A retro-inverso α-melanocyte stimulating hormone analog with MC1R-binding selectivity

α-melanocyte stimulating hormone (α-MSH) is a tridecapeptide fragment of pro-opiomelanocortin (POMC) with broad effects on appetite, skin pigmentation, hormonal regulation, and potential roles in both inflammation and autoimmunity. The use of this peptide as an anti-inflammatory agent is limited by its low selectivity between the melanocortin receptors, susceptibility to proteolytic degradation, and rapid clearance from circulation. A retro-inverso (RI) sequence of α-MSH was characterized for receptor activity and resistance to protease. This peptide demonstrated surprisingly high selectivity for binding the melanocortin receptor 1 (MC1R). However, RI-α-MSH exhibited a diminished binding affinity for MC1R compared to α-MSH. Mapping of the residues critical for agonist activity, receptor binding, and selectivity by alanine scanning, identified the same critical core tetrapeptide required for the native peptide. Modest improvements in affinity were obtained by conservative changes employing non-natural amino acids and substitution of the C-terminal sequence with a portion of a MC1R ligand peptide previously identified by phage display. Recombination of these elements yielded a peptide with an identical K(i) as α-MSH at MC1R and a lower EC(50) in Mel-624 melanoma cells. A number of other structural modifications of the RI peptide were found to differ in effect from those reported for the L-form α-MSH, suggesting a significantly altered interaction with the MC1R.