dextrorphan and Opioid-Related-Disorders

To investigate the influence of different cytochrome P450 (CYP) activities and other potential covariates on the disposition of methadone in patients on methadone maintenance therapy (MMT).. Eighty-eight patients (58 male; 21-55 years; 84 White) on MMT were studied. CYP2D6 activity [3 h plasma metabolic ratio of dextromethorphan (DEX) to dextrorphan (DOR)] was determined in 44 patients (29 male; 24-55 years), CYP1A2 activity (salivary caffeine elimination half-life) in 44 patients (21 male; 24-55 years) and CYP3A activity (oral clearance of midazolam) in 49 patients (33 male; 23-55 years). Data on all three CYPs were obtained from 32 subjects. Total plasma concentrations of (RS)-methadone and total and unbound plasma concentrations of both enantiomers were measured by LC/MS. Population pharmacokinetics and subsequent multiple regression analysis were used to calculate methadone oral clearance and to identify its covariates.. Between 61 and 68% of the overall variation in total plasma trough concentrations of (RS)-, (R)- and (S)-methadone was explained by methadone dose, duration of addiction before starting MMT, CYP3A activity and illicit morphine use. CYP3A activity explained 22, 16, 15 and 23% of the variation in unbound (R)-, unbound (S)-, total (RS)- and total (S)-methadone clearances, respectively. Neither CYP2D6 nor CYP1A2 activity was related to methadone disposition.. CYP3A activity has a modest influence on methadone disposition. Inhibitors and inducers of this enzyme should be monitored in patients taking methadone.

Topics: Adult; Aryl Hydrocarbon Hydroxylases; Biomarkers; Caffeine; Cytochrome P-450 CYP1A2; Cytochrome P-450 CYP2D6; Cytochrome P-450 CYP3A; Dextromethorphan; Dextrorphan; Female; Humans; Isomerism; Male; Methadone; Midazolam; Middle Aged; Narcotics; Opioid-Related Disorders; Saliva; Young Adult

Met-enkephalin and beta-endorphin levels were determined in the pituitary and brain of rats after treatment for several weeks with either agonists of high receptor affinity, such as levorphanol and etorphine, or with the narcotic antagnoist naloxone. Long-term activation of opiate receptors failed to change the endorphin levels in restricted areas of brain and pituitary, although a high degree of tolerance/dependence is apparent in those animals. Chronic blockade of opiate receptors by naloxone also fails to affect endorphin levels in the pituitary, but selectively increases metenkephalin levels in the striatum. The present data do not support the notion of negative feedback mechanisms to regulate endorphingergic functions during the development of opiate tolerance/dependence.

Topics: Animals; Brain; Dextrorphan; Drug Tolerance; Endorphins; Etorphine; Feedback; Humans; Levorphanol; Loperamide; Male; Naloxone; Opioid-Related Disorders; Pituitary Gland; Rats; Receptors, Opioid