Influence of CYP2B6 and CYP3A4 polymorphisms on the virologic and immunologic responses of patients treated with efavirenz-containing regimen.

OBJECTIVES: The main objective of this study was to evaluate the effect of CYP2B6 and CYP3A4 polymorphisms on the virological and immunologic responses of HIV patients. A total of 153 HIV-positive patients were enlisted for the study. PATIENTS AND METHODS: Viral load and median CD4 T cell counts were evaluated at baseline and month 6 (M6). Samples were identified using TaqMan genotyping assays. RESULTS: The AG in CYP2B6 rs2279343 was associated with VLS compared to homozygous AA. In the dominant model, the AG/GG genotypes were associated with VLS compared to the AA genotype. Moreover, in overdominant model, the AG genotype was associated with VLS compared to AA/GG. Regarding immunological response, only the AG in SNP rs2279343 CYP2B6 was associated with an increase in CD4 cell count between baseline and M6. In CYP2B6 rs3745274, the CD4 cell count at M6 was higher than that of baseline for GG carriers and for GT carriers. In CYP3A4 rs2740574, the TC carriers showed a higher median CD4 count at M6 compared to that of the baseline count, as well as for CC carriers. The best genotypes combination associated with CD4 cell count improvement were AA/AG in SNP rs2279343 and GG/GT in SNP rs3745274. CONCLUSION: Our findings support the fact that CYP2B6 rs2279343 could help in the prediction of VLS and both SNPs rs3745274 and rs2279343 in CYP2B6 and CYP3A4 rs2740574 were associated with immune recovery in Malian HIV-positive patients.

Inhibition of cytochrome P450 enzymes and uridine 5'-diphospho-glucuronosyltransferases by vicagrel in human liver microsomes: A prediction of potential drug-drug interactions.

Vicagrel, an antiplatelet drug candidate targeting platelet P2Y12 receptor and has finished its phase II clinical trial. The inhibition of six major cytochrome P450 enzymes (P450) (CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP3A4) and six UDP-glucuronosyltransferases (UGT) (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, and UGT2B7) by vicagrel was evaluated using pooled human liver microsomes and specific probe substrates. Physiology-based pharmacokinetic (PBPK) simulation was further applied to predict the in vivo drug-drug interaction (DDI) potential between vicagrel and bupropion as well as S-mephenytoin. The results suggested that vicagrel inhibited CYP2B6 and CYP2C19 potently with apparent IC50 values of 1.6 and 2.0 µM, respectively. In terms of mode of reversible inhibition, vicagrel exhibited mixed-type inhibition of CYP2B6-catalyzed bupropion hydroxylation and noncompetitive inhibition of CYP2C19-mediated S-mephenytoin 4'-hydroxylation with Ki values of 0.19 µM and 1.2 µM, respectively. Vicagrel displayed profound time-dependent inhibition towards CYP2B6 with maximal rate constant of inactivation (kinact) and half-maximal inactivator concentration (KI) values of 0.062 min-1 and 1.52 µM, respectively. No time-dependent inhibition by vicagrel was noted for CYP2C19. For UGT, negligible to moderate inhibition by vicagrel was observed with IC50 values of >50.0, >50.0, 28.2, 8.7, >50.0 and 28.2 µM for UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9 and UGT2B7, respectively. In terms of mode of reversible inhibition, vicagrel exhibited mixed-type inhibition of UGT1A6-catalyzed N-Acetylserotonin ß-D-glucuronidation with a Ki value of 5.6 µM. No time-dependent inhibition by vicagrel was noted for UGT1A6. PBPK simulation indicated that neither altered AUC nor Cmax of bupropion and S-mephenytoin was observed in the presence of vicagrel. Our study provides inhibitory constants for future DDI prediction between vicagrel and drug substrates of CYP2B6, CYP2C19 and UGT1A6. In addition, our simulation suggests the lack of clinically important DDI between vicagrel and bupropion or S-mephenytoin.

Oxypeucedanin is a Mechanism-based Inactivator of CYP2B6 and CYP2D6.

BACKGROUND: Herbal medicine Angelica dahurica is widely employed for the treatment of rheumatism and pain relief in China. Oxypeucedanin is a major component in the herb. OBJECTIVES: The objectives of this study are aimed at the investigation of mechanism-based inactivation of CYP2B6 and CYP2D6 by oxypeucedanin, characterization of the reactive metabolites associated with the enzyme inactivation, and identification of the P450s participating in the bioactivation of oxypeucedanin. METHODS: Oxypeucedanin was incubated with liver microsomes or recombinant CYPs2B6 and 2D6 under designed conditions, and the enzyme activities were measured by monitoring the generation of the corresponding products. The resulting reactive intermediates were trapped with GSH and analyzed by LC-MS/MS. RESULTS: Microsomal incubation with oxypeucedanin induced a time-, concentration-, and NADPH-dependent inhibition of CYPs2B6 and 2D6 with kinetic values of KI/kinact 1.82 µM/0.07 min-1 (CYP2B6) and 8.47 µM/0.044 min-1 (CYP2D6), respectively. Ticlopidine and quinidine attenuated the observed time-dependent enzyme inhibitions. An epoxide and/or γ-ketoenal intermediate(s) derived from oxypeucedanin was/were trapped in microsomal incubations. CYP3A4 was the primary enzyme involved in the bioactivation of oxypeucedanin. CONCLUSION: Oxypeucedanin was a mechanism-based inactivator of CYP2B6 and CYP2D6. An epoxide and/or γ- ketoenal intermediate(s) may be responsible for the inactivation of the two enzymes.

Investigation of CYP2B6, 3A4 and ß-esterase interactions of Withania somnifera (L.) dunal in human liver microsomes and HepG2 cells.

ETHNOPHARMACOLOGICAL RELEVANCE: Withania somnifera (L.) Dunal (Solanaceae) is a traditional herb, used in African indigenous systems of medicine for the treatment of various diseases (including HIV/AIDS and tuberculosis). The relevance of clinically significant interactions of Withania with ARVs and anti-TB drugs needs to be investigated. AIM OF THE STUDY: This study evaluated the effects of its roots on cytochromes P450 (CYPs) 2B6, 3A4, and rifampicin metabolism pathway, using methanol, ethanol, aqueous, and ethyl acetate solvent extractions. MATERIALS AND METHODS: The extracts were tested on human liver microsomes (HLM) for CYP inhibition, mRNA expression in HepG2 cells for CYP induction. Biochemical qualitative tests and LC-MS/MS methodology were used to determine active phytoconstituents. RESULTS: The methanolic and ethyl acetate extracts inhibited CYP2B6 with IC50s 79.16 and 57.96 µg/ml respectively, while none of the extracts had any effect on rifampicin metabolism or showed time-dependant inhibition (TDI). All extracts were moderate inducers of CYP3A4; the aqueous extract exhibited 38%-fold shift induction of CYP3A4 compared to the control. The methanolic extract had the lowest CTC50 (50% of cytotoxicity inhibition) (67.13 ± 0.83 µg/ml). LC-MS/MS-PDA full scans were consistent with the presence of flavone salvigenin (m/z 327), alkaloid isopelletierine (m/z 133), steroidal lactone 2,3-dihydrowithaferin-A (m/z 472), and other withanolides including withaperuvin I (m/z 533), withaferin derivative (m/z 567), some of these compounds likely being responsible for the observed CYP2B6 inhibition and CYP3A4 induction. The putative gastrointestinal tract (GIT) concentration for the active extracts was 1800 µg/ml and the hepatic circulation concentrations were estimated at about 220 µg/ml and 13.5 µg/ml for the methanolic and ethyl acetate extracts, respectively. The extrapolated in vivo percentage of inhibition was at 85% for the methanolic extract against CYP2B6. CONCLUSIONS: The findings reported in this study suggest that W. somnifera extracts have the potential of causing clinically significant herb-drug interactions (HDI) as moderate inducer of CYP3A4 and inhibitor of CYP2B6 metabolism pathway (methanol and ethyl acetate extracts).

Deoxyshikonin reversibly inhibits cytochrome P450 2B6.

Deoxyshikonin, a natural shikonin derivative, is the major component of Lithospermum erythrorhizon and exhibits various pharmacological effects such as lymphangiogenetic, antibacterial, wound healing, and anticancer effects. To investigate the herb-drug interaction potential associated with deoxyshikonin, the inhibitory effects of deoxyshikonin on eight major cytochrome P450 (CYP) enzymes were examined using cocktail substrate-incubated human liver microsomes. Deoxyshikonin strongly inhibited CYP2B6-catalyzed bupropion hydroxylation, with a Ki value of 3.5 µM, and the inhibition was confirmed using purified human CYP2B6. The inhibition was reversible because the inhibitory effect of deoxyshikonin was not dependent on the preincubation time. The results indicated that deoxyshikonin-induced drug-drug interaction should be considered when any herb containing deoxyshikonin is used for conventional medications.

Zebrafish (Danio rerio) water tank model for the investigation of drug metabolism: Progress, outlook, and challenges.

Zebrafish (Danio rerio) water tank (ZWT) approach was investigated as an alternative model for metabolism studies based on six different experiments with four model compounds. Sibutramine was applied for the multivariate optimization of ZWT conditions, also for the comparison of the metabolism among ZWT, humans and mice, beyond for the role of CYP2B6 in ZWT. After the optimization, 18 fish and 168 hours of experiments is the minimum requirement for a relevant panel of biotransformation products. A comparison among the species resulted in the observation of the same hydroxylated metabolites, with differences in metabolites concentration ratio. However, the ZWT allowed tuning of the conditions to obtain a specific metabolic profile, depending on the need. In addition, by utilizing CYP2B6 inhibition, a relevant ZWT pathway for the demethylation of drugs was determined. The stereospecificity of the ZWT metabolism was investigated using selegiline and no racemization or inversion transformations were observed. Moreover, the investigation of metabolism of cannabimimetics was performed using JWH-073 and the metabolites observed are the same described for humans, except for the hydroxylation at the indol group, which was explained by the absence of CYP2C9 orthologs in zebrafish. Finally, hexarelin was used as a model to evaluate studies by ZWT for drugs with low stability. As a result, hexarelin displays a very fast metabolization in ZWT conditions and all the metabolites described for human were observed in ZWT. Therefore, the appropriate conditions, merits, and relevant limitations to conduct ZWT experiments for the investigation of drug metabolism are described.

Inhibitory effect of α-terpinyl acetate on cytochrome P450 2B6 enzymatic activity.

Human cytochrome P450 2B6 is an important hepatic enzyme for the metabolism of xenobiotics and clinical drugs. Recently, more attention has been paid to P450 2B6 because of the increasing number of drugs it metabolizes. It has been known to interact with terpenes, the major constituents of the essential oils used for various medicinal purposes. In this study, the effect of monoterpenes on P450 2B6 catalytic activity was investigated. Recombinant P450 2B6 was expressed in Escherichia coli and purified using Ni-affinity chromatography. The purified P450 2B6 enzyme displayed bupropion hydroxylation activity in gas-mass spectrometry (GC-MS) analysis with a kcat of 0.5 min-1 and a Km of 47 µM. Many terpenes displayed the type I binding spectra to purified P450 2B6 enzyme and α-terpinyl acetate showed strong binding affinity with a Kd value of 5.4 µM. In GC-MS analysis, P450 2B6 converted α-terpinyl acetate to a putative oxidative product. The bupropion hydroxylation activity of P450 2B6 was inhibited by α-terpinyl acetate and its IC50 value was 10.4 µM α-Terpinyl acetate was determined to be a competitive inhibitor of P450 2B6 with a Ki value of 7.6 µM. The molecular docking model of the binding site of the P450 2B6 complex with α-terpinyl acetate was constructed. It showed the tight binding of α-terpinyl acetate in the active site of P450 2B6, which suggests that it could be a competitive substrate for P450 2B6.

Use of Phenoxyaniline Analogues To Generate Biochemical Insights into the Interactio n of Polybrominated Diphenyl Ether with CYP2B Enzymes.

Human hepatic cytochromes P450 (CYP) are integral to xenobiotic metabolism. CYP2B6 is a major catalyst of biotransformation of environmental toxicants, including polybrominated diphenyl ethers (PBDEs). CYP2B substrates tend to contain halogen atoms, but the biochemical basis for this selectivity and for species specific determinants of metabolism has not been identified. Spectral binding titrations and inhibition studies were performed to investigate interactions of rat CYP2B1, rabbit CYP2B4, and CYP2B6 with a series of phenoxyaniline (POA) congeners that are analogues of PBDEs. For most congeners, there was a <3-fold difference between the spectral binding constants (KS) and IC50 values. In contrast, large discrepancies between these values were observed for POA and 3-chloro-4-phenoxyaniline. CYP2B1 was the enzyme most sensitive to POA congeners, so the Val-363 residue from that enzyme was introduced into CYP2B4 or CYP2B6. This substitution partially altered the protein-ligand interaction profiles to make them more similar to that of CYP2B1. Addition of cytochrome P450 oxidoreductase (POR) to titrations of CYP2B6 with POA or 2'4'5'TCPOA decreased the affinity of both ligands for the enzyme. Addition of cytochrome b5 to a recombinant enzyme system containing POR and CYP2B6 increased the POA IC50 value and decreased the 2'4'5'TCPOA IC50 value. Overall, the inconsistency between KS and IC50 values for POA versus 2'4'5'TCPOA is largely due to the effects of redox partner binding. These results provide insight into the biochemical basis of binding of diphenyl ethers to human CYP2B6 and changes in CYP2B6-mediated metabolism that are dependent on POA congener and redox partner identity.

Inhibitory Effects of Garcinia cambogia Extract on CYP2B6 Enzyme Activity.

This study assessed the inhibitory effects of Garcinia cambogia extract on the cytochrome P450 enzymes in vitro. G. cambogia extract was incubated with cytochrome P450 isozyme-specific substrates in human liver microsomes and recombinant CYP2B6 isozyme, and the formation of the marker metabolites was measured to investigate the inhibitory potential on cytochrome P450 enzyme activities. The results showed that G. cambogia extract has significant inhibitory effects on CYP2B6 activity in a concentration-dependent manner. Furthermore, the inhibition was potentiated following preincubation with NADPH, indicating that G. cambogia extract is a time-dependent inhibitor of CYP2B6. Meanwhile, hydroxycitric acid, the major bioactive ingredient of G. cambogia extract, did not exhibit significant inhibition effects on cytochrome P450 enzyme activities. G. cambogia extract could modulate the pharmacokinetics of CYP2B6 substrate drugs and lead to interactions with those drugs. Therefore, caution may be required with respect to concomitant intake of dietary supplements containing G. cambogia extract with CYP2B6 substrates.

Inhibition of Cytochrome P450 2B6 Activity by Voriconazole Profiled Using Efavirenz Disposition in Healthy Volunteers.

Cytochrome P450 2B6 (CYP2B6) metabolizes clinically important drugs and other compounds. Its expression and activity vary widely among individuals, but quantitative estimation is hampered by the lack of safe and selective in vivo probes of CYP2B6 activity. Efavirenz, a nonnucleoside HIV-1 reverse transcriptase inhibitor, is mainly cleared by CYP2B6, an enzyme strongly inhibited in vitro by voriconazole. To test efavirenz metabolism as an in vivo probe of CYP2B6 activity, we quantified the inhibition of CYP2B6 activity by voriconazole in 61 healthy volunteers administered a single 100-mg oral dose of efavirenz with and without voriconazole administration. The kinetics of efavirenz metabolites demonstrated formation rate-limited elimination. Compared to control, voriconazole prolonged the elimination half-life (t1/2) and increased both the maximum concentration of drug in serum (Cmax) and the area under the concentration-time curve from 0 h to t (AUC0-t) of efavirenz (mean change of 51%, 36%, and 89%, respectively) (P < 0.0001) with marked intersubject variability (e.g., the percent change in efavirenz AUC0-t ranged from 0.4% to ∼224%). Voriconazole decreased efavirenz 8-hydroxylation by greater than 60% (P < 0.0001), whereas its effect on 7-hydroxylation was marginal. The plasma concentration ratio of efavirenz to 8-hydroxyefavirenz, determined 1 to 6 h after dosing, was significantly increased by voriconazole and correlated with the efavirenz AUC0-t (Pearson r = >0.8; P < 0.0001). This study demonstrates the mechanisms of voriconazole-efavirenz interaction, establishes the use of a low dose of efavirenz as a safe and selective in vivo probe for phenotyping CYP2B6 activity, and identifies several easy-to-use indices that should enhance understanding of the mechanisms of CYP2B6 interindividual variability. (This study is registered at ClinicalTrials.gov under identifier NCT01104376.).

MicroRNA hsa-miR-25-3p suppresses the expression and drug induction of CYP2B6 in human hepatocytes.

Cytochrome P450 2B6 (CYP2B6), mainly expressed in the liver and brain, is important for processing a number of widely used drugs. Variations in CYP2B6 expression are associated with decreased drug efficacy or adverse effects in some patients. Although CYP2B6 genetic variants are associated with its differential expression, epigenetic mechanisms affecting CYP2B6 gene regulation have not been established. Sequence analysis identified 29 domains in the CYP2B6 mRNA transcript that could be subject to regulation by microRNAs. Inverse correlations were found in human hepatocytes for the levels of the microRNAs hsa-miR-504-5p and hsa-miR-25-3p compared with CYP2B6 mRNA. Reporter gene assays showed that hsa-miR-25-3p suppresses CYP2B6 expression by targeting a specific sequence in the 3'-untranslated region of the mRNA transcript. Electrophoretic mobility shift assays confirmed that hsa-miR-25-3p forms stable complexes with its cognate mRNA sequence and that it recruits cellular factors, including Ago-4. Transfection of HepaRG cells with hsa-miR-25-3p mimics inhibited expression of the endogenous CYP2B6 gene and it also decreased rifampicin-dependent induction of CYP2B6 at the mRNA and protein levels. In summary, in silico and in vitro analyses show that hsa-miR-25-3p suppresses CYP2B6 expression in human liver cells via an epigenetic mechanism.

Inhibition of CYP2B6 by Medicinal Plant Extracts: Implication for Use of Efavirenz and Nevirapine-Based Highly Active Anti-Retroviral Therapy (HAART) in Resource-Limited Settings.

Highly active antiretroviral therapy (HAART) has greatly improved health parameters of HIV infected individuals. However, there are several challenges associated with the chronic nature of HAART administration. For populations in health transition, dual use of medicinal plant extracts and conventional medicine poses a significant challenge. There is need to evaluate interactions between commonly used medicinal plant extracts and antiretroviral drugs used against HIV/AIDS. Efavirenz (EFV) and nevirapine (NVP) are the major components of HAART both metabolized by CYP2B6, an enzyme that can potentially be inhibited or induced by compounds found in medicinal plant extracts. The purpose of this study was to evaluate the effects of extracts of selected commonly used medicinal plants on CYP2B6 enzyme activity. Recombinant human CYP2B6 was used to evaluate inhibition, allowing the assessment of herb-drug interactions (HDI) of medicinal plants Hyptis suaveolens, Myrothamnus flabellifolius, Launaea taraxacifolia, Boerhavia diffusa and Newbouldia laevis. The potential of these medicinal extracts to cause HDI was ranked accordingly for reversible inhibition and also classified as potential time-dependent inhibitor (TDI) candidates. The most potent inhibitor for CYP2B6 was Hyptis suaveolens extract (IC50 = 19.09 ± 1.16 µg/mL), followed by Myrothamnus flabellifolius extract (IC50 = 23.66 ± 4.86 µg/mL), Launaea taraxacifolia extract (IC50 = 33.87 ± 1.54 µg/mL), and Boerhavia diffusa extract (IC50 = 34.93 ± 1.06 µg/mL). Newbouldia laevis extract, however, exhibited weak inhibitory effects (IC50 = 100 ± 8.71 µg/mL) on CYP2B6. Launaea taraxacifolia exhibited a TDI (3.17) effect on CYP2B6 and showed a high concentration of known CYP450 inhibitory phenolic compounds, chlorogenic acid and caffeic acid. The implication for these observations is that drugs that are metabolized by CYP2B6 when co-administered with these herbal medicines and when adequate amounts of the extracts reach the liver, there is a high likelihood of standard doses affecting drug plasma concentrations which could lead to toxicity.

Common and Distinct Interactions of Chemical Inhibitors with Cytochrome P450 CYP1A2, CYP2A6 and CYP2B6 Enzymes.

BACKGROUND: Tobacco smoking is a leading cause of preventable disease and death globally. Nicotine is the main addictive component in tobacco. Nicotine is eliminated from the body by biotransformation in the liver to inactive metabolites. This reaction is catalyzed by the cytochrome P450 2A6 (CYP2A6) enzyme. Administering chemical inhibitors of CYP2A6 has been shown to slow down the elimination of nicotine with consequent reduction in number of cigarettes smoked. We have systematically developed small molecule CYP2A6 inhibitors with good balance between potency and CYP selectivity. OBJECTIVE: During this process we have noticed that many potent CYP2A6 inhibitors also inhibit other human liver CYP forms, most notably CYP1A2 and CYP2B6. This study aimed at defining common and distinct features of ligand binding to CYP1A2, CYP2A6 and CYP2B6 active sites. METHODS: We used our previous chemical inhibitor databases to construct improved 3-dimensional quantitative structureactivity relationship (3D-QSAR) models for CYP1A2, CYP2A6 and CYP2B6. RESULTS: Combined 3D-QSAR and docking procedures yielded precise information about the common and distinct interactions of inhibitors and the enzyme active sites. Positioning of hydrogen bond donor/acceptor atoms and the shape and volume of the compound defined the potency and specificity of inhibition. A novel potent and selective CYP1A2 inhibitor was found. CONCLUSION: This in silico approach will provide a means for very rapid and high throughput prediction of cross-inhibition of these three CYP enzymes.

Psoralen, a mechanism-based inactivator of CYP2B6.

Furanocoumarin compound psoralen (PRN) is a major active ingredient found in herbaceous plants. PRN has been used for the treatment of various dermal diseases in China. We evaluated the inhibitory effect of PRN on cytochrome P450 2B6 (CYP2B6) and found that PRN induced a time-, concentration-, and NADPH-dependent inactivation of CYP2B6 with the values of KI and kinact being 110.2 µM and 0.200 min(-1), respectively. Ticlopidine, a CYP2B6 substrate, prevented the enzyme from the inactivation induced by PRN. Exogenous nucleophile glutathione (GSH) and catalase/superoxide dismutase showed limited protection of CYP2B6 from the inactivation. The estimated partition ratio of the inactivation was approximately 400. GSH trapping experiments indicates that an epoxide or/and γ-ketoenal intermediate was formed in microsomal incubations with PRN. In summary, PRN was characterized as a mechanism-based inactivator of CYP2B6.

Evaluation of metabolism dependent inhibition of CYP2B6 mediated bupropion hydroxylation in human liver microsomes by monoamine oxidase inhibitors and prediction of potential as perpetrators of drug interaction.

The objective of the study was to evaluate the metabolism dependent inhibition of CYP2B6 catalyzed bupropion hydroxylation in human liver microsomes by monoamine oxidase (MAO) inhibitors and to predict the drug-drug interaction potential of monoamine oxidase inhibitors as perpetrators of drug interaction. Human liver microsomal CYP2B6 activities were investigated using bupropion hydroxylation as probe substrate marker. The results from single point time dependent inhibition and shift assays suggest that clorgyline, pargyline, phenelzine, and selegiline were metabolism based inhibitors of CYP2B6. In IC50 shift assays, clorgyline, pargyline, phenelzine and selegiline are metabolism based inhibitors of CYP2B6 with fold shit of 3.0-, 3.7-, 2.9-, and 11.4-fold respectively. The inactivation of clorgyline was characterized by KI value of 2.5 ± 0.3 and k(inact) value of 0.045 ± 0.001 min(-1). Phenelzine inactivated CYP2B6 with KI and k(inact) values of 44.9 ± 6.9 µM and 0.085 ± 0.003 min(-1) respectively. Inactivation of selegiline was characterized with KI and k(inact) values of 22.0 ± 3.3 and 0.074 ± 0.002 min(-1) respectively. The inactivation caused by these inhibitors was not reversed by dialysis indicating irreversible inhibition. Based on the mechanistic static model, selegiline showed an increase in the area under the curve (AUC) of efavirenz and bupropion by 1.01-fold. Phenelzine predicted to cause an increase in the AUC of efavirenz and bupropion by 9.4- and 2.4-fold respectively considering unbound hepatic inlet concentrations of phenelzine. In conclusion, the results from this study demonstrated that MAO inhibitors can inactivate human liver microsomal CYP2B6. The likelihood of drug interaction when selegiline co-administered with CYP2B6 substrates is remote. Caution is required while co-administering phenelzine with substrates that are exclusively metabolized by CYP2B6 enzyme and substrates that have narrow therapeutic index.

Characterization of CYP2B6 in a CYP2B6-humanized mouse model: inducibility in the liver by phenobarbital and dexamethasone and role in nicotine metabolism in vivo.

The aim of this study was to further characterize the expression and function of human CYP2B6 in a recently generated CYP2A13/2B6/2F1-transgenic (TG) mouse model, in which CYP2B6 is expressed selectively in the liver. The inducibility of CYP2B6 by phenobarbital (PB) and dexamethasone (DEX), known inducers of CYP2B6 in human liver, was examined in the TG mice, as well as in TG/Cyp2abfgs-null (or "CYP2B6-humanized") mice. Hepatic expression of CYP2B6 mRNA and protein was greatly induced by PB or DEX treatment in both TG and TG/Cyp2abfgs-null mice. Function of the transgenic CYP2B6 was first studied using bupropion as a probe substrate. In PB-treated mice, the rates of hepatic microsomal hydroxybupropion formation (at 50 µM bupropion) were >4-fold higher in TG/Cyp2abfgs-null than in Cyp2abfgs-null mice (for both male and female mice); the rate difference was accompanied by a 5-fold higher catalytic efficiency in the TG/Cyp2abfgs-null mice and was abolished by an antibody to CYP2B6. The ability of CYP2B6 to metabolize nicotine was then examined, both in vitro and in vivo. The rates of hepatic microsomal cotinine formation from nicotine were significantly higher in TG/Cyp2abfgs-null than in Cyp2abfgs-null mice, pretreated with PB or DEX. Furthermore, systemic nicotine metabolism was faster in TG/Cyp2abfgs-null than in Cyp2abfgs-null mice. Thus, the transgenic CYP2B6 was inducible and functional, and, in the absence of mouse CYP2A and CYP2B enzymes, it contributed to nicotine metabolism in vivo. The CYP2B6-humanized mouse will be valuable for studies on in vivo roles of hepatic CYP2B6 in xenobiotic metabolism and toxicity.