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

Abstract

Fentanyl and its congeners are of interest not only because of their clinical applications, but also because certain members of this series of opioid analgesics exhibit unique properties, such as acting as pseudoirreversible 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 [3H]ohmefentanyl with the mu binding site, as well as an increased dissociation rate. The present study was undertaken to determine the stereochemical requirements for pseudoirreversible inhibition of mu receptor binding using the methylfentanyl congeners, (+-)-cis-N-[1-(2-hydroxy-2-phenylethyl)-3-methyl-4-piperidyl]-N- phenylpropanamide HCl (RTI-4614-4) and its four resolved enantiomers. AR configuration of the 2-hydroxy group was essential for high affinity binding and pseudoirreversible inhibition. The two enantiomers with this configuration, 1b((2R,3R,4S)-N-[1-(2-hydroxy-2-phenylethyl)-3-methyl-4-piperidyl]-N- phenylpropanamide oxolate) and 1c 1c ((2R,3S,4R)-N-[1-(2-hydroxy-2-phenylethyl)-3-methyl-4-piperidyl]-N- phenylpropanamide HCl), acted as pseudoirreversible inhibitors of the mu receptor as labeled with [3H][D-Ala2-MePhe4,Gly-ol5]enkaphalin, [3H]fentanyl or [3H]etorphine. RTI-4614-4, 1b, and 1c decreased the Bmax of [3H][D-Ala2-MaePhe4,Gly-ol5]enkepalin binding sites without altering the dissociation rate. These drugs had a lesser effect on steady-state [3H]fentanyl and [3H]etorphine binding but did produce statistically significant changes in the parameters of the two-component dissociation model, which accurately described the dissociation of these [3H]ligands. Viewed collectively, these data indicate that the mechanism of the pseudoirreversible inhibition appears to depend on the radioligand used to label the mu receptor. To explain these data, a pseudoallosteric model is proposed that postulates that certain mu ligands bind to different domains of the drug recognition site of the mu receptor and that the prebinding of pseudoirreversible inhibitors to the recognition site changes the domains available to a radioligand, leading to alterations in steady-state binding levels and dissociation kinetics.

Opioid peptide receptor studies. 12. Buprenorphine is a potent and selective ?/? antagonist in the [35S]-GTP-?-S functional binding assay

“…and 6␤-125 iodo-3,14-dihydroxy-17-cyclopropylmethyl-4,5␣-epoxymorphinan ([125I]IOXY) as described elsewhere (Ni et al, 1993. In some experiments (Fig.…”

Opioid peptide receptor studies. 7. The methylfentanyl congener RTI-4614-4 and its four enantiomers bind to different domains of the rat ? opioid receptor

“…Our data consistently showed that isomer 1c had a much lower Ki value than isomer 1a, whereas those of Wang et al (1995) showed that isomer 1a had a much lower Ki value than isomer 1c. The reanalysis showed that samples used in our binding studies in previously published work (Brine et al, 1992(Brine et al, , 1995Ni et al, 1993b) were mislabeled. The sample labeled 1a actually contained 1c, and the sample labeled 1c actually contained 1a.…”

“…The availability of the four resolved enantiomers allowed us to determine the stereochemical requirements for the pseudoirreversible binding described above as well as their in vivo actions (see below). The results (Ni et al, 1993b) showed that the (3R,4S) configuration was essential for both high affinity binding and pseudoirreversible inhibition, while the 2S configuration of the 2-OH provides additional enhancement of binding affinity and potency. Interestingly, although 1b and 1a both produced pseudoirreversible inhibition of µ receptor binding, there were in fact subtle differences between these ligands.…”

“…Ohmefentanyl consists of a mixture of isomers 1a and 1c (Brine et al, 1995). Before some published data is summarized, the reader should note that inconsistencies between our previously published binding data (Brine et al, 1992(Brine et al, , 1995Ni et al, 1993b) and those reported by Wang et al (1995) regarding the Ki values of isomer 1a and isomer 1c for the µ receptor prompted us to reanalyze our samples of the isomers. Our data consistently showed that isomer 1c had a much lower Ki value than isomer 1a, whereas those of Wang et al (1995) showed that isomer 1a had a much lower Ki value than isomer 1c.…”

“…Interestingly, although 1b and 1a both produced pseudoirreversible inhibition of µ receptor binding, there were in fact subtle differences between these ligands. For example, pretreating membranes with 1b but not 1a altered [ 3 H]fentanyl dissociation, and 1a was more effective at reducing the Bmax of [ 3 H]DAMGO binding sites than 1b (Ni et al, 1993b).…”