Side chain methyl substitution in the delta-opioid receptor antagonist TIPP has an important effect on the activity profile

The delta-opioid antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP-OH) or its C-termina l amide analogue was systematically modified topologically by substitution of each amino acid residue by all stereoisomers of the corresponding beta-m ethyl amino acid. The potency and selectivity (delta- vs mu- and kappa-opio id receptor) were evaluated by radioreceptor binding assays. Agonist or ant agonist potency were assayed in the mouse vas deferens and in the guinea pi g ileum. In the TIPP analogues containing L-beta-methyl amino acids the inf luence on delta-receptor affinity and on delta-antagonist potency is limite d, the [(2S,3R)-beta-MePhe(3)]TIPP-OH analogue being among the most potent delta-antagonists reported. In the D-beta-methyl amino acid series, the [D- beta-MeTic(2)] analogues are delta-selective antagonists whereas [D-Tic(2)] 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 [(2 R,3S)-beta-MePhe(3)]TIPP-NH2 is a potent delta-agonist, its C-terminal carb oxylic acid analogue being more delta-selective but displaying partial agon ism in both the delta- and mu-bioassay. These results constitute further ex amples of a profound influence of beta-methyl substitution on the potency, selectivity, and signal transduction properties of a peptide.