Molecular Docking, Molecular Dynamics, and Structure-Activity Relationship Explorations of 14-Oxygenated N-Methylmorphinan-6ones as Potent μ-Opioid Receptor Agonists

Among opioids, morphinans are of major importance as the most effective analgesic drugs acting primarily via p-opioid receptor (mu-OR) activation. Our long-standing efforts in the field of opioid analgesics from the class of morphinans led to N-methylmorphinan-6-ones differently substituted at positions 5 and 14 as mu-OR agonists inducing potent analgesia and fewer undesirable effects. Herein we present the first thorough molecular modeling study and structure activity relationship (SAR) explorations aided by docking and molecular dynamics (MD) simulations of 14 oxygenated N-methylmorphinan-6-ones to gain insights into their mode of binding to the mu-OR and interaction mechanisms. The structure of activated mu-OR provides an essential model for how ligand/mu-OR binding is encoded within small chemical differences in otherwise structurally similar morphinans. We reveal important molecular interactions that these mu-agonists share and distinguish them. The molecular docking outcomes indicate the crucial role of the relative orientation of the ligand in the mu-OR binding site, influencing the propensity of critical non-covalent interactions that are required to facilitate ligand/mu-OR interactions and receptor activation. The MD simulations point out minor differences in the tendency to form hydrogen bonds by the 4,5 alpha-epoxy group, along with the tendency to affect the 3-7 lock switch. The emerged SARs reveal the subtle interplay between the substituents at positions 5 and 14 in the morphinan scaffold by enabling the identification of key structural elements that determine the distinct pharmacological profiles. This study provides a significant structural basis for understanding ligand binding and mu-OR activation by the 14-oxygenated N-methylmorphinan-6-ones, which should be useful for guiding drug design.