Brain penetration of methadone (R)- and (S)-enantiomers is greatly increased by P-glycoprotein deficiency in the blood-brain barrier of Abcb1a gene knockout mice.

Rationale: Methadone maintenance treatment is complicated by the wide variability of efficacy among patients. The large interindividual variability of the plasma concentrations of methadone was previously thought to be responsible for the variable therapeutic efficacy. However, recent studies suggested that methadone may be a substrate of P-glycoprotein (P-gp). Therefore, the function of P-gp in blood-brain barrier (BBB) may affect the concentration of methadone at its site(s) of action in the central nervous system, thereby contributing to its therapeutic efficacy and/or adverse events.
Objective: To investigate the effect of P-gp on brain penetration of methadone (R)- and (S)-enantiomers and their major oxidative metabolite 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP).
Methods: We compared the tissue distribution of methadone (R)- and (S)-enantiomers and EDDP in the Abcb1a-/- gene knockout mice and the Abcb1a+/+ wild-type mice 1 h following intraperitoneal administration of 15 microg Rac-methadone/g mouse.
Results: Plasma concentrations of (R)- and (S)-methadone were similar between the two animal groups. However, the brain concentrations of (R)- and (S)-methadone in the Abcb1a-/- mice were markedly higher (15- and 23-fold, respectively, P<0.0001) than those of the Abcb1a+/+ wild-type mice. No statistically significant difference was found for other organs between the mutants and controls. No organ difference was found for EDDP between the mutants and controls.
Conclusions: (R)- and (S)-methadone are substrates of P-gp. The P-gp in BBB greatly limits the brain entry of (R)- and (S)-methadone to their central nervous system acting sites. The interindividual variation in expression of P-gp in BBB may represent a source of variation for the access and effects of methadone in the brain.