Cyclorphan

Cyclorphan is an opioid analgesic of the morphinan family that was never marketed. It acts as a μ-opioid receptor (MOR) weak partial agonist or antagonist, κ-opioid receptor (KOR) full agonist, and, to a much lesser extent, δ-opioid receptor (DOR) agonist (75-fold lower affinity relative to the KOR). The drug was first synthesized in 1964 by scientists at Research Corporation.^: 232 In clinical trials, it had relatively long duration, good absorption, and provided strong pain relief but produced psychotomimetic effects via KOR activation, so its development was not continued.^{: 232, 237}