Selective inhibition of CYP2B6-catalyzed bupropion hydroxylation in human liver microsomes in vitro

Evaluation of reversible cytochrome P450 inhibition by Withania somnifera leaf and root extracts

It is important to understand the potential of botanical-drug interactions to ensure the safe use of botanical dietary supplements (BDS). Cytochrome P450 (P450) is one of the most abundant phase 1 drug-metabolizing enzymes and is accountable for a great deal of pharmacokinetic botanical-drug interactions. This problem is particularly acute for older adults who often consume BDS with multiple prescription medicines. The consequences of botanical-drug interactions can lead to lack of prodrug efficacy or drug toxicity from reduced drug clearance through inhibition of P450 metabolizing enzymes. In this study, a 7-in-1 cocktail P450 inhibition assay with 7 Food and Drug Administration-recommended P450s (CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2D6, and CYP3A4/5) including CYP2B6 recombinant enzyme was performed, minimizing substrate interactions with respect to specificity while maximizing assay sensitivity. High-performance liquid chromatography-mass spectrometry was used for quantitative determination of probe substrate metabolism. Withania somnifera L. Dunal (ashwagandha), a popular BDS in the United States with sales of ∼$16 million in 2021, is used to promote sleep and relieve stress and anxiety, especially in older adults. However, comprehensive studies of pharmacokinetic drug interactions with ashwagandha, especially with leaf extracts, have not been reported. Four extracts from ashwagandha root or leaf were evaluated for P450 inhibition, and no reversible inhibition was detected at IC50 > 100 μg/mL extract. SIGNIFICANCE STATEMENT: Ashwagandha is often consumed by older adults, who also often use multiple prescribed medications concomitantly. Polypharmacy, combined with age-related decline of drug metabolism and other changes in drug disposition in this population, increases the risk of adverse events due to botanical inhibition of drug metabolism, indicating the significance of evaluating ashwagandha for potential pharmacokinetic drug interactions. This study will support our understanding for the safe use of ashwagandha.

A Review of CYP-Mediated Drug Interactions: Mechanisms and In Vitro Drug-Drug Interaction Assessment

Drug metabolism is a major determinant of drug concentrations in the body. Drug-drug interactions (DDIs) caused by the co-administration of multiple drugs can lead to alteration in the exposure of the victim drug, raising safety or effectiveness concerns. Assessment of the DDI potential starts with in vitro experiments to determine kinetic parameters and identify risks associated with the use of comedication that can inform future clinical studies. The diverse range of experimental models and techniques has significantly contributed to the examination of potential DDIs. Cytochrome P450 (CYP) enzymes are responsible for the biotransformation of many drugs on the market, making them frequently implicated in drug metabolism and DDIs. Consequently, there has been a growing focus on the assessment of DDI risk for CYPs. This review article provides mechanistic insights underlying CYP inhibition/induction and an overview of the in vitro assessment of CYP-mediated DDIs.

Pharmacokinetic-Interactions of BI 425809, a Novel Glycine Transporter 1 Inhibitor, With Cytochrome P450 and P-Glycoprotein Substrates: Findings From In Vitro Analyses and an Open-Label, Single-Sequence Phase I Study

PURPOSE/BACKGROUND

Glycine transporter-1 inhibitors may ameliorate cognitive deficits in schizophrenia. This study evaluated potential drug-drug interactions with the glycine transporter-1 inhibitor BI 425809.

METHODS/PROCEDURES

Interactions with cytochromes P450 (CYP) and P-glycoprotein (P-gp) were assessed in in vitro assays using human hepatocytes and Caco-2 cells, respectively. Pharmacokinetic characteristics of probe drugs were subsequently assessed in a Phase I, open-label, single-sequence crossover study in healthy male participants. Participants received a probe-drug cocktail containing midazolam (CYP3A4), warfarin (CYP2C9), and omeprazole (CYP2C19) and a separate dose of digoxin (P-gp), alone and on a background of steady-state BI 425809 25 mg once daily in 2 treatment periods. Adverse events were monitored.

FINDINGS/RESULTS

In vitro assays revealed concentration-dependent induction of CYP3A4 and inhibition of P-gp by BI 425809. In the clinical study, 12 of 13 participants completed both periods. With BI 425809, area under the plasma concentration curve from administration to the last measurement (AUC 0-tz ) and maximum plasma concentration ( Cmax ) for midazolam were lower than when administered alone. Adjusted geometric mean ratios (90% confidence interval) were 70.6% (63.9%-78.1%) for AUC 0-tz and 77.6% (67.3%-89.4%) for Cmax . For warfarin and digoxin, AUC 0-tz and Cmax were similar with and without BI 425809. For omeprazole, BI 425809 slightly reduced AUC 0-tz but not Cmax versus omeprazole alone. No new safety signals were identified.

IMPLICATIONS/CONCLUSIONS

These findings indicate induction of CYP3A4 by once-daily BI 425809 25 mg (the assumed highest therapeutic dose) and no meaningful effects on CYP2C9, CYP2C19, or P-gp in vivo.

Comprehensive Predictions of Cytochrome P450 (P450)-Mediated In Vivo Cannabinoid-Drug Interactions Based on Reversible and Time-Dependent P450 Inhibition in Human Liver Microsomes

We previously reported the unbound reversible (IC50,u) and time-dependent (KI,u) inhibition potencies of cannabidiol (CBD), delta-9-tetrahydrocannabinol (THC), and THC metabolites 11-hydroxy THC (11-OH THC) and 11-nor-9-carboxy-delta-9-THC (11-COOH THC) against the major cytochrome P450 (P450) enzymes (1A2, 2C9, 2C19, 2D6, and 3A). Here, using human liver microsomes, we determined the CYP2A6, 2B6, and 2C8 IC50,u values of the aforementioned cannabinoids and the IC50,u and KI,u of the circulating CBD metabolites 7-hydroxy CBD (7-OH CBD) and 7-carboxy CBD (7-COOH CBD), against all the P450s listed above. The IC50,u of CBD, 7-OH CBD, THC, and 11-OH THC against CYP2B6 was 0.05, 0.34, 0.40, and 0.32 μM, respectively, and against CYP2C8 was 0.28, 1.02, 0.67, and 3.66 μM, respectively. 7-COOH CBD, but not 11-COOH THC, was a weak inhibitor of CYP2B6 and 2C8. All tested cannabinoids except 11-COOH THC were weak inhibitors of CYP2A6. 7-OH CBD inhibited all P450s examined (IC50,u<2.5 μM) except CYP1A2 and inactivated CYP2C19 and CYP3A, with inactivation efficiencies (kinact/KI,u) of 0.10 and 0.14 minutes-1 μM-1, respectively. Using several different static models, we predicted the following maximum pharmacokinetic interactions (affected P450 probe drug and area under the plasma concentration-time curve ratio) between oral CBD (700 mg) and drugs predominantly metabolized by CYP3A (midazolam, 14.8) > 2C9 (diclofenac, 9.6) > 2C19 (omeprazole, 7.3) > 1A2 (theophylline, 4.0) > 2B6 (ticlopidine, 2.2) > 2D6 (dextromethorphan, 2.1) > 2C8 (repaglinide, 1.6). Oral (130 mg) or inhaled (75 mg) THC was predicted to precipitate interactions with drugs predominately metabolized by CYP2C9 (diclofenac, 6.6 or 2.3, respectively) > 3A (midazolam, 1.8) > 1A2 (theophylline, 1.4). In vivo drug interaction studies are warranted to verify these predictions. SIGNIFICANCE STATEMENT: This study, combined with our previous findings, provides for the first time a comprehensive analysis of the potential for cannabidiol, delta-9-tetrahydrocannabinol, and their metabolites to inhibit cytochrome P450 enzymes in a reversible or time-dependent manner. These analyses enabled us to predict the potential of these cannabinoids to produce drug interactions in vivo at clinical or recreational doses.

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.

Highly productive and scalable approach to synthesize ticlopidine: A potent thienopyridine anti-platelet aggregation drug

Ticlopidine (trade name Ticlid), an acidic thienopyridine derivative, is an effective, well-known and long-acting inhibitor of platelet aggregation. Because of its potent inhibitory activity for treating a variety of diseases, the development of efficient approaches for accessing ticlopidine represents an important endeavour. Therefore, in this research work, we developed a promising novel five-step synthetic approach for synthesizing ticlopidine. This method provides ticlopidine in 60% overall yield from readily available starting material viz. thiophene. In this methodology, all steps afforded excellent yields and are operationally simple and environmentally acceptable. This approach also offers various attractive advantages, for example, it's applicable for large-scale synthesis, has simple work-up procedures and short reaction times, and uses inexpensive and readily available reagents. Furthermore, 4,5,6,7-tetrahydrothieno[3,2-c]pyridine is a key precursor for the synthesis of numerous bioactive compounds such as prasugrel and clopidogrel. This protocol provides 4,5,6,7-tetrahydrothieno[3,2-c]pyridine in 62% overall yield via a 4-step synthetic approach.

CYP enzymes, expressed within live human suspension cells, are superior to widely-used microsomal enzymes in identifying potent CYP1A1/CYP1B1 inhibitors: Identification of quinazolinones as CYP1A1/CYP1B1 inhibitors that efficiently reverse B[a]P toxicity and cisplatin resistance

Microsomal cytochrome P450 (CYP) enzymes, isolated from recombinant bacterial/insect/yeast cells, are extensively used for drug metabolism studies. However, they may not always portray how a developmental drug would behave in human cells with intact intracellular transport mechanisms. This study emphasizes the usefulness of human HEK293 kidney cells, grown in 'suspension' for expression of CYPs, in finding potent CYP1A1/CYP1B1 inhibitors, as possible anticancer agents. With live cell-based assays, quinazolinones 9i/9b were found to be selective CYP1A1/CYP1B1 inhibitors with IC₅₀ values of 30/21 nM, and > 150-fold selectivity over CYP2/3 enzymes, whereas they were far less active using commercially-available CYP1A1/CYP1B1 microsomal enzymes (IC₅₀, >10/1.3-1.7 μM). Compound 9i prevented CYP1A1-mediated benzo[a]pyrene-toxicity in normal fibroblasts whereas 9b completely reversed cisplatin resistance in PC-3/prostate, COR-L23/lung, MIAPaCa-2/pancreatic and LS174T/colon cancer cells, underlining the human-cell-assays' potential. Our results indicate that the most potent CYP1A1/CYP1B1 inhibitors would not have been identified if one had relied merely on microsomal enzymes.

Insights into CYP2B6-mediated drug-drug interactions

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

High-Throughput Cytochrome P450 Cocktail Inhibition Assay for Assessing Drug-Drug and Drug-Botanical Interactions

Detection of drug-drug interactions is essential during the early stages of drug discovery and development, and the understanding of drug-botanical interactions is important for the safe use of botanical dietary supplements. Among the different forms of drug interactions that are known, inhibition of cytochrome P450 (P450) enzymes is the most common cause of drug-drug or drug-botanical interactions. Therefore, a rapid and comprehensive mass spectrometry-based in vitro high-throughput P450 cocktail inhibition assay was developed that uses 10 substrates simultaneously against nine CYP isoforms. Including probe substrates for CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and two probes targeting different binding sites of CYP3A4/5, this cocktail simultaneously assesses at least as many P450 enzymes as previous assays while remaining among the fastest due to short incubation times and rapid analysis using ultrahigh pressure liquid chromatography-tandem mass spectrometry. The method was validated using known inhibitors of each P450 enzyme and then shown to be useful not only for single-compound testing but also for the evaluation of potential drug-botanical interactions using the botanical dietary supplement licorice (Glycyrrhiza glabra) as an example.

Applicability of second-generation upcyte® human hepatocytes for use in CYP inhibition and induction studies

Human upcyte® hepatocytes are proliferating hepatocytes that retain many characteristics of primary human hepatocytes. We conducted a comprehensive evaluation of the application of second-generation upcyte® hepatocytes from four donors for inhibition and induction assays using a selection of reference inhibitors and inducers. CYP1A2, CYP2B6, CYP2C9, and CYP3A4 were reproducibly inhibited in a concentration-dependent manner and the calculated IC50 values for each compound correctly classified them as potent inhibitors. Upcyte® hepatocytes were responsive to prototypical CYP1A2, CYP2B6, CYP2C9, and CYP3A4 inducers, confirming that they have functional AhR-, CAR-, and PXR-mediated CYP regulation. A panel of 11 inducers classified as potent, moderate or noninducers of CYP3A4 and CYP2B6 were tested. There was a good fit of data from upcyte® hepatocytes to three different predictive models for CYP3A4 induction, namely the Relative Induction Score (RIS), AUCu/F2, and C max,u/Ind50. In addition, PXR (rifampicin) and CAR-selective (carbamazepine and phenytoin) inducers of CYP3A4 and CYP2B6 induction, respectively, were demonstrated. In conclusion, these data support the use of second-generation upcyte® hepatocytes for CYP inhibition and induction assays. Under the culture conditions used, these cells expressed CYP activities that were equivalent to or higher than those measured in primary human hepatocyte cultures, which could be inhibited or induced by prototypical CYP inhibitors and inducers, respectively. Moreover, they can be used to predict in vivo CYP3A4 induction potential using three prediction models. Bulk availability of cells from multiple donors makes upcyte® hepatocytes suitable for DDI screening, as well as more in-depth mechanistic investigations.

Hepatotoxicity of piperazine designer drugs: Comparison of different in vitro models

Piperazine derived drugs emerged on the drug market in the last decade. The aim of this study was to investigate in vitro the potential hepatotoxicity of the designer drugs N-benzylpiperazine (BZP), 1-(3-trifluoromethylphenyl)piperazine (TFMPP), 1-(4-methoxyphenyl)piperazine (MeOPP) and 1-(3,4-methylenedioxybenzyl)piperazine (MDBP) in two human hepatic cell lines (HepaRG and HepG2) and in primary rat hepatocytes. Cell death was evaluated by the MTT assay, after 24 h-incubations. Among the tested drugs, TFMPP was the most cytotoxic. HepaRG cells and primary hepatocytes revealed to be the most and the least resistant cellular models, respectively. To ascertain whether the CYP450 metabolism could explain their higher susceptibility, primary hepatocytes were co-incubated with the piperazines and the CYP450 inhibitors metyrapone and quinidine, showing that CYP450-mediated metabolism contributes to the detoxification of these drugs. Additionally, the intracellular contents of reactive species, ATP, reduced (GSH) and oxidized (GSSG) glutathione, changes in mitochondrial membrane potential (Δψm) and caspase-3 activation were further evaluated in primary cells. Overall, an increase in reactive species formation, followed by intracellular GSH and ATP depletion, loss of Δψm and caspase-3 activation was observed for all piperazines, in a concentration-dependent manner. In conclusion, piperazine designer drugs produce hepatic detrimental effects that can vary in magnitude among the different analogues.

Gene variants in CYP2C19 are associated with altered in vivo bupropion pharmacokinetics but not bupropion-assisted smoking cessation outcomes

Bupropion is used clinically to treat depression and to promote smoking cessation. It is metabolized by CYP2B6 to its active metabolite hydroxybupropion, yet alterations in CYP2B6 activity have little impact on bupropion plasma levels. Furthermore, less than 10% of a bupropion dose is excreted as urinary bupropion and its characterized metabolites hydroxybupropion, threohydrobupropion, and erythrohydrobupropion, suggesting that alternative metabolic pathways may exist. In vitro data suggested CYP2C19 could metabolize bupropion. The current study investigated the impact of functional CYP2C19 genetic variants on bupropion pharmacokinetics and treatment outcomes. In 42 healthy volunteers, CYP2C19*2 (a reduced activity allele) was associated with higher bupropion area under the plasma concentration-time curve (AUC), but similar hydroxybupropion AUC. The mean bupropion AUC was 771 versus 670 hours⋅ng/ml in individuals with and without CYP2C19*2, respectively (P = 0.017). CYP2C19*2 was also associated with higher threohydrobupropion and erythrohydrobupropion AUC (P < 0.005). Adjusting for CYP2B6 genotype did not alter these associations, and CYP2C19 variants did not alter the utility of the hydroxybupropion/bupropion ratio as a measure of CYP2B6 activity. Finally, in a clinical trial of 540 smokers, CYP2C19 genotype was not associated with smoking cessation outcomes, supporting the hypothesis that bupropion response is mediated by hydroxybupropion, which is not altered by CYP2C19. In conclusion, our study reports the first in vivo evidence that reduced CYP2C19 activity significantly increases the steady-state exposure to bupropion and its reductive metabolites threohydrobupropion and erythrohydrobupropion. These pharmacokinetic changes were not associated with differences in bupropion's ability to promote smoking cessation in smokers, but may influence the side effects and toxicity associated with bupropion.

Bupropion: pharmacology and therapeutic applications

A total of 17 years after its introduction, bupropion remains a safe and effective antidepressant, suitable for first-line use. Bupropion undergoes metabolic transformation to an active metabolite, 4-hydroxybupropion, through hepatic cytochrome P450-2B6 (CYP2B6) and has inhibitory effects on cytochrome P450-2D6 (CYP2D6), thus raising concern for clinically-relevant drug interactions. Common side effects are nervousness and insomnia. Nausea appears slightly less common than with the SSRI drugs and sexual dysfunction is probably the least of any antidepressant. Bupropion is relatively safe in overdose with seizures being the predominant concern. The mechanism of action of bupropion is still uncertain but may be related to inhibition of presynaptic dopamine and norepinephrine reuptake transporters. The activity of vesicular monoamine transporter-2, the transporter pumping dopamine, norepinephrine and serotonin from the cytosol into presynaptic vesicles, is increased by bupropion and may be a component of its mechanism of action. Bupropion is approved for use in major depression and seasonal affective disorder and has demonstrated comparable efficacy to other antidepressants in clinical trials. Bupropion is also useful in augmenting a partial response to selective serotonin reuptake inhibitor antidepressants, although bupropion should not be combined with monoamine oxidase inhibitors. It may be less likely to provoke mania than antidepressants with prominent serotonergic effects. Bupropion is effective in helping people quit tobacco smoking. Anecdotal reports indicate bupropion may lower inflammatory mediators such as tumor necrosis factor-alpha, may lower fatigue in cancer and may help reduce concentration problems.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Cytochrome P450 2B6: function, genetics, and clinical relevance

Cytochrome P450 (CYP) 2B6 belongs to the set of important hepatic drug-metabolizing CYPs. It makes up roughly 3%-6% of total hepatic CYP content and metabolizes several pharmaceuticals including bupropion, efavirenz, cyclophosphamide, pethidine, ketamine and propofol. The enzyme is susceptible to drug-drug interactions by enzyme induction and inhibition. In addition to drugs, CYP2B6 is able to both detoxify and bioactivate a number of procarcinogens and environmental agents including pesticides and herbicides. There is an extensive interindividual variability in the expression of CYP2B6, which is in part explained by extensive genetic polymorphism. CYP2B6 is one of the most polymorphic CYP genes in humans with over 100 described SNPs, numerous complex haplotypes and distinct ethnic and racial frequencies. This review summarizes the basic properties of CYP2B6 and the main characteristics of clinical relevance.

[(3)H]metyrapol and 4-[(131)i]iodometomidate label overlapping, but not identical, binding sites on rat adrenal membranes

Metyrapone, metyrapol, and etomidate are competitive inhibitors of 11-deoxycorticosterone hydroxylation by 11β-hydroxylase. [(3)H]Metyrapol and 4-[(131)I]iodometomidate bind with high affinity to membranes prepared from bovine and rat adrenals. Here we report inhibitory potencies of several compounds structurally related to one or both of these adrenostatic drugs, against the binding of both radioligands to rat adrenal membranes. While derivatives of etomidate inhibited the binding of both radioligands with similar potencies, derivatives of metyrapone inhibited the binding of 4-[(131)I]iodometomidate about 10 times weaker than the binding of [(3)H]metyrapol. By X-ray structure analysis the absolute configuration of (+)-1-(2-fluorophenyl)-2-methyl-2-(pyridin-3-yl)-1-propanol [(+)-11, a derivative of metyrapol] was established as (R). We introduce 1-(2-fluorophenyl)-2-methyl-2-(pyridin-3-yl)-1-propanone (9; Ki = 6 nM), 2-(1-imidazolyl)-2-methyl-1-phenyl-1-propanone (13; 2 nM), and (R)-(+)-[1-(4-iodophenyl)ethyl]-1H-imidazole (34; 4 nM) as new high affinity ligands for the metyrapol binding site on 11β-hydroxylase and discuss our results in relation to a proposed active site model of 11β-hydroxylase.

Ticlopidine inhibits both O-demethylation and renal clearance of tramadol, increasing the exposure to it, but itraconazole has no marked effect on the ticlopidine-tramadol interaction

PURPOSE

We assessed possible drug interactions of tramadol given concomitantly with the potent CYP2B6 inhibitor ticlopidine, alone or together with the potent CYP3A4 and P-glycoprotein inhibitor itraconazole.

METHODS

In a randomized, placebo-controlled cross-over study, 12 healthy subjects ingested 50 mg of tramadol after 4 days of pretreatment with either placebo, ticlopidine (250 mg twice daily) or ticlopidine plus itraconazole (200 mg once daily). Plasma and urine concentrations of tramadol and its active metabolite O-desmethyltramadol (M1) were monitored over 48 h and 24 h, respectively.

RESULTS

Ticlopidine increased the mean area under the plasma concentration-time curve (AUC0-∞) of tramadol by 2.0-fold (90 % confidence interval (CI) 1.6-2.4; p < 0.001) and Cmax by 1.4-fold (p < 0.001), and reduced its oral and renal clearance (p < 0.01). Ticlopidine reduced the AUC0-3 of M1 (p < 0.001) and the ratio of the AUC0-∞ of M1 to that of tramadol, but did not influence the AUC0-∞ of M1. Tramadol or M1 pharmacokinetics did not differ between the ticlopidine alone and ticlopidine plus itraconazole phases.

CONCLUSIONS

Ticlopidine increased exposure to tramadol, reduced its renal clearance and inhibited the formation of M1, most likely via inhibition of CYP2B6 and/or CYP2D6. The addition of itraconazole to ticlopidine did not modify the outcome of the drug interaction. Concomitant clinical use of ticlopidine and tramadol may enhance the risk of serotonergic effects, especially when higher doses of tramadol are used.

Clinically significant drug interactions with newer antidepressants

After the introduction of selective serotonin reuptake inhibitors (SSRIs), other newer antidepressants with different mechanisms of action have been introduced in clinical practice. Because antidepressants are commonly prescribed in combination with other medications used to treat co-morbid psychiatric or somatic disorders, they are likely to be involved in clinically significant drug interactions. This review examines the drug interaction profiles of the following newer antidepressants: escitalopram, venlafaxine, desvenlafaxine, duloxetine, milnacipran, mirtazapine, reboxetine, bupropion, agomelatine and vilazodone. In general, by virtue of a more selective mechanism of action and receptor profile, newer antidepressants carry a relatively low risk for pharmacodynamic drug interactions, at least as compared with first-generation antidepressants, i.e. monoamine oxidase inhibitors (MAOIs) and tricyclic antidepressants (TCAs). On the other hand, they are susceptible to pharmacokinetic drug interactions. All new antidepressants are extensively metabolized in the liver by cytochrome P450 (CYP) isoenzymes, and therefore may be the target of metabolically based drug interactions. Concomitant administration of inhibitors or inducers of the CYP isoenzymes involved in the biotransformation of specific antidepressants may cause changes in their plasma concentrations. However, due to their relatively wide margin of safety, the consequences of such kinetic modifications are usually not clinically relevant. Conversely, some newer antidepressants may cause pharmacokinetic interactions through their ability to inhibit specific CYPs. With regard to this, duloxetine and bupropion are moderate inhibitors of CYP2D6. Therefore, potentially harmful drug interactions may occur when they are coadministered with substrates of these isoforms, especially compounds with a narrow therapeutic index. The other new antidepressants are only weak inhibitors or are not inhibitors of CYP isoforms at usual therapeutic concentrations and are not expected to affect the disposition of concomitantly administered medications. Although drug interactions with newer antidepressants are potentially, but rarely, clinically significant, the use of antidepressants with a more favourable drug interaction profile is advisable. Knowledge of the interaction potential of individual antidepressants is essential for safe prescribing and may help clinicians to predict and eventually avoid certain drug combinations.

Metabolism of bupropion by baboon hepatic and placental microsomes

The aim of this investigation was to determine the biotransformation of bupropion by baboon hepatic and placental microsomes, identify the enzyme(s) catalyzing the reaction(s) and determine its kinetics. Bupropion was metabolized by baboon hepatic and placental microsomes to hydroxybupropion (OH-BUP), threo- (TB) and erythrohydrobupropion (EB). OH-bupropion was the major metabolite formed by hepatic microsomes (Km 36±6 μM, Vmax 258±32 pmol mg protein(-1) min(-1)), however the formation of OH-BUP by placental microsomes was below the limit of quantification. The apparent Km values of bupropion for the formation of TB and EB by hepatic and placental microsomes were similar. The selective inhibitors of CYP2B6 (ticlopidine and phencyclidine) and monoclonal antibodies raised against human CYP2B6 isozyme caused 80% inhibition of OH-BUP formation by baboon hepatic microsomes. The chemical inhibitors of aldo-keto reductases (flufenamic acid), carbonyl reductases (menadione), and 11β-hydroxysteroid dehydrogenases (18β-glycyrrhetinic acid) significantly decreased the formation of TB and EB by hepatic and placental microsomes. Data indicate that CYP2B of baboon hepatic microsomes is responsible for biotransformation of bupropion to OH-BUP, while hepatic and placental short chain dehydrogenases/reductases and to a lesser extent aldo-keto reductases are responsible for the reduction of bupropion to TB and EB.

Chemical inhibitors of cytochrome P450 isoforms in human liver microsomes: a re-evaluation of P450 isoform selectivity

The majority of marketed small-molecule drugs undergo metabolism by hepatic Cytochrome P450 (CYP) enzymes (Rendic 2002). Since these enzymes metabolize a structurally diverse number of drugs, metabolism-based drug-drug interactions (DDIs) can potentially occur when multiple drugs are coadministered to patients. Thus, a careful in vitro assessment of the contribution of various CYP isoforms to the total metabolism is important for predicting whether such DDIs might take place. One method of CYP phenotyping involves the use of potent and selective chemical inhibitors in human liver microsomal incubations in the presence of a test compound. The selectivity of such inhibitors plays a critical role in deciphering the involvement of specific CYP isoforms. Here, we review published data on the potency and selectivity of chemical inhibitors of the major human hepatic CYP isoforms. The most selective inhibitors available are furafylline (in co-incubation and pre-incubation conditions) for CYP1A2, 2-phenyl-2-(1-piperidinyl)propane (PPP) for CYP2B6, montelukast for CYP2C8, sulfaphenazole for CYP2C9, (-)-N-3-benzyl-phenobarbital for CYP2C19 and quinidine for CYP2D6. As for CYP2A6, tranylcypromine is the most widely used inhibitor, but on the basis of initial studies, either 3-(pyridin-3-yl)-1H-pyrazol-5-yl)methanamine (PPM) and 3-(2-methyl-1H-imidazol-1-yl)pyridine (MIP) can replace tranylcypromine as the most selective CYP2A6 inhibitor. For CYP3A4, ketoconazole is widely used in phenotyping studies, although azamulin is a far more selective CYP3A inhibitor. Most of the phenotyping studies do not include CYP2E1, mostly because of the limited number of new drug candidates that are metabolized by this enzyme. Among the inhibitors for this enzyme, 4-methylpyrazole appears to be selective.

Identification of human liver cytochrome P450 enzymes involved in the metabolism of SCH 530348 (Vorapaxar), a potent oral thrombin protease-activated receptor 1 antagonist

Vorapaxar (SCH 530348), a potent oral thrombin protease-activated receptor 1 antagonist, is being developed as an antiplatelet agent for patients with established vascular disease. The objective of this study was to identify the human liver cytochrome P450 (P450) enzyme(s) responsible for the metabolism of SCH 530348. Human liver microsomes metabolized SCH 530348 to M19, an amine metabolite formed via carbamate cleavage, and M20 (monohydroxy-SCH 530348). Recombinant human CYP3A4 exhibited the most activity (11.5% profiled radioactivity) for the formation of M19, followed by markedly less substrate conversion with CYP1A1 and CYP2C19. Trace levels of M19, a major excreted human metabolite, were detected with CYP1A2, CYP3A5, and CYP4F3A. Formation of M19 by human liver microsomes was inhibited 89% by ketoconazole (IC(50), 0.73 μM), 34% by tranylcypromine, and 89% by anti-CYP3A4 monoclonal antibody. There was a significant correlation between the rate of M19 formation and midazolam 1'-hydroxylation (r = 0.75) or M19 formation and testosterone 6β-hydroxylation (r = 0.92). The results of screening, inhibition, and correlation studies confirmed that CYP3A4 is the major P450 enzyme responsible for M19 formation from SCH 530348. In contrast, formation of M20, a major circulating human metabolite at steady state, was primarily catalyzed by CYP3A4 and CYP2J2. M20 is pharmacologically equipotent to SCH 530348, whereas M19 is an inactive metabolite. Formation of M20 by human liver microsomes was inhibited 89% by ketoconazole, 75% by astemizole (a CYP2J2 inhibitor), and 43% by CYP3A4 monoclonal antibody. These results suggest that CYP3A4 and CYP2J2 are both involved in the formation of M20 metabolite.

Metabolism of α-thujone in human hepatic preparations in vitro

This study aims to characterize the metabolism of α-thujone in human liver preparations in vitro and to identify the role of cytochrome P450 (CYP) and possibly other enzymes catalyzing α-thujone biotransformations. With a liquid chromatography-mass spectrometry (LC-MS) method developed for measuring α-thujone and four potential metabolites, it was demonstrated that human liver microsomes produced two major (7- and 4-hydroxy-thujone) and two minor (2-hydroxy-thujone and carvacrol) metabolites. Glutathione and cysteine conjugates were detected in human liver homogenates, but not quantified. No glucuronide or sulphate conjugates were detected. Major hydroxylations accounted for more than 90% of the primary microsomal metabolism of α-thujone. Screening of α-thujone metabolism with CYP recombinant enzymes indicated that CYP2A6 was principally responsible for the major 7- and 4-hydroxylation reactions, although CYP3A4 and CYP2B6 participated to a lesser extent and CYP3A4 and CYP2B6 catalyzed minor 2-hydroxylation. Based on the intrinsic efficiencies of different recombinant CYP enzymes and average abundances of these enzymes in human liver microsomes, CYP2A6 was calculated to be the most active enzyme in human liver microsomes, responsible for 70-80% of the metabolism on average. Inhibition screening indicated that α-thujone inhibited both CYP2A6 and CYP2B6, with 50% inhibitory concentration values of 15.4 and 17.5 µM, respectively.

Structures of cytochrome P450 2B4 complexed with the antiplatelet drugs ticlopidine and clopidogrel

Prior X-ray crystal structures of rabbit cytochrome P450 2B4 (2B4) in complexes with various imidazoles have demonstrated markedly different enzyme conformations depending on the size of the inhibitor occupying the active site. In this study, structures of 2B4 were determined with the antiplatelet drugs clopidogrel and ticlopidine, which were expected to have greater freedom of movement in the binding pocket. Ticlopidine could be modeled into the electron density maps in two distinct orientations, both of which are consistent with metabolic data gathered with other mammalian P450 enzymes. Results of ligand docking and heme-induced NMR relaxation of drug protons showed that ticlopidine was preferentially oriented with the chlorophenyl group closest to the heme. Because of its stereocenter, clopidogrel was easier to fit in the electron density and exhibited a single orientation, which points the chlorophenyl ring toward the heme. The C(α) traces of both complexes aligned very well with each other and revealed a compact, closed structure that resembles the conformation observed in two previously determined 2B4 structures with the small molecule inhibitors 4-(4-chlorophenyl)imidazole and 1-(4-chlorophenyl)imidazole. The 2B4 active site is able to accommodate small ligands by moving only a small number of side chains, suggesting that ligand reorientation is energetically favored over protein conformational changes for binding of these similarly sized molecules. Adjusting both protein conformation and ligand orientation in the active site gives 2B4 the flexibility to bind to the widest range of molecules, while also being energetically favorable.

Stereo-selective metabolism of methadone by human liver microsomes and cDNA-expressed cytochrome P450s: a reconciliation

In vitro metabolism of methadone was investigated in cytochrome P450 (CYP) supersomes and phenotyped human liver microsomes (HLMs) to reconcile past findings on CYP involvement in stereo-selective metabolism of methadone. Racaemic methadone was used for incubations; (R)- and (S)-methadone turnover and (R)- and (S)-EDDP formation were determined using chiral liquid chromatography-tandem mass spectrometry. CYP supersome activity for methadone use and EDDP formation ranked CYP2B6 > 3A4 > 2C19 > 2D6 > 2C18, 3A7 > 2C8, 2C9, 3A5. After abundance scaling, CYP3A4, 2B6 and 2C19 accounted for 63-74, 12-32 and 1. 4-14% of respective activity. CYP2B6, 2D6 and 2C18 demonstrated a preference for (S)-EDDP formation; CYP2C19, 3A7 and 2C8 for (R)-EDDP; 3A4 none. Correlation analysis with 15 HLMs supported the involvement of CYP2B6 and 3A. The significant correlation of S/R ratio with CYP2B6 activity confirmed its stereo-selectivity. CYP2C19 and 2D6 inhibitors and monoclonal antibody (mAb) did not inhibit EDDP formation in HLM. Chemical and mAb inhibition of CYP3A in high 3A activity HLM reduced EDDP formation by 60-85%; inhibition of CYP2B6 in 2B6 high-activity HLM reduced (S)-EDDP formation by 80% and (R)-EDDP formation by 55%. Inhibition changed methadone metabolism in a stereo-selective manner. When CYP3A was inhibited, 2B6 mediated (S)-EDDP formation predominated; S/R stereo-selectivity increased. When 2B6 was inhibited (S)-EDDP formation fell and stereo-selectivity decreased. The results confirmed the primary roles of CYPs 3A4 and 2B6 in methadone metabolism; CYP2C8 and 2C9 did not appear involved; 2C19 and 2D6 have minimal roles. CYP2B6 is the primary determinant of stereo-selective metabolism; stereo-selective inhibition might play a role in varied plasma concentrations of the two enantiomers.

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

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

Bupropion metabolism by human placenta

Smoking during pregnancy is the largest modifiable risk factor for pregnancy-related morbidity and mortality. The success of bupropion for smoking cessation warrants its investigation for the treatment of pregnant patients. Nevertheless, the use of bupropion for the treatment of pregnant smokers requires additional data on its bio-disposition during pregnancy. Therefore, the aim of this investigation was to determine the metabolism of bupropion in placentas obtained from nonsmoking and smoking women, identify metabolites formed and the enzymes catalyzing their formation, as well as the kinetics of the reaction. Data obtained revealed that human placentas metabolized bupropion to hydroxybupropion, erythro- and threohydrobupropion. The rates for formation of erythro- and threohydrobupropion exceeded that for hydroxybupropion by several folds, were dependent on the concentration of bupropion and exhibited saturation kinetics with an apparent K(m) value of 40microM. Human placental 11beta-hydroxysteroid dehydrogenases were identified as the major carbonyl-reducing enzymes responsible for the reduction of bupropion to threo- and erythrohydrobupropion in microsomal fractions. On the other hand, CYP2B6 was responsible for the formation of OH-bupropion. These data suggest that both placental microsomal carbonyl-reducing and oxidizing enzymes are involved in the metabolism of bupropion.

Metabolism and disposition of the thienopyridine antiplatelet drugs ticlopidine, clopidogrel, and prasugrel in humans

Ticlopidine, clopidogrel, and prasugrel are thienopyridine prodrugs that inhibit adenosine-5'-diphosphate (ADP)-mediated platelet aggregation in vivo. These compounds are converted to thiol-containing active metabolites through a corresponding thiolactone. The 3 compounds differ in their metabolic pathways to their active metabolites in humans. Whereas ticlopidine and clopidogrel are metabolized to their thiolactones in the liver by cytochromes P450, prasugrel proceeds to its thiolactone following hydrolysis by carboxylesterase 2 during absorption, and a portion of prasugrel's active metabolite is also formed by intestinal CYP3A. Both ticlopidine and clopidogrel are subject to major competing metabolic pathways to inactive metabolites. Thus, varying efficiencies in the formation of active metabolites affect observed effects on the onset of action and extent of inhibition of platelet aggregation (IPA). Knowledge of the CYP-dependent formation of ticlopidine and clopidogrel thiolactones helps explain some of the observed drug-drug interactions with these molecules and, more important, the role of CYP2C19 genetic polymorphism on the pharmacokinetics of and pharmacodynamic response to clopidogrel. The lack of drug interaction potential and the absence of CYP2C19 genetic effect result in a predictable response to thienopyridine antiplatelet therapy with prasugrel. Current literature shows that greater ADP-mediated IPA is associated with significantly better clinical outcomes for patients with acute coronary syndrome.

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

Cytochrome P450 2B6 (CYP2B6) is expressed predominantly in human liver. It catalyzes the oxidative biotransformation of various drugs, including bupropion, which is an antidepressant and a tobacco use cessation agent. Serious adverse effects of high dosages of bupropion have been reported, including the onset of seizure. As Ginkgo biloba extract may be consumed with bupropion or another CYP2B6 drug substrate, potential exists for an herb-drug interaction. Therefore, we investigated the effect of G. biloba extract and some of its chemical constituents (terpene trilactones and flavonols) on the in vitro catalytic activity of CYP2B6 as assessed by the bupropion hydroxylation assay with recombinant enzyme and hepatic microsomes. The amount of hydroxybupropion was quantified by a novel and validated ultraperformance liquid chromatography/mass spectrometry method. Enzyme kinetic analysis indicated that G. biloba extract competitively inhibited hepatic microsomal CYP2B6-catalyzed bupropion hydroxylation (apparent K(i) was 162 +/- 14 microg/ml). Bilobalide and ginkgolides A, B, C, and J were not responsible for the inhibition of CYP2B6 catalytic activity by the extract. Whereas the 3-O-glucoside and 3-O-rutinoside of quercetin, kaempferol, and isorhamnetin had no effect, the corresponding aglycones (10 and 50 microg/ml) decreased hepatic microsomal bupropion hydroxylation. The inhibition of CYP2B6 by kaempferol was competitive (apparent K(i) was 10 +/- 1 microg/ml). In summary, G. biloba extract and its flavonol aglycones are naturally occurring inhibitors of in vitro CYP2B6 catalytic activity and bupropion hydroxylation. Future studies are needed to investigate whether G. biloba extract interacts in vivo with bupropion or other clinically important CYP2B6 drug substrates.

An evaluation of the cytochrome P450 inhibition potential of selected pesticides in human hepatic microsomes

The goal of this work was to study the ability of 18 pesticides to inhibit selective model activities for all major xenobiotic-metabolizing enzymes, namely CYP1A1/2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4. Generally organophosphorus insecticides were the most potent and extensive inhibitors, especially towards CYP1A1/2 (IC(50) values of chlorpyrifos, fenitrothion and profenofos approximately 3 micro M), CYP2B6 (IC(50) values of chlorpyrifos and fenitrothion 2.5 micro M), CYP2C8 (fenitrothion 4.3 micro M), CYP2C9 (fenitrothion and malathion 4.8 and 2.5 micro M, respectively), CYP2D6 (chlorpyrifos and phenthoate approximately 3 micro M) and CYP3A4 (chlorpyrifos, fenitrothion and phenthoate 3-4 micro M). Otherwise there were quite considerable differences in potency and extent of inhibition between different organophosphates. Pyrethroids were in general very weak or inactive. Deltamethrin and fenvalerate were potent inhibitors of CYP2D6 (IC(50) values of approximately 3 micro M) while lambda-cyhalothrin potently inhibited both CYP2D6 and CYP3A4-mediated activities (IC(50)'s about 3-4 micro M). Some pesticides caused relatively potent inhibitions sporadically (carbendazim, CYP2D6, IC(50) = 12 micro M; atrazine, CYP3A4, IC(50) = 2.8 micro M; glyphosate, CYP2C9, IC(50) = 3.7 micro M; hexaflumuron, IC(50) = 6.0 micro M). With the exceptions of alpha-cypermethrin, cypermethrin, isoproturon, carbaryl and abamectin, most pesticides inhibited relatively potently at least one CYP-selective activity, which may have relevance for potential interactions in occupational exposures and for further studies on the CYP-associated metabolism of respective pesticides.

Mechanism-based inhibition of human cytochrome P450 2B6 by ticlopidine, clopidogrel, and the thiolactone metabolite of prasugrel

Mechanism-based inhibition of CYP2B6 in human liver microsomes by thienopyridine antiplatelet agents ticlopidine and clopidogrel and the thiolactone metabolites of those two agents plus that of prasugrel were investigated by determining the time- and concentration-dependent inhibition of the activity of bupropion hydroxylase as the typical CYP2B6 activity. By comparing the ratios of k(inact) (maximal inactivation rate constant)/K(I) (the inactivator concentration producing a half-maximal rate of inactivation), it was found that the thiolactone metabolite of prasugrel is 10- and 22-fold less potent, respectively, in the mechanism-based inhibition of CYP2B6 than ticlopidine and clopidogrel. The k(inact)/K(I) ratio of the thiolactone metabolite of ticlopidine was comparable with that of the parent compound, whereas this ratio for the thiolactone metabolite of clopidogrel was significantly smaller than that of clopidogrel. In conclusion, ticlopidine, its thiolactone metabolite, and clopidogrel were more potent mechanism-based inhibitors of CYP2B6 than the thiolactone metabolite of prasugrel.

Measurement of in vitro cytochrome P450 2B6 activity

Cytochrome P450 2B6 (CYP2B6) is responsible for the metabolism of a number of therapeutically relevant drugs and environmental chemicals. While not as frequently involved in xenobiotic metabolism as other more commonly studied P450 enzymes such as CYP3A, CYP2D6, or CYP2C9, its importance is becoming increasingly realized. Bupropion hydroxylase activity is an excellent indicator of in vitro CYP2B6 activity, and the metabolite formed is easily measured by HPLC/MS/MS. This assay allows the use of very low concentrations of human liver microsomes or recombinant enzyme, and it represents a selective, sensitive approach to assessing in vitro CYP2B6 activity, with minimal sample preparation.

Comparison of human cytochrome P450 inhibition by the thienopyridines prasugrel, clopidogrel, and ticlopidine

Differences in the inhibition of cytochrome P450 activities among thienopyridine antiplatelet agents, ticlopidine, clopidogrel, prasugrel, and the metabolites, 2-oxo-clopidogrel, clopidogrel acid metabolite, deacetylated metabolite of prasugrel (R-95913) and the pharmacologically active metabolites of clopidogrel and prasugrel, were examined using recombinant cytochromes P450 and fluorescent probe substrates. Ticlopidine and clopidogrel inhibited CYP2B6 with IC(50) values of 0.0517+/-0.0323 microM and 0.0182+/-0.0069 microM, respectively, and inhibited CYP2C19 with IC(50) values of 0.203+/-0.124 microM and 0.524+/-0.160 microM, respectively. Ticlopidine also inhibited CYP2D6 (IC(50) of 0.354+/-0.158 microM). In contrast, 2-oxo-clopidogrel, prasugrel and R-95913 were much weaker inhibitors of CYP2B6, CYP2C19 and CYP2D6. The inhibitory effects of all the compounds tested were much weaker on the isoforms other than those indicated above. The active metabolites of clopidogrel and prasugrel and clopidogrel acid metabolite also did not affect the activities of the P450s examined.

CYP2B6: new insights into a historically overlooked cytochrome P450 isozyme

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