Discovery of a Selective Small-Molecule Melanocortin-4 Receptor Agonist with Efficacy in a Pilot Study of Sexual Dysfunction in Humans

Functional coupling between MC4R and Kir7.1 contributes to clozapine-induced hyperphagia

Most antipsychotic drugs (APDs) induce hyperphagia and weight gain. However, the neural mechanisms are poorly understood, partly due to challenges replicating their metabolic effects in rodents. Here, we report a new mouse model that recapitulates overeating induced by clozapine, a widely prescribed APD. Our study shows that clozapine boosts food intake by inhibiting melanocortin 4 receptor (MC4R) expressing neurons in the paraventricular nucleus of the hypothalamus. Interestingly, neither clozapine nor risperidone, another commonly used APD, affects receptor-ligand binding or the canonical Gαs signaling of MC4R. Instead, they inhibit neuronal activity by enhancing the coupling between MC4R and Kir7.1, leading to the open state of the inwardly rectifying potassium channel. Deletion of Kir7.1 in Mc4r-Cre neurons prevents clozapine-induced weight gain, while treatment with a selective Kir7.1 blocker mitigates overeating in clozapine-fed mice. Our findings unveil a molecular pathway underlying the effect of APDs on feeding behavior and suggest its potential as a therapeutic target.

Methyl-Containing Pharmaceuticals

This Special Issue, which focused on methyl-containing pharmaceuticals, collected different papers and reviews on this topic [...].

A review on pharmacological options for the treatment of erectile dysfunction: state of the art and new strategies

INTRODUCTION

Erectile dysfunction (ED) affects between 12.9% and 28.1% of men worldwide, presenting a strong aged-correlated prevalence. Several pharmacological treatments are currently available for ED, which can be classified into oral, injection, and topical/intraurethral therapies.

AREAS COVERED

Extensive research on PubMed/MEDLINE until February 2023 was performed. For each of the aforementioned drug classes, available molecules, and formulations, their efficacy and most common adverse events as well as general guidelines on prescription were investigated and extensively described. A glimpse into future directions regarding ED pharmacotherapy is also present.

EXPERT OPINION

In recent years, there have been significant developments in pharmacological treatments for ED. It is essential for physicians to identify the best treatment option for patients based on their preferences and sexual habits. The treatment approach for ED has shifted from a sequential to a parallel paradigm, where all treatment options are available as first-line therapies. While there are promising regenerative therapies for ED, such as shockwaves and platelet-rich plasma injections, pharmacological treatment is still the most effective option for most patients.

Discovery of the Potent and Selective MC4R Antagonist PF-07258669 for the Potential Treatment of Appetite Loss

The melanocortin-4 receptor (MC4R) is a centrally expressed, class A GPCR that plays a key role in the regulation of appetite and food intake. Deficiencies in MC4R signaling result in hyperphagia and increased body mass in humans. Antagonism of MC4R signaling has the potential to mitigate decreased appetite and body weight loss in the setting of anorexia or cachexia due to underlying disease. Herein, we report on the identification of a series of orally bioavailable, small-molecule MC4R antagonists using a focused hit identification effort and the optimization of these antagonists to provide clinical candidate 23. Introduction of a spirocyclic conformational constraint allowed for simultaneous optimization of MC4R potency and ADME attributes while avoiding the production of hERG active metabolites observed in early series leads. Compound 23 is a potent and selective MC4R antagonist with robust efficacy in an aged rat model of cachexia and has progressed into clinical trials.

Ligands for Melanocortin Receptors: Beyond Melanocyte-Stimulating Hormones and Adrenocorticotropin

The discovery of melanocortins in 1916 has resulted in more than 100 years of research focused on these peptides. Extensive studies have elucidated well-established functions of melanocortins mediated by cell surface receptors, including MSHR (melanocyte-stimulating hormone receptor) and ACTHR (adrenocorticotropin receptor). Subsequently, three additional melanocortin receptors (MCRs) were identified. Among these five MCRs, MC3R and MC4R are expressed primarily in the central nervous system, and are therefore referred to as the neural MCRs. Since the central melanocortin system plays important roles in regulating energy homeostasis, targeting neural MCRs is emerging as a therapeutic approach for treating metabolic conditions such as obesity and cachexia. Early efforts modifying endogenous ligands resulted in the development of many potent and selective ligands. This review focuses on the ligands for neural MCRs, including classical ligands (MSH and agouti-related peptide), nonclassical ligands (lipocalin 2, β-defensin, small molecules, and pharmacoperones), and clinically approved ligands (ACTH, setmelanotide, bremelanotide, and several repurposed drugs).

Alpha-melanocyte-stimulating hormone (αMSH) modulates the rewarding properties of social interactions in an oxytocin receptor-dependent manner in Syrian hamsters (Mesocricetus Auratus)

A reduction in the rewarding properties of social interactions is frequently a key contributor to neuropsychiatric disorders. Although much remains to be learned about the neural mechanisms governing social reward, numerous studies have found that oxytocin can enhance the salience of rewarding social interactions. As a result, oxytocin has been suggested as a pharmacotherapy for disorders characterized by a dampening of social motivation. However, exogenous oxytocin does not cross the blood-brain barrier effectively, which has led to the investigation of alternative approaches to induce central oxytocin release, such as pharmaceuticals targeting melanocortins. Although oxytocin treatment is widely viewed to increase social reward, there is also recent evidence that high concentrations of oxytocin can decrease social reward. In the present study we tested the hypothesis that alpha-melanocyte-stimulating hormone (αMSH) influences the rewarding properties of social interactions by acting on oxytocin receptors. Male and female Syrian hamsters were given intracerebroventricular infusions of saline, αMSH, or a cocktail containing αMSH and an oxytocin receptor antagonist during social conditioning with a same-sex hamster and then tested for a conditioned place preference. αMSH decreased preference for the socially-paired chamber compared to saline treatment, and administration of the oxytocin antagonist concurrent with αMSH administration returned subjects' preference to control levels. Importantly, αMSH treatments did not affect any measures of body composition or the specific social behaviors displayed during conditioning. These data suggest that melanocortin-targeting drugs should be administered carefully to avoid the possibility of decreasing the rewarding properties of social interactions.

MC4R biased signalling and the conformational basis of biological function selections

The MC4R, a GPCR, has long been a major target for obesity treatment. As the most well‐studied melanocortin receptor subtype, the evolutionary knowledge pushes the drug development and structure–activity relationship (SAR) moving forward. The past decades have witnessed the evolution of scientists' view on GPCRs gradually from the control of a single canonical signalling pathway via a bilateral ‘active‐inactive’ model to a multi‐state alternative model where the ligands' binding affects the selection of the downstream signalling. This evolution brings the concept of biased signalling and the beginning of the next generation of peptide drug development, with the aim of turning from receptor subtype specificity to signalling pathway selectivity. The determination of the value structures of the MC4R revealed insights into the working mechanism of MC4R activation upon binding of agonists. However, new challenge has risen as we seek to unravel the mystery of MC4R signalling selection. Thus, more biased agonists and ligands with representative biological functions are needed to solve the rest of the puzzle.

Sexual incentive motivation, sexual behavior, and general arousal: Do rats and humans tell the same story?

Sexual incentive stimuli activate sexual motivation and heighten the level of general arousal. The sexual motive may induce the individual to approach the incentive, and eventually to initiate sexual acts. Both approach and the ensuing copulatory interaction further enhance general arousal. We present data from rodents and humans in support of these assertions. We then suggest that orgasm is experienced when the combined level of excitation surpasses a threshold. In order to analyze the neurobiological bases of sexual motivation, we employ the concept of a central motive state. We then discuss the mechanisms involved in the long- and short-term control of that state as well as those mediating the momentaneous actions of sexual incentive stimuli. This leads to an analysis of the neurobiology behind the interindividual differences in responsivity of the sexual central motive state. Knowledge is still fragmentary, and many contradictory observations have been made. Nevertheless, we conclude that the basic mechanisms of sexual motivation and the role of general arousal are similar in rodents and humans.

The Deuterated "Magic Methyl" Group: A Guide to Site-Selective Trideuteromethyl Incorporation and Labeling by Using CD3 Reagents

In the field of medicinal chemistry, the precise installation of a trideuteromethyl group is gaining ever-increasing attention. Site-selective incorporation of the deuterated "magic methyl" group can provide profound pharmacological benefits and can be considered an important tool for drug optimization and development. This review provides a structured overview, according to trideuteromethylation reagent, of currently established methods for site-selective trideuteromethylation of carbon atoms. In addition to CD3 , the selective introduction of CD2 H and CDH2 groups is also considered. For all methods, the corresponding mechanism and scope are discussed whenever reported. As such, this review can be a starting point for synthetic chemists to further advance trideuteromethylation methodologies. At the same time, this review aims to be a guide for medicinal chemists, offering them the available C-CD3 formation strategies for the preparation of new or modified drugs.

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.

Installing the “magic methyl” – C–H methylation in synthesis

Following notable cases of remarkable potency increases in methylated analogues of lead compounds, this review documents the state-of-the-art in C–H methylation technology.

Pharmacology and perspectives in erectile dysfunction in man

Penile erection is a perfect example of microcirculation modulated by psychological factors and hormonal status. It is the result of a complex neurovascular process that involves the integrative synchronized action of vascular endothelium; smooth muscle; and psychological, neuronal, and hormonal systems. Therefore, the fine coordination of these events is essential to maintain penile flaccidity or allow erection; an alteration of these events leads to erectile dysfunction (ED). ED is defined as the consistent or recurrent inability of a man to attain and/or maintain a penile erection sufficient for sexual activity. A great boost to this research field was given by commercialization of phosphodiesterase-5 (PDE5) inhibitors. Indeed, following the discovery of sildenafil, research on the mechanisms underlying penile erection has had an enormous boost, and many preclinical and clinical papers have been published in the last 10 years. This review is structured to provide an overview of the mediators and peripheral mechanism(s) involved in penile function in men, the drugs used in therapy, and the future prospective in the management of ED. Indeed, 30% of patients affected by ED are classified as "nonresponders," and there is still an unmet need for therapeutic alternatives. A flowchart suggesting the guidelines for ED evaluation and the ED pharmacological treatment is also provided.

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.

Structure⁻Activity Relationships of the Tetrapeptide Ac-His-Arg-(pI)DPhe-Tic-NH2 at the Mouse Melanocortin Receptors: Modification at the (pI)DPhe Position Leads to mMC3R Versus mMC4R Selective Ligands

The five melanocortin receptors (MC1R–MC5R) are involved in numerous biological pathways, including steroidogenesis, pigmentation, and food intake. In particular, MC3R and MC4R knockout mice suggest that the MC3R and MC4R regulate energy homeostasis in a non-redundant manner. While MC4R-selective agonists have been utilized as appetite modulating agents, the lack of MC3R-selective agonists has impeded progress in modulating this receptor in vivo. In this study, the (pI)DPhe position of the tetrapeptide Ac-His-Arg-(pI)DPhe-Tic-NH₂ (an MC3R agonist/MC4R antagonist ligand) was investigated with a library of 12 compounds. The compounds in this library were found to have higher agonist efficacy and potency at the mouse (m) MC3R compared to the MC4R, indicating that the Arg-DPhe motif preferentially activates the mMC3R over the mMC4R. This observation may be used in the design of new MC3R-selective ligands, leading to novel probe and therapeutic lead compounds that will be useful for treating metabolic disorders.

Small Molecule Neuropeptide S and Melanocortin 4 Receptor Ligands as Potential Treatments for Substance Use Disorders

There is a vital need for novel approaches and biological targets for drug discovery and development. Treatment strategies for substance use disorders (SUDs) to date have been mostly ineffective other than substitution-like therapeutics. Two such targets are the peptide G-protein-coupled receptors neuropeptide S (NPS) and melanocortin 4 (MC4). Preclinical evidence suggests that antagonists, inverse agonists, or negative allosteric modulators of these receptors might be novel therapeutics for SUDs. NPS is a relatively unexplored receptor with high potential for treating SUD. MC4 has a strong link to early-onset obesity, and emerging evidence suggests significant overlap between food-maintained and drug-maintained behaviors making MC4 an intriguing target for SUD. This chapter provides an overview of the literature in relation to the roles of NPS and MC4 in drug-seeking behaviors and then provides a medicinal chemistry-based survey of the small molecule ligands for each receptor.

Binding, Thermodynamics, and Selectivity of a Non-peptide Antagonist to the Melanocortin-4 Receptor

The melanocortin-4 receptor (MC4R) is a potential drug target for treatment of obesity, anxiety, depression, and sexual dysfunction. Crystal structures for MC4R are not yet available, which has hindered successful structure-based drug design. Using microsecond-scale molecular-dynamics simulations, we have investigated selective binding of the non-peptide antagonist MCL0129 to a homology model of human MC4R (hMC4R). This approach revealed that, at the end of a multi-step binding process, MCL0129 spontaneously adopts a binding mode in which it blocks the agonistic-binding site. This binding mode was confirmed in subsequent metadynamics simulations, which gave an affinity for human hMC4R that matches the experimentally determined value. Extending our simulations of MCL0129 binding to hMC1R and hMC3R, we find that receptor subtype selectivity for hMC4R depends on few amino acids located in various structural elements of the receptor. These insights may support rational drug design targeting the melanocortin systems.

Bench-top to clinical therapies: A review of melanocortin ligands from 1954 to 2016

The discovery of the endogenous melanocortin agonists in the 1950s have resulted in sixty years of melanocortin ligand research. Early efforts involved truncations or select modifications of the naturally occurring agonists leading to the development of many potent and selective ligands. With the identification and cloning of the five known melanocortin receptors, many ligands were improved upon through bench-top in vitro assays. Optimization of select properties resulted in ligands adopted as clinical candidates. A summary of every melanocortin ligand is outside the scope of this review. Instead, this review will focus on the following topics: classic melanocortin ligands, selective ligands, small molecule (non-peptide) ligands, ligands with sex-specific effects, bivalent and multivalent ligands, and ligands advanced to clinical trials. Each topic area will be summarized with current references to update the melanocortin field on recent progress. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Drugs in preclinical to phase II clinical development for the treatment of erectile dysfunction

INTRODUCTION

Erectile function is an important aspect in the quality of life of many men. For men with erectile dysfunction (ED), a spectrum of treatment options exists. Novel therapies for ED are currently being developed in order to delay surgical placement of a penile prosthesis - the final step in the management of treatment-refractory ED. Areas covered: This review examines innovative treatments such as alternative vasoactive agents, trophic factors and bio-compounds as well as gene and stem cell therapy. All therapies are currently in some phase of development for the management of ED. Using the MedLine and FDA Clinical Trials Registry, recent developments in treatment of ED were queried. Expert opinion: Recent studies have demonstrated the potential for multiple, novel therapies in the treatment of ED. Much of the work requires further experimentation in large-scale, blinded, placebo-controlled studies. This will require a concerted effort to bring these products to market.

Melanocortin Receptor Agonists Facilitate Oxytocin-Dependent Partner Preference Formation in the Prairie Vole

The central melanocortin (MC) system has been widely studied for its effects on food intake and sexual behavior. However, the MC system, and more specifically the MC4 receptor (MC4R), also interacts with neurochemical systems that regulate socioemotional behaviors, including oxytocin (OT) and dopamine. In monogamous prairie voles, OT and dopamine interact to promote partner preference formation, a laboratory measure of an enduring social bond between mates. Here we investigated the effects of MC receptor activation on partner preference formation in prairie voles, as well as the interaction between the MC and OT systems during this process. Peripheral administration of the brain penetrant MC3/4R receptor peptide agonist, Melanotan II (MTII), and the highly selective, small-molecule MC4R agonist, Pf-446687, enhanced partner preference formation in the prairie vole, but not in the non-monogamous meadow vole. MTII-induced partner preferences were enduring, as they were present 1 week after drug manipulation. The prosocial effects of MCR agonists may be mediated, in part, through modulation of OT, as coadministration of an OT receptor antagonist prevented MTII-induced partner preferences. MTII also selectively activated hypothalamic OT neurons and potentiated central OT release. As OT has been shown to enhance some aspects of social cognition in humans, our data suggest that the MC4R may be a viable therapeutic target for enhancing social function in psychiatric disorders, including autism spectrum disorders and schizophrenia, potentially through activation of the OT system.

Reduced melanocortin production causes sexual dysfunction in male mice with POMC neuronal insulin and leptin insensitivity

Proopiomelanocortin (POMC)-derived peptides like α-melanocyte-stimulating hormone (MSH) substantially improve hepatic insulin sensitivity and regulate energy expenditure. Melanocortinergic agents are also powerful inducers of sexual arousal that are being investigated for a possible therapeutic role in erectile dysfunction. It is currently unclear whether reduced melanocortin (MC) activity may contribute to the sexual dysfunction accompanying obesity and type 2 diabetes. Male rodents with leptin and insulin resistance targeted to POMC neurons (leptin receptor [LepR]/insulin receptor [IR]POMC mice) exhibit obesity, hyperinsulinemia, hyperglycemia, and systemic insulin resistance. In this study, we demonstrate that LepR/IRPOMC males are also subfertile due to dramatic alterations in sexual behavior. Remarkably, these reproductive changes are accompanied by decreased α-MSH production not present when a single receptor type is deleted. Unexpectedly, behavioral sensitivity to α-MSH and MC receptor expression are also reduced in LepR/IRPOMC males, a potential adaptation of the MC system to altered α-MSH production. Together, these results suggest that concurrent insulin and leptin resistance in POMC neurons in individuals with obesity or type 2 diabetes can reduce endogenous α-MSH levels and impair sexual function.

Neonatal melanocortin receptor agonist treatment reduces play fighting and promotes adult attachment in prairie voles in a sex-dependent manner

The melanocortin receptor (MCR) system has been studied extensively for its role in feeding and sexual behavior, but effects on social behavior have received little attention. α-MSH interacts with neural systems involved in sociality, including oxytocin, dopamine, and opioid systems. Acute melanotan-II (MTII), an MC3/4R agonist, potentiates brain oxytocin (OT) release and facilitates OT-dependent partner preference formation in socially monogamous prairie voles. Here we examined the long-term impact of early-life MCR stimulation on hypothalamic neuronal activity and social development in prairie voles. Male and female voles were given daily subcutaneous injections of 10 mg/kg MTII or saline between postnatal days (PND) 1-7. Neonatally-treated males displayed a reduction in initiated play fighting bouts as juveniles compared to control males. Neonatal exposure to MTII facilitated partner preference formation in adult females, but not males, after a brief cohabitation with an opposite-sex partner. Acute MTII injection elicited a significant burst of the immediate early gene EGR-1 immunoreactivity in hypothalamic OT, vasopressin, and corticotrophin releasing factor neurons, when tested in PND 6-7 animals. Daily neonatal treatment with 1 mg/kg of a more selective, brain penetrant MC4R agonist, PF44687, promoted adult partner preferences in both females and males compared with vehicle controls. Thus, developmental exposure to MCR agonists lead to a persistent change in social behavior, suggestive of structural or functional changes in the neural circuits involved in the formation of social relationships.

A new class of prolylcarboxypeptidase inhibitors, part 2: the aminocyclopentanes

A series of potent inhibitors of prolylcarboxypeptidase (PrCP) was developed by modifying a lead structure that was discovered by high-throughput screening. The tert-butyl pyrrolidine was replaced by an aminocyclopentane to reduce the metabolic liabilities of the original lead. The compounds demonstrated sub-nanomolar in vitro IC(50) values, minimal activity shifts in pure plasma and improved pharmacokinetics. Complete ex vivo plasma target engagement was achieved with low brain exposure at the 20 h time point following p.o. dosing in a mouse. The results indicate that the aminocyclopentanes are useful tools for studying the therapeutic potential of peripheral (non-CNS) PrCP inhibition.

A new class of prolylcarboxypeptidase inhibitors, part 1: discovery and evaluation

A new structural class of potent prolylcarboxypeptidase (PrCP) inhibitors was discovered by high-throughput screening. The series possesses a tractable SAR profile with sub-nanomolar in vitro IC(50) values. Compared to prior inhibitors, the new series demonstrated minimal activity shifts in pure plasma and complete ex vivo plasma target engagement in mouse plasma at the 20 h post-dose time point (po). In addition, the in vivo level of CNS and non-CNS drug exposure was measured.