PROBING THE STEREOCHEMICAL REQUIREMENTS FOR RECEPTOR RECOGNITION OF DELTA-OPIOID AGONISTS THROUGH TOPOGRAPHIC MODIFICATIONS IN POSITION-1

A series of side-chain constrained tyrosine derivatives, 2',6'-dimethy l-beta-methyltyrosines (TMT), has been designed and incorporated into position 1 of the highly selective delta opioid agonists DPDPE (Tyr-D- Pen(2)-Gly-Phe-D-Pen(5)-OH) and deltorphin I (DELT I, Tyr-D-Ala-Phe-As p-Val-Val-Gly-NH2). Molecular mechanics calculations on isolated TMT r esidues and nuclear magnetic resonance (NMR) studies of the TMT(1)-con taining peptides in DMSO showed that each of the four stereoisomers of TMT favors one particular rotamer of the side-chain chi(1) torsional angle. Therefore, substitution of four TMT isomers for Tyr(1) allows u s 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 relat ion to the side chain conformation. The potency and selectivity of fou r isomers of [TMT(1)]DPDPE and four isomers of [TMT(1)]DELT I were eva luated by radioreceptor binding assays in the rat brain using mu- and delta-selective radiolabeled ligands and by bioassays with guinea pig ileum (GPI, mu receptor) and mouse vas deferens (MVD, delta receptor). In the DPDPE series only one isomer, [(2S,3R)-TMT(1)]-DPDPE showed hi gh potency and selectivity for the delta opioid receptors. The favorab le side-chain rotamers found for this analogue, i.e., the trans rotame r of TMT(1) and the gauche (-) rotamer of Phe(4), were proposed as the most probable delta receptor-binding conformations of DPDPE analogues . Two [TMT(1)]DELT I isomers possessed considerable delta receptor pot encies. The (2S,3R)-TMT(1) isomer appeared to be a superpotent, but mo derately delta-selective agonist, while the (2S,3R)-TMT(1) isomer show ed the highest selectivity for the delta receptors in this series. Sur prisingly, [(2R,3R)TMT(1)]DELT I also was moderately potent at the del ta receptor. These results suggest that the delta receptor requirement s for the linear DELT I analogues may be satisfied with two different modes of binding of the (2S,3S)- and (2S,3R)-TMT(1) isomers. This stud y provides important guidance for the design of peptide and non-peptid e ligands selective for the delta opioid receptor.