We focused our investigation on methadone (MTD) because its uses are now gaining importance in the treatment of chronic pain. Six women and six men were given a single oral dose of 10 mg of MTD under fasting conditions. Following the administration of MTD, two meals were given at 5 and 13 hours post-dose. Plasma and urine samples were collected, and MTD and its main metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP, only in urine) were stereoselectively monitored by means of high-performance liquid chromatography (HPLC) method. Both in men and in women, R-MTD showed a lower plasma exposure than S-MTD because of its lower plasma protein binding. Urinary exposure showed an inverse relationship. The higher R-MTD urinary concentrations found in women could explain the more intense side effects they experienced in relation with men. Secondary peaks were observed in both sexes after meal intake, revealing that MTD follows blood-gastrointestinal tract recirculation. Its basic properties enable its secretion to the gastric juice, and thereafter its reabsorption from the intestinal lumen. Interestingly, a significant decrease in the R-to-S EDDP excretion rate ratio after food intake was observed. This finding could be due to a difference in the stereoselective enantiomer metabolism between the enterocytes and the hepatocytes, in favour of S-MTD, due to a plausible difference in enzyme ratio between intestine and liver. Men showed a non-significant higher decrease in the R-to-S EDDP ratio. These findings evidence not only that the intestinal metabolism of MTD is relevant, but also that it is stereoselectively different from the hepatic one. MTD would not be favoured from gastrointestinal tract recycling, especially the S-enantiomer, because every time it passes through the enterocytes its elimination increases.
View Full Article