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 <SCP>l</SCP>- or <SCP>d</SCP>-Phe⁶ or <SCP>d</SCP>-Nal(2‘)⁶, and with <SCP>d</SCP>-Trp⁸ or <SCP>d</SCP>-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 <SCP>d</SCP>-Nal(2‘)⁸ and Phe⁶ with <SCP>d</SCP>-Phe⁶ in γ-MSH-NH<INF>2</INF> forms a selective antagonist for the hMC3R, whereas, substitution of Phe⁶ with <SCP>d</SCP>-Nal(2‘)⁶ and replacing Trp⁸ with <SCP>d</SCP>-Trp⁸ at γ-MSH-NH<INF>2</INF> yields a selective partial agonist for the hMC1R. Finally, substitution of His⁵ with Pro⁵ and Trp⁸ with <SCP>d</SCP>-Nal(2‘)⁸ in γ-MSH-NH<INF>2</INF> leads to a highly potent and selective agonist for the hMC1R. Molecular modeling showed that, at the C-terminal of Nle³-γ-MSH-NH<INF>2</INF>, 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 Nle³-γ-MSH-NH<INF>2</INF> 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.