Thienorphine is a very potent, extremely long-acting, orally-active opioid analgesic with mixed agonistâÂÂantagonist properties which was developed by the Beijing Institute of Pharmacology and Toxicology as a potential non-addictive analgesic and treatment for opioid dependence. It is a high-affinity, balanced ligand of the ü- (K<sub>i</sub> = 0.22 nM), ô- (K<sub>i</sub> = 0.69 nM), and ú-opioid receptors (K<sub>i</sub> = 0.14 nM), behaving as a partial agonist of the ü- (E<sub>max</sub> = 19%âÂÂ28%) and ú-opioid receptors (E<sub>max</sub> = 65âÂÂ75%) and as an antagonist of the ô-opioid receptor. It also possesses relatively low affinity for the nociceptin receptor (K<sub>i</sub> = 36.5 nM), where it acts as an antagonist.