Ψ and χ Angle Constrains at the C-Terminus Trp Position of the Melanotropin Tetrapeptide Ac-His-d-Phe-Arg-Trp-NH Lead to Potent and Selective Agonists at hMC1R and hMC4R.

Melanocortin receptors (MCRs) are a family of G protein-coupled receptors that regulate important physiological functions. Yet, drug development targeting MCRs is hindered by potential side effects due to a lack of receptor subtype-selective ligands with bioavailability. Here, we report novel synthetic pathways to introduce Ψ and χ angle constraints at the C-terminus Trp position of the nonselective prototype tetrapeptide agonist Ac-His-d-Phe-Arg-Trp-NH. With these conformational constraints, peptide (Ac-His-d-Phe-Arg-Aia) shows improved selectivity at hMC1R, with an EC of 11.2 nM for hMC1R and at least 15-fold selectivity compared to other MCR subtypes. Peptide (Ac-His-CF-d-Phe-Arg-Aia) is a potent and selective hMC4R agonist with an EC of 4.1 nM at hMC4R and at least ninefold selectivity. Molecular docking studies reveal that the Ψ and χ angle constraints force the C-terminal Aia residue to flip and interact with TM6 and TM7, a feature that we hypothesize leads to the receptor subtype selectivity.