The outstanding properties of methadone are its analgesic activity, its efficacy by oral route, its extended duration of action in suppressing of withdrawal symptoms in physically dependent individuals, and its tendency to show persistent effects with repeated administration. The analgesic activity of methadone, a racemate, is almost entirely the result of its R-methadone content. Respiratory depression is the chief hazard associated with methadone, and its peak respiratory depressant effects typically occur later, and persist for longer than its peak analgesic effect, particularly in the early dosing period. Methadone undergoes extensive metabolism in the liver and the major metabolic pathway is N-demethylation by CYP3A4 and CYP2B6. There is a remarkable interindividual variability in the rate of methadone metabolism. Mothers with opioid addiction are often placed on methadone before delivery in an attempt to reduce illicit opioid usage. Methadone is useful because it can be taken orally, only requires one or two daily doses and has a long-lasting effect. Weaning should not be attempted during pregnancy and, because of increased clearance, the dose of methadone may need to be increased in the last 3 months of pregnancy. Significant positive correlations were found for umbilical cord methadone concentrations, methadone mean daily dose, mean dose during the third trimester, and methadone cumulative daily dose. In conclusion, umbilical cord methadone concentrations were correlated to methadone dose. A total of 55-94% of infants born to opioid-dependent mothers in US show signs of opioid withdrawal. Methadone is useful to avoid use of opioid illicit drugs.
KEYWORDS:
methadone; neonates; metabolism; pharmacokinetic side-effects