Hold on tight: the kinetic profiling of opioid receptor ligands using the CORAL-MD

Opioid receptors, particularly the μ-opioid receptor, are central targets in pain management, yet opioid therapeutics remain limited by adverse effects. Recent work indicates that ligand residence time may contribute to signaling bias (a potential avenue to dissociate therapeutic effects from deleterious side effects) by stabilizing distinct receptor conformations. Here, we investigated molecular determinants underlying prolonged residence time of μ-opioid receptor ligands. Seven compounds with experimentally measured residence times and diverse signaling profiles were studied using conventional MD, random accelerated MD, and metadynamics to characterize their unbinding behavior and contact patterns with the receptor. To enable the systematic extraction and qualitative exploration of interaction features associated with residence time, we developed CORAL-MD (Correlational Analysis of Ligand–Protein Interactions)–an automated platform that analyzes MD-derived interaction profiles in relation to compound kinetics and other experimental outcomes (activity/properties). Application of CORAL-MD revealed specific interaction motifs associated with extended ligand retention, offering valuable guidance for designing safer, more selective opioid therapeutics. The tool is freely accessible at http://coralmd.if-pan.krakow.pl and it facilitates biological interpretation of MD simulations without requiring any programming expertise. Scientific contribution We identified key ligand-receptor interaction patterns and structural determinants that contribute to prolonged complex stability, including persistent interactions with Asp147 3.32 and Tyr148 3.33 , as well as additional stabilizing contacts in the extracellular vestibule and at the TM5/TM6 interface. However, the interaction patterns can differ depending on the initial receptor conformation, which should be carefully selected on the basis of the functional effect triggered by the examined compounds. Our findings deepen the mechanistic understanding of binding kinetics in opioid receptors and offer valuable guidance for the rational design of next-generation opioid therapeutics with improved efficacy and reduced side effects. We also share CORAL-MD, an on-line tool (freely-available at http://coralmd.if-pan.krakow.pl ), enabling analysis of dependencies between ligand-interaction patterns from molecular dynamics simulations and compound activities, kinetic properties or other experimentally verified features.