Substitution of aromatic and nonaromatic amino acids for the Phe3 residue in the δ-selective opioid peptide deltorphin I: Effects on binding affinity and selectivity

Deltorphins I and II (Tyr-D-Ala-Phe-Asp-Val-Val-Gly NH₂ and Tyr-D-Ala-Phe-Glu-Val-Val-Gly NH₂) display a high degree of 6-opioid receptor selectivity. Since they lack the intervening Gly3 residue found between the Tyr and Phe aromatic moieties in pentapeptide enkephalins, deltorphins I and II resemble a previously described series of cyclic tetrapep-tides based on Tyr-c[D-Cys-Phe-D-Pen] (JOM-13). With the goal of development of structure-activity relationships for deltorphins and comparison with that of the cyclic tetrapep-tides, ten analogs of deltorphin I were synthesized in which Phe³ was replaced with specific aromatic and nonaromatic amino acids with varying physicochemical properties. Results indicated that analogs containing the bicyclic aromatic amino acids 3-(l-naphthyl)-L-alanine [1-Nal; K_i(μ) = 767 nM, K_i(§) = 7.70 nM], 3-(2-naphthyl)-L-alanine [2-Nal; K_i(μ)= 1910 nM, K_i(§) = 49.2 nM], tryptophan [K_i(μ)= 1250 nM, K_i(§) = 23.9nM], and 3-(3-benzothienyl)-L-alanine [Bth; K_i(μ)= 112nM, K_i(§) = 3.36 nM] were fairly well tolerated at μ- and §-receptors, though affinity was compromised to varying degrees relative to deltorphin I. Shortening the Phe side chain by incorporation of phenylglycine (Pgl) was detrimental to both μ (K_i= 4710 nM) and § (K_i= 15.6 nM) binding, while extension of the side chain with homophenylalanine (Hfe) enhanced μ binding (K_i= 67.8 nM), leaving § affinity unaffected (K_i= 2.64 nM). Substitution with nonaromatic amino acids valine and isoleucine led expectedly to poor opioid binding [K_i(μ) =≥ 10000 nM for each, K_i(§) = 160 and 94.7 nM, respectively], while peptides containing cyclohexylalanine (Cha) and leucine surprisingly retained affinity at both μ (K_i= 322 and 1240 nM, respectively) and § (K_i= 10.5 and 12.4 nM, respectively) sites. In general, these trends mirror those observed for similar modification in Tyr-c[D-Cys-Phe-D-Pen].