Abstract
Methadone (MD) is the most established substance abuse pharmacotherapy of choice for the management of heroin dependence. To date, drug-drug interactions involving MD have been characterized asymmetrically among existing reports, which describe how other drugs affect the metabolic or pharmacokinetic profiles of MD; however, limited information is available regarding the potential for MD to influence similar fates of coadministered drugs. Moreover, little to no mechanistic evidence has been explored. Here, we show that MD induces hepatic drug-metabolizing enzymes (DMEs) through the activation of pregnane X receptor (PXR) and constitutive androstane receptor (CAR). Real-time polymerase chain reaction analysis of human hepatocyte cultures revealed that MD induces the mRNA expression of CYP2B6, CYP3A4, UGT1A1, and multidrug resistance 1 in a concentration-related manner, with the maximal induction of CYP2B6 challenging that of the induction by rifampicin. Furthermore, MD-mediated induction of CYP2B6 and CYP3A4 proteins was observed in Western blot analysis. In cell-based reporter assays, MD significantly increased human (h) PXR-mediated CYP2B6 reporter activities but exhibited minimal effect on hCAR activation as a result of the constitutive activity of hCAR in HepG2 cells. Further studies revealed that treatment with MD resulted in significant nuclear accumulation of adenovirus/enhanced yellow fluorescent protein tagged-hCAR in human hepatocytes, which has been regarded as the initial step of CAR activation. Additional analysis of the two enantiomers of MD, R-(-)-MD (active) and S-(+)-MD (inactive), indicates the lack of stereoselectivity pertaining to MD-mediated DME induction. Overall, our results show that MD induces the hepatic expression of multiple DMEs by activating PXR- and CAR-mediated pathways.