A novel cyclic enkephalin analogue with potent opioid antagonist activity

2′,6′-Dimethyl substitution of the Tyr1 residue in opioid agonist peptides and deletion of the N-terminal amino group, as achieved by replacement of Tyr1 with 3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid (Dhp), have been shown to produce opioid antagonists. To examine the effect of β-methylation of Dhp1 in opioid peptides on the activity profile, stereoselective syntheses of (3S)- and (3R)-3-methyl-3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid [(3S)- and (3R)-Mdp] were carried out. In comparison with the cyclic parent antagonist peptide Dhp-c[D-Cys-Gly-Phe(pNO2)-D-Cys]NH2, the methylated analogue (3S)-Mdp-c[D-Cys-Gly-Phe(pNO2)-D-Cys]NH2 showed higher μ, δ and κ antagonist potencies in functional assays and higher binding affinities for μ, δ and κ opioid receptors (Kiμ = 2.03 nM; Kiδ = 2.34 nM; Kiκ = 49.5 nM), whereas the corresponding (3R)-Mdp1-analogue was less potent by 1–2 orders of magnitude.