Structural modifications of the N-terminal tetrapeptide segment of [D-Ala2]deltorphin I: effects on opioid receptor affinities and activities in vitro and on antinociceptive potency.
A series of deltorphin I analogs containing D- or L-N-methylalanine (MeAla), D- or L-proline (Pro), alpha-aminoisobutyric acid (Aib), sarcosine (Sar) or D-tert-leucine (Tle) in place of D-Ala2, or phenylalanine in place of Tyr1, was synthesized. The opioid activity profiles of these peptides were determined in mu and delta 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-MeAla2-analog were slightly more potent delta agonists in the mouse vas deferens (MDV) assay, and the D-MeAla2-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 delta opioid potency of [L-MeAla2]deltorphin I is postulated to be due to the demonstrated presence of a conformer with a cis Tyr1-MeAla2 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-NH2, which displayed lower delta receptor affinity than the parent peptide but higher delta selectivity and, surprisingly, three times higher antinociceptive potency. The D- and L-Pro2-, Sar2- and D-Tle2-analogs showed much reduced delta receptor affinities and were inactive in the tail flick test. Replacement of Tyr1 in deltorphin I with Phe produced a 32-fold decrease in delta receptor affinity but only a 7-fold drop in antinociceptive potency.
Schmidt
R
R
Laboratory of Chemical Biology and Peptide Research, Clinical Research Institute of Montreal, Quebec, Canada. schillp@IRCM.UMontreal.CA
Menard
D
D
Mrestani-Klaus
C
C
Chung
N N
NN
Lemieux
C
C
Schiller
P W
PW
D000701:Analgesics, Opioid / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D001921:Brain / Q000378:metabolism; D004594:Electrophysiology; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D051379:Mice; D009119:Muscle Contraction / Q000187:drug effects; D009842:Oligopeptides / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D010147:Pain Measurement; D011485:Protein Binding; D051381:Rats; D017465:Receptors, Opioid, delta / Q000819:agonists / Q000145:classification / Q000378:metabolism*; D014649:Vas Deferens / Q000187:drug effects
D000701:Analgesics, Opioid; D009842:Oligopeptides; D017465:Receptors, Opioid, delta; C060208:deltorphin I, Ala(2)-