New Endomorphin-2 Analogues Incorporating beta2-hPhe-beta3-hPhe Scaffolds

A common strategy adopted in medicinal chemistry to study the structure-activity relationships of native bioactive peptides is the alteration of their backbone sequence, through incorporation of unnatural amino acids or conformational constraints. In particular, -amino acids containing hybrid peptides seem to show great potential as peptidomimetics.1 Our goal concerns the design and synthesis of new conformationally constrained scaffolds, based on “proteinogenic” -amino acids, and their incorporation into endogenous peptides for biological evaluation. Endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2, EM-2) is a highly potent and selective endogenous mu-opioid receptor agonist neuropeptide. Since the mu-opioid receptor agonists display the most potent antinociceptive activity, EM-2 is an important model in the search toward new analgesics.2 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, ranging from beta-turns to extended conformations. In particular, the relevant role of the proper spatial orientation of the aromatic rings and in particular of the benzylic side chains at position 3 and 4 is well established. We planned the synthesis of a small library of EM-2 analogues, characterized by various substitutions of Phe residues at position 3 and 4 with the corresponding beta2-Phe and beta3-Phe unnatural amino acids. From preliminary studies of molecular modelling, we also identified the constrained beta2-Phe-beta3-Phe dipeptide mimics as valuable turn inducer scaffolds. Compounds have been synthesized by condensation of the (S)-Tic (Tic = tetrahydroisoquinoline carboxylic acid) methylester analogue with the Boc-protected beta2-(S)-Phe and beta2-(R)-Phe. Four EM-2 tetrapeptide mimics are now under conformational investigation and biological evaluation. We confide that results of these studies could provide valuable insights into the effects on bioactivity of beta-amino acid substitution, stereochemistry and conformational constraint in the Phe-Phe portion of EM-2.