Opioid peptide receptor studies. 14. Stereochemistry determines agonist efficacy and intrinsic efficacy in the [S-35]GTP-gamma-S functional binding assay

Previous data obtained with the cloned rat mu opioid receptor demonstrated that stereochemistry affects the four parameters of the ligand-receptor int eraction: potency (ED50), efficacy (maximal stimulation), intrinsic efficac y (effect as a function of receptor occupation), and binding affinity. This study evaluated the activities of structurally diverse opioid receptor lig ands in the [S-35]GTP-gamma -S binding assay, comparing the relationship be tween receptor binding, activation, efficacy, and intrinsic efficacy. The d ata, obtained with cloned rat mu receptors, demonstrated that an analgetic, (-)-5-m-hydroxyphenyl-2-methylmorphan (NIH8508), and its (+)-isomer (NIH85 09), behave as partial agonists, but had different intrinsic efficacy in th e [S-35]GTP-gamma -S binding assay. Replacement of the methyl group with th e phenethyl group on the piperidine nitrogen of NIH8508 and NIH8509 [(1R,5S )-AH019 and (1S,5R)-AH019] increased affinity for the mu receptor and elimi nated any agonist effect, supporting the hypothesis that certain structural features make these compounds antagonists. These study also show that all of the fully efficacious mu agonists studied here had high levels of intrin sic efficacy, producing a 50% response at about 10% receptor occupancy. Com parison of the binding K-i in competitively inhibiting [I-125]IOXY binding to the functional K-i for opioid antagonists [K-i(IOXY)/K-i(GTP-gamma -S)] provides more detailed evidence that the [S-35]GTP-gamma -S binding assay c an be used to reliably determine apparent functional antagonist K-i values in addition to agonist ED50, efficacy and intrinsic efficacy. Published 200 1 Wiley-Liss, Inc.