(+)- Cis-3-methylfentanyl and its analogs bind pseudoirreversibly to the mu opioid binding site: Evidence for pseudoallosteric modulation

Previous studies demonstrated that preincubation of membranes from the brain of the rat with 1μM (+)- cis-3-methylfentanyl produced a wash-resistant inhibition of μ receptor binding. The present study was designed to: (1) determine the mechanism by which (+)- cis-3-methylfentanyl produced a wash-resistant inhibition of μ receptor binding, and (2) to generate a structure-activity study, using wash-resistant inhibition as the end-point. Pretreatment of membranes with 500 nM (+)- cis-3-methylfentanyl increased the K d of binding sites for [ 3H]ohmefentanyl, without altering the B max,. The increase in the K d was only partially due to the presence of residual drug and was accompanied by an increase in the dissociation rate of the binding of [ 3H]ohmefentanyl. Therefore, pretreatment of membranes with (+)- cis-3-methylfentanyl resulted in a lower affinity interaction of [ 3H]ohmefentanyl with the μ binding site, consistent with a model postulating pseudoallosteric modulation of μ binding sites by (+)- cis-3-methylfentanyl and its analogs. The rank order of potencies for wash-resistant inhibition of the binding of [ 3H]6β-fluoro-6-desoxyoxymorphone or [ 3H]ohmefentanyl, was lofentanil >(+)- cis-3-methylfentanyl > ohmefentanyl > sufentanil. All other opioids tested (1 μM morphine, 1 μM naloxone, 1μM fentanyl, 1 μM (+)-cyclazocine, 1 μM (−)- cis-3-methylfentanyl) did not act as wash-resistant inhibitors of μ binding sites. Although the pseudoirreversible IC 50, of these agents did not correlate with their ED 50 values for producing antinociception, after intravenous administration, the authors speculate that this property, termed “pseudoirreversible inhibition”, might contribute to the extraordinary potency of (+)- cis-3-methylfentanyl and its analogs as antinociceptive agents.