Probing the stereochemical requirements for receptor recognition of δ opioid agonists through topographic modifications in position 1

A series of side-chain constrained tyrosine derivatives, 2',6'-dimethyl-β-methyltyrosines (TMT), has been designed and incorporated into position 1 of the highly selective δ opioid agonists DPDPE (Tyr-D-Pen²-Gly-Phe-D-Pen⁵-OH) and deltorphin I (DELT I, Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH₂). Molecular mechanics calculations on isolated TMT residues and nuclear magnetic resonance (NMR) studies of the TMT¹-containing peptides in DMSO showed that each of the four stereoisomers of TMT favors one particular rotamer of the side-chain χ₁ torsional angle. Therefore, substitution of four TMT isomers for Tyr¹ allows us to perform a systematic conformational scan through three staggered rotamers of the aromatic side chain, gauche (-), trans, and gauche (+), and to explore specific binding requirements of the receptor in relation to the side chain conformation. The potency and selectivity of four isomers of [TMT¹]DPDPE and four isomers of [TMT¹]DELT I were evaluated by radioreceptor binding assays in the rat brain using μ- and δ-selective radiolabeled ligands and by bioassays with guinea pig ileum (GPI, μ receptor) and mouse vas deferens (MVD, δ receptor). In the DPDPE series only one isomer, [(2S,3R)-TMT¹ ]DPDPE showed high potency and selectivity for the δ opioid receptors. The favorable side-chain rotamers found for this analogue, i.e., the trans rotamer of TMT¹ and the gauche (-) rotamer of Phe⁴, were proposed as the most probable δ receptor-binding conformations of DPDPE analogues. Two [TMT¹]DELT I isomers possessed considerable δ receptor potencies. The (2S,3R)-TMT¹ isomer appeared to be a superpotent, but moderately δ-selective agonist, while the (2S,3S)-TMT¹ isomer showed the highest selectivity for the δ receptors in this series. Surprisingly, [(2R,3R)TMT¹]DELT I also was moderately potent at the δ receptor. These results suggest that the δ receptor requirements for the linear DELT I analogues may be satisfied with two different modes of binding of the (2S,3S)- and (2S,3R)TMT¹ isomers. This study provides important guidance for the design of peptide and non-peptide ligands selective for the δ opioid receptor.