Structure-activity relationships of melanocortin agonists containing the benzimidazole scaffold

Discovery of Polypharmacological Melanocortin-3 and -4 Receptor Probes and Identification of a 100-Fold Selective nM MC3R Agonist versus a μM MC4R Partial Agonist

The centrally expressed melanocortin-3 and melanocortin-4 receptors (MC3R and MC4R, respectively) are established targets to treat diseases of positive- and negative-energy homeostasis. We previously reported [ Doering , S. R. ; J. Med. Chem. 2017 , 60 , 4342 - 4357 ] mixture-based positional scanning approaches to identify dual MC3R agonist and MC4R antagonist tetrapeptides. Herein, 46 tetrapeptides were chosen for MC3R agonist screening selectivity profiles, synthesized, and pharmacologically characterized at the mouse melanocortin-1, -3, -4, and -5 receptors. Substitutions to the tetrapeptide template were selected solely based on MC3R agonist potency from the mixture-based screen. This study resulted in the discovery of compound 42 (Ac-Val-Gln-(pI)DPhe-DTic-NH₂), a full MC3R agonist that is 100-fold selective for the MC3R over the μM MC4R partial agonist pharmacology. This compound represents a first-in-class MC3R selective agonist. This ligand will serve as a useful in vivo molecular probe for the investigation of the roles of the MC3R and MC4R in diseases of dysregulated energy homeostasis.

A rearrangement involving a solid-state melt reaction for the synthesis of multifunctional tetrasubstituted olefins

A new catalyst- and solvent-free intramolecular rearrangement sequence leading towards benzimidazole-tethered tetrasubstituted olefins through a solid-state melt reaction (SSMR).

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.

Synthesis and Structure-Activity Relationships of Substituted Urea Derivatives on Mouse Melanocortin Receptors

The melanocortin system is involved in the regulation of several complex physiological functions. In particular, the melanocortin-3 and -4 receptors (MC3R/MC4R) have been demonstrated to regulate body weight, energy homeostasis, and feeding behavior. Synthetic and endogenous melanocortin agonists have been shown to be anorexigenic in rodent models. Herein, we report synthesis and structure-activity relationship (SAR) studies of 27 nonpeptide small molecule ligands based on an unsymmetrical substituted urea core. Three templates containing key residues from the lead compounds, showing diversity at three positions (R(1), R(2), R(3)), were designed and synthesized. The syntheses were optimized for efficient microwave-assisted chemistry that significantly reduced total syntheses time compared to a previously reported room temperature method. The pharmacological characterization of the compounds on the mouse melanocortin receptors identified compounds 1 and 12 with full agonist activity at the mMC4R, but no activity was observed at the mMC3R when tested up to 100 μM concentrations. The SAR identified compounds possessing aliphatic or saturated cyclic amines at the R(1) position, bulky aromatic groups at the R(2) position, and benzyl group at the R(3) position resulted in mMC4R selectivity over the mMC3R. The small molecule template and SAR knowledge from this series may be helpful in further design of MC3R/MC4R selective small molecule ligands.

Triazine-benzimidazole hybrids: anticancer activity, DNA interaction and dihydrofolate reductase inhibitors

A new series of triazine-benzimidazole hybrids has been synthesized with different substitution of primary and secondary amines at one of the position of triazine in moderate to good yields. These compounds were evaluated for their inhibitory activities over 60 human tumor cell lines at one dose and five dose concentrations. Compounds 6b, 8 and 9 showed broad spectrum of antitumor activities with GI50 values of 9.79, 2.58 and 3.81μM, respectively. DNA binding studies also indicated strong interaction properties of these compounds. These synthesized compounds also showed inhibition of mammalian dihydrofolate reductase (DHFR). Compound 6b was depicted as the most active member of DHFR inhibitor with IC50 value of 1.05μM. Molecular modelling studies were used to identify the stabilized interactions of Compound 6b within the active site of enzyme for DHFR.

Divergent synthesis of unsymmetrical annulated biheterocyclic compound libraries: benzimidazole linked indolo-benzodiazepines/quinoxaline

Diversity-oriented synthesis of novel benzimidazole linked indolo-benzodiazepine/quinoxaline ring systems using poly(ethylene glycol) as soluble polymer support is described. Commercially available 4-fluoro-3-nitrobenzoic acid and indoline were utilized for the construction of these annulated biheterocyclic compound libraries having multiple privileged structures with three-point structural diversity. A reagent based diversification approach coupled with the Pictet-Spengler-type condensation was used to construct the tetracyclic indolo-benzodiazepines/quinoxalines on substituted benzimidazoles.

γ₂-Melanocyte stimulation hormone (γ₂-MSH) truncation studies results in the cautionary note that γ₂-MSH is not selective for the mouse MC3R over the mouse MC5R

The melanocortin system has been implicated in a multitude of physiological pathways including obesity, satiety, energy homeostasis, sexual behavior, pigmentation, sodium regulation, hypertension, and many others. Based upon studies of the endogenous melanocortin receptor agonists at the cloned human melanocortin receptor proteins, it was concluded that the γ-MSH related agonist ligands are selective for the MC3 versus the MC4 and MC5 receptors. In attempts to understand and identify the specific amino acids of γ₂-MSH important for MC3R selectivity, we have performed N- and C-terminal truncation studies and pharmacologically characterized twenty-eight ligands at the mouse MC1 and MC3-5 melanocortin receptors. The C-terminal Trp-Asp⁹-Arg¹⁰-Phe¹¹ residues are important for nM potency at the mMC3R and the Arg⁷-Trp⁸ residues are important for mMC5R nM potency. We observed the unanticipated results that several of the C-terminal truncated analogs possessed nM agonist potency at the mMC3 and mMC5Rs which lead us to perform a comparative side-by-side study of the mouse and human MC5R. These data resulted in μM γ₂-MSH analog potency at the hMC5R, consistent with previous reports, however at the mMC5R, nM γ₂-MSH analog potency was observed. Thus, these data support the hypothesis of important species specific differences in γ-MSH related ligand potency at the rodent versus human MC5R subtype that is critical for the interpretation of in vivo rodent physiological studies. These results prompted us to examine the affects of a peripherally administered melanocortin agonist on hypothalamic gene expression levels of the MC3R, MC4R, and MC5R. The super potent non-selective NDP-MSH agonist was administered i.p. and resulted in significantly decreased levels of mMC3R and mMC5R hypothalamic mRNA versus saline control. These data provide for the first time data demonstrating peripherally administered NDP-MSH can modify hypothalamic melanocortin receptor expression levels.

Effect of serotonergic anorectics on food intake and induction of Fos in brain of mice with disruption of melanocortin 3 and/or 4 receptors

Previous studies have indicated that type 3 or 4 melanocortin receptors (MCR) are downstream of the critical anorectic action of drugs that stimulate 5-HT(2C) receptors. To characterize further the receptor types involved, we have studied the effect of serotonergic anorectics in mice with genomic disruption of either MC3R or MC4R, or their combined knockout. In a first experiment, we showed that wild type (WT) and MC4R-/- mice showed comparable inhibition of food intake following acute treatment with dexnorfenfluramine. In a second experiment using WAY-161503, a 5-HT receptor full agonist with selectivity for 2B and 2C subtypes, we found that MC4R-/- responded comparably to WT, while MC3R-/- had reduced sensitivity. Double receptor knockout (DKO) mice responded comparably to WT and MC4R-/-. Surprisingly, brain Fos-ir was not strongly induced in any brain region by WAY-16103 with the exception of the paraventricular nucleus of DKO. These data suggest that MC3Rs may be involved in the response to serotonergic anorectic agents, and more generally in control of food intake.