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

Pharmacogenomic associations of cyclophosphamide pharmacokinetic candidate genes with 4hydroxycyclophosphamide formation in children with Cancer

PURPOSE

4-hydroxycyclophosphamide (4HCY) is the principal precursor to the cytotoxic metabolite of cyclophosphamide (CY), which is often used as first-line treatment of children with cancer. There is conflicting data regarding the relationship between CY efficacy, toxicity, and pharmacokinetics with the genes encoding proteins involved in 4HCY pharmacokinetics, specifically its formation and elimination.

METHODS

We evaluated germline pharmacogenetics in children with various malignancies receiving their first CY dose. Using linear regression, we analyzed the associations between two pharmacokinetic outcomes - how fast a child cleared CY (i.e., CY clearance) and the ratio of the 4HCY/CY exposure, specifically area under the plasma concentration-time curve (AUC), and 372 single nucleotide polymorphisms (SNP) in 14 drug-metabolizing transporters or enzymes involved in 4HCY formation or elimination.

RESULTS

Age was associated with the ratio of 4HCY/CY AUC (P = 0.004); Chemotherapy regimen was associated with CY clearance (P = 0.003). No SNPs were associated with CY clearance or the ratio of 4HCY/CY AUC after controlling for a false discovery rate.

CONCLUSION

Age and chemotherapy regimen, but not germline pharmacogenomics, were associated with CY clearance or the ratio of 4HCY/CY AUC. Other methods, such as metabolomics or lipidomics, should be explored.

Parthenolide enhances the metronomic chemotherapy effect of cyclophosphamide in lung cancer by inhibiting the NF-kB signaling pathway

BACKGROUND

Parthenolide (PTL), a sesquiterpene lactone derived from the medicinal herb Chrysanthemum parthenium, exhibits various biological effects by targeting NF-kB, STAT3, and other pathways. It has emerged as a promising adjunct therapy for multiple malignancies.

AIM

To evaluate the in vitro and in vivo effect of PTL on cyclophosphamide (CTX) metronomic chemotherapy.

METHODS

The cytotoxicity of PTL and CTX on Lewis lung cancer cells (LLC cells) was assessed by measuring cell activity and apoptosis. The anti-tumor efficiency was evaluated using a tumor xenograft mice model, and the survival of mice and tumor volume were monitored. Additionally, the collected tumor tissues were analyzed for tumor microenvironment indicators and inflammatory factors.

RESULTS

In vitro, PTL demonstrated a synergistic effect with CTX in inhibiting the growth of LLC cells and promoting apoptosis. In vivo, metronomic chemotherapy combined with PTL and CTX improved the survival rate of tumor-bearing mice and reduced tumor growth rate. Furthermore, metronomic chemotherapy combined with PTL and CTX reduced NF-κB activation and improved the tumor immune microenvironment by decreasing tumor angiogenesis, reducing Transforming growth factor β, and α-SMA positive cells.

CONCLUSION

PTL is an efficient compound that enhances the metronomic chemotherapy effects of CTX both in vitro and in vivo, suggesting its potential as a supplementary therapeutic strategy in metronomic chemotherapy to improve the chemotherapy effects.

Cyclophosphamide Pharmacogenomic Variation in Cancer Treatment and Its Effect on Bioactivation and Pharmacokinetics

Cyclophosphamide (Cy) is a prodrug that is mainly bioactivated by cytochrome P450 (CYP) 2B6 enzyme. Several other enzymes are also involved in its bioactivation and affect its kinetics. Previous studies have shown the effect of the enzymes' genetic polymorphisms on Cy kinetics and its clinical outcome. These results were controversial primarily because of the involvement of several interacting enzymes in the Cy metabolic pathway, which can also be affected by several clinical factors as well as other drug interactions. In this review article, we present the effect of CYP2B6 polymorphisms on Cy kinetics since it is the main bioactivating enzyme, as well as discussing all previously reported enzymes and clinical factors that can alter Cy efficacy. Additionally, we present explanations for key Cy side effects related to the nature and site of its bioactivation. Finally, we discuss the role of busulphan in conditioning regimens in the Cy metabolic pathway as a clinical example of drug-drug interactions involving several enzymes. By the end of this article, our aim is to have provided a comprehensive summary of Cy pharmacogenomics and the effect on its kinetics. The utility of these findings in the development of new strategies for Cy personalized patient dose adjustment will aid in the future optimization of patient specific Cy dosages and ultimately in improving clinical outcomes. In conclusion, CYP2B6 and several other enzyme polymorphisms can alter Cy kinetics and consequently the clinical outcomes. However, the precise quantification of Cy kinetics in any individual patient is complex as it is clearly under multifactorial genetic control. Additionally, other clinical factors such as the patient's age, diagnosis, concomitant medications, and clinical status should also be considered.

Association between CYP2B6 genetic variability and cyclophosphamide therapy in pediatric patients with neuroblastoma

Cyclophosphamide, an oxazaphosphorine prodrug is frequently used in treatment of neuroblastoma, which is one of the most prevalent solid organ malignancies in infants and young children. Cytochrome P450 2B6 (CYP2B6) is the major catalyst and CYP2C19 is the minor enzyme in bioactivation and inactivation pathways of cyclophosphamide. CYP-mediated metabolism may contribute to the variable pharmacokinetics of cyclophosphamide and its toxic byproducts leading to insufficient response to the therapy and development of clinically significant side effects. The aim of the study was to reveal the contribution of pharmacogenetic variability in CYP2B6 and CYP2C19 to the treatment efficacy and cyclophosphamide-induced side effects in pediatric neuroblastoma patients under cyclophosphamide therapy (N = 50). Cyclophosphamide-induced hematologic toxicities were pivotal in all patients, whereas only moderate hepatorenal toxicity was developed. The patients' CYP2B6 metabolizer phenotypes were associated with the occurrence of lymphopenia, thrombocytopenia, and monocytopenia as well as of liver injury, but not with kidney or urinary bladder (hemorrhagic cystitis) toxicities. Furthermore, the patients' age (< 1.5 years, P = 0.03) and female gender (P ≤ 0.02), but not CYP2B6 or CYP2C19 metabolizer phenotypes appeared as significant prognostic factors in treatment outcomes. Our results may contribute to a better understanding of the impact of CYP2B6 variability on cyclophosphamide-induced side effects.

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.

Methadone pharmacogenetics in vitro and in vivo: Metabolism by CYP2B6 polymorphic variants and genetic variability in paediatric disposition

AIMS

Methadone metabolism and clearance are determined principally by polymorphic cytochrome P4502B6 (CYP2B6). Some CYP2B6 allelic variants affect methadone metabolism in vitro and disposition in vivo. We assessed methadone metabolism by CYP2B6 minor variants in vitro. We also assessed the influence of CYP2B6 variants, and P450 oxidoreductase (POR) and CYP2C19 variants, on methadone clearance in surgical patients in vivo.

METHODS

CYP2B6 and P450 oxidoreductase variants were coexpressed with cytochrome b5 . The metabolism of methadone racemate and enantiomers was measured at therapeutic concentrations and intrinsic clearances were determined. Adolescents receiving methadone for surgery were genotyped for CYP2B6, CYP2C19 and POR, and methadone clearance and metabolite formation clearance were determined.

RESULTS

In vitro, CYP2B6.4 was more active than wild-type CYP2B6.1. CYPs 2B6.5, 2B6.6, 2B6.7, 2B6.9, 2B6.17, 2B6.19 and 2B6.26 were less active. CYPs 2B6.16 and 2B6.18 were inactive. CYP2B6.1 expressed with POR variants POR.28, POR.5 and P228L had lower rates of methadone metabolism than wild-type reductase. In vivo, methadone clinical clearance decreased linearly with the number of CYP2B6 slow metabolizer alleles, but was not different in CYP2C19 slow or rapid metabolizer phenotypes, or in carriers of the POR*28 allele.

CONCLUSIONS

Several CYP2B6 and POR variants were slow metabolizers of methadone in vitro. Polymorphisms in CYP2B6, but not CYP2C19 or P450 reductase, affected methadone clearance in vivo. CYP2B6 polymorphisms 516G>T and 983T>C code for canonical loss of function variants and should be assessed when considering genetic influences on clinical methadone disposition. These complementary translational in vitro and in vivo results inform on pharmacogenetic variability affecting methadone disposition in patients.

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.

Pharmacogenomic associations of cyclophosphamide pharmacokinetic candidate genes with event-free survival in intermediate-risk rhabdomyosarcoma: A report from the Children's Oncology Group

BACKGROUND

In vitro data suggest that the growth of rhabdomyosarcoma (RMS) cells is suppressed in a concentration-dependent manner by 4-hydroxycyclophosphamide (4HCY), the principal precursor to the cytotoxic metabolite of cyclophosphamide (CY). Various retrospective studies on the relationship between genes encoding proteins involved in the formation and elimination of 4HCY (i.e., 4HCY pharmacokinetics) and cyclophosphamide (CY) efficacy and toxicity have been conflicting.

PROCEDURES

We evaluated germline pharmacogenetics in 262 patients with newly diagnosed intermediate-risk RMS who participated in one prospective Children's Oncology Group clinical trial, ARST0531. Patients were treated with either vincristine/actinomycin/cyclophosphamide (VAC) or VAC alternating with vincristine/irinotecan (VAC/VI). We analyzed the associations between event-free survival and 394 single-nucleotide polymorphisms (SNP) in 14 drug metabolizing enzymes or transporters involved in 4HCY pharmacokinetics.

RESULTS

Eight SNPs were associated (p-value < .05 by univariate analysis) with 3-year event-free survival; no SNPs survived a false discovery rate < 0.05.

CONCLUSIONS

Our data suggest that a pharmacogenomic approach to therapy personalization of cyclophosphamide in intermediate-risk rhabdomyosarcoma is not viable. Other methods to personalize therapy should be explored.

CYP2B6 Functional Variability in Drug Metabolism and Exposure Across Populations-Implication for Drug Safety, Dosing, and Individualized Therapy

Adverse drug reactions (ADRs) are one of the major causes of morbidity and mortality worldwide. It is well-known that individual genetic make-up is one of the causative factors of ADRs. Approximately 14 million single nucleotide polymorphisms (SNPs) are distributed throughout the entire human genome and every patient has a distinct genetic make-up which influences their response to drug therapy. Cytochrome P450 2B6 (CYP2B6) is involved in the metabolism of antiretroviral, antimalarial, anticancer, and antidepressant drugs. These drug classes are commonly in use worldwide and face specific population variability in side effects and dosing. Parts of this variability may be caused by single nucleotide polymorphisms (SNPs) in the CYP2B6 gene that are associated with altered protein expression and catalytic function. Population variability in the CYP2B6 gene leads to changes in drug metabolism which may result in adverse drug reactions or therapeutic failure. So far more than 30 non-synonymous variants in CYP2B6 gene have been reported. The occurrence of these variants show intra and interpopulation variability, thus affecting drug efficacy at individual and population level. Differences in disease conditions and affordability of drug therapy further explain why some individuals or populations are more exposed to CYP2B6 pharmacogenomics associated ADRs than others. Variabilities in drug efficacy associated with the pharmacogenomics of CYP2B6 have been reported in various populations. The aim of this review is to highlight reports from various ethnicities that emphasize on the relationship between CYP2B6 pharmacogenomics variability and the occurrence of adverse drug reactions. In vitro and in vivo studies evaluating the catalytic activity of CYP2B6 variants using various substrates will also be discussed. While implementation of pharmacogenomic testing for personalized drug therapy has made big progress, less data on pharmacogenetics of drug safety has been gained in terms of CYP2B6 substrates. Therefore, reviewing the existing evidence on population variability in CYP2B6 and ADR risk profiles suggests that, in addition to other factors, the knowledge on pharmacogenomics of CYP2B6 in patient treatment may be useful for the development of personalized medicine with regards to genotype-based prescription.

Pharmacokinetics and Pharmacogenetics of Cyclophosphamide in a Neonate and Infant Childhood Cancer Patient Population

Infants and young children represent an important but much understudied childhood cancer patient population. The pharmacokinetics and pharmacogenetics of the widely used anticancer prodrug cyclophosphamide were investigated in children <2 years of age. Concentrations of cyclophosphamide and selected metabolites were determined in patients administered cyclophosphamide at doses ranging from 100–1500 mg/m² (5–75 mg/kg), with various infusion times as determined by the standard treatment regimen that each patient was receiving. Polymorphisms in genes including CYP2B6 and CYP2C19 were investigated. Data generated for cyclophosphamide were analysed using a previously published population pharmacokinetic model. Cyclophosphamide pharmacokinetics was assessed in 111 samples obtained from 25 patients ranging from 4–23 months of age. The average cyclophosphamide clearance for the patients was 46.6 mL/min/m² (ranging from 9.4–153 mL/min/m²), with marked inter-patient variability observed (CV 41%). No significant differences in cyclophosphamide clearance or exposure (AUC) were observed between patient groups as separated by age or body weight. However, marked differences in drug clearance and metabolism were noted between the current data in children <2 years of age and recently published results from a comparable study conducted by our group in older children, which reported significantly lower cyclophosphamide clearance values and metabolite exposures using the same population pharmacokinetic model for analysis. Whilst this study demonstrates no significant differences in cyclophosphamide clearance in patients <2 years, it highlights large differences in dosing protocols across tumour types. Furthermore, the study suggests marked differences in cyclophosphamide clearance in children less than two years of age as compared to older patients.

Population Pharmacokinetic, Pharmacogenetic, and Pharmacodynamic Analysis of Cyclophosphamide in Ethiopian Breast Cancer Patients

Cyclophosphamide (CPA) containing chemotherapy regimen is the standard of care for breast cancer treatment in sub-Saharan Africa. Wide inter-individual variations in pharmacokinetics (PK) of cyclophosphamide (CPA) influence the efficacy and toxicity of CPA containing chemotherapy. Data on the pharmacokinetics (PK) profile of CPA and its covariates among black African patients is lacking. We investigated population pharmacokinetic/pharmacogenetic/pharmacodynamic (PK-PG-PD) of CPA in Ethiopian breast cancer patients. During the first cycle of CPA-based chemotherapy, the population PK parameters for CPA were determined in 267 breast cancer patients. Absolute neutrophil count was recorded at baseline and day 20 post-CPA administration. A population PK and covariate model analysis was performed using non-linear mixed effects modeling. Semi-mechanistic and empiric drug response models were explored to describe the relationship between the area under concentration-time curve (AUC), and neutrophil toxicity. One compartment model better described CPA PK with population clearance and apparent volume of distribution (V_D) of 5.41 L/h and 46.5 L, respectively. Inter-patient variability in CPA clearance was 54.5%. Patients carrying CYP3A5*3 or *6 alleles had lower elimination rate constant and longer half-life compared to wild type carriers. CYP2C9 *2 or *3 carriers were associated with increased clearance of CPA. Patients who received 500 mg/m² based CPA regimen were associated with a 32.3% lower than average clearance and 37.1% lower than average V_D compared to patients who received 600 mg/m². A 0.1 m² unit increase in body surface area (BSA) was associated with a 5.6% increment in V_D. The mean V_D (33.5 L) in underweight group (BMI < 18.5 kg/m²) was significantly lower compared to those of overweight (48.1 L) or obese patients (51.9 L) (p < 0.001). AUC of CPA was positively correlated with neutropenic toxicity. In conclusion, we report large between-patient variability in clearance of CPA. CYP3A5 and CYP2C9 genotypes, BSA, BMI, and CPA dosage regimen influence PK of CPA. Plasma CPA exposure positively predicts chemotherapy-associated neutropenic toxicity.

Stereoselective Bupropion Hydroxylation by Cytochrome P450 CYP2B6 and Cytochrome P450 Oxidoreductase Genetic Variants

Bioactivation of the antidepressant and smoking cessation drug bupropion is catalyzed predominantly by CYP2B6. The metabolite hydroxybupropion derived from t-butylhydroxylation is considered to contribute to the antidepressant and smoking-cessation effects of the parent drug. Bupropion hydroxylation is the canonical in vitro and in vivo probe for CYP2B6 activity. P450 also requires obligate partnership with P450 oxidoreductase (POR). Human CYP2B6 and POR genes are highly polymorphic. Some CYP2B6 variants affect bupropion disposition. This investigation evaluated the influence of several human CYP2B6 and POR genetic variants on stereoselective bupropion metabolism, using an insect cell coexpression system containing CYP2B6, POR, and cytochrome b ₅ Based on intrinsic clearances (Cl_ints), relative activities for S,S-hydroxybupropion formation were in the order CYP2B6.4 > CYP2B6.1 > CYP2B6.17 > CYP2B6.5 > CYP2B6.6 ≈ CYP2B6.26 ≈ CYP2B6.19 > CYP2B6.7 > CYP2B6.9 > > CYP2B6.16 and CYP2B6.18; relative activities for R,R-hydroxybupropion formation were in the order CYP2B6.17 > CYP2B6.4 > CYP2B6.1 > CYP2B6.5 ≈ CYP2B6.19 ≈ CYP2B6.26 > CYP2B6.6 > CYP2B6.7 ≈ CYP2B6.9 > > CYP2B6.16 and CYP2B6.18. Bupropion hydroxylation was not influenced by POR variants. CYP2B6-catalyzed bupropion hydroxylation is stereoselective. Though V_max and K_m varied widely among CYP2B6 variants, stereoselectivity was preserved, reflected by similar Cl_int(S,S-hydroxybupropion)/Cl_int(R,R-hydroxybupropion) ratios (1.8-2.9), except CYP2B6.17, which was less enantioselective. Established concordance between human bupropion hydroxylation in vitro and in vivo, together with these new results, suggests additional CYP2B6 variants may influence human bupropion disposition. SIGNIFICANCE STATEMENT: Bupropion pharmacokinetics, metabolism, and clinical effects are affected by the CYP2B6*6 polymorphism. Other expressed CYP2B6 polymorphisms had diminished (*5, *6, *7, *9, *19, *26) or defective (*16, *18) in vitro bupropion hydroxylation. P450 oxidoreductase genetic variants had no effect on metabolism, suggesting no clinical consequence of this polymorphism. These CYP2B6 polymorphisms may portend diminished in vivo bupropion hydroxylation and predict additional clinically important variant alleles.

Genetic Polymorphism of GSTP-1 Affects Cyclophosphamide Treatment of Autoimmune Diseases

Cyclophosphamide is one of the most potent and reliable anti-cancer and immunosuppressive drugs. In our study, 33 individuals with different autoimmune diseases were treated with cyclophosphamide according to standard protocols. The responses to the treatments were determined by measuring the alteration of several typical parameters characterizing the given autoimmune diseases over time. We concluded that about 45% of the patients responded to the treatment. Patients were genotyped for polymorphisms of the CYP3A4, CYP2B6, GSTM1, GSTT1, and GSTP1 genes and disease remission cases were compared to the individual polymorphic genotypes. It was found that the GSTP1 I105V allelic variation significantly associated with the cyclophosphamide treatment-dependent disease-remissions. At the same time the GSH content of the erythrocytes in the patients with I105V allelic variation did not change. It appears that the individuals carrying the Ile105Val SNP in at least one copy had a significantly higher response rate to the treatment. Since this variant of GSTP1 can be characterized by lower conjugation capacity that results in an elongated and higher therapeutic dose of cyclophosphamide, our data suggest that the decreased activity of this variant of GSTP1 can be in the background of the more effective disease treatment.

The importance of both CYP2C19 and CYP2B6 germline variations in cyclophosphamide pharmacokinetics and clinical outcomes

Cyclophosphamide is an alkylating agent used in the treatment of solid and haematological malignancies and as an immunosuppressive agent. As a prodrug, it is dependent on bioactivation to the active phosphoramide mustard metabolite to elicit its therapeutic effect. This focused review will highlight the evidence for the role of germline pharmacogenetic variation in both plasma pharmacokinetics and clinical outcomes. There is a substantial indication from 13 pharmacokinetic and 17 therapeutic outcome studies, in contexts as diverse as haematological malignancy, breast cancer, systemic lupus erythematosus and myeloablation, that pharmacogenetic variation in both CYP2C19 and CYP2B6 influence the bioactivation of cyclophosphamide. An additional role for pharmacogenetic variation in ALDH1A1 has also been reported. Future studies should comprehensively assess these 3 pharmacogenes and undertake appropriate statistical analysis of gene-gene interactions to confirm these findings and may allow personalised treatment regimens.

Association of Antiepileptic Medications with Outcomes after Allogeneic Hematopoietic Cell Transplantation with Busulfan/Cyclophosphamide Conditioning

High-dose busulfan (BU) followed by high-dose cyclophosphamide (CY) before allogeneic hematopoietic cell transplantation (HCT) has long been used as treatment for hematologic malignancies. Administration of phenytoin or newer alternative antiepileptic medications (AEMs) prevents seizures caused by BU. Phenytoin induces enzymes that increase exposure to active CY metabolites in vivo, whereas alternative AEMs do not have this effect. Lower exposure to active CY metabolites with the use of alternative AEMs could decrease the risk of toxicity but might increase the risk of recurrent malignancy after HCT. Previous studies have not determined whether outcomes with alternative AEMs differ from those with phenytoin in patients treated with BU/CY before allogeneic HCT. We studied a cohort of 2155 patients, including 1460 treated with phenytoin and 695 treated with alternative AEMs, who received BU/CY before allogeneic HCT between 2004 and 2014. We found no differences suggesting decreased overall survival or relapse-free survival or increased risks of relapse, nonrelapse mortality, acute or chronic graft-versus-host disease, or regimen-related toxicity associated with the use of alternative AEMs compared with phenytoin. The risk of dialysis was lower in the alternative AEM group than in the phenytoin group. Alternative AEMs are safe for prevention of seizures after BU administration and can avoid the undesirable toxicities and drug interactions caused by phenytoin.

Nicotine oxidation by genetic variants of CYP2B6 and in human brain microsomes

Common variation in the CYP2B6 gene, encoding the cytochrome P450 2B6 enzyme, is associated with substrate-specific altered clearance of multiple drugs. CYP2B6 is a minor contributor to hepatic nicotine metabolism, but the enzyme has been proposed as relevant to nicotine-related behaviors because of reported CYP2B6 mRNA expression in human brain tissue. Therefore, we hypothesized that CYP2B6 variants would be associated with altered nicotine oxidation, and that nicotine metabolism by CYP2B6 would be detected in human brain microsomes. We generated recombinant enzymes in insect cells corresponding to nine common CYP2B6 haplotypes and demonstrate genetically determined differences in nicotine oxidation to nicotine iminium ion and nornicotine for both (S) and (R)-nicotine. Notably, the CYP2B6.6 and CYP2B6.9 variants demonstrated lower intrinsic clearance relative to the reference enzyme, CYP2B6.1. In the presence of human brain microsomes, along with nicotine-N-oxidation, we also detect nicotine oxidation to nicotine iminium ion. However, unlike N-oxidation, this activity is NADPH independent, does not follow Michaelis-Menten kinetics, and is not inhibited by NADP or carbon monoxide. Furthermore, metabolism of common CYP2B6 probe substrates, methadone and ketamine, is not detected in the presence of brain microsomes. We conclude that CYP2B6 metabolizes nicotine stereoselectively and common CYP2B6 variants differ in nicotine metabolism activity, but did not find evidence of CYP2B6 activity in human brain.

Common Polymorphisms of CYP2B6 Influence Stereoselective Bupropion Disposition

Bupropion hydroxylation is a bioactivation and metabolic pathway, and the standard clinical CYP2B6 probe. This investigation determined the influence of CYP2B6 allelic variants on clinical concentrations and metabolism of bupropion enantiomers. Secondary objectives evaluated the influence of CYP2C19 and P450 oxidoreductase variants. Healthy volunteers in specific cohorts (CYP2B6*1/*1, CYP2B6*1/*6, CYP2B6*6/*6, and also CYP2B6*4 carriers) received single-dose oral bupropion. Plasma and urine bupropion and hydroxybupropion was quantified. Subjects were also genotyped for CYP2C19 and P450 oxidoreductase variants. Hydroxylation of both bupropion enantiomers, assessed by plasma hydroxybupropion/bupropion AUC ratios and urine hydroxybupropion formation clearances, was lower in CYP2B6*6/*6 but not CYP2B6*1/*6 compared with CYP2B6*1/*1 genotypes, and numerically greater in CYP2B6*4 carriers. CYP2C19 and P450 oxidoreductase variants did not influence bupropion enantiomers hydroxylation or plasma concentrations. The results show that clinical hydroxylation of both bupropion enantiomers was equivalently influenced by CYP2B6 allelic variation. CYP2B6 polymorphisms affect S-bupropion bioactivation, which may affect therapeutic outcomes.

High-throughput metabolism-induced toxicity assays demonstrated on a 384-pillar plate

The US Environmental Protection Agency (EPA) launched the Transform Tox Testing Challenge in 2016 with the goal of developing practical methods that can be integrated into conventional high-throughput screening (HTS) assays to better predict the toxicity of parent compounds and their metabolites in vivo. In response to this need and to retrofit existing HTS assays for assessing metabolism-induced toxicity of compounds, we have developed a 384-pillar plate that is complementary to traditional 384-well plates and ideally suited for culturing human cells in three dimensions at a microscale. Briefly, human embryonic kidney (HEK) 293 cells in a mixture of alginate and Matrigel were printed on the 384-pillar plates using a microarray spotter, which were coupled with 384-well plates containing nine model compounds provided by the EPA, five representative Phase I and II drug metabolizing enzymes (DMEs), and one no enzyme control. Viability and membrane integrity of HEK 293 cells were measured with the calcein AM and CellTiter-Glo^® kit to determine the IC₅₀ values of the nine parent compounds and DME-generated metabolites. The Z' factors and the coefficient of variation measured were above 0.6 and below 14%, respectively, indicating that the assays established on the 384-pillar plate are robust and reproducible. Out of nine compounds tested, six compounds showed augmented toxicity with DMEs and one compound showed detoxification with a Phase II DME. This result indicates that the 384-pillar plate platform can be used to measure metabolism-induced toxicity of compounds in high-throughput with individual DMEs. As xenobiotics metabolism is a complex process with a variety of DMEs involved, the predictivity of our approach could be further improved with mixtures of DMEs.

Flavin-containing monooxygenase 3 (FMO3) role in busulphan metabolic pathway

Busulphan (Bu) is an alkylating agent used in the conditioning regimen prior to hematopoietic stem cell transplantation (HSCT). Bu is extensively metabolized in the liver via conjugations with glutathione to form the intermediate metabolite (sulfonium ion) which subsequently is degraded to tetrahydrothiophene (THT). THT was reported to be oxidized forming THT-1-oxide that is further oxidized to sulfolane and finally 3-hydroxysulfolane. However, the underlying mechanisms for the formation of these metabolites remain poorly understood. In the present study, we performed in vitro and in vivo investigations to elucidate the involvement of flavin-containing monooxygenase-3 (FMO3) and cytochrome P450 enzymes (CYPs) in Bu metabolic pathway. Rapid clearance of THT was observed when incubated with human liver microsomes. Furthermore, among different recombinant microsomal enzymes, the highest intrinsic clearance for THT was obtained via FMO3 followed by several CYPs including 2B6, 2C8, 2C9, 2C19, 2E1 and 3A4. In Bu- or THT-treated mice, inhibition of FMO3 by phenylthiourea significantly suppressed the clearance of both Bu and THT. Moreover, the simultaneous administration of a high dose of THT (200μmol/kg) to Bu-treated mice reduced the clearance of Bu. Consistently, in patients undergoing HSCT, repeated administration of Bu resulted in a significant up-regulation of FMO3 and glutathione-S-transfrase -1 (GSTA1) genes. Finally, in a Bu-treated patient, additional treatment with voriconazole (an antimycotic drug known as an FMO3-substrate) significantly altered the Bu clearance. In conclusion, we demonstrate for the first time that FMO3 along with CYPs contribute a major part in busulphan metabolic pathway and certainly can affect its kinetics. The present results have high clinical impact. Furthermore, these findings might be important for reducing the treatment-related toxicity of Bu, through avoiding interaction with other concomitant used drugs during conditioning and hence improving the clinical outcomes of HSCT.

Association of Liver Injury From Specific Drugs, or Groups of Drugs, With Polymorphisms in HLA and Other Genes in a Genome-Wide Association Study

BACKGROUND & AIMS

We performed a genome-wide association study (GWAS) to identify genetic risk factors for drug-induced liver injury (DILI) from licensed drugs without previously reported genetic risk factors.

METHODS

We performed a GWAS of 862 persons with DILI and 10,588 population-matched controls. The first set of cases was recruited before May 2009 in Europe (n = 137) and the United States (n = 274). The second set of cases were identified from May 2009 through May 2013 from international collaborative studies performed in Europe, the United States, and South America. For the GWAS, we included only cases with patients of European ancestry associated with a particular drug (but not flucloxacillin or amoxicillin-clavulanate). We used DNA samples from all subjects to analyze HLA genes and single nucleotide polymorphisms. After the discovery analysis was concluded, we validated our findings using data from 283 European patients with diagnosis of DILI associated with various drugs.

RESULTS

We associated DILI with rs114577328 (a proxy for A*33:01 a HLA class I allele; odds ratio [OR], 2.7; 95% confidence interval [CI], 1.9-3.8; P = 2.4 × 10^-8) and with rs72631567 on chromosome 2 (OR, 2.0; 95% CI, 1.6-2.5; P = 9.7 × 10^-9). The association with A*33:01 was mediated by large effects for terbinafine-, fenofibrate-, and ticlopidine-related DILI. The variant on chromosome 2 was associated with DILI from a variety of drugs. Further phenotypic analysis indicated that the association between DILI and A*33:01 was significant genome wide for cholestatic and mixed DILI, but not for hepatocellular DILI; the polymorphism on chromosome 2 was associated with cholestatic and mixed DILI as well as hepatocellular DILI. We identified an association between rs28521457 (within the lipopolysaccharide-responsive vesicle trafficking, beach and anchor containing gene) and only hepatocellular DILI (OR, 2.1; 95% CI, 1.6-2.7; P = 4.8 × 10^-9). We did not associate any specific drug classes with genetic polymorphisms, except for statin-associated DILI, which was associated with rs116561224 on chromosome 18 (OR, 5.4; 95% CI, 3.0-9.5; P = 7.1 × 10^-9). We validated the association between A*33:01 terbinafine- and sertraline-induced DILI. We could not validate the association between DILI and rs72631567, rs28521457, or rs116561224.

CONCLUSIONS

In a GWAS of persons of European descent with DILI, we associated HLA-A*33:01 with DILI due to terbinafine and possibly fenofibrate and ticlopidine. We identified polymorphisms that appear to be associated with DILI from statins, as well as 2 non-drug-specific risk factors.

Enantioselectivity in the Metabolism of Cyclophosphamide in Patients With Multiple or Systemic Sclerosis

The aim of this study was to evaluate the enantioselective pharmacokinetics of cyclophosphamide and its metabolites 4-hydroxycyclophosphamide and carboxyethylphosphoramide mustard in patients with systemic or multiple sclerosis. Patients with systemic sclerosis (n = 10) or multiple sclerosis (n = 10), genotyped for the allelic variants of CYP2C9*2 and CYP2C9*3 and of the CYP2B6 G516T polymorphism, were treated with 50 mg cyclophosphamide/kg daily for 4 days. Serial blood samples were collected up to 24 hours after administration of the last cyclophosphamide dose. Cyclophosphamide, 4-hydroxycyclophosphamide, and carboxyethylphosphoramide enantiomers were analyzed in plasma samples using liquid chromatography-tandem mass spectrometry coupled to chiral column Chiralcel OD-R or Chiralpak AD-RH. Cytokines IL-2, IL-4, IL-6, IL-8, IL-10, IL- 12p70, IL-17, TNF-α, and INT-δ in the plasma samples collected before cyclophosphamide infusion were analyzed by Milliplex MAP human cytokine/chemokine. Pharmacokinetic parameters showed higher plasma concentrations of (S)-(-)-cyclophosphamide (AUC 215.0 vs 186.2 μg·h/mL for multiple sclerosis patients and 219.1 vs 179.2 μg·h/mL for systemic sclerosis patients) and (R)-4-hydroxycyclophosphamide (AUC 5.6 vs 3.7 μg·h/mL for multiple sclerosis patients and 6.3 vs 5.6 μg·h/mL for systemic sclerosis patients) when compared to their enantiomers in both groups of patients, whereas the pharmacokinetics of the carboxyethylphosphoramide metabolite was not enantioselective. Cytokines' plasma concentrations were similar between multiple and systemic sclerosis groups. The pharmacokinetics of cyclophosphamide is enantioselective in patients with systemic sclerosis and multiple sclerosis, with higher plasma concentrations of the (S)-(-)-cyclophosphamide enantiomer due to the preferential formation of the (R)-4-hydroxycyclophosphamide metabolite.

CYP2B6rs2279343 Is Associated with Improved Survival of Pediatric Rhabdomyosarcoma Treated with Cyclophosphamide

Background

Rhabdomyosarcoma (RMS) is a small round blue cell malignant tumor, representing 7% of childhood malignancies, and over 50% of all soft tissue sarcomas. Cyclophosphamide (CPA) is a prodrug and is the mainstay of RMS treatment. CYP2B6 is a highly polymorphic drug metabolizing enzyme involved in CPA bioactivation. The influence of CYP2B6 single nucleotide polymorphisms (SNPs) on the survival of RMS is still unknown.

Methods

We genotyped CYP2B6SNPs rs2279343, rs3745274, and rs3211371 by restriction fragment polymorphism (RFLP) after PCR amplification in a cohort of 73 pediatric RMS patients treated with CPA-based first line treatment. We then analyzed the association between those genotypes and survival outcome of RMS.

Results

The frequencies of CYP2B6 rs2279343, rs3745274, and rs3211371 were 63%, 45.2%, and 5.5%, respectively. There was no association between rs3745274, rs3211371 genotypes and survival outcomes of RMS. However, the carriers of at least one mutant allele CYP2B6rs2279343 had significantly longer event-free survival (p-value = 0.03).

Conclusion

Our results demonstrated that CYP2B6 rs2279343 may predict EFS in RMS patients and warrants future studies to clarify the pharmacogenetics of CPA in pediatrics. If validated, integration of genetic factors with clinical and molecular characteristics could be used for a composite algorithm to better stratify risk prior to treatment.

Cytochrome P450 Oxidoreductase Influences CYP2B6 Activity in Cyclophosphamide Bioactivation

Introduction

Cyclophosphamide is commonly used as an important component in conditioning prior to hematopoietic stem cell transplantation, a curative treatment for several hematological diseases. Cyclophosphamide is a prodrug activated mainly by cytochrome P450 2B6 (CYP2B6) in the liver. A high degree of inter- and intra-individual variation in cyclophosphamide kinetics has been reported in several studies.

Materials and Methods

Hydroxylation of cyclophosphamide was investigated in vitro using three microsomal batches of CYP2B6*1 with different ratios of POR/CYP expression levels. Twenty patients undergoing hematopoietic stem cell transplantation were also included in the study. All patients received an i.v. infusion of cyclophosphamide (60 mg/kg/day, for two days) as a part of their conditioning. Blood samples were collected from each patient before cyclophosphamide infusion, 6 h after the first dose and before and 6 h after the second dose. POR gene expression was measured by mRNA analysis and the pharmacokinetics of cyclophosphamide and its active metabolite were determined.

Results

A strong correlation between the in vitro intrinsic clearance of cyclophosphamide and the POR/CYP ratio was found. The apparent K_m for CYP2B6.1 was almost constant (3-4 mM), while the CL_int values were proportional to the POR/CYP ratio (3-34 μL/min/nmol CYP). In patients, the average expression of the POR gene in blood was significantly (P <0.001) up-regulated after cyclophosphamide infusion, with high inter-individual variations and significant correlation with the concentration ratio of the active metabolite 4-hydroxy-cyclophosphamide/cyclophosphamide. Nine patients were carriers for POR*28; four patients had relatively high POR expression.

Conclusions

This investigation shows for the first time that POR besides CYP2B6 can influence cyclophosphamide metabolism. Our results indicate that not only CYPs are important, but also POR expression and/or activity may influence cyclophosphamide bioactivation, affecting therapeutic efficacy and treatment related toxicity and hence on clinical outcome. Thus, both POR and CYP genotype and expression levels may have to be taken into account when personalizing treatment schedules to achieve optimal therapeutic drug plasma concentrations of cyclophosphamide.

Cytochrome P450 2B6*5 Increases Relapse after Cyclophosphamide-Containing Conditioning and Autologous Transplantation for Lymphoma

Cyclophosphamide (Cy) is a prodrug that depends on bioactivation by hepatic cytochrome P450 (CYP) enzymes for its cytotoxicity. We evaluated the influence of single nucleotide polymorphisms (SNPs) of CYP enzymes on the efficacy of autologous hematopoietic cell transplantation (HCT) for lymphoma. SNPs of 22 genes were analyzed in 93 patients with Hodgkin (n = 52) and non-Hodgkin lymphoma (n = 41) treated with high-dose Cy followed by autologous HCT between 2004 and 2012. Preparative regimens contained Cy (120 mg/kg) combined with carmustine/etoposide (n = 61) or Cy (6000 mg/m(2)) with total body irradiation (n = 32). Lack of complete remission as measured by pretransplant positron emission tomography was the sole clinical factor associated with increased risk of relapse (HR, 2.1). In genomic analysis, we identified a single SNP (rs3211371) in exon 9 (C > T) of the CYP2B6 gene (allele designation 2B6*5) that significantly impacted patient outcomes. After adjusting for disease status and conditioning regimen, patients with the CYP2B6*1/*5 genotype had a higher 2-year relapse rate (HR, 3.3; 95% CI, 1.6 to 6.5; P = .041) and decreased overall survival (HR, 13.5; 95% CI, 3.5 to 51.9; P = .008) than patients with the wild-type allele. Two-year progression-free survival for patients with 2 hypofunctional CYP2B6 variant genotypes (*5 and *6) was only 11% (95% CI, 1% to 39%) compared with 67% (95% CI, 55% to 77%) for patients with the wild-type CYP2B6*1 allele in exon 9. Our results suggest that CYP2B6 SNPs influence the efficacy of high-dose Cy and significantly reduce the success of autologous HCT for lymphoma patients with the CYP2B6*5 variant.

Pharmacogenomics and personalized medicine: a review focused on their application in the Chinese population

The field of pharmacogenomics was initiated in the 1950s and began to thrive after the completion of the human genome project 10 years ago. Thus far, more than 100 drug labels and clinical guidelines referring to pharmacogenomic biomarkers have been published, and several key pharmacogenomic markers for either drug safety or efficacy have been identified and subsequently adopted in clinical practice as pre-treatment genetic tests. However, a tremendous variation of genetic backgrounds exists between different ethnic groups. The application of pharmacogenomics in the Chinese population is still a long way off, since the published guidelines issued by the organizations such as US Food and Drug Administration require further confirmation in the Chinese population. This review highlights important pharmacogenomic discoveries in the Chinese population and compares the Chinese population with other nations regarding the pharmacogenomics of five most commonly used drugs, ie, tacrolimus, cyclosporine A, warfarin, cyclophosphamide and azathioprine.

Differences in Methadone Metabolism by CYP2B6 Variants

Methadone is a long-acting opioid with considerable unexplained interindividual variability in clearance. Cytochrome P450 2B6 (CYP2B6) mediates clinical methadone clearance and metabolic inactivation via N-demethylation to 2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP). Retrospective studies suggest that individuals with the CYP2B6*6 allelic variant have higher methadone plasma concentrations. Catalytic activities of CYP2B6 variants are highly substrate- and expression-system dependent. This investigation evaluated methadone N-demethylation by expressed human CYP2B6 allelic variants in an insect cell coexpression system containing P450 reductase. Additionally, the influence of coexpressing cytochrome b5, whose role in metabolism can be inhibitory or stimulatory depending on the P450 isoform and substrate, on methadone metabolism, was evaluated. EDDP formation from therapeutic (0.25-1 μM) R- and S-methadone concentrations was CYP2B6.4 ≥ CYP2B6.1 ≥ CYP2B6.5 >> CYP2B6.9 ≈ CYP2B6.6, and undetectable from CYP2B6.18. Coexpression of b5 had small and variant-specific effects at therapeutic methadone concentrations but at higher concentrations stimulated EDDP formation by CYP2B6.1, CYP2B6.4, CYP2B6.5, and CYP2B6.9 but not CYP2B6.6. In vitro intrinsic clearances were generally CYP2B6.4 ≥ CYP2B6.1 > CYP2B6.5 > CYP2B6.9 ≥ CYP2B6.6. Stereoselective methadone metabolism (S>R) was maintained with all CYP2B6 variants. These results show that methadone N-demethylation by CYP2B6.4 is greater compared with CYP2B6.1, whereas CYP2B6.9 and CYP2B6.6 (which both contain the 516G>T, Q172H polymorphism), are catalytically deficient. The presence or absence of b5 in expression systems may explain previously reported disparate catalytic activities of CYP2B6 variants for specific substrates. Differences in methadone metabolism by CYP2B6 allelic variants provide a mechanistic understanding of pharmacogenetic variability in clinical methadone metabolism and clearance.

Cytochrome P450 2J2, a new key enzyme in cyclophosphamide bioactivation and a potential biomarker for hematological malignancies

The role of cytochrome P450 2J2 (CYP2J2) in cyclophosphamide (Cy) bioactivation was investigated in patients, cells and microsomes. Gene expression analysis showed that CYP2J2 mRNA expression was significantly (P<0.01) higher in 20 patients with hematological malignancies compared with healthy controls. CYP2J2 expression showed significant upregulation (P<0.05) during Cy treatment before stem cell transplantation. Cy bioactivation was significantly correlated to CYP2J2 expression. Studies in HL-60 cells expressing CYP2J2 showed reduced cell viability when incubated with Cy (half maximal inhibitory concentration=3.6 mM). Inhibition of CYP2J2 using telmisartan reduced Cy bioactivation by 50% and improved cell survival. Cy incubated with recombinant CYP2J2 microsomes has resulted in apparent Km and Vmax values of 3.7-6.6 mM and 2.9-10.3 pmol/(min·pmol) CYP, respectively. This is the first study demonstrating that CYP2J2 is equally important to CYP2B6 in Cy metabolism. The heart, intestine and urinary bladder express high levels of CYP2J2; local Cy bioactivation may explain Cy-treatment-related toxicities in these organs.

Optimizing drug therapy in pediatric SCT: focus on pharmacokinetics

Given age-related differences in drug metabolism and indications for hematopoietic SCT (HSCT), personalized drug dosing of the conditioning regimen and post-transplant immunosuppression may reduce graft rejection, relapse rates and toxicity in pediatric HSCT recipients. This manuscript summarizes the pharmacokinetic/dynamic data of HSCT conditioning and post-grafting immunosuppression, presented at the First Annual Pediatric Bone Marrow Transplant Consortium (PBMTC) meeting in April 2013. Personalized dosing of BU to a target plasma exposure reduces graft rejection in children and improves relapse/toxicity rates in adults. Current weight-based dosing achieves the target BU exposure in only a minority (24.3%) of children. The initial BU dose should be based on the European Medicines Agency nomogram or population pharmacokinetic models to improve the numbers of children achieving the target exposure. There are limited pharmacokinetic data for treosulfan, CY, fludarabine and alemtuzumab as HSCT conditioning in children. For post-grafting immunosuppression, mycophenolic acid (MPA) clearance may be increased in younger children (<12 years). The preferred MPA pharmacokinetic monitoring parameters and target range are still evolving in HSCT recipients. Multi-institutional trials incorporating properly powered pharmacokinetic/dynamic studies are needed to assess the effect of variability in the plasma exposure of drugs/metabolites on clinical outcomes in pediatric HSCT recipients.

A pilot pharmacologic biomarker study in HLA-haploidentical hematopoietic cell transplant recipients

PURPOSE

Eleven patients diagnosed with various hematologic malignancies receiving an HLA-haploidentical hematopoietic cell transplant (HCT) participated in an ancillary biomarker trial. The goal of the trial was to evaluate potential pharmacologic biomarkers pertinent to the conditioning regimen [fludarabine monophosphate (fludarabine) and cyclophosphamide (CY)] or postgrafting immunosuppression [CY and mycophenolate mofetil (MMF)] in these patients.

METHODS

We characterized the interpatient variability of nine pharmacologic biomarkers. The biomarkers evaluated were relevant to fludarabine (i.e., area under the curve (AUC) of 2-fluoro-ara-A or F-ara-A), CY (i.e., AUCs of CY and four of its metabolites), and MMF (i.e., total mycophenolic acid (MPA) AUC, unbound MPA AUC, and inosine monophosphate dehydrogenase (IMPDH) activity).

RESULTS

Interpatient variability in the pharmacologic biomarkers was high. Among those related to HCT conditioning, the interpatient variability ranged from 1.5-fold (CY AUC) to 4.0-fold (AUC of carboxyethylphosphoramide mustard, a metabolite of CY). Among biomarkers evaluated as part of postgrafting immunosuppression, the interpatient variability ranged from 1.7-fold (CY AUC) to 4.9-fold (IMPDH area under the effect curve). There was a moderate correlation (R (2) = 0.441) of within-patient 4-hydroxycyclophosphamide formation clearance.

CONCLUSIONS

Considerable interpatient variability exists in the pharmacokinetic and drug-specific biomarkers potentially relevant to clinical outcomes in HLA-haploidentical HCT recipients. Pharmacodynamic studies are warranted to optimize the conditioning regimen and postgrafting immunosuppression administered to HLA-haploidentical HCT recipients.

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.

Methadone N-demethylation by the common CYP2B6 allelic variant CYP2B6.6

The long-acting opioid methadone displays considerable unexplained interindividual pharmacokinetic variability. Methadone metabolism clinically occurs primarily by N-demethylation to 2-ethyl-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), catalyzed predominantly by CYP2B6. Retrospective studies suggest that the common allele variant CYP2B6*6 may influence methadone plasma concentrations. The catalytic activity of CYP2B6.6, encoded by CYP2B6*6, is highly substrate-dependent. This investigation compared methadone N-demethylation by CYP2B6.6 with that by wild-type CYP2B6.1. Methadone enantiomer and racemate N-demethylation by recombinant-expressed CYP2B6.6 and CYP2B6.1 was determined. At substrate concentrations (0.25-2 µM) approximating plasma concentrations occurring clinically, rates of methadone enantiomer N-demethylation by CYP2B6.6, incubated individually or as the racemate, were one-third to one-fourth those by CYP2B6.1. For methadone individual enantiomers and metabolism by CYP2B6.6 compared with CYP2B6.1, Vmax was diminished, Ks was greater and the in vitro intrinsic clearance was diminished 5- to 6-fold. The intrinsic clearance for R- and S-EDDP formation from racemic methadone was diminished approximately 6-fold and 3-fold for R- and S-methadone, respectively. Both CYP2B6.6 and CYP2B6.1 showed similar stereoselectivity (S>R-methadone). Human liver microsomes with diminished CYP2B6 content due to a CYP2B6*6 allele had lower rates of methadone N-demethylation. Results show that methadone N-demethylation catalyzed by CYP2B6.6, the CYP2B6 variant encoded by the CYP2B6*6 polymorphism, is catalytically deficient compared with wild-type CYP2B6.1. Diminished methadone N-demethylation by CYP2B6.6 may provide a mechanistic explanation for clinical observations of altered methadone disposition in individuals carrying the CYP2B6*6 polymorphism.

Role of pharmacogenetics in busulfan/cyclophosphamide conditioning therapy prior to hematopoietic stem cell transplantation

Hematopoietic stem cell transplantation (HSCT) is a curative treatment for several malignant and nonmalignant disorders. Busulfan (Bu) and cyclophosphamide (Cy) are the most commonly used alkylators in high-dose pretransplant conditioning for HSCT; a treatment that is correlated with drug-related toxicity and relapse. Pharmacogenetic investigations have shown that CYP450, as well as aldehyde dehydrogenase, are clearly involved with Cy metabolism and are associated with altered treatment response, Cy metabolism and the unique stem-cell sparing capacity. Moreover, glutathione-S-transferase isoenzymes have been associated with cellular outward transport of various alkylating agents, including Cy metabolites, melphalan, Bu and chlorambucil. A shift from genetic-based studies to whole-genome-based investigations of Cy- and Bu-associated markers may contribute to personalizing the conditioning therapy and enhancing the clinical outcome of HSCT.