Structure–Activity Relationships of Peptides Incorporating a Bioactive Reverse-Turn Heterocycle at the Melanocortin Receptors: Identification of a 5800-fold Mouse Melanocortin-3 Receptor (mMC3R) Selective Antagonist/Partial Agonist versus the Mouse Melanocortin-4 Receptor (mMC4R)

Abstract

The melanocortin-3 (MC3) and melanocortin-4 (MC4) receptors regulate energy homeostasis, food intake, and associated physiological conditions. The MC4R has been studied extensively. Less is known about specific physiological roles of the MC3R. A major obstacle to this lack of knowledge is attributed to a limited number of identified MC3R selective ligands. We previously reported a spatial scanning approach of a 10-membered thioether-heterocycle ring incorporated into a chimeric peptide template that identified a lead nM MC4R ligand. Based upon those results, 17 compounds were designed and synthesized that focused upon modification in the pharmacophore domain. Notable results include the identification of a 0.13 nM potent 5800-fold mMC3R selective antagonist/slight partial agonist versus a 760 nM mMC4R full agonist (ligand 11). Biophysical experiments (2D 1H NMR and computer assisted molecular modeling) of this ligand resulted in the identification of an inverse γ-turn secondary structure in the ligand pharmacophore domain.

Synthesis, Biophysical, and Pharmacological Evaluation of the Melanocortin Agonist AST3-88: Modifications of Peptide Backbone at Trp 7 Position Lead to a Potent, Selective, and Stable Ligand of the Melanocortin 4 Receptor (MC4R)

“…The thioether ring was formed on the resin by mild on-bead cyclization method. 34 , 41 After completion of the synthesis, the ligand was cleavage and the crude peptide was dissolved in a 20% DMSO/water solution and stirred at room temperature to form the disulfide-bridge. The peptides AST3-88 and AMW6103 possessed the correct molecular weights as determined by mass spectrometry.…”

“… 33 In 2002, the heterocyclic moiety 44 (Figure 1 ) was reported to possess nanomolar MCR functional agonist activity and initial studies were performed incorporating this moiety into the AMW3-130 peptide template. 34 , 41 The first study identified the AMW610 template (Figure 1 ) as possessing the most potent nanomolar MCR full agonist functionality of the SAR at the time. 34 Biophysical experiments, similar to the ones reported herein, were performed in attempts to correlate SAR and identify solution-phase structural differences between the peptides containing different orientations of the heterocycle.…”

“…The MC4R to MC3R selectivity of the control compounds AMW610, AMW3-130, and heterocyclic ring itself ranged from 7- to 12-fold (Table 1 ). 34 , 41 …”

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

“…Of the agonist compounds selective for the hMC1R (Table 1), one was selective for the hMC1R over the hMC4R [119], three were selective for the hMC1R over the hMC3R and hMC4R [119-121], two were selective for the hMC1R over the hMC3R and hMC5R [122], and three possessed at least 100-fold selectivity for the hMC1R over the hMC3R, hMC4R, and hMC5R [121, 123]. For agonist compounds selective for the mMC1R (Table 2), one ligand was selective for the mMC1R over the mMC3R [124], three were selective for the mMC1R over the mMC3R and mMC4R [124, 125], and one compound was at least 100-fold selective for the mMC1R over the three remaining receptors [126]. Of the ligands selective for the hMC1R or mMC1R, three were based upon the linear structure of α-MSH [119, 123], four were substitutions of the MTII/SHU9119 scaffold [120, 121], five were small molecules [122, 124], and two were cyclic analogues of AGRP possessing a thioether heterocyclic [125, 126].…”

“…For agonist compounds selective for the mMC1R (Table 2), one ligand was selective for the mMC1R over the mMC3R [124], three were selective for the mMC1R over the mMC3R and mMC4R [124, 125], and one compound was at least 100-fold selective for the mMC1R over the three remaining receptors [126]. Of the ligands selective for the hMC1R or mMC1R, three were based upon the linear structure of α-MSH [119, 123], four were substitutions of the MTII/SHU9119 scaffold [120, 121], five were small molecules [122, 124], and two were cyclic analogues of AGRP possessing a thioether heterocyclic [125, 126]. …”