Studies on Analgesic Oligopeptides. VI. : Further Studies of Synthesis and Biological Properties of Tripeptide Alkylamides, Try-D-Arg-Phe-X

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Nine analogs based on a structure of Tyr-D-Arg-Phe-X (X=alkylamides or alkylhydrazide containing electoron-withdrawing atoms or groups) were newly synthesized and their biological properties were examined by the opioid receptor binding properties of μ-, δ- and κ-receptors, guinea-pig ileum (GPI) assay and analgesic activity in the tail pinch test after subcutaneous administration in mice. Analogs with X=NHCH₂CF₃, Sar-ol, or NH(CH₂)₂CN showed potent activities in the GPI and analgesic assays and high affinity for μ-receptor. An analog with X=taurinamide was found to possess 4-fold higher μ-receptor selectivity than that of [D-Ala², MePhe⁴, Gly-ol⁵]enkephalin (DAGO). The receptor binding properties of previously reported analogs [Chem. Pharm. Bull., 33, 1528 (1985); ibid., 33, 4865 (1985); ibid., 36, 4834 (1988)] were also examined for overall discussion of the structure-activity relationships of this series of tripeptide amides.