Inhibition of Human CYP2B6-Catalyzed Bupropion Hydroxylation by Ginkgo biloba Extract: Effect of Terpene Trilactones and Flavonols

Faculty of Pharmaceutical Sciences, The University of British Columbia, 2146 East Mall, Vancouver, BC, V6T 1Z3, Canada.
Drug metabolism and disposition: the biological fate of chemicals (Impact Factor: 3.25). 07/2009; 37(9):1931-7. DOI: 10.1124/dmd.109.028118
Source: PubMed


Cytochrome P450 2B6 (CYP2B6) is expressed predominantly in human liver. It catalyzes the oxidative biotransformation of various drugs, including bupropion, which is an antidepressant and a tobacco use cessation agent. Serious adverse effects of high dosages of bupropion have been reported, including the onset of seizure. As Ginkgo biloba extract may be consumed with bupropion or another CYP2B6 drug substrate, potential exists for an herb-drug interaction. Therefore, we investigated the effect of G. biloba extract and some of its chemical constituents (terpene trilactones and flavonols) on the in vitro catalytic activity of CYP2B6 as assessed by the bupropion hydroxylation assay with recombinant enzyme and hepatic microsomes. The amount of hydroxybupropion was quantified by a novel and validated ultraperformance liquid chromatography/mass spectrometry method. Enzyme kinetic analysis indicated that G. biloba extract competitively inhibited hepatic microsomal CYP2B6-catalyzed bupropion hydroxylation (apparent K(i) was 162 +/- 14 microg/ml). Bilobalide and ginkgolides A, B, C, and J were not responsible for the inhibition of CYP2B6 catalytic activity by the extract. Whereas the 3-O-glucoside and 3-O-rutinoside of quercetin, kaempferol, and isorhamnetin had no effect, the corresponding aglycones (10 and 50 microg/ml) decreased hepatic microsomal bupropion hydroxylation. The inhibition of CYP2B6 by kaempferol was competitive (apparent K(i) was 10 +/- 1 microg/ml). In summary, G. biloba extract and its flavonol aglycones are naturally occurring inhibitors of in vitro CYP2B6 catalytic activity and bupropion hydroxylation. Future studies are needed to investigate whether G. biloba extract interacts in vivo with bupropion or other clinically important CYP2B6 drug substrates.

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    • "Bupropion 4-hydroxylase activity is a widely used catalytic marker of CYP2B6 activity in human liver microsomes (Faucette et al., 2000; Hesse et al., 2000). Hepatic microsomal bupropion 4-hydroxylase activity was measured using UHPLC/MS/MS analysis as described by Lau and Chang (2009). Reaction mixtures contained human liver microsomes (0.5 mg/ml final concentration), a saturating concentration of bupropion (100µM final concentration), and 50mM phosphate buffer with 3mM magnesium chloride (pH 7.4), in a volume of 0.49 ml. "
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    ABSTRACT: Hydroxylated polybrominated diphenyl ethers (PBDEs) have been found in human serum, suggesting that they are formed by in vivo oxidative metabolism of PBDEs. However, the biotransformation of 2,2',4,4',5-pentabromodiphenyl ether (BDE-99), a major PBDE detected in human tissue and environmental samples, is poorly understood. In the present study, the oxidative metabolism of BDE-99 was assessed using pooled and single-donor human liver microsomes, a panel of human recombinant cytochrome P450 (CYP) enzymes, and CYP-specific antibodies. Hydroxylated metabolites were quantified using a liquid chromatography/tandem mass spectrometry-based method. In total, 10 hydroxylated metabolites of BDE-99 were produced by human liver microsomes. Six metabolites were identified as 2,4,5-tribromophenol (2,4,5-TBP), 4-OH-BDE-90, 5'-OH-BDE-99, 6'-OH-BDE-99, 4'-OH-BDE-101, and 2-OH-BDE-123 using authentic standards. Three monohydroxy- and one dihydroxy-pentabrominated metabolites were unidentified. Rates of formation of the three major metabolites (2,4,5-TBP, 5'-OH-BDE-99, and 4'-OH-BDE-101) by human liver microsomes ranged from 24.4 to 44.8 pmol/min/mg protein. Additional experiments demonstrated that the dihydroxylated metabolite was a primary metabolite of BDE-99 and was not produced by hydroxylation of a monohydroxy metabolite. Among the panel of recombinant CYP enzymes tested, formation of all 10 hydroxylated metabolites was catalyzed solely by CYP2B6. A combined approach using antibodies to CYP2B6 and single-donor liver microsomes expressing a wide range of CYP2B6 levels confirmed that CYP2B6 was responsible for the biotransformation of BDE-99. Collectively, the results show that the oxidative metabolism of BDE-99 by human liver microsomes is catalyzed solely by CYP2B6 and is an important determinant of the toxicity and bioaccumulation of BDE-99 in humans.
    Toxicological Sciences 06/2012; 129(2):280-92. DOI:10.1093/toxsci/kfs215 · 3.85 Impact Factor
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    • "Bupropion ((±)-2-(tert-butylamino)-30-chloropropiophenone hydrochloride) is a second-generation antidepressant agent with neurochemical properties different from common tricyclic antidepressants (Yeniceli and Dogrukol-Ak 2009). It is also used as a non-nicotine drug for smoking cessation (Lau and Chang 2009). Bupropion (BUP) is metabolized to three metabolites: hydroxybupropion (HBUP), erythrohydrobupropion, and threohydroburpropion (Golden et al. 1988; Hesse et al. 2000; Schroeder 1983). "
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    ABSTRACT: A sensitive and selective liquid chromatograpy-mass spectrometry method for the determination of bupropion and its main metabolite, hydroxubupropion, in rat plasma was developed and validated. After addition of carbamazepine as internal standard (IS) and precipitation of protein with acetonitrile, the plasma samples were analyzed on an Agilent Zorbax SB-C18 (2.1 mm x 50 mm, 3.5 microm) column at 30 degrees C, with acetonitrile-0.1% formic acid as mobile phase at a flow rate of 0.4 mL min(-1). The detection was carried out in the selective ion monitoring mode with a positive electrospray ionization interface. The calibration curve was linear over the 10-2000 ng mL(-1) for bupropion and 5-1000 ng mL(-1) for hydroxybupropion in plasma. RSD of inter-day and intra-day precision was less than 7% for bupropion, 9% for hydroxybupropion. The developed method was successfully applied to pharmacokinetic studies after single intragastric administration of bupropion 15 mg kg(-1) to rats.
    Pharmazie 12/2011; 66(12):924-8. DOI:10.1691/ph.2011.1078 · 1.05 Impact Factor
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    • "Incubations were performed with 0.5 ml of Hanks' balanced salt solution containing 250 ␮M bupropion for 15 min for each preparation of hepatocytes after 72-h treatment with 0.1% DMSO or 10 ␮M RIF. After incubations were complete, supernatants were transferred to deep well blocks and frozen at Ϫ80°C for subsequent liquid chromatography/ tandem mass spectrometry analysis of hydroxybuprion formation using standard analytical methods as described previously (Lau and Chang, 2009). "
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    ABSTRACT: CYP2B6 is a highly inducible and polymorphic enzyme involved in the metabolism of an increasing number of clinically important drugs. Significant interindividual variability in CYP2B6 expression has been attributed to either genetic polymorphisms or chemical-mediated induction through the activation of constitutive androstane receptor and/or pregnane X receptor (PXR). It was reported that the -82T→C substitution within the CYP2B6*22 allele creates a functional CCAAT/enhancer-binding protein (C/EBP) binding site and enhances the basal expression of the CYP2B6 gene. Here, we explored whether this polymorphic mutation could affect drug-mediated induction of CYP2B6. Cell-based promoter reporter assays demonstrated that CYP2B6 luciferase activity was synergistically enhanced in the presence of both -82T→C mutation and rifampicin (RIF)-activated PXR. On the other hand, this synergism was attenuated by disrupting the C/EBP binding site or knocking down C/EBPα expression. Mechanistic studies revealed that C/EBPα plays an important role in such synergism by directly interacting with PXR; enhancing RIF-mediated recruitment of PXR to the -82T→C harboring CYP2B6 promoter; and looping the PXR-bound distal phenobarbital-responsive enhancer module toward the proximal C/EBP binding site. Furthermore, the genotype-phenotype association was evaluated in cultured human primary hepatocytes from 44 donors. Interestingly, RIF-mediated induction of CYP2B6 in four -82T/C carriers was higher compared with that in the reference -82T/T homozygotes. Together, our results demonstrate, for the first time, a synergistic interplay between a CYP2B6 polymorphism and PXR-mediated induction, which may contribute to the large individual variations and inducibility of CYP2B6 in humans.
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