beta-methylation of the Phe(7) and Trp(9) melanotropin side chain pharmacophores affects ligand-receptor interactions and prolonged biological activity

Topographically modified melanotropin side chain pharmacophore residues Phe(7) and Trp(9) in a cyclic peptide template (Ac-Nle(4)-c[Asp-His-Xaa(7)-Arg-Yaa(9)-Lys]-NH2) and Phe(7) in a linear peptide template (Ac-Ser-Tyr-Ser-Nle(4)-Glu-His-Xaa(7)-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 beta-methylphenylalanine (beta-MePhe)(7) and beta-methyltryptophan (beta-MeTrp)(9) and the two isomers of 1,2,3,4-tetrahydro-beta-carboline (Tea)(9) Modifications in the cyclic template resulted in up to a 1000-fold difference in potency for the beta-MePhe(7) stereoisomeric peptides; up to a 476-fold difference in potency resulted for the beta-MeTrp(9) peptides, and about a 50-fold difference between the Tca(9)-containing peptides. Up to a 40-fold difference in potency resulted for the beta-MePhe(7)? stereoisomeric peptides using the linear template in these assays. The relative potency racing for modifications in the cyclic template of beta-MePhe(7) were 2R,3S > 2S,3S = 2S,3R 2 > 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 beta-MeTrp(9) were 2R,3S > 2R,3R > 2S,3R > 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 Tca(9) 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 beta-methyl amino acid, peptide template, and bioassay system. Furthermore, comparisons of beta-MeTrp(9) 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.