Discovery of an Ortho-Substituted N‑Cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaine Derivative as a Selective and Potent Kappa Opioid Receptor Agonist with Subsided Sedative Effect

Research into kappa opioid receptor (KOR) agonists with attenuated central-nervous-system side effects is a critical focus for developing productive and safe analgesics. Herein, a series of ortho-substituted N-cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaines were designed, synthesized, and subjected to bioassays. Compound 7a exhibited high subtype selectivity and potent agonistic activity toward KOR (KOR, K i = 3.9 nM, MOR/KOR = 270, DOR/KOR = 1075; [35S]­GTPγS binding, EC50 = 3.4 nM). Additionally, this compound exhibited robust and persistent antinociceptive effects in rodent models with different animal strains (hot plate test, ED50 = 0.20–0.30 mg/kg, i.p.; abdominal constriction test, ED50 = 0.20–0.60 mg/kg, i.p.), with its KOR-mediated mechanism for antinociception firmly established. Notably, compound 7a, unlike conventional KOR agonists, displayed minimal sedation and aversion at the antinociceptive ED50 dose. This feature addresses a crucial limitation in existing KOR agonists, positioning compound 7a as a promising novel therapeutic agent.