New Endomorphin-2 Hybrid Mimics Incorporating beta-hPhenylalanine Scaffold as Potential Pain Suppressors

Endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2, EM-2) is an endogenous neuropeptide active and selective towards mu-opioid receptor (MOR), it plays a relevant role in the regulation of pain perception and in analgesia. Structural investigation of EM-2 reveals the high conformational freedom of the Phe side chains and also the inherent flexibility of the peptide backbone, indicating many probable bioactive conformations. A common strategy adopted to study the structure-activity relationship of native peptides is the incorporation of unnatural amino acids and/or conformational constraints. Advantages may come from homologated amino acids and mixed (alfa- and beta-) backbone peptidomimetics. We planned the synthesis of a small library of analogues, characterized by various substitutions on Phe residues at position 3 and 4, with the aim of better establishing the relevant role on bioactivity of the proper spatial orientation of the benzyl side chains. EM-2 tetrapeptide mimics were synthesized starting from constrained and linear beta-hPhe-beta-hPhe dipeptide mimics. They were subjected to conformational investigation and biological evaluation. From preliminary results, we observed the maintenance of MOR affinity in the nanomolar scale in most compounds, even if the C-terminal Phe-Phe dipeptide is simultaneously substituted by a peptidomimetic structure.