Xorphanol

Xorphanol (INN) (developmental code name TR-5379 or TR-5379M), also known as xorphanol mesylate (USAN), is an opioid analgesic of the morphinan family that was never marketed. It is a mixed agonist–antagonist of opioid receptors, acting preferentially as a high-efficacy partial agonist/near-full agonist of the κ-opioid receptor (K_i = 0.4 nM; EC₅₀ = 3.3 nM; E_max = 84%) and to a lesser extent as a partial agonist of the μ-opioid receptor with lower relative intrinsic activity and marked antagonistic potential (including the ability to antagonize morphine-induced effects and induce opioid withdrawal in opioid-dependent individuals). The drug has also been found to act as a weak partial agonist of the δ-opioid receptor (IC₅₀ = 8 nM; I_max = 76%).