The role of CYP2B6 in human xenobiotic metabolism

Pharmacogenetic Variation in Neanderthals and Denisovans and Implications for Human Health and Response to Medications

Modern humans carry both Neanderthal and Denisovan (archaic) genome elements that are part of the human gene pool and affect the life and health of living individuals. The impact of archaic DNA may be particularly evident in pharmacogenes-genes responsible for the processing of exogenous substances such as food, pollutants, and medications-as these can relate to changing environmental effects, and beneficial variants may have been retained as modern humans encountered new environments. However, the health implications and contribution of archaic ancestry in pharmacogenes of modern humans remain understudied. Here, we explore 11 key cytochrome P450 genes (CYP450) involved in 75% of all drug metabolizing reactions in three Neanderthal and one Denisovan individuals and examine archaic introgression in modern human populations. We infer the metabolizing efficiency of these 11 CYP450 genes in archaic individuals and find important predicted phenotypic differences relative to modern human variants. We identify several single nucleotide variants shared between archaic and modern humans in each gene, including some potentially function-altering mutations in archaic CYP450 genes, which may result in altered metabolism in living people carrying these variants. We also identified several variants in the archaic CYP450 genes that are novel and unique to archaic humans as well as one gene, CYP2B6, that shows evidence for a gene duplication found only in Neanderthals and modern Africans. Finally, we highlight CYP2A6, CYP2C9, and CYP2J2, genes which show evidence for archaic introgression into modern humans and posit evolutionary hypotheses that explain their allele frequencies in modern populations.

Machine Learning Models to Predict Cytochrome P450 2B6 Inhibitors and Substrates

Cytochrome P450 2B6 (CYP2B6) is responsible for the metabolism of ∼7% of marketed drugs. The in vitro drug interaction studies guidance for industry issued by the FDA stipulates that drug sponsors need to evaluate whether the investigated drugs interact with the major drug-metabolizing P450s including CYP2B6. Therefore, there has been greater attention to the development of predictive models for CYP2B6 inhibitors and substrates. In this study, conventional machine learning and deep learning models were developed to predict CYP2B6 inhibitors and substrates. Our results showed that the best CYP2B6 inhibitor model yielded the AUC values of 0.95 and 0.75 with the 10-fold cross-validation and the test set, respectively, and the best CYP2B6 substrate model produced the AUC values of 0.93 and 0.90 with the 10-fold cross-validation and the test set, respectively. The generalization ability of the CYP2B6 inhibitor and substrate models was assessed by using the external validation sets. Several significant substructural fragments relevant to CYP2B6 inhibitors and substrates were detected via frequency substructure analysis and information gain. In addition, the applicability domain of the models was defined by employing a nonparametric method based on the probability density distribution. We anticipate that our results would be useful for the prediction of potential CYP2B6 inhibitors and substrates in the early stage of drug discovery.

Multidisciplinary Insights into the Structure-Function Relationship of the CYP2B6 Active Site

Cytochrome P450 2B6 (CYP2B6) is a highly polymorphic human enzyme involved in the metabolism of many clinically relevant drugs, environmental toxins, and endogenous molecules with disparate structures. Over the last 20-plus years, in silico and in vitro studies of CYP2B6 using various ligands have provided foundational information regarding the substrate specificity and structure-function relationship of this enzyme. Approaches such as homology modeling, X-ray crystallography, molecular docking, and kinetic activity assays coupled with CYP2B6 mutagenesis have done much to characterize this originally neglected monooxygenase. However, a complete understanding of the structural details that make new chemical entities substrates of CYP2B6 is still lacking. Surprisingly little in vitro data has been obtained about the structure-function relationship of amino acids identified to be in the CYP2B6 active site. Since much attention has already been devoted to elucidating the function of CYP2B6 allelic variants, here we review the salient findings of in silico and in vitro studies of the CYP2B6 structure-function relationship with a deliberate focus on the active site. In addition to summarizing these complementary approaches to studying structure-function relationships, we note gaps/challenges in existing data such as the need for more CYP2B6 crystal structures, molecular docking results with various ligands, and data coupling CYP2B6 active site mutagenesis with kinetic parameter measurement under standard expression conditions. Harnessing in silico and in vitro techniques in tandem to understand the CYP2B6 structure-function relationship will likely offer further insights into CYP2B6-mediated metabolism. SIGNIFICANCE STATEMENT: The apparent importance of cytochrome P450 2B6 (CYP2B6) in the metabolism of various xenobiotics and endogenous molecules has grown since its discovery with many in silico and in vitro studies offering a partial description of its structure-function relationship. Determining the structure-function relationship of CYP2B6 is difficult but may be aided by thorough biochemical investigations of the CYP2B6 active site that provide a more complete pharmacological understanding of this important enzyme.

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.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Metabolites of Tobacco- and E-Cigarette-Related Nitrosamines Can Drive Cu2+-Mediated DNA Oxidation

Nitrosamine metabolites resulting from cigarette smoking and E-cigarette (E-cig) vaping cause DNA damage that can lead to genotoxicity. While DNA adducts of metabolites of nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosonornicotine (NNN) are well-known tobacco-related cancer biomarkers, only a few studies implicate NNN and NNK in DNA oxidation in humans. NNK and NNN were found in the urine of E-cigarette users who never smoked cigarettes. This paper proposes the first chemical pathways of DNA oxidation driven by NNK and NNN metabolites in redox reactions with Cu2+ and NADPH leading to reactive oxygen species (ROS). A microfluidic array with thin films of DNA and metabolic enzymes that make metabolites of NNN and NNK in the presence of Cu2+ and NADPH was used to estimate relative rates of DNA oxidation. Detection by electrochemiluminescence (ECL) employed a new ECL dye [Os(tpy-benz-COOH)2]2+ that is selective for and sensitive to the primary DNA oxidation product 8-oxo-7,8-dihydro-2-deoxyguanosine (8-oxodG) in DNA. Enzyme-DNA films on magnetic beads were used to produce nitrosamine metabolites that enter ROS-forming redox cycles with Cu2+ and NADPH, and liquid chromatography-mass spectrometry (LC-MS) was used to quantify 8-oxodG and identify metabolites. ROS were detected by optical sensors. Metabolites of NNK and NNN + Cu2+ + NADPH generated relatively high rates of DNA oxidation. Lung is the exposure route in smoking and vaping, human lung tissue contains Cu2+ and NADPH, and lung microsomal enzymes gave the highest rates of DNA oxidation in this study. Also, E-cigarette vapor contains 6-fold more copper than that in cigarette smoke, which could exacerbate DNA oxidation.

Inhibition of cytochrome P450 2B6 by Astragalus extract mixture HT042

Astragalus extract mixture (AEM) HT042 is a functional food approved by the MFDS (Korean FDA) for increasing height. It comprises a mixture of three standardized extracts from Astragalus membranaceus root, Eleutherococcus senticosus stem, and Phlomis umbrosa root. In this study, drug-functional food interaction was analyzed using six major human cytochrome P450 enzymes. The inhibitory effect of AEM HT042 on P450 activities was studied using a P450-NADPH P450 reductase reconstitution system. Among the six P450 enzymes (1A2, 2A6, 2B6, 2D6, 2C9, and 3A4), only P450 2B6 activity was markedly decreased by AEM HT042 addition. The bupropion hydroxylation activity of P450 2B6 was analyzed using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). A calculated IC₅₀ value of 10.62 µg/ml was obtained. To identify the inhibitory compounds in the mixture, four active compounds in AEM HT042 were analyzed. Shanzhiside methylester exhibited inhibitory effects on P450 2B6, whereas formononetin, eleutheroside E, and sesamoside did not affect P450 2B6 activity at all. Our results suggest that shanzhiside methylester in AEM HT042 is responsible for the inhibitory effect on P450 2B6 metabolism. Characterization of the inhibitory effect on P450 can help determine the safe administration of functional foods along with many clinical drugs that are metabolized by P450.

One Hundred Faces of Geraniol

Geraniol is a monoterpenic alcohol with a pleasant rose-like aroma, known as an important ingredient in many essential oils, and is used commercially as a fragrance compound in cosmetic and household products. However, geraniol has a number of biological activities, such as antioxidant and anti-inflammatory properties. In addition, numerous in vitro and in vivo studies have shown the activity of geraniol against prostate, bowel, liver, kidney and skin cancer. It can induce apoptosis and increase the expression of proapoptotic proteins. The synergy of this with other drugs may further increase the range of chemotherapeutic agents. The antibacterial activity of this compound was also observed on respiratory pathogens, skin and food-derived strains. This review discusses some of the most important uses of geraniol.

Predation threats for a 24-h period activated the extension of axons in the brains of Xenopus tadpoles

The threat of predation is a driving force in the evolution of animals. We have previously reported that Xenopus laevis enhanced their tail muscles and increased their swimming speeds in the presence of Japanese larval salamander predators. Herein, we investigated the induced gene expression changes in the brains of tadpoles under the threat of predation using 3′-tag digital gene expression profiling. We found that many muscle genes were expressed after 24 h of exposure to predation. Ingenuity pathway analysis further showed that after 24 h of a predation threat, various signal transduction genes were stimulated, such as those affecting the actin cytoskeleton and CREB pathways, and that these might increase microtubule dynamics, axonogenesis, cognition, and memory. To verify the increase in microtubule dynamics, DiI was inserted through the tadpole nostrils. Extension of the axons was clearly observed from the nostril to the diencephalon and was significantly increased (P ≤ 0.0001) after 24 h of exposure to predation, compared with that of the control. The dynamic changes in the signal transductions appeared to bring about new connections in the neural networks, as suggested by the microtubule dynamics. These connections may result in improved memory and cognition abilities, and subsequently increase survivability.

The effect of chronic kidney disease on CYP2B expression and activity in male Wistar rats

Chronic kidney disease (CKD) is characterized by progressive reduction in kidney function over time. CKD affects greater than 10% of the population and its incidence is on the rise due to the growing prevalence of its risk factors. Previous studies demonstrated CKD alters nonrenal clearance of drugs in addition to reducing renal clearance. We assessed the function and expression of hepatic CYP2B enzymes using a rat model of CKD. CKD was induced in Wistar rats by supplementing their chow with adenine and confirmed through the detection of elevated uremic toxins in plasma. Liver enzymes AST and ALT were unchanged by the adenine diet. Bupropion was used as a probe substrate for hepatic CYP2B function using rat liver microsomes. The resulting metabolite, hydroxy-bupropion, and bupropion were quantified by ultra-performance liquid chromatography coupled to time-of-flight mass spectrometry. Level of mRNA and protein were determined by RT-PCR and Western blot, respectively. The results of our study demonstrate that CYP2B1 is downregulated in a rat model of CKD. CYP2B1 mRNA level was significantly decreased (88%, P < 0.001) in CKD relative to control. Similarly, maximal enzymatic velocity (Vmax) for CYP2B was decreased by 46% in CKD relative to control (P < 0.0001). Previous studies involving patients with CKD demonstrated altered bupropion pharmacokinetics compared to control. Hence, our results suggest that these alterations may be mediated by attenuated CYP2B hepatic metabolism. This finding may partially explain the alterations in pharmacokinetics and nonrenal drug clearance frequently observed in patients with CKD.

Xenobiotica-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

Studies on the metabolic fate of medical drugs, skin care products, cosmetics and other chemicals intentionally or accidently applied to the human skin have become increasingly important in order to ascertain pharmacological effectiveness and to avoid toxicities. The use of freshly excised human skin for experimental investigations meets with ethical and practical limitations. Hence information on xenobiotic-metabolizing enzymes (XME) in the experimental systems available for pertinent studies compared with native human skin has become crucial. This review collects available information of which—taken with great caution because of the still very limited data—the most salient points are: in the skin of all animal species and skin-derived in vitro systems considered in this review cytochrome P450 (CYP)-dependent monooxygenase activities (largely responsible for initiating xenobiotica metabolism in the organ which provides most of the xenobiotica metabolism of the mammalian organism, the liver) are very low to undetectable. Quite likely other oxidative enzymes [e.g. flavin monooxygenase, COX (cooxidation by prostaglandin synthase)] will turn out to be much more important for the oxidative xenobiotic metabolism in the skin. Moreover, conjugating enzyme activities such as glutathione transferases and glucuronosyltransferases are much higher than the oxidative CYP activities. Since these conjugating enzymes are predominantly detoxifying, the skin appears to be predominantly protected against CYP-generated reactive metabolites. The following recommendations for the use of experimental animal species or human skin in vitro models may tentatively be derived from the information available to date: for dermal absorption and for skin irritation esterase activity is of special importance which in pig skin, some human cell lines and reconstructed skin models appears reasonably close to native human skin. With respect to genotoxicity and sensitization reactive-metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the Conclusions section in the end of this review.

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 k_cat of 0.5 min^-1 and a K_m 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 K_d 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 K_i 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.

Human cytochrome P450 2B6 genetic variability in Botswana: a case of haplotype diversity and convergent phenotypes

Identification of inter-individual variability for drug metabolism through cytochrome P450 2B6 (CYP2B6) enzyme is important for understanding the differences in clinical responses to malaria and HIV. This study evaluates the distribution of CYP2B6 alleles, haplotypes and inferred metabolic phenotypes among subjects with different ethnicity in Botswana. A total of 570 subjects were analyzed for CYP2B6 polymorphisms at position 516 G > T (rs3745274), 785 A > G (rs2279343) and 983 T > C (rs28399499). Samples were collected in three districts of Botswana where the population belongs to Bantu (Serowe/Palapye and Chobe) and San-related (Ghanzi) ethnicity. The three districts showed different haplotype composition according to the ethnic background but similar metabolic inferred phenotypes, with 59.12%, 34.56%, 2.10% and 4.21% of the subjects having, respectively, an extensive, intermediate, slow and rapid metabolic profile. The results hint at the possibility of a convergent adaptation of detoxifying metabolic phenotypes despite a different haplotype structure due to the different genetic background. The main implication is that, while there is substantial homogeneity of metabolic inferred phenotypes among the country, the response to drugs metabolized via CYP2B6 could be individually associated to an increased risk of treatment failure and toxicity. These are important facts since Botswana is facing malaria elimination and a very high HIV prevalence.

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 (t_1/2) and increased both the maximum concentration of drug in serum (C_max) and the area under the concentration-time curve from 0 h to t (AUC_0-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 AUC_0-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 AUC_0-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.).

Insights into CYP2B6-mediated drug-drug interactions

Mounting evidence demonstrates that CYP2B6 plays a much larger role in human drug metabolism than was previously believed. The discovery of multiple important substrates of CYP2B6 as well as polymorphic differences has sparked increasing interest in the genetic and xenobiotic factors contributing to the expression and function of the enzyme. The expression of CYP2B6 is regulated primarily by the xenobiotic receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR) in the liver. In addition to CYP2B6, these receptors also mediate the inductive expression of CYP3A4, and a number of important phase II enzymes and drug transporters. CYP2B6 has been demonstrated to play a role in the metabolism of 2%–10% of clinically used drugs including widely used antineoplastic agents cyclophosphamide and ifosfamide, anesthetics propofol and ketamine, synthetic opioids pethidine and methadone, and the antiretrovirals nevirapine and efavirenz, among others. Significant inter-individual variability in the expression and function of the human CYP2B6 gene exists and can result in altered clinical outcomes in patients receiving treatment with CYP2B6-substrate drugs. These variances arise from a number of sources including genetic polymorphism, and xenobiotic intervention. In this review, we will provide an overview of the key players in CYP2B6 expression and function and highlight recent advances made in assessing clinical ramifications of important CYP2B6-mediated drug–drug interactions.

Systematic tracking of disrupted modules identifies significant genes and pathways in hepatocellular carcinoma

The objective of the present study is to identify significant genes and pathways associated with hepatocellular carcinoma (HCC) by systematically tracking the dysregulated modules of re-weighted protein-protein interaction (PPI) networks. Firstly, normal and HCC PPI networks were inferred and re-weighted based on Pearson correlation coefficient. Next, modules in the PPI networks were explored by a clique-merging algorithm, and disrupted modules were identified utilizing a maximum weight bipartite matching in non-increasing order. Then, the gene compositions of the disrupted modules were studied and compared with differentially expressed (DE) genes, and pathway enrichment analysis for these genes was performed based on Expression Analysis Systematic Explorer. Finally, validations of significant genes in HCC were conducted using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis. The present study evaluated 394 disrupted module pairs, which comprised 236 dysregulated genes. When the dysregulated genes were compared with 211 DE genes, a total of 26 common genes [including phospholipase C beta 1, cytochrome P450 (CYP) 2C8 and CYP2B6] were obtained. Furthermore, 6 of these 26 common genes were validated by RT-qPCR. Pathway enrichment analysis of dysregulated genes demonstrated that neuroactive ligand-receptor interaction, purine and drug metabolism, and metabolism of xenobiotics mediated by CYP were significantly disrupted pathways. In conclusion, the present study greatly improved the understanding of HCC in a systematic manner and provided potential biomarkers for early detection and novel therapeutic methods.

Pharmacogenetics of the Cytochrome P450 Enzyme System: Review of Current Knowledge and Clinical Significance

Genetic variation in drug metabolizing enzymes is an important contributor to interindividual variation in drug disposition and response and is associated with significant clinical consequences. Many commonly used drugs are dependent on the cytochrome P450 monooxygenase enzymes (CYP450) for their metabolism and elimination. At present, more than 57 active human CYP450 genes are known, and the majority of these genes are polymorphic. Despite the large number of CYP450 genes, only the CYP1, CYP2, and CYP3 families of enzymes have a major role in drug metabolism. Approximately 10 CYP450s are responsible for the metabolism of a large number of pharmacologic agents in human beings. The polymorphic forms of the CYP450s are responsible for the development of a significant number of adverse drug reactions and may also contribute to drug response. Genetic polymorphisms have now been identified in the genes encoding all the main CYP450s that contribute to drug and other xenobiotic metabolism, and there are marked interethnic differences in the distribution and frequency of variant alleles. A review of the progress in the pharmacogenetics of P450s that are important for drug metabolism is presented with particular emphasis on the clinical relevance of this research.

Coumarin Derivatives as Substrate Probes of Mammalian Cytochromes P450 2B4 and 2B6: Assessing the Importance of 7-Alkoxy Chain Length, Halogen Substitution, and Non-Active Site Mutations

Using a combined structural and biochemical approach, the functional importance of a recently described peripheral pocket bounded by the E-, F-, G-, and I-helices in CYP2B4 and 2B6 was probed. Three series of 4-substituted-7-alkoxycoumarin derivatives with -H, -CH3, or -CF3 at the 4 position of the coumarin core were used initially to monitor functional differences between CYP2B4 and 2B6. 7-Ethoxy-4-(trifluoromethyl)coumarin (7-EFC) displayed the highest catalytic efficiency among these substrates. Mutants were made to alter side-chain polarity (V/E194Q) or bulk (F/Y244W) to alter access to the peripheral pocket. Modest increases in catalytic efficiency of 7-EFC O-deethylation by the mutants were magnified considerably by chlorination or bromination of the substrate ethoxy chain. A structure of CYP2B6 Y244W in complex with (+)-α-pinene was solved at 2.2 Å and showed no CYMAL-5 in the peripheral pocket. A ligand free structure of CYP2B4 F244W was solved at 3.0 Å with CYMAL-5 in the peripheral pocket. In both instances, comparison of the respective wild-type and mutant CYP2B enzymes revealed that CYMAL-5 occupancy of the peripheral pocket had little effect on the topology of active site residue side-chains, despite the fact that the peripheral pocket and active site are located on opposite sides of the I-helix. Analysis of available CYP2B structures suggest that the effect of the amino acid substitutions within the peripheral pocket derive from altered interactions between the F and G helices.

Rifampin enhances cytochrome P450 (CYP) 2B6-mediated efavirenz 8-hydroxylation in healthy volunteers

The effect of rifampin on the in vivo metabolism of the antiretroviral drug efavirenz was evaluated in healthy volunteers. In a cross-over placebo control trial, healthy subjects (n = 20) were administered a single 600 mg oral dose of efavirenz after pretreatment with placebo or rifampin (600 mg/day for 10 days). Plasma and urine concentrations of efavirenz, 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz were measured by LC-MS/MS. Compared to placebo treatment, rifampin increased the oral clearance (by ∼2.5-fold) and decreased maximum plasma concentration (Cmax) and area under the plasma concentration-time curve (AUC0-∞) of efavirenz (by ∼1.6- and ∼2.5-fold respectively) (p < 0.001). Rifampin treatment substantially increased the Cmax and AUC0-12h of 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz, metabolic ratio (AUC0-72h of metabolites to AUC0-72h efavirenz) and the amount of metabolites excreted in urine (Ae0-12hr) (all, p < 0.01). Female subjects had longer elimination half-life (1.6-2.2-fold) and larger weight-adjusted distribution volume (1.6-1.9-fold) of efavirenz than male subjects (p < 0.05) in placebo and rifampin treated groups respectively. In conclusion, rifampin enhances CYP2B6-mediated efavirenz 8-hydroxylation in vivo. The metabolism of a single oral dose of efavirenz may be a suitable in vivo marker of CYP2B6 activity to evaluate induction drug interactions involving this enzyme.

Xenobiotic-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

The exposure of the skin to medical drugs, skin care products, cosmetics, and other chemicals renders information on xenobiotic-metabolizing enzymes (XME) in the skin highly interesting. Since the use of freshly excised human skin for experimental investigations meets with ethical and practical limitations, information on XME in models comes in the focus including non-human mammalian species and in vitro skin models. This review attempts to summarize the information available in the open scientific literature on XME in the skin of human, rat, mouse, guinea pig, and pig as well as human primary skin cells, human cell lines, and reconstructed human skin models. The most salient outcome is that much more research on cutaneous XME is needed for solid metabolism-dependent efficacy and safety predictions, and the cutaneous metabolism comparisons have to be viewed with caution. Keeping this fully in mind at least with respect to some cutaneous XME, some models may tentatively be considered to approximate reasonable closeness to human skin. For dermal absorption and for skin irritation among many contributing XME, esterase activity is of special importance, which in pig skin, some human cell lines, and reconstructed skin models appears reasonably close to human skin. With respect to genotoxicity and sensitization, activating XME are not yet judgeable, but reactive metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the “Overview and Conclusions” section in the end of this review.

The G⁵¹⁶T CYP2B6 germline polymorphism affects the risk of acute myeloid leukemia and is associated with specific chromosomal abnormalities

The etiology of acute myeloid leukemia (AML) underlies the influence of genetic variants in candidate genes. The CYP2B6 enzyme detoxifies many genotoxic xenobiotics, protecting cells from oxidative damage. The CYP2B6 gene is subjected to a single-nucleotide polymorphism (G⁵¹⁶T) with heterozygotes (GT) and homozygotes (TT) presenting decreased enzymatic activity. This case-control study aimed to investigate the association of CYP2B6 G⁵¹⁶T polymorphism with the susceptibility of AML and its cytogenetic and clinical characteristics. Genotyping was performed on 619 AML patients and 430 healthy individuals using RCR-RFLP and a novel LightSNip assay. The major finding was a statistically higher frequency of the variant genotypes (GT and TT) in patients compared to the controls (GT:38.8% vs 29.8% and TT:9.3% vs 5.3% respectively) (p<0.001). More specifically, a significantly higher frequency of GT+TT genotypes in de novo AML patients (46.6%) and an immensely high frequency of TT in secondary AML (s-AML) (20.5%) were observed. The statistical analysis showed that the variant T allele was approximately 1.5-fold and 2.4-fold higher in de novo and s-AML respectively than controls. Concerning FAB subtypes, the T allele presented an almost 2-fold increased in AML-M2. Interestingly, a higher incidence of the TT genotype was observed in patients with abnormal karyotypes. In particular, positive correlations of the mutant allele were found in patients carrying specific chromosomal aberrations [-7/del(7q), -5/del(5q), +8, +21 or t(8;21)], complex or monosomal karyotypes. Finally, a strikingly higher frequency of TT genotype was also observed in patients stratified to the poor risk group. In conclusion, our results provide evidence for the involvement of the CYP2B6 polymorphism in AML susceptibility and suggest a possible role of the CYP2B6 genetic background on the development of specific chromosomal aberrations.

Allele and genotype frequencies of CYP2B6 in a Turkish population

Increasing interest in cytochrome P450 2B6 (CYP2B6) genetic polymorphism was stimulated by revelations of a specific CYP2B6 genotype significantly affecting the metabolism of various drugs in common clinical use in terms of increasing drug efficacy and avoiding adverse drug reactions. The present study aimed to determine the frequencies of CYP2B6*4 CYP2B6*5, CYP2B6*6, CYP2B6*7 and CYP2B6*9 alleles in healthy Turkish individuals (n = 172). Frequencies of three single nucleotide polymorphisms were 516G>T (28%), 785A>G (33%), and 1459C>T (12%). The frequencies of CYP2B6*1, *4, *5, *6, *7, and *9 alleles were 54.3 (95% CI 49.04-59.56), 6.4% (95% CI 3.81-8.99), 11% (95% CI 7.69-14.31), 25.3% (95% CI 20.71-29.89), 0.87% (95% CI -0.11-1.85) and 2.0% (95% CI 0.52-3.48), respectively. Allele *6 was more frequent (25.3%) than the other variant alleles in Turkish subjects. The frequencies of CYP2B6*4, *5, *6, *7, and *9 alleles were similar to European populations but significantly different from that reported for Asian populations. This is the first study to document the frequencies of the CYP2B6*4, *5, *6, *7, *9 alleles in the healthy Turkish individuals and our results could provide clinically useful information on drug metabolism by CYP2B6 in Turkish population.

UGT1A1*28 polymorphism in chronic lymphocytic leukemia: the first investigation of the polymorphism in disease susceptibility and its specific cytogenetic abnormalities

Chronic lymphocytic leukemia (CLL) has been recently attributed to a combination of genetic predisposition and exposure to environmental factors. UDP-glucuronosyltransferase (UGT)1A1*28 is an inborn polymorphism that results in significant downregulation of uridine diphosphate glucuronyltransferase 1-1 (UGT1A1) activity, one of the most critical metabolizing enzymes involved in the detoxification of toxic substances, some of which contribute to CLL pathogenesis. Here, for the first time, we investigated the putative impact of UGT1A1*28 on CLL incidence and on the formation of the most common chromosomal abnormalities of CLL. UGT1A1*28 was investigated in 109 CLL patients and 108 healthy controls, and was associated with karyotypic and fluorescence in situ hybridization (FISH) results. A significant high frequency of the mutant genotype was observed in patients carrying abnormal FISH patterns, especially del(11q) and +12, which are CLL-specific abnormalities. We also observed a significant association between UGT1A1*28 and the intermediate to unfavorable cytogenetic CLL risk groups. No difference, though, was observed in genotypes between patients and controls. Therefore, we could suggest that UGT-deficient individuals may be at a greater risk for developing CLL-specific abnormalities. Our study might serve as a starting point to consider UGT1A1*28 polymorphism as one of the possible predisposing factors of CLL pathogenesis.

Functional polymorphism of CYP2B6 G15631T is associated with hematologic and cytogenetic response in chronic myeloid leukemia patients treated with imatinib

In the spite of the impressive results achieved with imatinib in chronic myeloid leukemia (CML) patients, differences in patient's response are observed, which may be explained by interindividual genetic variability. It is known that cytochrome P450 enzymes play a major role in the metabolism of imatinib. The present study aimed to understand the functional impact of CYP2B6 15631G>T polymorphism on the response of imatinib in CML patients and its relation to CML susceptibility. We have genotyped CYP2B6 G15631T in 48 CML patients and 64 controls by PCR-RFLP. CYP2B6 15631G>T was not found to be a risk factor for CML (OR 95 % CI, 1.12, 0.6-2, p > 0.05). Hematologic response loss was higher in patients with 15631GG/TT genotype when compared with 15631GT (36.8 vs. 13.8 %; X (2) = 3.542, p = 0.063). Complete cytogenetic response was higher in 15631GG/GT genotype groups when compared with 15631TT (X (2) = 3.298, p = 0.024). Primary cytogenetic resistance was higher in patients carrying 15631GG/TT genotype when compared with 15631GT carriers (52.6 vs. 17.2 %; X (2) = 6.692, p = 0.010). Furthermore, side effects were more common for patients carrying 15631GG genotypes when compared with GT/TT carriers (36 vs. 13.8 %; X (2) = 8.3, p = 0.004). In light of our results, identification of 15631G>T polymorphism in CML patients might be helpful to predict therapeutic response to imatinib.

Effects of clopidogrel and itraconazole on the disposition of efavirenz and its hydroxyl metabolites: exploration of a novel CYP2B6 phenotyping index

AIMS

To evaluate the effects of clopidogrel and itraconazole on the disposition of efavirenz and its hydroxyl metabolites in relation to the CYP2B6*6 genotype and explore potential phenotyping indices for CYP2B6 activity in vivo using a low dose of oral efavirenz.

METHODS

We conducted a randomized three phase crossover study in 17 healthy Korean subjects pre-genotyped for the CYP2B6*6 allele (CYP2B6*1/*1, n = 6; *1/*6, n = 6; *6/*6, n = 5). Subjects were pretreated with clopidogrel (75 mg day(-1) for 4 days), itraconazole (200 mg day(-1) for 6 days), or placebo and then given a single dose of efavirenz (200 mg). The plasma (0-120 h) and urine (0-24 h) concentrations of efavirenz and its metabolites (7- and 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz) were determined by LC/MS/MS.

RESULTS

This study is the first to delineate quantitatively the full (phase I and II) metabolic profile of efavirenz and its three hydroxyl metabolites in humans. Clopidogrel pretreatment markedly decreased AUC(0,48 h), C(max) and Ae(0,24 h) for 8,14-dihydroxyefavirenz, compared with placebo; 95% CI of the ratios were 0.55, 0.73, 0.30, 0.45 and 0.25, 0.47, respectively. The 8,14-dihydroxyefavirenz : efavirenz AUC(0,120 h) ratio was significantly correlated with the weight-adjusted CL/F of efavirenz (r(2) ≈ 0.4, P < 0.05), differed with CYP2B6*6 genotype and was affected by clopidogrel pretreatment (P < 0.05) but not by itraconazole pretreatment.

CONCLUSIONS

The disposition of 8,14-dihydroxy-EFV appears to be sensitive to CYP2B6 activity alterations in human subjects. The 8,14-dihydroxyefaviremz : efavirenz AUC(0,120 h) ratio is attractive as a candidate phenotyping index for CYP2B6 activity in vivo.

Pharmacogenetics of cytochrome P450 2B6 (CYP2B6): advances on polymorphisms, mechanisms, and clinical relevance

Cytochrome P450 2B6 (CYP2B6) belongs to the minor drug metabolizing P450s in human liver. Expression is highly variable both between individuals and within individuals, owing to non-genetic factors, genetic polymorphisms, inducibility, and irreversible inhibition by many compounds. Drugs metabolized mainly by CYP2B6 include artemisinin, bupropion, cyclophosphamide, efavirenz, ketamine, and methadone. CYP2B6 is one of the most polymorphic CYP genes in humans and variants have been shown to affect transcriptional regulation, splicing, mRNA and protein expression, and catalytic activity. Some variants appear to affect several functional levels simultaneously, thus, combined in haplotypes, leading to complex interactions between substrate-dependent and -independent mechanisms. The most common functionally deficient allele is CYP2B6*6 [Q172H, K262R], which occurs at frequencies of 15 to over 60% in different populations. The allele leads to lower expression in liver due to erroneous splicing. Recent investigations suggest that the amino acid changes contribute complex substrate-dependent effects at the activity level, although data from recombinant systems used by different researchers are not well in agreement with each other. Another important variant, CYP2B6*18 [I328T], occurs predominantly in Africans (4-12%) and does not express functional protein. A large number of uncharacterized variants are currently emerging from different ethnicities in the course of the 1000 Genomes Project. The CYP2B6 polymorphism is clinically relevant for HIV-infected patients treated with the reverse transcriptase inhibitor efavirenz, but it is increasingly being recognized for other drug substrates. This review summarizes recent advances on the functional and clinical significance of CYP2B6 and its genetic polymorphism, with particular emphasis on the comparison of kinetic data obtained with different substrates for variants expressed in different recombinant expression systems.

Nuclear receptors PXR and CAR: implications for drug metabolism regulation, pharmacogenomics and beyond

INTRODUCTION

'Orphan' nuclear receptors belong to the nuclear receptor (NR) superfamily of transcriptional factors. Binding of ligands to these receptors results in the recruitment of the co-activators, thereby regulating the expression of cognate target genes.

AREAS COVERED

This review discusses the transcriptional regulation of P450 genes by two major xenobiotic nuclear receptors, pregnane X receptor (PXR) and constitutive androstane receptor (CAR). Additional PXR and CAR target genes include those encoded for UDP-glucuronosyltransferases, glutathione S-transferases, sulfotransferases and drug transporters. The authors discuss the involvement of PXR and CAR in endobiotic metabolism. They also review the polymorphisms of PXR and CAR.

EXPERT OPINION

PXR and CAR are both xenobiotic and endobiotic receptors. A remarkably diverse set of chemicals can activate PXR and CAR. There is significant cross-talk among xenobiotic receptors. Future studies are needed to focus on the polymorphisms of the nuclear receptors and the complex regulatory networks among nuclear receptors. Considerations should be given while designing PXR- or CAR-targeting pharmaceutics to avoid adverse drug effects. In the meantime, due to the diverse functions of PXR and CAR, agonists or antagonists for these receptors may have therapeutic potentials in managing certain diseases and enhancing therapeutic indexes.

The influence of sex, ethnicity, and CYP2B6 genotype on bupropion metabolism as an index of hepatic CYP2B6 activity in humans

The effects of sex, ethnicity, and genetic polymorphism on hepatic CYP2B6 (cytochrome P450 2B6) expression and activity were previously demonstrated in vitro. Race/ethnic differences in CYP2B6 genotype and phenotype were observed only in women. To identify important covariates associated with interindividual variation in CYP2B6 activity in vivo, we evaluated these effects in healthy volunteers using bupropion (Wellbutrin SR GlaxoSmithKline, Research Triangle Park, NC) as a CYP2B6 probe substrate. A fixed 150-mg oral sustained-release dose of bupropion was administered to 100 healthy volunteers comprising four sex/ethnicity cohorts (n = 25 each): Caucasian men and Caucasian, African American, and Hispanic women. Blood samples were obtained at 0 and 6 hours postdose for the measurement of serum bupropion (BU) and hydroxybupropion (HB) concentrations. Whole blood was obtained at baseline for CYP2B6 genotyping. To characterize the relationship between CYP2B6 activity and ethnicity, sex, and genotype when accounting for serum BU concentrations (dose-adjusted log(10)-transformed), analysis of covariance model was fitted in which the dependent variable was CYP2B6 activity represented as the log(10)-transformed, metabolic ratio of HB to BU concentrations. Several CYP2B6 polymorphisms were associated with CYP2B6 activity. Evidence of dependence of CYP2B6 activity on ethnicity or genotype-by-ethnicity interactions was not detected in women. These results suggest that CYP2B6 genotype is the most important patient variable for predicting the level of CYP2B6 activity in women, when measured by the metabolism of bupropion. The bupropion metabolic ratio appears to detect known differences in CYP2B6 activity associated with genetic polymorphism, across different ethnic groups. Prospective studies will be needed to validate the use of bupropion as a probe substrate for clinical use.

Bioactivation of the cancer chemopreventive agent tamoxifen to quinone methides by cytochrome P4502B6 and identification of the modified residue on the apoprotein

The nonsteroidal antiestrogen tamoxifen was introduced as a treatment for breast cancer 3 decades ago. It has also been approved as a chemopreventive agent and is prescribed to women at high risk for this disease. However, several studies have shown that use of tamoxifen leads to increased risk of endometrial cancer in humans. One potential pathway of tamoxifen toxicity could involve metabolism via hydroxylation to give 4-hydroxytamoxifen (4OHtam), which may be further oxidized to form a quinone methide. CYP2B6 is a highly polymorphic drug-metabolizing enzyme, and it metabolizes a number of clinically important drugs. Earlier studies from our laboratory have shown that tamoxifen is a mechanism-based inactivator of CYP2B6. The aim of the current study was to investigate the possible formation of reactive intermediates through detection of protein covalent binding and glutathione ethyl ester adduct (GSHEE) formation. The incubation of tamoxifen with 2B6 gave rise to an adduct of 4OHtam with glutathione, which was characterized as the 4OHtam quinone methide + GSHEE with an m/z value of 719, and the structure was characterized by liquid chromatography-tandem mass spectrometry. The metabolic activation of tamoxifen in the CYP2B6 reconstituted system also resulted in the formation of an adduct to the P4502B6 apoprotein, which was identified using liquid chromatography mass spectrometry. The site responsible for the inactivation of CYP2B6 was determined by proteolytic digestion and identification of the labeled peptide. This revealed a tryptic peptide ¹⁸⁸FHYQDQE¹⁹⁴ with the site of adduct formation localized to Gln193 as the site modified by the reactive metabolite formed during tamoxifen metabolism.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Efavirenz primary and secondary metabolism in vitro and in vivo: identification of novel metabolic pathways and cytochrome P450 2A6 as the principal catalyst of efavirenz 7-hydroxylation

Efavirenz primary and secondary metabolism was investigated in vitro and in vivo. In human liver microsome (HLM) samples, 7- and 8-hydroxyefavirenz accounted for 22.5 and 77.5% of the overall efavirenz metabolism, respectively. Kinetic, inhibition, and correlation analyses in HLM samples and experiments in expressed cytochrome P450 show that CYP2A6 is the principal catalyst of efavirenz 7-hydroxylation. Although CYP2B6 was the main enzyme catalyzing efavirenz 8-hydroxylation, CYP2A6 also seems to contribute. Both 7- and 8-hydroxyefavirenz were further oxidized to novel dihydroxylated metabolite(s) primarily by CYP2B6. These dihydroxylated metabolite(s) were not the same as 8,14-dihydroxyefavirenz, a metabolite that has been suggested to be directly formed via 14-hydroxylation of 8-hydroxyefavirenz, because 8,14-dihydroxyefavirenz was not detected in vitro when efavirenz, 7-, or 8-hydroxyefavirenz were used as substrates. Efavirenz and its primary and secondary metabolites that were identified in vitro were quantified in plasma samples obtained from subjects taking a single 600-mg oral dose of efavirenz. 8,14-Dihydroxyefavirenz was detected and quantified in these plasma samples, suggesting that the glucuronide or the sulfate of 8-hydroxyefavirenz might undergo 14-hydroxylation in vivo. In conclusion, efavirenz metabolism is complex, involving unique and novel secondary metabolism. Although efavirenz 8-hydroxylation by CYP2B6 remains the major clearance mechanism of efavirenz, CYP2A6-mediated 7-hydroxylation (and to some extent 8-hydroxylation) may also contribute. Efavirenz may be a valuable dual phenotyping tool to study CYP2B6 and CYP2A6, and this should be further tested in vivo.

Haplotype structure and allele frequencies of CYP2B6 in Spaniards and Central Americans

This study was aimed to investigate the potential differences in allele frequencies of the CYP2B6 gene between Spaniards and Central Americans. Three single nucleotide polymorphisms of the CYP2B6 gene 516 G>T, 785 A>G and 1459 C>T were assayed by a polymerase chain reaction in 180 Spaniards and 182 Central Americans. The allele frequencies for CYP2B6*1, CYP2B6*4, CYP2B6*5, CYP2B6*6, CYP2B6*9 in Spaniards and Central Americans were 0.593 and 0.642, 0.062 and 0.073, 0.113 and 0.030, 0.215 and 0.230, 0.014 and 0.023, respectively. CYP2B6*5 was less prevalent among Central Americans than in Spaniards (P < 0.001). In comparison to other previously studied populations, the CYP2B6*5 allele frequency among Spaniards was similar to other Caucasian or African groups, and higher than that in Asian populations. The CYP2B6*5 allele frequency in Central Americans was lower than that in Africans or Caucasian groups and higher than in Asians. The results indicate the presence of ethnic differences in CYP2B6 genetic variants between Spaniards and Central Americans, and support the need for further investigations to explore whether these differences significantly alter the efficacy or toxicity of CYP2B6 substrate drugs.

Cooperative properties of cytochromes P450

Cytochromes P450 form a large and important class of heme monooxygenases with a broad spectrum of substrates and corresponding functions, from steroid hormone biosynthesis to the metabolism of xenobiotics. Despite decades of study, the molecular mechanisms responsible for the complex non-Michaelis behavior observed with many members of this superfamily during metabolism, often termed 'cooperativity', remain to be fully elucidated. Although there is evidence that oligomerization may play an important role in defining the observed cooperativity, some monomeric cytochromes P450, particularly those involved in xenobiotic metabolism, also display this behavior due to their ability to simultaneously bind several substrate molecules. As a result, formation of distinct enzyme-substrate complexes with different stoichiometry and functional properties can give rise to homotropic and heterotropic cooperative behavior. This review aims to summarize the current understanding of cooperativity in cytochromes P450, with a focus on the nature of cooperative effects in monomeric enzymes.

Brain drug-metabolizing cytochrome P450 enzymes are active in vivo, demonstrated by mechanism-based enzyme inhibition

Individuals vary in their response to centrally acting drugs, and this is not always predicted by drug plasma levels. Central metabolism by brain cytochromes P450 (CYPs) may contribute to interindividual variation in response to drugs. Brain CYPs have unique regional and cell-type expression and induction patterns, and they are regulated independently of their hepatic isoforms. In vitro, these enzymes can metabolize endogenous and xenobiotic substrates including centrally acting drugs, but there is no evidence to date of their in vivo function. This has been difficult to demonstrate in the presence of hepatically derived metabolites that may cross the blood-brain barrier. In addition, because of the membrane location of brain CYPs and the rate limiting effect of endogenous heme levels on the activity and appropriate membrane insertion of some induced CYPs, it has been unclear whether sufficient cofactors and coenzymes are present for constitutive and induced CYP forms to be enzymatically active. We have developed a method using a radiolabeled mechanism-based inhibitor of CYP2B1, (3)H-8-methoxypsoralen, to demonstrate for the first time that both the constitutive and induced forms of this enzyme are active in situ in the living rat brain. This methodology provides a novel approach to assess the function of enzymes in extrahepatic tissues, where expression levels are often low. Selective induction of metabolically active drug metabolizing enzymes in the brain may also provide ways to control prodrug activation in specific brain regions as a novel therapeutic avenue.

CYP2B6: new insights into a historically overlooked cytochrome P450 isozyme

Human CYP2B6 has been thought to account for a minor portion (<1%) of total hepatic cytochrome P450 (CYP) content and to have a minor function in human drug metabolism. Recent studies, however, indicate that the average relative contribution of CYP2B6 to total hepatic CYP content ranges from 2% to 10%. An increased interest in CYP2B6 research has been stimulated by the identification of an ever-increasing substrate list for this enzyme, polymorphic and ethnic variations in expression levels, and evidence for cross-regulation with CYP3A4, UGT1A1 and several hepatic drug transporters by the nuclear receptors pregnane X receptor and constitutive androstane receptor. Moreover, 20- to 250-fold interindividual variation in CYP2B6 expression has been demonstrated, presumably due to transcriptional regulation and polymorphisms. These individual differences may result in variable systemic exposure to drugs metabolized by CYP2B6, including the antineoplastics cyclophosphamide and ifosfamide, the antiretrovirals nevirapine and efavirenz, the anesthetics propofol and ketamine, the synthetic opioid methadone, and the anti-Parkinsonian selegiline. The potential clinical significance of CYP2B6 further enforces the need for a comprehensive review of this xenobiotic metabolizing enzyme. This communication summarizes recent advances in our understanding of this traditionally neglected enzyme and provides an overall picture of CYP2B6 with respect to expression, localization, substrate-specificity, inhibition, regulation, polymorphisms and clinical significance. Emphasis is given to nuclear receptor mediated transcriptional regulation, genetic polymorphisms, and their clinical significance.