The opioid peptide TIPP (H-Tyr-Tic-Phe-Phe-OH, Tic:1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) was substituted with Dmt (2‘,6‘-dimethyltyrosine) and a new unnatural amino acid, β-MeCha (β-methyl-cyclohexylalanine). This double substitution led to a new series of opioid peptides displaying subnanomolar δ antagonist activity and μ agonist or antagonist properties depending on the configuration of the β-MeCha residue. The most promising analog, H-Dmt-Tic-(2S,3S)-β-MeCha-Phe-OH was a very selective δ antagonist both in the mouse vas deferens (MVD) assay (Ke = 0.241 ± 0.05 nM) and in radioligand binding assay (Kiδ = 0.48 ± 0.05 nM, Kiμ/Kiδ = 2800). The epimeric peptide H-Dmt-Tic-(2S,3R)-β-MeCha-Phe-OH and the corresponding peptide amide turned out to be mixed partial μ agonist/δ antagonists in the guinea pig ileum and MVD assays. Our results constitute further examples of the influence of Dmt and β-methyl substitution as well as C-terminal amidation on the potency, selectivity, and signal transduction properties of TIPP related peptides. Some of these compounds represent valuable pharmacological tools for opioid research.
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