Design, synthesis, and evaluation of proline based melanocortin receptor ligands

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.

Alpha-melanocyte stimulating hormone: production and degradation

Proopiomelanocortin (POMC) is a polypeptide hormone precursor that is expressed in the brain and in peripheral tissues such as in the pituitary gland, immune system, and skin. In the brain, POMC is processed to form several peptides including alpha-melanocyte stimulating hormone (α-MSH). alpha-MSH is expressed in the hypothalamic arcuate nucleus and in the nucleus tractus solitarius of the brainstem where it has a crucial role in the regulation of metabolic functions. Specifically, α-MSH is an anorexigenic peptide. Its production and maturation processes have been shown to be regulated according to the metabolic condition of the organism. This review summarizes our current knowledge on α-MSH processing including its maturation and degradation processes and pharmacological aspects of its manipulation.

Targeting melanocortin receptors: an approach to treat weight disorders and sexual dysfunction

The melanocortin system has multifaceted roles in the control of body weight homeostasis, sexual behaviour and autonomic functions, and so targeting this pathway has immense promise for drug discovery across multiple therapeutic areas. In this Review, we first outline the physiological roles of the melanocortin system, then discuss the potential of targeting melanocortin receptors by using MC3 and MC4 agonists for treating weight disorders and sexual dysfunction, and MC4 antagonists to treat anorectic and cachectic conditions. Given the complexity of the melanocortin system, we also highlight the challenges and opportunities for future drug discovery in this area.

Identification of arginine analogues as antagonists and agonists for the melanocortin-4 receptor

In the present study, conducted to explore potent and small molecular melanocortin-4 (MC4) receptor ligands, we found that tripeptide 3a, containing a D-Phe-Arg-2-Nal (Nal; naphthylalanine) sequence, exhibited a moderate affinity for the MC4 receptor. Structural optimization led to the identification of a compound with a high affinity for the MC4 receptor, namely, tripeptide 3e, which showed a 70-fold higher affinity for the MC4 receptor than the lead compound 3a. Moreover, in an effort to further reduce the peptidic characters of tripeptide 3e, we found that dipeptide 3g exhibited a relatively high affinity for the MC4 receptor. Furthermore, in these analogues, the substituted position (1' vs. 2') of the naphthyl ring of Nal residue at position 7 was found to be important for the differentiation of agonist and antagonist activity. The synthesis and structure-activity relationships of the arginine analogues as MC4 receptor ligands were described in this paper.

The MC4 receptor and control of appetite

Mutations in the human melanocortin (MC)4 receptor have been associated with obesity, which underscores the relevance of this receptor as a drug target to treat obesity. Infusion of MC4R agonists decreases food intake, whereas inhibition of MC receptor activity by infusion of an MC receptor antagonist or with the inverse agonist AgRP results in increased food intake. This review addresses the role of the MC system in different aspects of feeding behaviour. MC4R activity affects meal size and meal choice, but not meal frequency, and the type of diet affects the efficacy of MC4R agonists to reduce food intake. The central sites involved in the different aspects of feeding behaviour that are affected by MC4R signalling are being unravelled. The paraventricular nucleus plays an important role in food intake per se, whereas MC signalling in the lateral hypothalamus is associated with the response to a high fat diet. MC4R signalling in the brainstem has been shown to affect meal size. Further genetic, behavioural and brain-region specific studies need to clarify how the MC4R agonists affect feeding behaviour in order to determine which obese individuals would benefit most from treatment with these drugs. Application of MCR agonists in humans has already revealed side effects, such as penile erections, which may complicate introduction of these drugs in the treatment of obesity.

Design, synthesis, and biological evaluation of a new class of small molecule peptide mimetics targeting the melanocortin receptors

A new bicyclic template has been developed for the synthesis of peptide mimetics. Straightforward synthetic steps, starting from amino acids, allow the facile construction of a wide range of analogs. This system was designed to target the melanocortin receptors (MCRs), with functional group selection based on a known pharmacophore and guidance from molecular modeling to rationally identify positional and stereochemical isomers likely to be active. The functions of hMCRs are critical to myriad biological activities, including pigmentation, steroidogenesis, energy homeostasis, erectile activity, and inflammation. These G-protein-coupled receptors (GPCRs) are targets for drug discovery in a number of areas, including cancer, pain, and obesity therapeutics. All compounds from this series tested to date are antagonists which bind with high affinity. Importantly, many are highly selective for a particular MCR subtype, including some of the first completely hMC5R-selective antagonists reported.

Design and synthesis of potent and selective 1,3,4-trisubstituted-2-oxopiperazine based melanocortin-4 receptor agonists

The design and synthesis of a series of potent 1,3,4-trisubstituted-2-oxopiperazine based MC4 agonists are described. The tripeptidomimetic analogs (12a,b and 23) and the dipeptidomimetic 27 displayed single-nanomolar binding affinity and agonist potency for MC4R and excellent selectivity for MC4R relative to MC1R.

Design, synthesis, and evaluation of proline and pyrrolidine based melanocortin receptor agonists. A conformationally restricted dipeptide mimic approach

The design, synthesis, and structure-activity relationships (SAR) of a series of novel proline and pyrrolidine based melanocortin receptor (MCR) agonists are described. To validate a conformationally constrained Arg-Nal dipeptide analogue strategy, we first synthesized and evaluated a test set of cis-(2R,4R)-proline analogues (21a-g). All of these compounds showed significant binding and agonist potency at the hMC1R, hMC3R, and hMC4R. Potent cis-(2S,4R)-pyrrolidine based MCR agonists (35a-g) were subsequently developed by means of this design approach. A SAR study directed toward probing the effect of the two chiral centers in the pyrrolidine ring on biological activity revealed the importance of the (S) absolute configuration at the 2-position for binding affinity, agonist potency, and receptor selectivity. Among the four sets of the pyrrolidine diastereomers investigated, analogues with the (2S,4R) configuration were the most potent agonists across the three receptors, followed by those possessing the (2S,4S) configuration.

Synthesis of Tic-D-Phe Psi[CH2-CH2] isostere and its use in the development of melanocortin receptor agonists

The first synthesis of Tic-D-Phe Psi[CH(2)-CH(2)] isostere is described, which features diastereoselective alkylation of the tricyclic lactam 14. The use of this novel dipeptide isostere in the development of melanocortin agonists has been demonstrated by the synthesis of peptidomimetic 7 and non-peptidic ligand 27. Both compounds displayed significant binding and agonist potency at the MC4R.