Levorphanol

Levorphanol /lɛvaʊrfɑːnɒl/ (brand name Levo-Dromoran) is an opioid medication used to treat moderate to severe pain. Chemically, it is (−)-3-hydroxy-N-methyl-morphinan. It is the levorotatory stereoisomer of racemorphan, first described in Germany in 1948 as an orally-active morphine-like analgesic. It has been in clinical use in the U.S. since 1953.

Levorphanol acts predominantly as an agonist of the μ-opioid receptor, but is also an agonist of the δ-opioid, κ-opioid, and nociceptin receptors, as well as an NMDA receptor antagonist and a serotonin-norepinephrine reuptake inhibitor. Levorphanol, similarly to certain other opioids, also acts as a glycine receptor antagonist and GABA receptor antagonist at very high concentrations. Levorphanol is 6-8 times as potent as morphine at the mu-opioid receptor.

Relative to morphine, levorphanol lacks complete cross-tolerance and possesses greater intrinsic activity at the MOR. The duration of action is generally long compared to other comparable analgesics and varies from 4 hours to as much as 15 hours. For this reason levorphanol is useful in palliation of chronic pain and similar conditions. Levorphanol has an oral to parenteral effectiveness ratio of 2:1, one of the most favourable of the strong narcotics. Its NMDA actions, similar to those of the phenylheptylamine open-chain narcotics such as methadone or the phenylpiperidine ketobemidone, make levorphanol useful for types of pain that other analgesics may not be as effective against, such as neuropathic pain. Levorphanol's exceptionally high analgesic efficacy in the treatment of neuropathy is also conferred by its action on serotonin and norepinephrine transporters, similar to the opioids tramadol and tapentadol, and mutually complements the analgesic effect of its NMDA antagonization.

Levorphanol shows a high rate of psychotomimetic effects such as hallucinations and delirium, which have been attributed to by its binding to and activation of the κ-opioid receptor. However, activation of this receptor as well as of the δ-opioid receptor have been determined to contribute to its analgesic effects.

Levorphanol is listed under the Single Convention On Narcotic Drugs 1961 and is regulated like morphine in most countries. In the United States, it is a Schedule II Narcotic controlled substance with a DEA ACSCN of 9220 and 2013 annual aggregate manufacturing quota of 4.5 kilos. The salts in use are the tartrate (free base conversion ratio 0.58) and hydrobromide (0.76).

Levorphanol is the left Isomer of Racemorphan, that is the racemic mixture of the two stereoisomers with differing pharmacology.