An in vitro profile of activity for the (+) and (-) enantiomers of spiradoline and PD117302

Kappa- and mu-opioid binding site affinities of the kappa-selective ligand U62066 and its optical isomers, (+)-U63639 and (-)-U63640 were compared with those of the structurally related ligand PD117302 and its respective isomers, (+)-PD123497 and (-)-PD123475. The relative efficacies of each compound were also established using the guinea-pig ileum, rat and rabbit vas deferens smooth muscle bioassays. The specific Opioid Receptor mediating the agonist behaviour was determined in the guinea-pig ileum bioassay by obtaining pKB values for naloxone and for the kappa-selective antagonist nor-binaltorphimine. Both racemic compounds and the (-)-enantiomers displayed high selectivity for the kappa-receptor with (-)-PD123475 the most selective. The (+)-enantiomer, PD123497, was approximately equipotent at mu-/kappa-sites while (+)-U63639 displayed a 140-fold mu-receptor selectivity. Bioassay studies showed each compound to be interacting at the kappa-receptor, with the exception of (+)-U63639 which displayed a profile consistent with that of a weak mu-receptor agonist.