STEREOCHEMICAL REQUIREMENTS FOR PSEUDOIRREVERSIBLE INHIBITION OF OPIOID MU RECEPTOR-BINDING BY THE 3-METHYLFENTANYL CONGENERS, RTI-46144 AND ITS ENANTIOMERS - EVIDENCE FOR DIFFERENT BINDING DOMAINS

Fentanyl and its congeners are of interest not only because of their c linical applications, but also because certain members of this series of opioid analgesics exhibit unique properties, such as acting as pseu doirreversible inhibitors of mu receptor binding, both in vitro and in vivo. Previous studies showed that pretreatment of membranes with (+) -cis-3-methylfentanyl resulted in a lower affinity interaction of [H-3 ]ohm-efentanyl with the mu binding site, as well as an increased disso ciation rate. The present study was undertaken to determine the stereo chemical requirements for pseudoirreversible inhibition of mu receptor binding using the methylfentanyl congeners, ylethyl)-3-methyl-4-piper idyl]-N-phenylpropanamide HCl (RTI-4614-4) and its four resolved enant iomers. A R configuration of the 2-hydroxy group was essential for hig h affinity binding and pseudoirreversible inhibition. The two enantiom ers with this configuration, 1b ylethyl)-3-methyl-4-piperidyl]-N-pheny lpropanamide oxalate) and 1c ylethyl)-3-methyl-4-piperidyl]-N-phenylpr opanamide HCl), acted as pseudoirreversible inhibitors of the mu recep tor as labeled with [H-3][D-Ala2-MePhe4,Gly-ol5]enkephalin, [H-3]fenta nyl or [H-3]etorphine. RTI-4614-4, 1b, and 1c decreased the Bmax of [H -3][D-Ala2-MePhe4,Gly-ol5]enkephalin binding sites without altering th e dissociation rate. These drugs had a lesser effect on steady-state [ H-3]fentanyl and [H-3]etorphine binding but did produce statistically significant changes in the parameters of the two-component dissociatio n model, which accurately described the dissociation of these [H-3]lig ands. Viewed collectively, these data indicate that the mechanism of t he pseudoirreversible inhibition appears to depend on the radioligand used to label the mu receptor. To explain these data, a pseudoalloster ic model is proposed that postulates that certain mu ligands bind to d ifferent domains of the drug recognition site of the mu receptor and t hat the prebinding of pseudoirreversible inhibitors to the recognition site changes the domains available to a radioligand, leading to alter ations in steady-state binding levels and dissociation kinetics. (C) 1 993 Wiley-Liss, Inc.