Design, synthesis, and evaluation of Phe-Gly mimetics: Heterocyclic building blocks for pseudopeptides

Enantiopure heterocyclic Boc-protected Phe-Gly dipeptidomimetics containing 1,3,4-oxadiazole, 1,2,4-oxadiazole, and 1,2,4-triazole ring systems have been synthesized as building blocks in the synthesis of pseudopeptides. Three derivatives (1-3) have the carboxylic acid function directly bound to the heterocyclic ring, and three derivatives (4-6) have an extra methylene group between the heterocyclic ring and the acid function to allow for an increased conformational flexibility. The mimetics were used as Phe Gly replacements in the biologically active peptides dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and substance P (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-MetNH(2), SP). The pseudopeptide synthesis was performed using solid-phase methodology on a MBHA-resin using Boc-chemistry. The biological evaluation was performed by testing the mu- and delta-opioid receptor affinities of the dermorphin pseudopeptides and the NK1 receptor affinities of the SP pseudopeptides. The results showed that all mimetics except 3 were excellent replacements of Phe-Gly in dermorphin since they displayed affinities for the mu-receptor (IC50 = 12-31 nM) in the same range as dermorphin itself (IC50 = 6.2 nM). The agonist activity of three pseudopeptides at human mu-receptors was also evaluated. It was shown that the tested compounds retained their agonist activity. The SP pseudopeptides showed considerably lower affinities (IC50 > 1 mu M) for the NK1 receptor than SP itself(IC50 = 15 nM) indicating that the Phe-Gly replacements prevent the pseudopeptides from adopting bioactive conformations.