Structure‐activity relationships of dermorphin analogues containing N‐substituted amino acids in the 2‐position of the peptide sequence

A series of dermorphin analogues containing an N ‐alkylated amino‐acid residue Xaa in the 2‐position of the peptide sequence was synthesized (Xaa = N ‐methylalanine, proline, pipecolic acid, N ‐methylphenylalanine, 1,2,3,4‐tetrahydroisoquinoline‐3‐carboxylic acid [Tic]). These peptides have the potential of assuming a cis Tyr l ‐Xaa 2 peptide bond. Their in vitro opioid activity profiles were determined in μ and δ‐receptor‐representative binding assays and bioassays. Aside from [D‐Pro 2 ]dermorphin, all analogues showed high affinity for μ and/or δ‐opioid receptors. Whereas most compounds were found to be full μ‐agonists in the guinea pig ileum (GPI) assay, [Tic 2 ]dermorphin (compound 7) was a partial μ‐agonist. Replacement of Gly 4 in 7 with Phe resulted in an analogue (8) with weak μ‐antagonist activity. Furthermore, analogues 7 and 8 both were potent § antagonists (K c = 3–40 nM) against the §‐agonists Leuenkephalin, DPDPE and deltorphin I in the mouse vas deferens (MVD) assay. Compound 3, containing l ‐Pro in the 2‐position, turned out to be one of the most μ receptor‐selective linear dermorphin analogues reported to date. Low‐temperature HPLC experiments using micropellicular octadecyl silica as stationary phase revealed conformational heterogeneity of the dermorphin analogues which was ascribed to cis‐trans isomerization around the Tyr l ‐Xaa 2 ‐and Tyr 5 ‐Pro 6 peptide bonds. In the case of analogue 7 four separate peaks corresponding to the four possible isomers were apparent at ‐5°C. Since opioid peptide analogues with a non‐ N ‐akylated l ‐amino acid residue in the 2‐position are nearly inactive and cannot assume a cis peptide bond at the 1–2 position, these results support the hypothesis that the bioactive conformation of opioid peptides containing an N ‐alkylated l ‐amino acid residue in position 2 is characterized by a cis Tyr l ‐Xaa 2 peptide bond.