The melanocortin pathway and energy homeostasis: From discovery to obesity therapy.

BACKGROUND: Over the past 20 years, insights from human and mouse genetics have illuminated the central role of the brain leptin-melanocortin pathway in controlling mammalian food intake, with genetic disruption resulting in extreme obesity, and more subtle polymorphic variations influencing the population distribution of body weight. At the end of 2020, the U.S. Food and Drug Administration (FDA) approved setmelanotide, a melanocortin 4 receptor agonist, for use in individuals with severe obesity due to either pro-opiomelanocortin (POMC), proprotein convertase subtilisin/kexin type 1 (PCSK1), or leptin receptor (LEPR) deficiency. SCOPE OF REVIEW: Herein, we chart the melanocortin pathway's history, explore its pharmacology, genetics, and physiology, and describe how a neuropeptidergic circuit became an important druggable obesity target. MAJOR CONCLUSIONS: Unravelling the genetics of the subset of severe obesity has revealed the importance of the melanocortin pathway in appetitive control; coupling this with studying the molecular pharmacology of compounds that bind melanocortin receptors has brought a new obesity drug to the market. This process provides a drug discovery template for complex disorders, which for setmelanotide took 25 years to transform from a single gene into an approved drug.

Current Mechanistic and Pharmacodynamic Understanding of Melanocortin-4 Receptor Activation.

In this work we summarize our understanding of melanocortin 4 receptor (MC4R) pathway activation, aiming to define a safe and effective therapeutic targeting strategy for the MC4R. Delineation of cellular MC4R pathways has provided evidence for distinct MC4R signaling events characterized by unique receptor activation kinetics. While these studies remain narrow in scope, and have largely been explored with peptidic agonists, the results provide a possible correlation between distinct ligand groups and differential MC4R activation kinetics. In addition, when a set of small-molecule and peptide MC4R agonists are compared, evidence of biased signaling has been reported. The results of such mechanistic studies are discussed.

MC4R agonism promotes durable weight loss in patients with leptin receptor deficiency.

Genetic defects underlying the melanocortin-4 receptor (MC4R) signaling pathway lead to severe obesity. Three severely obese LEPR-deficient individuals were administered the MC4R agonist setmelanotide, resulting in substantial and durable reductions in hyperphagia and body weight over an observation period of 45-61 weeks. Compared to formerly developed and tested MC4R agonists, setmelanotide has the unique capability of activating nuclear factor of activated T cell (NFAT) signaling and restoring function of this signaling pathway for selected MC4R variants. Our data demonstrate the potency of setmelanotide in treatment of individuals with diverse MC4R-related pathway deficiencies.

Evaluation of a melanocortin-4 receptor (MC4R) agonist (Setmelanotide) in MC4R deficiency.

OBJECTIVE: Pro-opiomelanocortin (POMC)-derived peptides act on neurons expressing the Melanocortin 4 receptor (MC4R) to reduce body weight. Setmelanotide is a highly potent MC4R agonist that leads to weight loss in diet-induced obese animals and in obese individuals with complete POMC deficiency. While POMC deficiency is very rare, 1-5% of severely obese individuals harbor heterozygous mutations in MC4R. We sought to assess the efficacy of Setmelanotide in human MC4R deficiency. METHODS: We studied the effects of Setmelanotide on mutant MC4Rs in cells and the weight loss response to Setmelanotide administration in rodent studies and a human clinical trial. We annotated the functional status of 369 published MC4R variants. RESULTS: In cells, we showed that Setmelanotide is significantly more potent at MC4R than the endogenous ligand alpha-melanocyte stimulating hormone and can disproportionally rescue signaling by a subset of severely impaired MC4R mutants. Wild-type rodents appear more sensitive to Setmelanotide when compared to MC4R heterozygous deficient mice, while MC4R knockout mice fail to respond. In a 28-day Phase 1b clinical trial, Setmelanotide led to weight loss in obese MC4R variant carriers. Patients with POMC defects upstream of MC4R show significantly more weight loss with Setmelanotide than MC4R deficient patients or obese controls. CONCLUSIONS: Setmelanotide led to weight loss in obese people with MC4R deficiency; however, further studies are justified to establish whether Setmelanotide can elicit clinically meaningful weight loss in a subset of the MC4R deficient obese population.

Preclinical gastrointestinal prokinetic efficacy and endocrine effects of the ghrelin mimetic RM-131.

AIMS: The 28 amino acid hormone ghrelin, the natural ligand for the growth hormone secretagogue, or ghrelin receptor (GHR), has diverse physiological functions, including a possible role as a gastrointestinal prokinetic. The synthetic ghrelin mimetic RM-131 is in Phase II clinical trials for treatment of diabetic gastroparesis and other gastrointestinal (GI) disorders. We aimed to determine the relative potency of RM-131, when compared to other GI ghrelin mimetics, to predict efficacy and determine the role of RM-131 in models of inflammatory bowel disease. MAIN METHODS: We evaluated and compared ghrelin, RM-131 and other synthetic ghrelin mimetics for their prokinetic potency in models of gastrointestinal disorders in the rat and we evaluated the endocrine (rats and dogs) and anti-inflammatory effects (mice) of the ghrelin mimetic RM-131. KEY FINDINGS: The pentapeptide RM-131 increased gastric emptying in rodent models of ileus. RM-131 is about 100-fold more potent than human ghrelin and is 600 to 1800-fold more potent, when compared to several investigational ghrelin mimetics tested in clinical trials. RM-131 has anti-inflammatory effects and significantly increases survival and reduces macroscopic markers of tissue damage in a TNBS model of inflammatory bowel disease. RM-131 treatment shows a transient increase in growth hormone levels in Beagle dogs and rats, returning to baseline upon chronic treatment. Significant effects on glucose and insulin are not observed in chronic studies. SIGNIFICANCE: RM-131's potency, efficacy and endocrine profile, are promising attributes for the treatment of diverse functional gastrointestinal disorders in humans.

Melanocortins in the treatment of male and female sexual dysfunction.

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 pre-copulatory 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.

Imaging thromboembolism with fibrin-avid 99mTc-peptide: evaluation in swine.

UNLABELLED: A pentapeptide, Gly-Pro-Arg-Pro-Pro, with high affinity for alpha-chain-fibrin was labeled with (99m)Tc ((99m)Tc-TP850) and evaluated in swine to image experimental venous thromboembolism (deep vein thrombosis [DVT]) and pulmonary embolism (PE). METHODS: Scatchard analysis was performed to determine fibrin affinity for TP850 and the number of binding sites (receptors) per milligram of fibrin. DVT was induced in the left jugular vein and PE was induced by introducing a preformed autologous blood clot into the right atrium using a 7-French introducer sheath inserted into the right jugular vein. (99m)Tc-TP850 was injected at 4, 24, 48, 72, 96, or 120 h later. Animals were imaged for up to 4 h after injection, heparinized, and sacrificed. Lungs were extirpated, radiographed, and imaged, and the PE was removed. Other tissues, including blood and normal lungs, were harvested and, concomitantly, (99m)Tc was counted for determination of target-to-tissue ratios and the percentage injected dose per gram of tissue. RESULTS: The affinity for human fibrin was 10(-9) mol/L and there were >10(15) receptors per milligram of fibrin. DVT and PE were visualized for up to 4 h after injection with high DVT/blood (7.9-22.6), DVT/muscle (31.1-89.4), PE/blood (1-155), and PE/lung (0.8-245) ratios. Thereafter, the PEs fragmented spontaneously below the spatial resolution of the gamma-camera and, despite the high associated radioactivity, could not be localized in vivo. The fragmented clots were detectable by scintigraphy on excised lungs and provided excellent concordance with radiograms. CONCLUSION: (99m)Tc-TP850 with its modest affinity (10(-9) mol/L), rapid blood clearance, and high DVT and PE uptake is a promising agent for imaging vascular thrombosis.