Role of P-glycoprotein in transplacental transfer of methadone.

Methadone is the therapeutic agent of choice for treatment of the pregnant opiate addict. However, little is known on the factors affecting its concentration in the fetal circulation during pregnancy and how it might relate to neonatal outcome. Therefore, a better understanding of the function of placental metabolic enzymes and transporters should add to the knowledge of the role of the tissue in the disposition of methadone and its relation to neonatal outcome. We hypothesized that the expression and activity of the placental efflux transporter P-glycoprotein (P-gp) would affect the transfer of methadone to the fetal circulation. Data obtained utilizing dual perfusion of placental lobule and monolayers of Be-Wo cell line indicated that methadone is extruded by P-gp. Transfer of methadone to the fetal circuit was increased by 30% in the presence of the P-gp inhibitor GF120918 while the transfer of paclitaxel, a typical substrate of the glycoprotein, was increased by 50%. In the Be-Wo cell line, methadone and paclitaxel uptake was also increased in the presence of the P-gp inhibitor cyclosporin A. Moreover, the expression of P-gp in placental brush-border membranes varied between term placentas. Taken together, these data strongly suggest that the concentration of methadone in the fetal circulation is affected by the expression and activity of P-gp. It is reasonable to speculate that placental disposition of methadone affects its concentration in the fetal circulation. If true, this may also be directly related to the incidence and intensity of neonatal abstinence syndrome (NAS).