Novel ligands lacking a positive charge for the delta- and mu-opioid receptors.
Recently we reported using minilibraries to replace Lys(9) [somatostatin (SRIF) numbering] of the potent somatostatin agonist L-363,301 (c[-Pro-Phe-D-Trp-Lys-Thr-Phe-]) to generate the potent neurokinin receptor (NK-1) antagonist c[-Pro-Phe-D-Trp-p-F-Phe-Thr-Phe-]. This novel cyclic hexapeptide did not bind the SRIF receptor. Thus, a single mutation converted L-363,301, a SRIF agonist with potency ca. 2-8 times the potency of SRIF in laboratory animals,(24) into a selective NK-1 receptor antagonist with an IC(50) of 2 nM in vitro. During the screening of the same libraries for ligands of the delta-opioid receptor, we identified four compounds (1-4) which represent a new class of delta-opioid antagonists, some of which were also NK-1 receptor antagonists. The most potent delta-opioid antagonist, c[-Pro-1-Nal-D-Trp-Tyr-Thr-Phe-] (2), showed a K(e) value of 128 nM in the mouse vas deferens assay and a delta-receptor binding affinity constant of 152 nM in the rat brain membrane binding assay. These results are of interest because they represent a novel class of delta-opioid antagonists and, like two previously reported delta-opioid antagonists, they lack a positive charge. To examine further the requirement for a positive charge in the delta-opioid ligands, we prepared two analogues of the beta-casomorphin-derived mixed mu-agonist/delta-antagonist, H-Dmt-c[-D-Orn-2-Nal-D-Pro-Gly-] (7), in which we eliminated the positive charge either through formylation of the primary amino group (5) or by the deletion of this N-terminal amino group (6). These latter compounds proved to be delta-opioid antagonists with K(e) values in the 16-120 nM range, as well as fairly potent mu-opioid antagonists (K(e) approximately 200 nM). These six compounds provide the most convincing evidence to date that there is no requirement for a positive charge in mu- and delta-opioid receptor antagonists. In addition, cyclic hexapeptide 4 lacks a phenolic hydroxyl group. Taken together, these data suggest that the prevailing assumptions about delta- and mu-opioid receptor binding need revision and that the receptors for these opioid ligands have much in common with the NK-1 and somatostatin receptors.
Schiller
P W
PW
Laboratory of Chemical Biology and Peptide Research, Clinical Research Institute of Montréal, 110 Pine Avenue West, Montréal, Quebec, Canada.
Berezowska
I
I
Nguyen
T M
TM
Schmidt
R
R
Lemieux
C
C
Chung
N N
NN
Falcone-Hindley
M L
ML
Yao
W
W
Liu
J
J
Iwama
S
S
Smith
A B
AB
Hirschmann
R
R
D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects; D066298:In Vitro Techniques; D008024:Ligands; D008297:Male; D051379:Mice; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects; D009292:Narcotic Antagonists / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D064729:Neurokinin-1 Receptor Antagonists; D009842:Oligopeptides / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D010456:Peptides, Cyclic / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D011869:Radioligand Assay; D051381:Rats; D018040:Receptors, Neurokinin-1 / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013329:Structure-Activity Relationship; D014649:Vas Deferens / Q000187:drug effects
D008024:Ligands; D009292:Narcotic Antagonists; D064729:Neurokinin-1 Receptor Antagonists; D009842:Oligopeptides; D010456:Peptides, Cyclic; D018040:Receptors, Neurokinin-1; D017450:Receptors, Opioid, mu; C405037:cyclo(prolyl-1-naphthylalanyl-tryptophyl-tyrosyl-threonyl-phenylalanyl)