• Identification of Drugs of Abuse as Modulators of Drug-Metabolizing Enzymes through Nuclear Receptor-Mediated Mechanisms

      Baaqee, Antonia Tolson; Eddington, Natalie D. (2012)
      To date, the majority of reports discussing opioid-drug interactions focus intensively on characterizing how other drugs affect the metabolic and pharmacokinetic (MPK) profile of opioids, however little has been published regarding the potential for opioids to modulate MPK-based drug-drug interactions (DDIs) involving other commonly co-administered or co-abused drugs. Moreover, virtually no mechanistic evidence has been explored. Thus, the objective of this work was to elucidate how opioids affect the MPK of other drugs, thereby undertaking research from a perspective that has been historically overlooked. Accordingly, the specific aims of this study were to: 1) Screen several different drugs of abuse for nuclear receptor (NR) activation potential, 2) Determine the expression profiles of key drug-metabolizing enzymes (DMEs) or drug transporters for selected drugs in human primary hepatocytes (HPHs), and 3) Characterize the mechanistic roles played by xenoreceptors Pregnane X Receptor (PXR) and Constitutive Androstane Receptor (CAR) underlying observed DME modulation. Results: Here we show that several opioids were identified as potential NR activators, and selected drugs of abuse exhibited differential induction profiles at the mRNA level for target genes CYP2B6 and CYP3A4. Overall, for opioid therapies MD and BUP: 1) MD induced the hepatic expression of multiple key DMEs by activating PXR- and CAR-mediated pathways; 2) More specifically, MD treatment resulted in significant nuclear accumulation of adenovirus/enhanced yellow fluorescent protein tagged-hCAR in HPHs, which has been regarded as the initial step of CAR activation, and additional analysis of the two enantiomers of racemic MD, R-(-)-MD (active) and S- (+)-MD (inactive), indicated a lack of stereoselectivity pertaining to MD-mediated DME induction; 3) For BUP, although hPXR-mediated CYP2B6 and CYP3A4 reporter activities were significantly increased in HepG2 cells, treatment with identical concentrations of buprenorphine in HPHs resulted in literally no induction of target gene expression. Taken together, these results provide much-needed mechanistic evidence which demonstrates that MD may be more likely than BUP to modulate CAR- and PXR- mediated DME perturbation during opioid-drug interactions. This research is of great importance to the overall public health industry, particularly to those clinicians and research scientists whom administer MD or BUP as part of opioid maintenance pharmacotherapy.