Structure-activity relationship of N-cyclopropylmethyl-7α-(m-methylaminophenyl)-6,14-endoethano-northebaine derivatives as G-protein-biased KOR-selective agonists.

4,5-Epoxymorphinan derivatives represent a privileged chemotype among opioid ligands, with well-established clinical utility, and continue to serve as a widely used scaffold for the development of novel opioid-like therapeutics. This work describes the design and synthesis of a series of N-cyclopropylmethyl-7α-(m-methylaminophenyl)-6,14-endoethano-northebaine derivatives and identifies compound 8a as a structurally optimized KOR agonist with high KOR affinity, pronounced MOR/KOR and DOR/KOR subtype selectivity, and marked G-protein bias. In vivo, compound 8a exhibited robust, dose-dependent antinociceptive activity in both the hot-plate and abdominal constriction assays, with ED50 values of 8.2 and 0.5 mg/kg, respectively. Nevertheless, the emergence of motor impairment and aversive effects at behaviorally relevant doses underscores a critical limitation of relying solely on cell-based G-protein bias as a predictor of improved central nervous system safety for KOR agonists.