Novel Ligands Lacking a Positive Charge for the δ- and μ-Opioid Receptors

Abstract

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.

Molecular Docking of Opiates and Opioid Peptides, a Tool for the Design of Selective Agonists and Antagonists, and for the Investigation of Atypical Ligand-Receptor Interactions

“…In most cases, the removal or derivatization of this group resulted in inactive derivatives or antagonists; some outstanding examples of antagonists are given: the cyclic β-casomorphin-derived opioid peptide CHO-Dmt-c[D-Orn-2-Nal-Pro-Gly] showed MOR and DOR antagonist activity [100]; the cyclic opioid peptides containing 3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid (Dhp), (2S)-2-methyl-3-(2,6-dimethyl-4-hydroxyphenyl)-propanoic acid [(2S)-Mdp], or (3S)-3-methyl-3-(2,6-dimethyl-4-hydroxyphenyl) propanoic acid [(3S)-Mdp] instead of the classic Tyr1 residue, resulted in opioid antagonists [101][102][103]; the MOR and KOR antagonist N-formylnormorphine [104]; the dynorphin A analogue cyclo(N,5)[Trp3,Trp4,Glu5] dynorphin A-(1-11)NH 2 , an Nterminal to-side chain cyclic peptide showed good KOR antagonism [105].…”

Cyclic Opioid Peptides

“…However, it has been demonstrated that cyclic analogues of β-casomorphin, (CHO-Dmt-c 2,5 [DOrn-2-Nal-DPro-Gly] and Dhp-c 2,5 [DOrn-2-Nal-DPro-Gly]), lack the protonated nitrogen atom, and yet exhibit opioid activities as DOR ( K e = 16 and 120 nM, respectively, in the MVD) and MOR ( K e = 220 and 240 nM, respectively, in the GPI) antagonists, but did not interact with the KOR [46]. The Dhp-derivatives were the first neutral compounds not to possess the positively charged nitrogen atom for the opioid receptors.…”

Free radical scavenging abilities of flavonyl‐thiazolidine‐2,4‐dione compounds

“…In agreement with previous observations, joint D ‐Pro 2 substitution with other residues at positions 1 or 4 appeared to be better tolerated and provided ligands with biological activities similar to those of the EMs. The results on some dynorphin A analogues,128 and the assumption that certain opioid analogues lacking a cationizable N‐terminal amino group sustain receptor affinity,100, 129 led Cardillo et al. to synthesize a series of cyclic EM analogues containing D or L residues, and a Gly bridge connecting residues 1 and 4 by an amide bond 130.…”