Structural Modifications of the N-terminal Tetrapeptide Segment of [D-Ala 2]Deltorphin I: Effects on Opioid Receptor Affinities and Activities In Vitro and on Antinociceptive Potency

Abstract

Schmidt, R., D. Menard, C. Mrestani-Klaus, N. N. Chung, C. Lemieux and P. W. Schiller. Structural modifications of the N -terminal tetrapeptide segment of [ d-Ala 2 ]deltorphin I: effects on opioid receptor affinities and activities in vitro and on antinociceptive potency. Peptides 18(10) 1615–1621, 1997.—A series of deltorphin I analogs containing d- or l-N -methylalanine (MeAla), d- or l-proline (Pro), α-aminoisobutyric acid (Aib), sarcosine (Sar) or D-tert-leucine (Tle) in place of d-Ala 2, or phenylalanine in place of Tyr 1, was synthesized. The opioid activity profiles of these peptides were determined in μ and δ opioid receptor-representative binding assays and bioassays in vitro as well as in the rat tail flick test in vivo. In comparison with the deltorphin I parent, both the l- and the d-MeAla 2-analog were slightly more potent δ agonists in the mouse vas deferens (MDV) assay, and the d-MeAla 2-analog showed two-fold higher antinociceptive potency in the analgesic test. In view of the fact that deltorphin analogs with an unsubstituted l-amino acid residue in the 2-position generally lack opioid activity, the observed high δ opioid potency of [ l-MeAla 2]deltorphin I is postulated to be due to the demonstrated presence of a conformer with a cis Tyr 1-MeAla 2 peptide bond, since the cis conformer allows for a spatial arrangement of the pharmacophoric moieties in the N-terminal tripeptide segment similar to that in active deltorphin analogs containing a d-amino acid residue in the 2-position. Substitution of Aib in the 2-position led to a compound, H-Tyr-Aib-Phe-Asp-Val-Val-Gly-NH 2, which displayed lower δ receptor affinity than the parent peptide but higher δ selectivity and, surprisingly, three times higher antinociceptive potency. The d- and l- Pro 2-, Sar 2- and d-Tle 2-analogs showed much reduced δ receptor affinities and were inactive in the tail flick test. Replacement of Tyr 1 in deltorphin I with Phe produced a 32-fold decrease in δ receptor affinity but only a 7-fold drop in antinociceptive potency.