Related Experiment Videos
(+)-cis-3-methylfentanyl and its analogs bind pseudoirreversibly to the mu opioid binding site: evidence for
Abstract:
Previous studies demonstrated that preincubation of membranes from the brain of the rat with 1 microM (+)-cis-3-methylfentanyl produced a wash-resistant inhibition of mu receptor binding. The present study was designed to: (1) determine the mechanism by which (+)-cis-3-methylfentanyl produced a wash-resistant inhibition of mu 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 Kd of binding sites for [3H]ohmefentanyl, without altering the Bmax. The increase in the Kd 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 mu binding site, consistent with a model postulating pseudoallosteric modulation of mu binding sites by (+)-cis-3-methylfentanyl and its analogs. The rank order of potencies for wash-resistant inhibition of the binding of [3H]6 beta-fluoro-6-desoxyoxymorphone or [3H]ohmefentanyl, was lofentanil greater than (+)-cis-3-methylfentanyl greater than ohmefentanyl greater than sufentanil. All other opioids tested (1 microM morphine, 1 microM naloxone, 1 microM fentanyl, 1 microM (+)-cyclazocine, 1 microM (-)-cis-3-methylfentanyl) did not act as wash-resistant inhibitors of mu binding sites. Although the pseudoirreversible IC50 of these agents did not correlate with their ED50 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.
Insights
(+)-cis-3-methylfentanyl causes a wash-resistant inhibition of mu opioid receptor binding through pseudoallosteric modulation. This unique property may explain the potent antinociceptive effects of this drug and its analogs.
Area of Science:
- Pharmacology
- Neuroscience
- Medicinal Chemistry
Background:
- Previous studies indicated that (+)-cis-3-methylfentanyl inhibits mu opioid receptor binding in a manner resistant to washing.
- The precise mechanism underlying this persistent inhibition and its relationship to opioid potency were not fully understood.
Purpose of the Study:
- To elucidate the mechanism by which (+)-cis-3-methylfentanyl induces wash-resistant inhibition of mu opioid receptor binding.
- To conduct a structure-activity relationship (SAR) study, utilizing wash-resistant inhibition as the key endpoint for assessing opioid analogs.
Main Methods:
- Rat brain membranes were pretreated with (+)-cis-3-methylfentanyl.
- Mu opioid receptor binding assays were performed using radioligands ([3H]ohmefentanyl, [3H]6 beta-fluoro-6-desoxyoxymorphone).
- Analysis included determination of binding affinity (Kd), maximum binding capacity (Bmax), and dissociation rates.
Main Results:
- Pretreatment with (+)-cis-3-methylfentanyl increased the dissociation constant (Kd) for [3H]ohmefentanyl binding, indicating reduced affinity, without affecting the maximum binding capacity (Bmax).
- This affinity reduction was partly due to residual drug but primarily linked to an increased dissociation rate, suggesting a change in the receptor-ligand interaction.
- The rank order of potency for this wash-resistant inhibition was lofentanil > (+)-cis-3-methylfentanyl > ohmefentanyl > sufentanil; other tested opioids lacked this effect.
Conclusions:
- The findings support a model of pseudoallosteric modulation, where (+)-cis-3-methylfentanyl and its analogs interact with mu opioid receptors in a way that lowers the affinity for other ligands.
- This 'pseudoirreversible inhibition' property, though not directly correlating with antinociceptive ED50 values, is proposed as a potential contributor to the exceptional antinociceptive potency of (+)-cis-3-methylfentanyl and related compounds.