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-Giy-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)⁹. 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.