Methadone

Methadone is a synthetic opioid analgesic with full agonist activity at the µ-opioid receptor. While agonism of the µ-opioid receptor is the primary mechanism of action for the treatment of pain, methadone also acts as an agonist of κ- and σ-opioid receptors within the central and peripheral nervous systems. Interestingly, methadone differs from morphine (which is considered the gold standard reference opioid) in its antagonism of the N-methyl-D-aspartate (NMDA) receptor and its strong inhibition of serotonin and norepinephrine uptake, which likely also contributes to its antinociceptive activity.

Methadone is administered as a 50:50 racemic mixture of (R)- and (S)-stereoisomers, with (R)-methadone demonstrating ~10-fold higher affinity and potency for the µ-opioid receptor than the (S) stereoisomer. The analgesic activity of the racemate is almost entirely due to the (R)-isomer, while the (S)-isomer lacks significant respiratory depressant activity but does have antitussive effects.

While methadone shares similar effects and risks of other opioids such as morphine, hydromorphone, oxycodone, and fentanyl it has a number of unique pharmacokinetic and pharmacodynamic properties that distinguish it from them and make it a useful agent for the treatment of opioid addiction. For example, methadone abstinence syndrome, although qualitatively similar to that of morphine, differs in that the onset is slower, the course is more prolonged, and the symptoms are less severe.