The delta-opioid antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP-OH) or its C-terminal amide analogue was systematically modified topologically by substitution of each amino acid residue by all stereoisomers of the corresponding beta-methyl amino acid. The potency and selectivity (delta- vs mu- and kappa-opioid receptor) were evaluated by radioreceptor binding assays. Agonist or antagonist potency were assayed in the mouse vas deferens and in the guinea pig ileum. In the TIPP analogues containing L-beta-methyl amino acids the influence on delta-receptor affinity and on delta-antagonist potency is limited, the [(2S,3R)-beta-MePhe3]TIPP-OH analogue being among the most potent delta-antagonists reported. In the D-beta-methyl amino acid series, the [D-beta-MeTic2] analogues are delta-selective antagonists whereas [D-Tic2]TIPP-NH2 is a delta-agonist. NMR studies did not indicate any influence of the beta-methyl substituent on the conformation of the Tic residue. The [(2R,3S)-beta-MePhe3]TIPP-NH2 is a potent delta-agonist, its C-terminal carboxylic acid analogue being more delta-selective but displaying partial agonism in both the delta- and mu-bioassay. These results constitute further examples of a profound influence of beta-methyl substitution on the potency, selectivity, and signal transduction properties of a peptide.

In order to influence side-chain conformations and to increase the I.t-agonist properties of the 5-selective opioid receptor 8-antagonist H-Tyr-Tic-Phe-Phe-NH 2, residues Tyr l, Phe 3 and Phe 4 were replaced by their ]3-methylsubstituted stereoisomers. Synthesis of [3-Me-Tyr was carried out in a stereoselective way. Incorporation of the modified amino acids was performed by SPPS. Receptor binding data and GPI and MVD bioassays were obtained for all stereoisomers, in general showing equal or slightly increased potencies. In the [(R,S)[3-Me-Phe3]analogue,