β-Methylation of the Phe7 and Trp9 Melanotropin Side Chain Pharmacophores Affects Ligand−Receptor Interactions and Prolonged Biological Activity

Topographically modified melanotropin side chain pharmacophore residues Phe7 and Trp9 in a cyclic peptide template (Ac-Nle4-c[Asp-His-Xaa7-Arg-Yaa9-Lys]-NH2) and Phe7 in a linear peptide template (Ac-Ser-Tyr-Ser-Nle4-Glu-His-Xaa7-Arg-Trp-Gly-Lys-Pro-Val-NH2) result in differences in potency and prolonged biological activity in the frog and lizard skin bioassays. These topographic modifications included the four isomers of β-methylphenylalanine (β-MePhe)7 and β-methyltryptophan (β-MeTrp)9 and the two isomers of 1,2,3,4-tetrahydro-β-carboline (Tca). Modifications in the cyclic template resulted in up to a 1000-fold difference in potency for the β-MePhe7 stereoisomeric peptides; up to a 476-fold difference in potency resulted for the β-MeTrp9 peptides, and about a 50-fold difference between the Tca9-containing peptides. Up to a 40-fold difference in potency resulted for the β-MePhe7 stereoisomeric peptides using the linear template in these assays. The relative potency ranking for modifications in the cyclic template of β-MePhe7 were 2R,3S > 2S,3S = 2S,3R > 2R,3R in the frog assay and 2S,3R > 2R,3S > 2S,3S > 2R,3R in the lizard assay. The relative potencies for modifications in the cyclic template of β-MeTrp9 were 2R,3S > 2R,3R > 2S,3S ≫ 2S,3R in the frog assay and 2S,3S = 2R,3R > 2R,3S > 2S,3R in the lizard assay. The relative potencies for modifications in the cyclic template of Tca9 were dTca > lTca in both assays. Significant differences in prolonged (residual) activities were also observed for these modified peptides and were dependent upon stereochemistry of the β-methyl amino acid, peptide template, and bioassay system. Furthermore, comparisons of β-MeTrp9 stereoisomeric peptides on the frog, lizard, and human MC1 receptors suggest that structure−activity relationships on both the classical frog and lizard skin bioassays do not necessarily predict corresponding SAR profiles for the human melanocortin receptors, indicating a remarkable species specificity of the MC1 receptor requirements.