A General New Concept for the Development of Opioid Peptide Derived μ-, δ- and κ-Antagonists

For decades, it has been assumed that a positively charged nitrogen in opioid compounds is a necessary requirement for binding to their receptors. In a recent study, both somatostatin-derived cyclic hexapeptides lacking a positive charge and a neutral, cyclic des-amino analogue of β-casomorphin, containing 3-(2,6-dimethyl-4-hydroxy-phenyl)propanoic acid (Dhp) in place of Tyr1, were shown to be δ-opioid antagonists with moderate δ receptor binding affinity [1]. Subsequently, we prepared an analogue of the potent enkephalin analogue H-Dmt-D-Ala-Gly-Phe-Leu-NH2, in which the amino group was replaced with the neutral and almost isosteric methyl group [2]. This was achieved by replacement of Dmt with (2S)-2-methyl-3-(2,6-dimethyl-4-hydroxy-phenyl)propanoic acid ((2S)-Mdp), for which a stereospecific synthesis based on Evans chiral enol chemistry was developed [2]. The resulting peptide, (2S)-Mdp-D-Ala-Gly-Phe-Leu-NH2, turned out to be a quite potent o antagonist (Ke = 28 nM) in the mouse vas deferens (MVD) assay and a less potent μ antagonist (Ke =154 nM) in the guinea pig ileum (GPI) assay. In view of this interesting conversion of an agonist into an antagonist at both δ and μ receptors, it was of interest to prepare and examine analogues of a number of key opioid peptides that lack an N-terminal amino group and contain two methyl groups at the 2′,6′-positions of Tyr1.