Effects of pregnane X receptor (NR1I2) and CYP2B6 genetic polymorphisms on the induction of bupropion hydroxylation by rifampin

A genetic algorithm-based approach for the prediction of metabolic drug-drug interactions involving CYP2C8 or CYP2B6

BACKGROUND AND OBJECTIVES

A genetic algorithm (GA) approach was developed to predict drug-drug interactions (DDIs) caused by cytochrome P450 2C8 (CYP2C8) inhibition or cytochrome P450 2B6 (CYP2B6) inhibition or induction. Nighty-eight DDIs, obtained from published in vivo studies in healthy volunteers, have been considered using the area under the plasma drug concentration-time curve (AUC) ratios (i.e., ratios of AUC of the drug substrate administered in combination with a DDI perpetrator to AUC of the drug substrate administered alone) to describe the extent of DDI.

METHODS

The following parameters were estimated in this approach: the contribution ratios (CRCYP2B6 and CRCYP2C8, i.e., the fraction of the dose metabolized via CYP2B6 or CYP2C8, respectively) and the inhibitory or inducing potency of the perpetrator drug (IRCYP2B6, IRCYP2C8 and ICCYP2B6, for inhibition of CYP2B6 and CYP2C8, and induction of CYP2B6, respectively). The workflow consisted of three main phases. First, the initial estimates of the parameters were estimated through GA. Then, the model was validated using an external validation. Finally, the parameter values were refined via a Bayesian orthogonal regression using all data.

RESULTS

The AUC ratios of 5 substrates, 11 inhibitors and 19 inducers of CYP2B6, and the AUC ratios of 19 substrates and 23 inhibitors of CYP2C8 were successfully predicted by the developed methodology within 50-200% of observed values.

CONCLUSIONS

The approach proposed in this work may represent a useful tool for evaluating the suitable doses of a CYP2C8 or CYP2B6 substrates co-administered with perpetrators.

Pharmacogenetic Influence on Stereoselective Steady-State Disposition of Bupropion

Bupropion is used for treating depression, obesity, and seasonal affective disorder, and for smoking cessation. Bupropion is commonly prescribed, but has complex pharmacokinetics and interindividual variability in metabolism and bioactivation may influence therapeutic response, tolerability, and safety. Bupropion is extensively and stereoselectively metabolized, the metabolites are pharmacologically active, and allelic variation in cytochrome P450 (CYP) 2B6 affects clinical hydroxylation of single-dose bupropion. Genetic effects on stereoselective disposition of steady-state bupropion are not known. In this preplanned secondary analysis of a prospective, randomized, double-blinded, crossover study which compared brand and generic bupropion XL 300 mg drug products, we measured steady-state enantiomeric plasma and urine parent bupropion and primary and secondary metabolite concentrations. This investigation evaluated the influence of genetic polymorphisms in CYP2B6, CYP2C19, and P450 oxidoreductase on the disposition of Valeant Pharmaceuticals Wellbutrin brand bupropion in 67 participants with major depressive disorder. We found that hydroxylation of both bupropion enantiomers was lower in carriers of the CYP2B6*6 allele and in carriers of the CYP2B6 516G>T variant, with correspondingly greater bupropion and lesser hydroxybupropion plasma concentrations. Hydroxylation was 25-50% lower in CYP2B6*6 carriers and one-third to one-half less in 516T carriers. Hydroxylation of the bupropion enantiomers was comparably affected by CYP2B6 variants. CYP2C19 polymorphisms did not influence bupropion plasma concentrations or hydroxybupropion formation but did influence the minor pathway of 4'-hydroxylation of bupropion and primary metabolites. P450 oxidoreductase variants did not influence bupropion disposition. Results show that CYP2B6 genetic variants affect steady-state metabolism and bioactivation of Valeant brand bupropion, which may influence therapeutic outcomes. SIGNIFICANCE STATEMENT: Bupropion, used for depression, obesity, and smoking cessation, undergoes metabolic bioactivation, with incompletely elucidated interindividual variability. We evaluated cytochrome P450 (CYP) 2B6, CYP2C19 and P450 oxidoreductase genetic variants and steady-state bupropion and metabolite enantiomers disposition. Both enantiomers hydroxylation was lower in CYP2B6*6 and CYP2B6 516G>T carriers, with greater bupropion and lesser hydroxybupropion plasma concentrations. CYP2C19 polymorphisms did not affect bupropion or hydroxybupropion but did influence minor 4'-hydroxylation of bupropion and primary metabolites. CYP2B6 variants affect steady-state bupropion bioactivation, which may influence therapeutic outcomes.

Quantitative Prediction of Drug Interactions Caused by Cytochrome P450 2B6 Inhibition or Induction

BACKGROUND

Numerous drugs have the potential to be affected by cytochrome P450 (CYP) 2B6-mediated drug-drug interactions (DDIs).

OBJECTIVES

In this work, we extend a static approach to the prediction of the extent of pharmacokinetics DDIs between substrates and inhibitors or inducers of CYP2B6.

METHODS

This approach is based on the calculation of two parameters (the contribution ratio [CR], representing the fraction of dose of the substrate metabolized via this pathway and the inhibitory or inducing potency of the perpetrator [IR or IC, respectively]) calculated from the area under the concentration-time curve (AUC) ratios obtained in in-vivo DDI studies.

RESULTS

Forty-eight studies involving 5 substrates, 11 inhibitors and 18 inducers of CYP2B6 (overall 15 inhibition and 33 induction studies) were divided into test and validation sets and considered for estimation of the parameters. The proposed approach demonstrated a fair accuracy for predicting the extent of DDI related to CYP2B6 inhibition and induction, all predictions related to the validation test (N = 18) being 50-200% of the observed ratios.

CONCLUSIONS

This methodology can be used for proposing initial dose adaptations to be adopted, for example in clinical use or for designing DDI studies involving this enzyme.

Constitutive androstane receptor and pregnane X receptor genotype influence efavirenz plasma concentration and CYP2B6 enzyme activity

Efavirenz is metabolized by CYP2B6, an inducible enzyme whose expression is regulated by the constitutive androstane receptor and pregnane X receptor nuclear receptors. CAR and PXR are encoded by genetically polymorphic NR1I2 and NR1I3, respectively. We examined the impact of NR1I2 and NR1I3 genotype on plasma EFV concentration and CYP2B6 enzyme activity among TB-HIV co-infected patients in Ethiopia. Treatment-naïve HIV patients with TB co-infection (n = 80) were enrolled and received first-line EFV-based antiretroviral and rifampicin-based anti-TB therapy. Plasma EFV and 8-hydroxy-EFV concentrations at the 4th and 16th week of EFV treatment were determined using LC/MS/MS. EFV/8-hydroxy-EFVmetabolic ratio was used as CYP2B6 metabolic activity index. In multivariate regression analysis, NR1I3 rs3003596C or NR1I2 rs2472677T variant allele carriers had significantly lower plasma EFV concentrations than non-carriers. Patients with NR1I2 rs3814057C/C genotype or NR1I3 rs3003596C allele carriers had significantly lower mean log EFV MR. Among CYP2B6*6 allele carriers, patients with NR1I3 rs2502815T/T or NR1I2 rs3814057C/C genotype had significantly lower mean log EFV MR. In conclusion, genetic variants in NR1I2 and NR1I3 genes influence plasma EFV exposure and CYP2B6 enzyme activity in TB-HIV co-infected patients on drug treatment.

Association of CYP2B6 genetic polymorphisms with bupropion and hydroxybupropion exposure: A systematic review and meta-analysis

INTRODUCTION

Bupropion is metabolized to its active metabolite, hydroxybupropion (HB), by the genetically polymorphic cytochrome P450 2B6 (CYP2B6) enzyme. Despite its significant role in bupropion metabolism, the magnitude of the impact of CYP2B6 genotype on the exposure of bupropion has not been quantified.

OBJECTIVES

A systematic review and meta-analysis was conducted to quantify the association of bupropion and HB exposure with CYP2B6 variant alleles and genotype-defined metabolizer phenotypes.

METHODS

MEDLINE, EMBASE, Web of Science, Scifinder, PsycINFO, and CENTRAL were screened to identify studies that met the following inclusion criteria (search updated on February 2021): (1) area under the plasma drug concentration-time curve (AUC) of bupropion and/or HB in relation to CYP2B6 genotypes was studied, and (2) study participants were genotyped for common CYP2B6 variant alleles including at least CYP2B6*6. The Newcastle Ottawa Scale was used to assess risk of bias in each included study. The ratio of means (RoM) between CYP2B6 genotype or genotype-defined phenotype groups for bupropion exposure was calculated for each study and combined in a meta-analysis.

RESULTS

Eleven studies met the inclusion criteria for this systematic review, and 10 (including N = 413 participants) were included in the meta-analysis. All 10 studies involved healthy adult volunteers, where other medications were not allowed. The AUCs of HB and the active moiety (bupropion + HB) were significantly reduced in CYP2B6*6 carriers compared with the non-carriers (HB: RoM 0.77, 95% CI 0.71-0.83; active moiety: RoM 0.81, 95% CI 0.75-0.88). Both CYP2B6 poor and intermediate metabolizers had significantly decreased exposures to HB and the active moiety than normal metabolizers.

CONCLUSION

The CYP2B6*6 allele and genotype-determined CYP2B6 poor and intermediate metabolizer phenotypes are associated with significantly lower exposures to HB and the total active moiety. The findings of this study suggest opportunities to further study precision dosing strategies for bupropion therapy based on CYP2B6 genotype.

PharmVar GeneFocus: CYP2B6

The Pharmacogene Variation Consortium (PharmVar) catalogs star (*) allele nomenclature for the polymorphic human CYP2B6 gene. Genetic variation within the CYP2B6 gene locus impacts the metabolism or bioactivation of clinically important drugs. Of particular importance are efficacy and safety concerns regarding: efavirenz, which is used for the treatment of HIV type-1 infection; methadone, a mainstay in the treatment of opioid use disorder and as an analgesic; ketamine, used as an antidepressant and analgesic; and bupropion, which is prescribed to treat depression and for smoking cessation. This GeneFocus provides a comprehensive overview and summary of CYP2B6 and describes how haplotype information catalogued by PharmVar is utilized by the Pharmacogenomics Knowledgebase (PharmGKB) and the Clinical Pharmacogenetics Implementation Consortium (CPIC).

Physiologically Based Pharmacokinetic Modeling of Bupropion and Its Metabolites in a CYP2B6 Drug-Drug-Gene Interaction Network

The noradrenaline and dopamine reuptake inhibitor bupropion is metabolized by CYP2B6 and recommended by the FDA as the only sensitive substrate for clinical CYP2B6 drug-drug interaction (DDI) studies. The aim of this study was to build a whole-body physiologically based pharmacokinetic (PBPK) model of bupropion including its DDI-relevant metabolites, and to qualify the model using clinical drug-gene interaction (DGI) and DDI data. The model was built in PK-Sim® applying clinical data of 67 studies. It incorporates CYP2B6-mediated hydroxylation of bupropion, metabolism via CYP2C19 and 11β-HSD, as well as binding to pharmacological targets. The impact of CYP2B6 polymorphisms is described for normal, poor, intermediate, and rapid metabolizers, with various allele combinations of the genetic variants CYP2B6*1, *4, *5 and *6. DDI model performance was evaluated by prediction of clinical studies with rifampicin (CYP2B6 and CYP2C19 inducer), fluvoxamine (CYP2C19 inhibitor) and voriconazole (CYP2B6 and CYP2C19 inhibitor). Model performance quantification showed 20/20 DGI ratios of hydroxybupropion to bupropion AUC ratios (DGI AUCHBup/Bup ratios), 12/13 DDI AUCHBup/Bup ratios, and 7/7 DDGI AUCHBup/Bup ratios within 2-fold of observed values. The developed model is freely available in the Open Systems Pharmacology model repository.

Sources of Interindividual Variability

The efficacy, safety, and tolerability of drugs are dependent on numerous factors that influence their disposition. A dose that is efficacious and safe for one individual may result in sub-therapeutic or toxic blood concentrations in others. A significant source of this variability in drug response is drug metabolism, where differences in presystemic and systemic biotransformation efficiency result in variable degrees of systemic exposure (e.g., AUC, C_max, and/or C_min) following administration of a fixed dose.Interindividual differences in drug biotransformation have been studied extensively. It is recognized that both intrinsic factors (e.g., genetics, age, sex, and disease states) and extrinsic factors (e.g., diet , chemical exposures from the environment, and the microbiome) play a significant role. For drug-metabolizing enzymes, genetic variation can result in the complete absence or enhanced expression of a functional enzyme. In addition, upregulation and downregulation of gene expression, in response to an altered cellular environment, can achieve the same range of metabolic function (phenotype), but often in a less predictable and time-dependent manner. Understanding the mechanistic basis for variability in drug disposition and response is essential if we are to move beyond the era of empirical, trial-and-error dose selection and into an age of personalized medicine that will improve outcomes in maintaining health and treating disease.

Inhibition and induction of CYP enzymes in humans: an update

The cytochrome P450 (CYP) enzyme family is the most important enzyme system catalyzing the phase 1 metabolism of pharmaceuticals and other xenobiotics such as herbal remedies and toxic compounds in the environment. The inhibition and induction of CYPs are major mechanisms causing pharmacokinetic drug–drug interactions. This review presents a comprehensive update on the inhibitors and inducers of the specific CYP enzymes in humans. The focus is on the more recent human in vitro and in vivo findings since the publication of our previous review on this topic in 2008. In addition to the general presentation of inhibitory drugs and inducers of human CYP enzymes by drugs, herbal remedies, and toxic compounds, an in-depth view on tyrosine-kinase inhibitors and antiretroviral HIV medications as victims and perpetrators of drug–drug interactions is provided as examples of the current trends in the field. Also, a concise overview of the mechanisms of CYP induction is presented to aid the understanding of the induction phenomena.

Hepatocytic transcriptional signatures predict comparative drug interaction potential of rifamycin antibiotics

Current strategies to treat tuberculosis (TB) and co-morbidities involve multidrug combination therapies. Rifamycin antibiotics are a key component of TB therapy and a common source of drug–drug interactions (DDIs) due to induction of drug metabolizing enzymes (DMEs). Management of rifamycin DDIs are complex, particularly in patients with co-morbidities, and differences in DDI potential between rifamycin antibiotics are not well established. DME profiles induced in response to tuberculosis antibiotics (rifampin, rifabutin and rifapentine) were compared in primary human hepatocytes. We identified rifamycin induced DMEs, cytochrome P450 (CYP) 2C8/3A4/3A5, SULT2A, and UGT1A4/1A5 and predicted lower DDIs of rifapentine with 58 clinical drugs used to treat co-morbidities in TB patients. Transcriptional networks and upstream regulator analyses showed FOXA3, HNF4α, NR1I2, NR1I3, NR3C1 and RXRα as key transcriptional regulators of rifamycin induced DMEs. Our study findings are an important resource to design effective medication regimens to treat common co-conditions in TB patients.

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.

Association of POR and PPARα polymorphisms with risk of anti-tuberculosis drug-induced liver injury in Western Chinese Han population

OBJECTIVES

Anti-tuberculosis drug-induced liver injury (ATDILI) is a common and sometimes severe adverse drug reaction (ADR). This study was conducted to investigate the relationship between polymorphisms of two genes, cytochrome P450 oxidoreductase (POR) and peroxisome proliferator-activated receptor α (PPARα), and the risk of ATDILI in Western Chinese Han population.

METHODS

A total of 118 tuberculosis (TB) patients with ATDILI and 628 TB patients without ATDILI during anti-TB treatment were recruited from West China Hospital of Sichuan University. DNA was extracted from peripheral blood, and genotypes of the selected 12 single nucleotide polymorphisms (SNPs) (3 SNPs in the POR gene and 9 SNPs in the PPARα gene) were determined. Three genetic models (additive, dominant, and recessive), as well as a haplotype, were used to test the genetic risk of ATDILI. Extended subgroup analysis was conducted according to age, sex and different causality assessments.

RESULTS

The mutant allele, genotype and genetic model of rs3898649 in the POR gene were found to be associated with increased risk of ATDILI, especially in the younger (<50 years old), female and pulmonary tuberculosis subgroup. The other two SNPs rs28737229 and rs4728533 in the POR gene showed only a potential association with susceptibility to ATDILI after Bonferroni correction (P < .05 but P_Bonferroni > .05). The other 9 SNPs loci (rs135549, rs9626730, rs4253712, rs4823613, rs4253730, rs6007662, rs4253728, rs2024929 and rs135561) in the PPARα gene showed no significant differences between ATDILI and non-ATDILI in either allele frequencies or genotype (all P >.05).

CONCLUSIONS

The results demonstrated the strong correlation between POR gene SNP rs3898649 and ATDILI susceptibility, suggesting the importance of POR rs3898649 in the pathogenesis and development of ATDILI. Therefore, our results indicated that POR rs3898649 might be a valuable biomarker potentially involved in ATDILI.

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.

Donor Genotype and Intragraft Expression of CYP3A5 Reflect the Response to Steroid Treatment During Acute Renal Allograft Rejection

BACKGROUND

Glucocorticoid (GC)-refractory acute rejection (AR) is a risk factor for inferior renal allograft outcome. We investigated genetic predisposition to the response to steroid treatment of acute allograft rejection.

METHODS

Single nucleotide polymorphisms of genes involved in GC signaling (GR, GLCCI1) and drug metabolism and transport (CYP3A5, ABCB1, and PXR) were analyzed in kidney transplant recipients (1995-2005, Leiden cohort, n = 153) treated with methylprednisolone. Significant associations were verified in a second cohort (Berlin cohort, n = 66).

RESULTS

Patients who received a CYP3A5*1 allele expressing allograft had a lower risk of resistance to methylprednisolone during AR (odds ratio, 0.29; 95% confidence interval, 0.11-0.79; P = 0.016 in combined cohorts analysis). No differences were observed for GC signaling or other drug metabolism/transport-related genes. Both before transplantation (n = 69) and at time of AR (n = 88), tissue CYP3A5 mRNA expression was significantly higher in CYP3A5*1 allele expressing donor kidneys than in CYP3A5*3/*3 allografts (P < 0.00001). Moreover, steroid-responsive patients (n = 64) expressed significantly higher intragraft CYP3A5 mRNA levels compared to steroid-refractory patients (n = 42) in AR (P = 0.006).

CONCLUSIONS

CYP3A5 protein expression was detected in tubular epithelial cells and inflammatory cells within the grafts. Our findings show that steroid resistance during AR is associated with donor genotype and intragraft expression levels of CYP3A5.

Association of CYP2B6 gene polymorphisms and anti-tuberculosis drug-induced hepatotoxicity in a Chinese population

OBJECTIVES

Antituberculosis drug-induced hepatotoxicity (ATDH) remains a common and severe challenge in tuberculosis (TB) chemotherapy. A growing number of studies have revealed that genetic polymorphisms affect an individual's susceptibility to ATDH. The aim of this study was to explore the role of cytochrome P450 family 2 subfamily B member 6 (CYP2B6) gene polymorphisms in the development of ATDH in Chinese TB patients.

METHODS

CYP2B6*6 genotypes were determined in TB patients with and without ATDH. Association between polymorphisms and risk of ATDH was estimated by multiple logistic regression analysis.

RESULTS

A total of 343 eligible TB patients (166 with ATDH; 177 without ATDH) were included in this study. Analysis of all subjects revealed no statistical differences in genotype distribution between the two groups. However, the CYP2B6 *6/*6 genotype was significantly associated with decreased risk of ATDH in the male subgroup (P=0.039, OR=0.097, 95% CI: 0.011-0.885). Furthermore, in male patients, the presence of the CYP2B6*6 allele was significantly higher in the non-ATDH group compared with the ATDH group (26.2% vs. 15.5%, P=0.020, OR=0.522, 95% CI: 0.301-0.903).

CONCLUSIONS

This study is the first to demonstrate an association between CYP2B6 polymorphisms and the risk of ATDH in the Chinese population. We have shown that males who have the CYP2B6 *6/*6 genotype may be less susceptible to the development of ATDH. Further studies are required to confirm this genetic association result.

Pharmacokinetic Drug Interactions with Tobacco, Cannabinoids and Smoking Cessation Products

Tobacco smoke contains a large number of compounds in the form of metals, volatile gases and insoluble particles, as well as nicotine, a highly addictive alkaloid. Marijuana is the most widely used illicit drug of abuse in the world, with a significant increase in the USA due to the increasing number of states that allow medical and recreational use. Of the over 70 phytocannabinoids in marijuana, Δ9-tetrahydrocannabinol (Δ9THC), cannabidiol (CBD) and cannibinol are the three main constituents. Both marijuana and tobacco smoking induce cytochrome P450 (CYP) 1A2 through activation of the aromatic hydrocarbon receptor, and the induction effect between the two products is additive. Smoking cessation is associated with rapid downregulation of CYP1A enzymes. On the basis of the estimated half-life of CYP1A2, dose reduction of CYP1A drugs may be necessary as early as the first few days after smoking cessation to prevent toxicity, especially for drugs with a narrow therapeutic index. Nicotine is a substrate of CYP2A6, which is induced by oestrogen, resulting in lower concentrations of nicotine in females than in males, especially in females taking oral contraceptives. The significant effects of CYP3A4 inducers and inhibitors on the pharmacokinetics of Δ9THC/CBD oromucosal spray suggest that CYP3A4 is the primary enzyme responsible for the metabolism of Δ9THC and CBD. Limited data also suggest that CBD may significantly inhibit CYP2C19. With the increasing use of marijuana and cannabis products, clinical studies are needed in order to determine the effects of other drugs on pharmacokinetics and pharmacodynamics.

Evaluation of CYP2B6 Induction and Prediction of Clinical Drug-Drug Interactions: Considerations from the IQ Consortium Induction Working Group-An Industry Perspective

Drug-drug interactions (DDIs) due to CYP2B6 induction have recently gained prominence and clinical induction risk assessment is recommended by regulatory agencies. This work aimed to evaluate the potency of CYP2B6 versus CYP3A4 induction in vitro and from clinical studies and to assess the predictability of efavirenz versus bupropion as clinical probe substrates of CYP2B6 induction. The analysis indicates that the magnitude of CYP3A4 induction was higher than CYP2B6 both in vitro and in vivo. The magnitude of DDIs caused by induction could not be predicted for bupropion with static or dynamic models. On the other hand, the relative induction score, net effect, and physiologically based pharmacokinetics SimCYP models using efavirenz resulted in improved DDI predictions. Although bupropion and efavirenz have been used and are recommended by regulatory agencies as clinical CYP2B6 probe substrates for DDI studies, CYP3A4 contributes to the metabolism of both probes and is induced by all reference CYP2B6 inducers. Therefore, caution must be taken when interpreting clinical induction results because of the lack of selectivity of these probes. Although in vitro-in vivo extrapolation for efavirenz performed better than bupropion, interpretation of the clinical change in exposure is confounded by the coinduction of CYP2B6 and CYP3A4, as well as the increased contribution of CYP3A4 to efavirenz metabolism under induced conditions. Current methods and probe substrates preclude accurate prediction of CYP2B6 induction. Identification of a sensitive and selective clinical substrate for CYP2B6 (fraction metabolized > 0.9) is needed to improve in vitro-in vivo extrapolation for characterizing the potential for CYP2B6-mediated DDIs. Alternative strategies and a framework for evaluating the CYP2B6 induction risk are proposed.

Methadone Pharmacogenetics: CYP2B6 Polymorphisms Determine Plasma Concentrations, Clearance, and Metabolism

BACKGROUND

Interindividual variability in methadone disposition remains unexplained, and methadone accidental overdose in pain therapy is a significant public health problem. Cytochrome P4502B6 (CYP2B6) is the principle determinant of clinical methadone elimination. The CYP2B6 gene is highly polymorphic, with several variant alleles. CYP2B6.6, the protein encoded by the CYP2B6*6 polymorphism, deficiently catalyzes methadone metabolism in vitro. This investigation determined the influence of CYP2B6*6, and other allelic variants encountered, on methadone concentrations, clearance, and metabolism.

METHODS

Healthy volunteers in genotype cohorts CYP2B6*1/*1 (n = 21), CYP2B6*1/*6 (n = 20), and CYP2B6*6/*6 (n = 17), and also CYP2B6*1/*4 (n = 1), CYP2B6*4/*6 (n = 3), and CYP2B6*5/*5 (n = 2) subjects, received single doses of IV and oral methadone. Plasma and urine methadone and metabolite concentrations were determined by tandem mass spectrometry.

RESULTS

Average S-methadone apparent oral clearance was 35 and 45% lower in CYP2B6*1/*6 and CYP2B6*6/*6 genotypes, respectively, compared with CYP2B6*1/*1. R-methadone apparent oral clearance was 25 and 35% lower in CYP2B6*1/*6 and CYP2B6*6/*6 genotypes, respectively, compared with CYP2B6*1/*1. R- and S-methadone apparent oral clearance was threefold and fourfold greater in CYP2B6*4 carriers. IV and oral R- and S-methadone metabolism was significantly lower in CYP2B6*6 carriers compared with that of CYP2B6*1 homozygotes and greater in CYP2B6*4 carriers. Methadone metabolism and clearance were lower in African Americans in part because of the CYP2B6*6 genetic polymorphism.

CONCLUSIONS

CYP2B6 polymorphisms influence methadone plasma concentrations, because of altered methadone metabolism and thus clearance. Genetic influence is greater for oral than IV methadone and S- than R-methadone. CYP2B6 pharmacogenetics explains, in part, interindividual variability in methadone elimination. CYP2B6 genetic effects on methadone metabolism and clearance may identify subjects at risk for methadone toxicity and drug interactions.

Sources of interindividual variability

The efficacy, safety, and tolerability of drugs are dependent on numerous factors that influence their disposition. A dose that is efficacious and safe for one individual may result in sub-therapeutic or toxic blood concentrations in other individuals. A major source of this variability in drug response is drug metabolism, where differences in pre-systemic and systemic biotransformation efficiency result in variable degrees of systemic exposure (e.g., AUC, C max, and/or C min) following administration of a fixed dose.Interindividual differences in drug biotransformation have been studied extensively. It is well recognized that both intrinsic (such as genetics, age, sex, and disease states) and extrinsic (such as diet, chemical exposures from the environment, and even sunlight) factors play a significant role. For the family of cytochrome P450 enzymes, the most critical of the drug metabolizing enzymes, genetic variation can result in the complete absence or enhanced expression of a functional enzyme. In addition, up- and down-regulation of gene expression, in response to an altered cellular environment, can achieve the same range of metabolic function (phenotype), but often in a less reliably predictable and time-dependent manner. Understanding the mechanistic basis for drug disposition and response variability is essential if we are to move beyond the era of empirical, trial-and-error dose selection and into an age of personalized medicine that brings with it true improvements in health outcomes in the therapeutic treatment of disease.

Modeling of rifampicin-induced CYP3A4 activation dynamics for the prediction of clinical drug-drug interactions from in vitro data

Induction of cytochrome P450 3A4 (CYP3A4) expression is often implicated in clinically relevant drug-drug interactions (DDI), as metabolism catalyzed by this enzyme is the dominant route of elimination for many drugs. Although several DDI models have been proposed, none have comprehensively considered the effects of enzyme transcription/translation dynamics on induction-based DDI. Rifampicin is a well-known CYP3A4 inducer, and is commonly used as a positive control for evaluating the CYP3A4 induction potential of test compounds. Herein, we report the compilation of in vitro induction data for CYP3A4 by rifampicin in human hepatocytes, and the transcription/translation model developed for this enzyme using an extended least squares method that can account for inherent inter-individual variability. We also developed physiologically based pharmacokinetic (PBPK) models for the CYP3A4 inducer and CYP3A4 substrates. Finally, we demonstrated that rifampicin-induced DDI can be predicted with reasonable accuracy, and that a static model can be used to simulate DDI once the blood concentration of the inducer reaches a steady state following repeated dosing. This dynamic PBPK-based DDI model was implemented on a new multi-hierarchical physiology simulation platform named PhysioDesigner.

Genetic Variants of Pregnane X Receptor (PXR) and CYP2B6 Affect the Induction of Bupropion Hydroxylation by Sodium Ferulate

This study investigated the effects of pregnane X receptor (PXR/NR1I2) and CYP2B6 genetic variants on sodium ferulate (SF)-mediated induction of bupropion hydroxylation. The pharmacokinetics of bupropion and hydroxybupropion were evaluated after an oral dose of bupropion (150 mg) administered with and without SF pretreatment for 14 days in 33 healthy subjects. The area under the time-concentration curve (AUC) ratio of AUC_hyd (AUC(0-∞) of hydroxybupropion)/AUC_bup (AUC(0-∞) of bupropion) represents the CYP2B6 hydroxylation activity, which was significantly lower in CYP2B6*6 carriers (NR1I2 TGT noncarriers or carriers) than in noncarriers in both the basal and SF-induced states (p-value<0.05). AUC ratio and AUC_hyd of NR1I2 -24113AA variant were markedly lower than GA and GG genotypes (7.5±2.1 versus 14.5±3.3 and 20.6±1.1, and 8873±1431 versus 14,504±2218 and 17,586±1046) in the induced states. However, -24020(-)/(-) variant didn't show significant difference in the induction of CYP2B6 hydroxylation activity by SF compared with other -24020[GAGAAG]/(-) genotypes. NR1I2 TGT haplotype (-25385T+g.7635G+g.8055T) carriers exhibited a significantly decreased AUC ratio, compared with TGT noncarriers, in the basal states (7.6±1.0 versus 9.7±1.0), while this result wasn't observed in CYP2B6*6 noncarriers. Moreover, individuals with complete mutation-type [CYP2B6*6/*6+NR1I2 TGT+ -24113AA+ -24020 (-)/(-)] showed even lower percent difference of AUC ratio (8.7±1.2 versus 39.5±8.2) than those with complete wild-type. In conclusion, it is suggested that NR1I2 variants decrease the bupropion hydroxylation induced by SF treatment, particularly in CYP2B6*6 carriers.

TRIAL REGISTRATION

ChiCTR.org ChiCTR-TRC-11001285.

Rapid clinical induction of bupropion hydroxylation by metamizole in healthy Chinese men

AIMS

This study aimed to investigate the effect of metamizole on bupropion hydroxylation related to different CYP2B6 genotype groups in healthy volunteers.

METHODS

Sixteen healthy male volunteers (6 CYP2B6*1/*1, 6 CYP2B6*1/*6 and 4 CYP2B6*6/*6) received orally administered bupropion alone and during daily treatment with metamizole 1500 mg day(-1) (500 mg tablet taken three times daily) for 4 days. Serial blood samples were obtained up to 48 h after each bupropion dose.

RESULTS

After metamizole treatment relative to bupropion alone, the geometric mean ratios (GMRs) and 90% confidence interval (CI) of the AUC(0,∞) ratio of 4-hydroxybupropion over bupropion were 1.99 (1.57, 2.55) for the CYP2B6*1/*1 group, 2.15 (1.53, 3.05) for the CYP2B6*1/*6 group and 1.86 (1.36, 2.57) for the CYP2B6*6/*6 group. The GMRs and 90% CI of bupropion were 0.695 (0.622, 0.774) for AUC(0,∞) and 0.400 (0.353, 0.449) for C(max) , respectively. The corresponding values for 4-hydroxybupropion were 1.43 (1.28, 1.53) and 2.63 (2.07, 2.92). The t(1/2) value was significantly increased for bupropion and decreased for 4-hydroxybupropion. The t(max) values of bupropion and 4-hydroxybupropion were both significantly decreased. The mean percentage changes in pharmacokinetic parameters among the CYP2B6 genotype groups were not significantly different.

CONCLUSIONS

Oral administration of metamizole for 4 days significantly altered the pharmacokinetics of both bupropion and its active metabolite, 4-hydroxybupropion, and significantly increased the CYP2B6-catalyzed bupropion hydroxylation in all of the subjects. Cautions should be taken when metamizole is co-administered with CYP2B6 substrate drugs.

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.

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

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

Prevalence of poor and rapid metabolizers of drugs metabolized by CYP2B6 in North Indian population residing in Indian national capital territory

Identification of poor and rapid metabolizers for the category of drugs metabolized by cytochrome P450 2B6 (CYP2B6) is important for understanding the differences in clinical responses of drugs metabolized by this enzyme. This study reports the prevalence of poor and rapid metabolizers in North Indian population residing in the National Capital Territory. The prevalence of poor and rapid metabolizers was determined in the target population for the category of drugs metabolized by CYP2B6 by measuring plasma bupropion, a drug metabolized by CYP2B6, and its metabolite. Bupropion (75 mg) was administered to 107 volunteers, and the drug (bupropion) and its metabolite (hydroxybupropion) were determined simultaneously by LCMS/MS in the plasma. CYP2B6 activity was measured as hydroxybupropion/bupropion ratio, and volunteers were categorized as rapid or poor metabolizers on the basis of cutoff value of log (hydroxybupropion/bupropion). Significant differences were observed between the mean metabolite/drug ratio of rapid metabolizers (Mean = 0.59) and poor metabolizers (Mean = 0.26) with p<0.0001. Results indicate that 20.56% individuals in the target population were poor metabolizers for the category of drugs metabolized by CYP2B6. Cutoff value defined in this study can be used as a tool for evaluating the status of CYP2B6 using bupropion as a probe drug. The baseline information would be clinically useful before administering the drugs metabolized by this isoform.