications at the His position.J. Med. Chem. 2002; 45: 2801-2810Crossref PubMed Scopus (0) Google Scholar, 50Holder J.R. et al.Structure-activity relationships of the melanocortin tetrapeptide Ac-His-DPhe-Arg-Trp-NH2 at the mouse melanocortin receptors. Part 3: modifications at the Arg position.Peptides. 2003; 24: 73-82Crossref PubMed Scopus (37) Google Scholar, 51Holder J.R. et al.Structure–activity relationships of the melanocortin tetrapeptide Ac-His-DPhe-Arg-Trp-NH2 at the mouse melanocortin receptors: part 2 modifications at the Phe position.J. Med. Chem. 2002; 45: 3073-3081Crossref PubMed Scopus (0) Google Scholar]. Aside from the ligand-mediated activation, MC4R also shows constitutive activity, which is induced by its own N-terminal domain modulation and the ability of the TM domain to undergo spontaneous conformational transformations from the inactive to the active states [35Ersoy B.A. et al.Mechanism of N-terminal modulation of activity at the melanocortin-4 receptor GPCR.Nat. Chem. Biol. 2012; 8: 725-730Crossref PubMed Scopus (0) Google Scholar]. The amino acid sequence (His-Leu-Trp-Asn-Arg-Ser-Ser) responsible for the N-terminal modulation interacts with different residues than those the agonists bind to [35Ersoy B.A. et al.Mechanism of N-terminal modulation of activity at the melanocortin-4 receptor GPCR.Nat. Chem. Biol. 2012; 8: 725-730Crossref PubMed Scopus (0) Google Scholar, 52Srinivasan S. et al.Constitutive activity of the melanocortin-4 receptor is maintained by its N-terminal domain and plays a role in energy homeostasis in humans.J. Clin. Invest. 2004; 114: 1158-1164Crossref PubMed Scopus (148) Google Scholar]. This may suggest that there is an additional potential for positive allosteric modulation of the receptor as an additional avenue for the development of therapeutic or investigational compounds [53Cone, R.D. et al. (2013) Positive allosteric modulators for melanocortin receptors, Patent WO2013134376 A1Google Scholar, 54Montero-Melendez T. et al.Old drugs with new skills: fenoprofen as an allosteric enhancer at melanocortin receptor 3.Cell. Mol. Life Sci. 2017; 74: 1335-1345Crossref PubMed Scopus (1) Google Scholar, 55Pantel J. et al.Development of a high throughput screen for allosteric modulators of melanocortin-4 receptor signaling using a real time cAMP assay.Eur. J. Pharmacol. 2011; 660: 139-147Crossref PubMed Scopus (0) Google Scholar]. The melanocortins, unlike many other GPCRs, also have a unique layer of control exerted by endogenous antagonists. With the exception of MC2R, all the melanocortins can be antagonized by agouti signaling proteins (MC1R and MC4R) and agouti-related proteins (MC3R–MC5R) [45Abdel-Malek Z.A. Melanocortin receptors: their functions and regulation by physiological agonists and antagonists.Cell. Mol. Life Sci. 2001; 58: 434-441Crossref PubMed Google Scholar]. For a detailed review of the physiological roles of MCRs, the reader may consult the following publications [23Butler A.A. et al.Melanocortin-4 receptor is required for acute homeostatic responses to increased dietary fat.Nat. Neurosci. 2001; 4: 605-611Crossref PubMed Scopus (0) Google Scholar, 56Garfield A.S. et al.Role of central melanocortin pathways in energy homeostasis.Trends Endocrinol. Metab. 2009; 20: 203-215Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 57Tao Y.-X. The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology.Endocr. Rev. 2010; 31: 506-543Crossref PubMed Scopus (215) Google Scholar, 58Ramachandrappa S. et al.The melanocortin receptors and their accessory proteins.Front. Endocrinol. (Lausanne). 2013; 4: 9PubMed Google Scholar]. Obesity has been considered a world epidemic since 1997 by the World Health Organization, and the scientific community has been extending efforts to come with a solution [59Caballero B. The global epidemic of obesity: an overview.Epidemiol. Rev. 2007; 29: 1-5Crossref PubMed Scopus (543) Google Scholar]. Following the discovery of melanocortins and their cognate receptor(s), an exhaustive amount of work was performed in determining how these receptors were activated and how they influence the human phenotype [24Fan W. et al.Role of melanocortinergic neurons in feeding and the agouti obesity syndrome.Nature. 1997; 385: 165-168Crossref PubMed Scopus (1466) Google Scholar, 26Li S.J. et al.Melanocortin antagonists define two distinct pathways of cardiovascular control by alpha- and gamma-melanocyte-stimulating hormones.J. Neurosci. 1996; 16: 5182-5188PubMed Google Scholar, 60Sawyer T.K. et al.Comparative biological activities of highly potent active-site analogs of alpha-melanotropin.J. Med. Chem. 1982; 25: 1022-1027Crossref PubMed Google Scholar, 61Castrucci A.M. et al.Enzymological studies of melanotropins.Comp. Biochem. Physiol. B. 1984; 78: 519-524Crossref PubMed Scopus (0) Google Scholar, 62Haskell-Luevano C. et al.Discovery of prototype peptidomimetic agonists at the human melanocortin receptors MC1R and MC4R.J. Med. Chem. 1997; 40: 2133-2139Crossref PubMed Scopus (0) Google Scholar, 63Cone R.D. et al.The melanocortin receptors: agonists, antagonists, and the hormonal control of pigmentation.Recent Prog. Horm. Res. 1996; 51: 287-317PubMed Google Scholar, 64Mountjoy K.G. et al.Localization of the melanocortin-4 receptor (MC4-R) in neuroendocrine and autonomic control circuits in the brain.Mol. Endocrinol. 1994; 8: 1298-1308Crossref PubMed Scopus (895) Google Scholar, 65Hruby V.J. et al.Cyclic lactam alpha-melanotropin analogues of Ac-Nle4-cyclo[Asp5, d-Phe7,Lys10] alpha-melanocyte-stimulating hormone-(4-10)-NH2 with bulky aromatic amino acids at position 7 show high antagonist potency and selectivity at specific melanocortin receptors.J. Med. Chem. 1995; 38: 3454-3461Crossref PubMed Google Scholar, 66Lu D. et al.Agouti protein is an antagonist of the melanocyte-stimulating-hormone receptor.Nature. 1994; 71: 799-802Crossref Google Scholar, 67Gantz I. et al.Molecular cloning, expression, and gene localization of a fourth melanocortin receptor.J. Biol. Chem. 1993; 268: 15174-15179Abstract Full Text PDF PubMed Google Scholar]. While several melanocortin agonists were developed initially, with some of them even completing clinical trials as tanning agents, it was not until after the identification of the different MCRs that further testing for receptor selectivity took place [60Sawyer T.K. et al.Comparative biological activities of highly potent active-site analogs of alpha-melanotropin.J. Med. Chem. 1982; 25: 1022-1027Crossref PubMed Google Scholar, 61Castrucci A.M. et al.Enzymological studies of melanotropins.Comp. Biochem. Physiol. B. 1984; 78: 519-524Crossref PubMed Scopus (0) Google Scholar, 65Hruby V.J. et al.Cyclic lactam alpha-melanotropin analogues of Ac-Nle4-cyclo[Asp5, d-Phe7,Lys10] alpha-melanocyte-stimulating hormone-(4-10)-NH2 with bulky aromatic amino acids at position 7 show high antagonist potency and selectivity at specific melanocortin receptors.J. Med. Chem. 1995; 38: 3454-3461Crossref PubMed Google Scholar]. The fourth MCR was identified and located by northern blot analysis and in situ hybridization by Gantz et al. in 1993 [2Gantz I. et al.Molecular cloning of a novel melanocortin receptor.J. Biol. Chem. 1993; 268: 8246-8250Abstract Full Text PDF PubMed Google Scholar, 67Gantz I. et al.Molecular cloning, expression, and gene localization of a fourth melanocortin receptor.J. Biol. Chem. 1993; 268: 15174-15179Abstract Full Text PDF PubMed Google Scholar]. Shortly after, this work was corroborated by Mountjoy et al. [64Mountjoy K.G. et al.Localization of the melanocortin-4 receptor (MC4-R) in neuroendocrine and autonomic control circuits in the brain.Mol. Endocrinol. 1994; 8: 1298-1308Crossref PubMed Scopus (895) Google Scholar, 68Magenis R.E. et al.Mapping of the ACTH, MSH, and neural (MC3 and MC4) melanocortin receptors in the mouse and human.Mamm. Genome. 1994; 5: 503-508Crossref PubMed Google Scholar]. This important milestone stimulated the development of specific agonists and investigations into possible applications for the previously developed MCR agonists. A systematic literature search of the PubMed database yielded 1382 publications on ‘melanocortin 4 receptor’, up until August 2017, of which 43% were published in the last five years. Despite initial reports implicating MCRs in hypertensive patients, the increase in number of publications mirrors the importance of this specific receptor on metabolic regulation [14Ellacott K.L.J. et al.Obesity-induced inflammation in white adipose tissue is attenuated by loss of melanocortin-3 receptor signaling.Endocrinology. 2007; 148: 6186-6194Crossref PubMed Scopus (0) Google Scholar, 25Wannamethee S.G. et al.Hypertension, serum insulin, obesity and the metabolic syndrome.J. Hum. Hypertens. 1998; 12: 735-741Crossref PubMed Google Scholar]. The first reports on the influence of melanocortins over the feeding behavior date back over 30 years. A study conducted in 1986 by Poggioli et al. [44Poggioli R. et al.ACTH-(1-24) and alpha-MSH antagonize feeding behavior stimulated by kappa opiate agonists.Peptides. 1986; 7: 843-848Crossref PubMed Scopus (188) Google Scholar] suggests that melanocortin peptides inhibit food intake [44Poggioli R. et al.ACTH-(1-24) and alpha-MSH antagonize feeding behavior stimulated by kappa opiate agonists.Peptides. 1986; 7: 843-848Crossref PubMed Scopus (188) Google Scholar]. However, evidence supporting the critical role of MC4R in mediating this effect was only shown ten years later by Huszar et al. [7Huszar D. et al.Targeted disruption of the melanocortin-4 receptor results in obesity in mice.Cell. 1997; 88: 131-141Abstract Full Text Full Text PDF PubMed Google Scholar] in a study that reported obesity syndrome as a consequence of disruption of MC4R. In the years that followed this discovery, different mutations on the receptor were studied with the purpose of better defining the relation between the receptor and fatty tissue production, body weight, and energy expenditure [69S