STRUCTURE-ACTIVITY-RELATIONSHIPS OF DERMORPHIN ANALOGS 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 pepti des have the potential of assuming a cis Tyr(1)-Xaa(2) peptide bond. T heir in vitro opioid activity profiles were determined in mu- and delt a-receptor-representative binding assays and bioassays. Aside from [D- Pro(2)]dermorphin, all analogues showed high affinity for mu- and/or d elta-opioid receptors, Whereas most compounds were found to be full mu -agonists in the guinea pig ileum (GPI) assay, [Tic(2)]dermorphin (com pound 7) was a partial mu-agonist. Replacement of Gly(4) in 7 with Phe resulted in an analogue (8) with weak mu-antagonist activity. Further more, analogues 7 and 8 both were potent delta-antagonists (K-e = 3-40 nM) against the delta-agonists Leu-enkephalin, DPDPE and deltorphin I in the mouse vas deferens (MVD) assay. Compound 3, containing L-Pro i n the 2-position, turned out to be one of the most mu-receptor-selecti ve linear dermorphin analogues reported to date. Low-temperature HPLC experiments using micropellicular octadecyl silica as stationary phase revealed conformational heterogeneity of the dermorphin analogues whi ch was ascribed to cis-trans isomerization around the Tyr(1)-Xaa(2)- a nd Tyr(5)-Pro(6) peptide bonds. In the case of analogue 7 four separat e peaks corresponding to the four possible isomers were apparent at -5 degrees C. Since opioid peptide analogues with a non-N-alkylated L-am ino acid residue in the 2-position are nearly inactive and cannot assu me a cis peptide bond at the 1-2 position, these results support the h ypothesis that the bioactive conformation of opioid peptides containin g an N-alkylated L-amino acid residue in position 2 is characterized b y a cis Tyr(1)-Xaa(2) peptide bond. (C) Munksgaard 1995.