Atomic-Level Characterization of the Methadone-Stabilized Active Conformation of µ-Opioid Receptor

Abstract

Methadone is a synthetic opioid agonist with notoriously unique properties, such as lower abuse liability and induced relief of withdrawal symptoms and drug cravings, despite acting on the same opioid receptors triggered by classical opioids, in particular the µ-opioid receptor (MOR). Its distinct pharmacologic properties, which have recently been attributed to the preferential activation of β-arrestin over G proteins, make methadone a standard-of-care maintenance medication for opioid addiction. Although a recent biophysical study suggests that methadone stabilizes different MOR active conformations from those stabilized by classical opioid drugs or G protein-biased agonists, how this drug modulates the conformational equilibrium of MOR and what specific active conformation of the receptor it stabilizes is unknown. Here, we report the results of sub-millisecond adaptive sampling molecular dynamics (MD) simulations of a predicted methadone-bound MOR complex, and compare them with analogous data obtained for the classical opioid morphine and the G protein-biased ligand TRV130. The model, which is supported by existing experimental data, is analyzed using Markov State Models (MSMs) and transfer entropy analysis to provide testable hypotheses of methadone-specific conformational dynamics and activation kinetics of MOR.

Differential recognition of opioid analgesics by µ opioid receptors: Predicted interaction patterns correlate with ligand-specific voltage sensitivity

“…Moreover, morphine displayed several interactions with helix 6, which were mainly missing for methadone and fentanyl, also consistent with previous findings (Kapoor et al, 2020;Lipiński et al, 2019). The observed binding pose for methadone indicated a salt bridge with D147 3.32 as only direct interaction, comparable to the findings of Kapoor et al For fentanyl, we identified an H-bond with Y326 7.43 as critically important interaction.…”

“…Furthermore, we were able to identify important regions in the receptor which we correlated with the voltage effect on the MOR. Specifically, our molecular docking studies and subsequent fingerprint analysis, which calculated the interactions between a ligand and a receptor, revealed that morphine (or agonists with the morphinan scaffold) interacted with D147 3.32 , Y148 3.33 and the water networks between Y148 3.33 and H297 6.52 as described before (Huang et al, 2015;Kapoor et al, 2020;Lipiński et al, 2019;Vo et al, 2021).…”

A comprehensive evaluation of the potential binding poses of fentanyl and its analogs at the µ-opioid receptor

“…In recent years, high-resolution structures of the MOR have been acquired by both x-ray crystallography [14] , [15] and cryo-electron microscopy (cryo-EM) [16] , for an inactive conformation of the receptor bound with a morphinan antagonist, β-FNA [15] , and active conformations bound with agonists BU72 and DAMGO [14] , [16] . These structures have been leveraged to investigate the binding mechanisms, and related MOR conformations, of the opioids having therapeutic implications, such as methadone, morphine, and TRV130 [17] , [18] .…”

Editorial: 50 Years of Opioid Research and the International Narcotics Research Conference

“…One possibility that could explain bias is that agonists allow receptors to sample distinct sets of conformational spaces which correlate with specific effectors and signaling pathways. Kapoor et al use Molecular Dynamics simulations to predict the conformational dynamics of the mu opioid receptor bound to methadone in comparison to morphine and a G protein-biased ligand, as useful tools to navigate these conformational spaces (Kapoor et al, 2020).…”