Pharmacological characterization of endomorphin-2-based cyclic pentapeptides with methylated phenylalanine residues

As part of our continuing studies on the structure–activity relationships of cyclic pentapeptides based onthe structure of endomorphin-2, we report here the synthesis and biological activities of a new series ofanalogs incorporating 2, 3or 4-methylphenylalanine (MePhe) residues into positions 3 or 4 of the par-ent cyclopeptide, Dmt-c[d-Lys-Phe-Phe-Asp]NH2(Dmt = 2,6-dimethyltyrosine). Analogs with MePhe inposition 4 showed a row of magnitude increased -opioid receptor (MOP receptor) affinity as comparedwith a parent compound. The in vitro potencies of the new analogs were determined in calcium mobi-lization assay performed in Chinese Hamster Ovary (CHO) cells expressing human recombinant opioidreceptors and chimeric G proteins. All analogs were strong / (MOP/KOP) receptor agonists and weak (DOP) receptor agonists. In the in vivo hot-plate test in mice, the MePhe4-modified peptides showedremarkable antinociceptive activity after intracerebroventricular (i.c.v.) administration which was mostlikely due to the concomitant activation of more than one opioid receptor type