Decreased susceptibility of the cytochrome P450 2B6 variant K262R to inhibition by several clinically important drugs

Genetic variants, haplotype determination, and function of novel alleles of CYP2B6 in a Han Chinese population

Amino acid variants in protein may result in deleterious effects on enzymatic activity. In this study we investigate the DNA variants on activity of CYP2B6 gene in a Chinese Han population for potential use in precision medicine. All exons in CYP2B6 gene from 1483 Chinese Han adults (Zhejiang province) were sequenced using Sanger sequencing. The effects of nonsynonymous variants on recombinant protein catalytic activity were investigated in vitro with Sf12 system. The haplotype of novel nonsynonymous variants with other single nucleotide variants in the same allele was determined using Nanopore sequencing. Of 38 alleles listed on the Pharmacogene Variation Consortium, we detected 7 previously reported alleles and 18 novel variants, of which 11 nonsynonymous variants showed lower catalytic activity (0.00-0.60) on bupropion compared to CYP2B6*1. Further, these 11 novel star-alleles (CYP2B6*39-49) were assigned by the Pharmacogene Variation Consortium, which may be valuable for pharmacogenetic research and personalized medicine.

CYP2B6 allelic variants and non-genetic factors influence CYP2B6 enzyme function

Human CYP2B6 enzyme although constitutes relatively low proportion (1–4%) of hepatic cytochrome P450 content, it is the major catalyst of metabolism of several clinically important drugs (efavirenz, cyclophosphamide, bupropion, methadone). High interindividual variability in CYP2B6 function, contributing to impaired drug-response and/or adverse reactions, is partly elucidated by genetic polymorphisms, whereas non-genetic factors can significantly modify the CYP2B6 phenotype. The influence of genetic and phenoconverting non-genetic factors on CYP2B6-selective activity and CYP2B6 expression was investigated in liver tissues from Caucasian subjects (N = 119). Strong association was observed between hepatic S-mephenytoin N-demethylase activity and CYP2B6 mRNA expression (P < 0.0001). In less than one third of the tissue donors, the CYP2B6 phenotype characterized by S-mephenytoin N-demethylase activity and/or CYP2B6 expression was concordant with CYP2B6 genotype, whereas in more than 35% of the subjects, an altered CYP2B6 phenotype was attributed to phenoconverting non-genetic factors (to CYP2B6-specific inhibitors and inducers, non-specific amoxicillin + clavulanic acid treatment and chronic alcohol consumption, but not to the gender). Furthermore, CYP2B6 genotype–phenotype mismatch still existed in one third of tissue donors. In conclusion, identifying potential sources of CYP2B6 variability and considering both genetic variations and non-genetic factors is a pressing requirement for appropriate elucidation of CYP2B6 genotype–phenotype mismatch.

Genotype-sensitive reversible and time-dependent CYP2D6 inhibition in human liver microsomes

Cytochrome P450 (CYP) 2D6 metabolizes a wide range of xenobiotics and is characterized by a huge interindividual variability. A recent clinical study highlighted differential magnitude of CYP inhibition as a function of CYP2D6 genotype. The aim of this study was to investigate the effect of CYP2D6 genotype on the inhibition of dextromethorphan O-demethylation by duloxetine and paroxetine in human liver microsomes (HLMs). The study focused on genotypes defined by the combination of two fully functional alleles (activity score 2, AS 2, n = 6), of one fully functional and one reduced allele (activity score 1.5, AS 1.5, n = 4) and of one fully functional and one non-functional allele (activity score 1, AS 1, n = 6), which all predict extensive metabolizer phenotype. Kinetic experiments showed that maximal reaction velocity was affected by CYP2D6 genotype, with a decrease in 33% of V_max in AS 1 HLMs compared to AS 2 (P = 0.06). No difference in inhibition parameters K_i , K_I and k_inact was observed neither with the competitive inhibitor duloxetine nor with the time-dependent inhibitor paroxetine. Among the genotypes tested, we found no difference in absolute CYP2D6 microsomal levels with ELISA immunoquantification. Therefore, our results suggest that genotype-sensitive magnitude of drug-drug interactions recently observed in vivo is likely to be due to differential amounts of functional enzymes at the microsomal level rather than to a difference in inhibition potencies across genotypes, which motivates for further quantitative proteomic investigations of functional and variant CYP2D6 alleles.

The Prevalence of CYP2B6 Gene Polymorphisms in Malaria-endemic Population of Timor in East Nusa Tenggara Indonesia

Objectives

The CYP2B6 is one of the most polymorphic CYP genes in humans that has the potential to modify the pharmacological and toxicological responses to clinically important drugs such as antimalarial artemisinin and its derivatives. The aim of the study was to determine the frequency of CYP2B6 polymorphisms in Timor malaria endemic area, East Nusa Tenggara, Indonesia where Artemisin-based Combination Therapy (ACT) has been used to treat uncomplicated malaria.

Methods

A total of 109 healthy subjects were participated in this study. CYP2B6*4, *6 and *9 polymorphisms were analyzed using PCR-RFLP to confirm the SNPs prevalence of 516G>T and 785A>G in exon 4 and 5.

Results

There were 96 subjects included in the analysis. In the exon 4 of CYP2B6 516G>T, the frequency of the T mutation was 37.5% (39/96), and the wildtype 27.1% (26/96). In the exon 5, CYP2B6 785A>G mutant was detected in 29.2% (28/96) of individuals, and the wildtype allele in 35.4% (34/96). The frequency of CYP2B6*9 (516G>T), CYP2B6*4 (785A>G) and CYP2B6*6 (516G>T and 785A>G) were 40.6%, 29.2% and 22.9%, respectively. The prevalence of these CYP2B6 gene polymorphisms in Timorian ethnic were higher than that in Malay, Han Chinese, Indian, and Egyptian populations.

Conclusion

The prevalence of these CYP2B6 516G>T and 785A>G polymorphisms in Timorian ethnic is higher than that in other populations. These polymorphisms may affect the metabolism of artemisinin and its derivatives.

Functional importance of a peripheral pocket in mammalian cytochrome P450 2B enzymes

The functional importance of a peripheral pocket found in previously published X-ray crystal structures of CYP2B4 and CYP2B6 was probed using a biophysical approach. Introduction of tryptophan within the pocket of CYP2B4 at F202 or I241 leads to marked impairment of 7-ethoxy-4-(trifluoromethyl)coumarin (7-EFC) or 7-benzyloxyresorufin O-dealkylation efficiency; a similar substitution at F195, near the surface access to the pocket, does not affect these activities. The analogous CYP2B6 F202W mutant is inactive in the 7-EFC O-dealkylation assay. The stoichiometry of 7-EFC deethylation suggested that the decreased activity of F202W and I241W in CYP2B4 and lack of activity of F202W in CYP2B6 coincided with a sharp increase in the flux of reducing equivalents through the oxidase shunt to produce excess water. The results indicate that the chemical identity of residues within this peripheral pocket, but not at the mouth of the pocket, is important in substrate turnover and redox coupling, likely through effects on active site topology.

Isoniazid mediates the CYP2B6*6 genotype-dependent interaction between efavirenz and antituberculosis drug therapy through mechanism-based inactivation of CYP2A6

Efavirenz is commonly used to treat patients coinfected with human immunodeficiency virus and tuberculosis. Previous clinical studies have observed paradoxically elevated efavirenz plasma concentrations in patients with the CYP2B6*6/*6 genotype (but not the CYP2B6*1/*1 genotype) during coadministration with the commonly used four-drug antituberculosis therapy. This study sought to elucidate the mechanism underlying this genotype-dependent drug-drug interaction. In vitro studies were conducted to determine whether one or more of the antituberculosis drugs (rifampin, isoniazid, pyrazinamide, or ethambutol) potently inhibit efavirenz 8-hydroxylation by CYP2B6 or efavirenz 7-hydroxylation by CYP2A6, the main mechanisms of efavirenz clearance. Time- and concentration-dependent kinetics of inhibition by the antituberculosis drugs were determined using genotyped human liver microsomes (HLMs) and recombinant CYP2A6, CYP2B6.1, and CYP2B6.6 enzymes. Although none of the antituberculosis drugs evaluated at up to 10 times clinical plasma concentrations were found to inhibit efavirenz 8-hydroxylation by HLMs, both rifampin (apparent inhibition constant [Ki] = 368 μM) and pyrazinamide (Ki = 637 μM) showed relatively weak inhibition of efavirenz 7-hydroxylation. Importantly, isoniazid demonstrated potent time-dependent inhibition of efavirenz 7-hydroxylation in both HLMs (inhibitor concentration required for half-maximal inactivation [KI] = 30 μM; maximal rate constant of inactivation [kinact] = 0.023 min(-1)) and recombinant CYP2A6 (KI = 15 μM; kinact = 0.024 min(-1)) and also formed a metabolite intermediate complex consistent with mechanism-based inhibition. Selective inhibition of the CYP2B6.6 allozyme could not be demonstrated for any of the antituberculosis drugs using either recombinant enzymes or CYP2B6*6 genotype HLMs. In conclusion, the results of this study identify isoniazid as the most likely perpetrator of this clinically important drug-drug interaction through mechanism-based inactivation of CYP2A6.

Impact of CYP polymorphisms, ethnicity and sex differences in metabolism on dosing strategies: the case of efavirenz

PURPOSE

Differences in drug metabolism due to cytochrome P450 (CYP) polymorphisms may be significant enough to warrant different dosing strategies in carriers of specific cytochrome P450 (CYP) polymorphisms, especially for drugs with a narrow therapeutic index. The impact of such polymorphisms on drug plasma concentrations and the resulting dosing strategies are presented in this review, using the example of efavirenz (EFV).

METHODS

A structured literature search was performed to extract information pertaining to EFV metabolism and the influence of polymorphisms of CYP2B6, ethnicity, sex and drug interactions on plasma concentrations of EFV. The corresponding dosing strategies developed for carriers of specific CYP2B6 genotypes were also reviewed.

RESULTS

The polymorphic CYP2B6 enzyme, which is the major enzyme in the EFV metabolic pathway, is a key determinant for the significant inter-individual differences seen in EFV pharmacokinetics and pharmacodynamics (PKPD). Ethnic differences and the associated prevalence of CYP2B6 polymorphisms result in significant differences in the PKPD associated with a standard 600 mg per day dose of EFV, warranting dosage reduction in carriers of specific CYP2B6 polymorphisms. Drug interactions and auto-induction also influence EFV PKPD significantly.

CONCLUSION

Using EFV as an example of a drug with a narrow therapeutic index and a high inter-patient variability in plasma concentrations corresponding to a standard dose of the drug, this review demonstrates how genotyping of the primary metabolising enzyme can be useful for appropriate dosage adjustments in individuals. However, other variables such as drug interactions and auto-induction may necessitate plasma concentration measurements as well, prior to personalising the dose.

Functional characterization of cytochromes P450 2B from the desert woodrat Neotoma lepida

Mammalian detoxification processes have been the focus of intense research, but little is known about how wild herbivores process plant secondary compounds, many of which have medicinal value or are drugs. cDNA sequences that code for three enzymes of the cytochrome P450 (CYP) 2B subfamily, here termed 2B35, 2B36, and 2B37 have been recently identified from a wild rodent, the desert woodrat (Malenke et al., 2012). Two variant clones of each enzyme were engineered to increase protein solubility and to facilitate purification, as reported for CYP2B enzymes from multiple species. When expressed in Escherichia coli each of the woodrat proteins gave the characteristic maximum at 450nm in a reduced carbon monoxide difference spectrum but generally expressed at lower levels than rat CYP2B1. Two enzymes, 2B36 and 2B37, showed dealkylation activity with the model substrates 7-ethoxy-4-(trifluoromethyl)coumarin and 7-benzyloxyresorufin, whereas 2B35 was inactive. Binding of the monoterpene (+)-α-pinene produced a Type I shift in the absorbance spectrum of each enzyme. Mutation of 2B37 at residues 114, 262, or 480, key residues governing ligand interactions with other CYP2B enzymes, did not significantly change expression levels or produce the expected functional changes. In summary, two catalytic and one ligand-binding assay are sufficient to distinguish among CYP2B35, 2B36, and 2B37. Differences in functional profiles between 2B36 and 2B37 are partially explained by changes in substrate recognition site residue 114, but not 480. The results advance our understanding of the mechanisms of detoxification in wild mammalian herbivores and highlight the complexity of this system.

The effect of ritonavir on human CYP2B6 catalytic activity: heme modification contributes to the mechanism-based inactivation of CYP2B6 and CYP3A4 by ritonavir

The mechanism-based inactivation of human CYP2B6 by ritonavir (RTV) in a reconstituted system was investigated. The inactivation is time, concentration, and NADPH dependent and exhibits a K(I) of 0.9 μM, a k(inact) of 0.05 min⁻¹, and a partition ratio of approximately 3. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed that the protonated molecular ion of RTV exhibits an m/z at 721 and its two major metabolites are an oxidation product with MH⁺ at m/z 737 and a deacylated product with MH⁺ at m/z 580. Inactivation of CYP2B6 by incubation with 10 μM RTV for 10 min resulted in an approximately 50% loss of catalytic activity and native heme, but no modification of the apoprotein was observed. RTV was found to be a potent mixed-type reversible inhibitor (K(i) = 0.33 μM) and a type II ligand (spectral dissociation constant-K(s) = 0.85 μM) of CYP2B6. Although previous studies have demonstrated that RTV is a potent mechanism-based inactivator of CYP3A4, the molecular mechanism responsible for the inactivation has not been determined. Here, we provide evidence that RTV inactivation of CYP3A4 is due to heme destruction with the formation of a heme-protein adduct. Similar to CYP2B6, there is no significant modification of the apoprotein. Furthermore, LC-MS/MS analysis revealed that both CYP3A4 and human liver microsomes form an RTV-glutathione conjugate having a MH⁺ at m/z 858 during metabolism of RTV, suggesting the formation of an isocyanate intermediate leading to formation of the conjugate.

Structural and thermodynamic basis of (+)-α-pinene binding to human cytochrome P450 2B6

Despite recent advances in atomic-level understanding of drug and inhibitor interactions with human cytochromes P450, the decades-old questions of chemical and structural determinants of hydrocarbon binding are still unanswered. (+)-α-Pinene is a monoterpene hydrocarbon that is widely distributed in the environment and a potent P450 2B inhibitor. Therefore, a combined biophysical and structural analysis of human P450 2B6 interactions with (+)-α-pinene was undertaken to elucidate the basis of the very high affinity binding. Binding of (+)-α-pinene to the P450 active site was demonstrated by a Type I spectral shift. Thermodynamics of ligand binding were explored using isothermal titration calorimetry and compared to those of P450 2A6, which is much less flexible than 2B6 based on comparison of multiple X-ray crystal structures. Consistent with expectation, entropy is the major driving force for hydrocarbon binding to P450 2A6, as evidenced by the calorimetric results. However, formation of the 2B6-(+)-α-pinene complex has a significant enthalpic component. A 2.0 Å resolution crystal structure of this enzyme-ligand complex reveals that the highly plastic 2B6 utilizes previously unrecognized rearrangements of protein motifs. The results indicate that the specific components of enthalpic contribution to ligand binding are closely tied to the degree of enzyme flexibility.

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.

Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities, and impact of genetic variation

Cytochromes P450 (CYP) are a major source of variability in drug pharmacokinetics and response. Of 57 putatively functional human CYPs only about a dozen enzymes, belonging to the CYP1, 2, and 3 families, are responsible for the biotransformation of most foreign substances including 70-80% of all drugs in clinical use. The highest expressed forms in liver are CYPs 3A4, 2C9, 2C8, 2E1, and 1A2, while 2A6, 2D6, 2B6, 2C19 and 3A5 are less abundant and CYPs 2J2, 1A1, and 1B1 are mainly expressed extrahepatically. Expression of each CYP is influenced by a unique combination of mechanisms and factors including genetic polymorphisms, induction by xenobiotics, regulation by cytokines, hormones and during disease states, as well as sex, age, and others. Multiallelic genetic polymorphisms, which strongly depend on ethnicity, play a major role for the function of CYPs 2D6, 2C19, 2C9, 2B6, 3A5 and 2A6, and lead to distinct pharmacogenetic phenotypes termed as poor, intermediate, extensive, and ultrarapid metabolizers. For these CYPs, the evidence for clinical significance regarding adverse drug reactions (ADRs), drug efficacy and dose requirement is rapidly growing. Polymorphisms in CYPs 1A1, 1A2, 2C8, 2E1, 2J2, and 3A4 are generally less predictive, but new data on CYP3A4 show that predictive variants exist and that additional variants in regulatory genes or in NADPH:cytochrome P450 oxidoreductase (POR) can have an influence. Here we review the recent progress on drug metabolism activity profiles, interindividual variability and regulation of expression, and the functional and clinical impact of genetic variation in drug metabolizing P450s.

Alcohol consumption effect on antiretroviral therapy and HIV-1 pathogenesis: role of cytochrome P450 isozymes

INTRODUCTION

Alcohol consumption, which is highly prevalent in HIV-infected individuals, poses serious concerns in terms of rate of acquisition of HIV-1 infection, HIV-1 replication, response to highly active antiretroviral therapy (HAART) and AIDS/neuroAIDS progression. However, little is known about the mechanistic pathways by which alcohol exerts these effects, especially with respect to HIV-1 replication and the patient's response to HAART.

AREAS COVERED

In this review, the authors discuss the effects of alcohol consumption on HIV-1 pathogenesis and its effect on HAART. They also describe the role of cytochrome P450 2E1 (CYP2E1) in alcohol-mediated oxidative stress and toxicity, and the role of CYP3A4 in the metabolism of drugs used in HAART (i.e., protease inhibitors (PI) and non-nucleoside reverse transcriptase inhibitors (NNRTI)). Based on the most recent findings the authors discuss the role of CYP2E1 in alcohol-mediated oxidative stress in monocytes/macrophages and astrocytes, as well as the role of CYP3A4 in alcohol-PI interactions leading to altered metabolism of PI in these cells.

EXPERT OPINION

The authors propose that alcohol and PI/NNRTI interact synergistically in monocytes/macrophages and astrocytes through the CYP pathway leading to an increase in oxidative stress and a decrease in response to HAART. Ultimately, this exacerbates HIV-1 pathogenesis and neuroAIDS.

Effects of the CYP2B6*6 allele on catalytic properties and inhibition of CYP2B6 in vitro: implication for the mechanism of reduced efavirenz metabolism and other CYP2B6 substrates in vivo

The mechanism by which CYP2B6*6 allele alters drug metabolism in vitro and in vivo is not fully understood. To test the hypothesis that altered substrate binding and/or catalytic properties contribute to its functional consequences, efavirenz 8-hydroxylation and bupropion 4-hydroxylation were determined in CYP2B6.1 and CYP2B6.6 proteins expressed without and with cytochrome b5 (Cyt b5) and in human liver microsomes (HLMs) obtained from liver tissues genotyped for the CYP2B6*6 allele. The susceptibility of the variant protein to inhibition was also tested in HLMs. Significantly higher V(max) and K(m) values for 8-hydroxyefavirenz formation and ∼2-fold lower intrinsic clearance (Cl(int)) were noted in expressed CYP2B6.6 protein (-b5) compared with that of CYP2B6.1 protein (-b5); this effect was abolished by Cyt b5. The V(max) and Cl(int) values for 4-hydroxybupropion formation were significantly higher in CYP2B6.6 than in CYP2B6.1 protein, with no difference in K(m), whereas coexpression with Cyt b5 reversed the genetic effect on these kinetic parameters. In HLMs, CYP2B6*6/*6 genotype was associated with markedly lower V(max) (and moderate increase in K(m)) and thus lower Cl(int) values for efavirenz and bupropion metabolism, but no difference in catalytic properties was noted between CYP2B6*1/*1 and CYP2B6*1/*6 genotypes. Inhibition of efavirenz 8-hydroxylation by voriconazole was significantly greater in HLMs with the CYP2B6*6 allele (K(i) = 1.6 ± 0.8 μM) than HLMs with CYP2B6*1/*1 genotype (K(i) = 3.0 ± 1.1 μM). In conclusion, our data suggest the CYP2B6*6 allele influences metabolic activity by altering substrate binding and catalytic activity in a substrate- and Cyt b5-dependent manner. It may also confer susceptibility to inhibition.

Potential contribution of cytochrome P450 2B6 to hepatic 4-hydroxycyclophosphamide formation in vitro and in vivo

Results from retrospective studies on the relationship between cytochrome P450 (P450) 2B6 (CYP2B6) genotype and cyclophosphamide (CY) efficacy and toxicity in adult cancer patients have been conflicting. We evaluated this relationship in children, who have faster CY clearance and receive different CY-based regimens than adults. These factors may influence the P450s metabolizing CY to 4-hydroxycyclophosphamide (4HCY), the principal precursor to CY's cytotoxic metabolite. Therefore, we sought to characterize the in vitro and in vivo roles of hepatic CYP2B6 and its main allelic variants in 4HCY formation. CYP2B6 is the major isozyme responsible for 4HCY formation in recombinant P450 Supersomes. In human liver microsomes (HLM), 4HCY formation correlated with known phenotypic markers of CYP2B6 activity, specifically formation of (S)-2-ethyl-1,5-dimethyl-3,3-diphenyl pyrrolidine and hydroxybupropion. However, in HLM, CYP3A4/5 also contributes to 4HCY formation at the CY concentrations similar to plasma concentrations achieved in children (0.1 mM). 4HCY formation was not associated with CYP2B6 genotype at low (0.1 mM) or high (1 mM) CY concentrations potentially because CYP3A4/5 and other isozymes also form 4HCY. To remove this confounder, 4HCY formation was evaluated in recombinant CYP2B6 enzymes, which demonstrated that 4HCY formation was lower for CYP2B6.4 and CYP2B6.5 compared with CYP2B6.1. In vivo, CYP2B6 genotype was not directly related to CY clearance or ratio of 4HCY/CY areas under the curve in 51 children receiving CY-based regimens. Concomitant chemotherapy agents did not influence 4HCY formation in vitro. We conclude that CYP2B6 genotype is not consistently related to 4HCY formation in vitro or in vivo.

Investigation by site-directed mutagenesis of the role of cytochrome P450 2B4 non-active-site residues in protein-ligand interactions based on crystal structures of the ligand-bound enzyme

Residues located outside the active site of cytochromes P450 2B have exhibited importance in ligand binding, structural stability and drug metabolism. However, contributions of non-active-site residues to the plasticity of these enzymes are not known. Thus, a systematic investigation was undertaken of unique residue-residue interactions found in crystal structures of P450 2B4 in complex with 4-(4-chlorophenyl)imidazole (4-CPI), a closed conformation, or in complex with bifonazole, an expanded conformation. Nineteen mutants distributed over 11 sites were constructed, expressed in Escherichia coli and purified. Most mutants showed significantly decreased expression, especially in the case of interactions found in the 4-CPI structure. Six mutants (H172A, H172F, H172Q, L437A, E474D and E474Q) were chosen for detailed functional analysis. Among these, the K(s) of H172F for bifonazole was ∼ 20 times higher than for wild-type 2B4, and the K(s) of L437A for 4-CPI was ∼ 50 times higher than for wild-type, leading to significantly altered inhibitor selectivity. Enzyme function was tested with the substrates 7-ethoxy-4-(trifluoromethyl)coumarin, 7-methoxy-4-(trifluoromethyl)coumarin and 7-benzyloxyresorufin (7-BR). H172F was inactive with all three substrates, and L437A did not turn over 7-BR. Furthermore, H172A, H172Q, E474D and E474Q showed large changes in k(cat)/K(M) for each of the three substrates, in some cases up to 50-fold. Concurrent molecular dynamics simulations yielded distances between some of the residues in these putative interaction pairs that are not consistent with contact. The results indicate that small changes in the protein scaffold lead to large differences in solution behavior and enzyme function.

Polymorphic variants of cytochrome P450 2B6 (CYP2B6.4-CYP2B6.9) exhibit altered rates of metabolism for bupropion and efavirenz: a charge-reversal mutation in the K139E variant (CYP2B6.8) impairs formation of a functional cytochrome p450-reductase complex

In this study, metabolism of bupropion, efavirenz, and 7-ethoxy-4-trifluoromethylcoumarin (7-EFC) by CYP2B6 wild type (CYP2B6.1) and six polymorphic variants (CYP2B6.4 to CYP2B6.9) was investigated in a reconstituted system to gain a better understanding of the effects of the mutations on the catalytic properties of these naturally occurring variants. All six variants were successfully overexpressed in Escherichia coli, including CYP2B6.8 (the K139E variant), which previously could not be overexpressed in mammalian COS-1 cells (J Pharmacol Exp Ther 311:34-43, 2004). The steady-state turnover rates for the hydroxylation of bupropion and efavirenz and the O-deethylation of 7-EFC showed that these mutations significantly alter the catalytic activities of CYP2B6. It was found that CYP2B6.6 exhibits 4- and 27-fold increases in the K(m) values for the hydroxylation of bupropion and efavirenz, respectively, and CYP2B6.8 completely loses its ability to metabolize any of the substrates under normal turnover conditions. However, compared with CYP2B6.1, CYP2B6.8 retains 77% of its 7-EFC O-deethylase activity in the presence of tert-butyl hydroperoxide as an alternative oxidant, indicating that the heme and the active site are catalytically competent. Presteady-state measurements of the rate of electron transfer from NADPH-dependent cytochrome P450 reductase (CPR) to CYP2B6.8 using stopped-flow spectrophotometry revealed that CYP2B6.8 is incapable of accepting electrons from CPR. These observations provide conclusive evidence suggesting that the charge-reversal mutation in the K139E variant prevents CYP2B6.8 from forming a functional complex with CPR. Results from this work provide further insights to better understand the genotype-phenotype correlation regarding CYP2B6 polymorphisms and drug metabolism.

Differential effects of ethanol on spectral binding and inhibition of cytochrome P450 3A4 with eight protease inhibitors antiretroviral drugs

BACKGROUND

Cytochrome P450 3A4 (CYP3A4) is the most abundant CYP enzyme in the liver, which metabolizes approximately 50% of the marketed drugs including antiretroviral agents. CYP3A4 induction by ethanol and its impact on drug metabolism and toxicity is known. However, CYP3A4-ethanol physical interaction and its impact on drug binding, inhibition, or metabolism is not known, except that we have recently shown that ethanol facilitates the binding of a protease inhibitor (PI), nelfinavir, with CYP3A4. The current study was designed to examine the effect of ethanol on spectral binding and inhibition of CYP3A4 with all currently used PIs that differ in physicochemical properties.

METHODS

We performed type I and type II spectral binding with CYP3A4 at 0 and 20 mM ethanol and varying PIs' concentrations. We also performed CYP3A4 inhibition using 7-benzyloxy-4-trifluoromethylcoumarin substrate and NADPH at varying concentrations of PIs and ethanol.

RESULTS

Atazanavir, lopinavir, saquinavir, and tipranavir showed type I spectral binding, whereas indinavir and ritonavir showed type II. However, amprenavir and darunavir did not show spectral binding with CYP3A4. Ethanol at 20 mM decreased the maximum spectral change (δA(max)) with type I lopinavir and saquinavir, but it did not alter δA(max) with other PIs. Ethanol did not alter spectral binding affinity (K(D)) and inhibition constant (IC(50)) of type I PIs. However, ethanol significantly decreased the IC(50) of type II PIs, indinavir and ritonavir, and markedly increased the IC(50) of amprenavir and darunavir.

CONCLUSIONS

Overall, our results suggest that ethanol differentially alters the binding and inhibition of CYP3A4 with the PIs that have different physicochemical properties. This study has clinical relevance because alcohol has been shown to alter the response to antiretroviral drugs, including PIs, in HIV-1-infected individuals.

Effect of ethanol on spectral binding, inhibition, and activity of CYP3A4 with an antiretroviral drug nelfinavir

Cytochrome P450 3A4 (CYP3A4) is the most abundant CYP enzyme in the liver and metabolizes approximately 50% of the drugs, including antiretrovirals. Although CYP3A4 induction by ethanol and impact of CYP3A4 on drug metabolism and toxicity is known, CYP3A4-ethanol physical interaction and its impact on drug binding, inhibition, or metabolism is not known. Therefore, we studied the effect of ethanol on binding and inhibition of CYP3A4 with a representative protease inhibitor, nelfinavir, followed by the effect of alcohol on nelfinavir metabolism. Our initial results showed that methanol, ethanol, isopropanol, isobutanol, and isoamyl alcohol bind in the active site of CYP3A4 and exhibit type I spectra. Among these alcohol compounds, ethanol showed the lowest K(D) (5.9±0.34mM), suggesting its strong binding affinity with CYP3A4. Ethanol (20mM) decreased the K(D) of nelfinavir by >5-fold (0.041±0.007 vs. 0.227±0.038μM). Similarly, 20mM ethanol decreased the IC(50) of nelfinavir by >3-fold (2.6±0.5 vs. 8.3±3.1μM). These results suggest that ethanol facilitates binding of nelfinavir with CYP3A4. Furthermore, we performed nelfinavir metabolism using LCMS. Although ethanol did not alter k(cat), it decreased the K(m) of nelfinavir, suggesting a decrease in catalytic efficiency (k(cat)/K(m)). This is an important finding because alcoholism is prevalent in HIV-1-infected persons and alcohol is shown to decrease the response to antiretroviral therapy.

Crystal structure of a cytochrome P450 2B6 genetic variant in complex with the inhibitor 4-(4-chlorophenyl)imidazole at 2.0-A resolution

The structure of the K262R genetic variant of human cytochrome P450 2B6 in complex with the inhibitor 4-(4-chlorophenyl)imidazole (4-CPI) has been determined using X-ray crystallography to 2.0-A resolution. Production of diffraction quality crystals was enabled through a combination of protein engineering, chaperone coexpression, modifications to the purification protocol, and the use of unique facial amphiphiles during crystallization. The 2B6-4-CPI complex is virtually identical to the rabbit 2B4 structure bound to the same inhibitor with respect to the arrangement of secondary structural elements and the placement of active site residues. The structure supports prior P450 2B6 homology models based on other mammalian cytochromes P450 and is consistent with the limited site-directed mutagenesis studies on 2B6 and extensive studies on P450 2B4 and 2B1. Although the K262R genetic variant shows unaltered binding of 4-CPI, altered binding affinity, kinetics, and/or product profiles have been previously shown with several other ligands. On the basis of new P450 2B6 crystal structure and previous 2B4 structures, substitutions at residue 262 affect a hydrogen-bonding network connecting the G and H helices, where subtle differences could be transduced to the active site. Docking experiments indicate that the closed protein conformation allows smaller ligands such as ticlopidine to bind to the 2B6 active site in the expected orientation. However, it is unknown whether 2B6 undergoes structural reorganization to accommodate bulkier molecules, as previously inferred from multiple P450 2B4 crystal structures.

Inhibition of human CYP2B6-catalyzed bupropion hydroxylation by Ginkgo biloba extract: effect of terpene trilactones and flavonols

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.