Novel highly potent μ-opioid receptor antagonist based on endomorphin-2 structure

The μ-opioid agonists endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) exhibit an extremely high selectivity for the μ-opioid receptor and thus represent a potential framework for modification into μ-antagonists. Here we report on the synthesis and biological evaluation of novel [d-2-Nal4]endomorphin-2 analogs, [Sar2,d-2-Nal4]endomorphin-2 and [Dmt1,Sar2,d-2-Nal4]endomorphin-2 (Dmt = 2′6′-dimethyltyrosine; Sar = N-methylglycine, sarcosine; d-2-Nal = 3-(2-naphthyl)-d-alanine). [Dmt1,Sar2,d-2-Nal4]endomorphin-2 possessed very high affinity for the μ-opioid receptor (IC50 = 0.01 ± 0.001 nM) and turned out to be a potent and extremely selective μ-opioid receptor antagonist, as judged by the in vitro aequorin luminescence-based calcium assay (pA2 = 9.19). However, in the in vivo hot plate test in mice this analog was less potent than our earlier μ-opioid receptor antagonist, [Dmt1,d-2-Nal4]endomorphin-2 (antanal-2). The exceptional μ-opioid receptor in vitro activity and selectivity of [Dmt1, Sar2,d-2-Nal4]endomorphin-2 makes this analog a valuable pharmacological tool, but further modifications are needed to improve its in vivo profile.