Novel 3D pharmacophore of α-MSH/γ-MSH hybrids leads to selective human MC1R and MC3R analogues

To further evaluate elements that could contribute to the 3D topographical structure of γ-MSH, we have systematically designed a group of linear γ-MSH analogues and evaluated their biological activities: without a N-terminal acetyl, with and without a C-terminal amide, with Nle³, with L- or D-Phe⁶ or D-Nal(2′)⁶, and with D-Trp ⁸ or D-Nal(2′)⁸. It was found that changing the C-terminal acid in γ-MSH to an amide and replacing Met with Nle leads to increased binding affinities at all four subtypes of melanocortin receptors (10-100 fold). Substitution of Trp⁸ with D-Nal(2′)⁸ and Phe⁶ with D-Phe⁶ in γ-MSH-NH₂ forms a selective antagonist for the hMC3R, whereas, substitution of Phe⁶ with D-Nal(2′)⁶ and replacing Trp⁸ with D-Trp ⁸ at γ-MSH-NH₂ yields a selective partial agonist for the hMC1R. Finally, substitution of His⁵ with Pro⁵ and Trp⁸ with D-Nal(2′)⁸ in γ-MSH-NH₂ leads to a highly potent and selective agonist for the hMC1R. Molecular modeling showed that, at the C-terminal of Nle³-γ-MSH-NH₂, there is a reverse-turn-like structure, suggesting that there might be a secondary binding site involved in ligand - receptor interaction for γ-MSH analogues that may explain the enhanced binding affinities of the Nle ³-γ-MSH-NH₂ analogues. Our results indicate that increasing the hydrophobicity and replacing Phe⁶ and Trp⁸ with bulkier aromatic amino acid residues is very important for selectivity of α-MSH/γ-MSH hybrids for hMCRs.