Synthesis and characterization of potent and selective μ-opioid receptor antagonists, [Dmt1, D-2-Nal4]endomorphin-1 (antanal-1) and [Dmt1, D-2-Nal4]endomorphin-2 (antanal-2)

To synthesize potent antagonists of the mu-opioid receptor, we prepared a series of endomorphin-1 and endomorphin-2 analogues with 3-(1-naphthyl)-D-alanine (D-1-Nal) or 3-(2-naphthyl)-D-alanine (D-2-Nal) in position 4. Some of these analogues displayed weak antagonist properties. We tried to strengthen these properties by introducing the structurally modified tyrosine residue 2,6-dimethyltyrosine (Dmt) in place of Tyr(1). Among the synthesized compounds, [Dmt(1), D-2-Nal(4)]endomorphin-1, designated antanal-1, and [Dmt(1), D-2-Nal(4)]endomorphin-2, designated antanal-2, turned out to be highly potent and selective mu-opioid receptor antagonists, as judged on the basis of two functional assays, the receptor binding assay and the hot plate test of analgesia. Interestingly, another analogue of this series, [Dmt(1), D-1-Nal(4)]endomorphin-1, turned out to be a moderately potent mixed mu-agonist/delta-antagonist.