Modulation of CYP2E1 activity by isoniazid in rapid and slow N-acetylators

Effect of NAT2, GSTM1 and CYP2E1 genetic polymorphisms on plasma concentration of isoniazid and its metabolites in patients with tuberculosis, and the assessment of exposure-response relationships

Objectives: Isoniazid is a key drug in the chemotherapy of tuberculosis (TB), however, interindividual variability in pharmacokinetic parameters and drug plasma levels may affect drug responses including drug induced hepatotoxicity. The current study investigated the relationships between isoniazid exposure and isoniazid metabolism-related genetic factors in the context of occurrence of drug induced hepatotoxicity and TB treatment outcomes. Methods: Demographic characteristics and clinical information were collected in a prospective TB cohort study in Latvia (N = 34). Time to sputum culture conversion (tSCC) was used as a treatment response marker. Blood plasma concentrations of isoniazid (INH) and its metabolites acetylisoniazid (AcINH) and isonicotinic acid (INA) were determined at three time points (pre-dose (0 h), 2 h and 6 h after drug intake) using liquid chromatography-tandem mass spectrometry. Genetic variations of three key INH-metabolizing enzymes (NAT2, CYP2E1, and GSTM1) were investigated by application PCR- and Next-generation sequencing-based methods. Depending on variables, group comparisons were performed by Student's t-test, one-way ANOVA, Mann-Whitney-Wilcoxon, and Kruskal-Wallis tests. Pearson correlation coefficient was calculated for the pairs of normally distributed variables; model with rank transformations were used for non-normally distributed variables. Time-to-event analysis was performed to analyze the tSCC data. The cumulative probability of tSCC was obtained using Kaplan-Meier estimators. Cox proportional hazards models were fitted to estimate hazard rate ratios of successful tSCC. Results: High TB treatment success rate (94.1%) was achieved despite the variability in INH exposure. Clinical and demographic factors were not associated with either tSCC, hepatotoxicity, or INH pharmacokinetics parameters. Correlations between plasma concentrations of INH and its metabolites were NAT2 phenotype-dependent, while GSTM1 genetic variants did not showed any effects. CYP2E1*6 (T > A) allelic variant was associated with INH pharmacokinetic parameters. Decreased level of AcINH was associated with hepatotoxicity, while decreased values of INA/INH and AcINH/INH were associated with month two sputum culture positivity. Conclusion: Our findings suggest that CYP2E1, but not GSTM1, significantly affects the INH pharmacokinetics along with NAT2. AcINH plasma level could serve as a biomarker for INH-related hepatotoxicity, and the inclusion of INH metabolite screening in TB therapeutic drug monitoring could be beneficial in clinical studies for determination of optimal dosing strategies.

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

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

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.

The Effects of Cruciferous Vegetable-Enriched Diets on Drug Metabolism: A Systematic Review and Meta-Analysis of Dietary Intervention Trials in Humans

Relatively few studies exist in the literature that discuss the effects of diet on drug metabolism and how this can affect interindividual differences in systemic drug exposure. Several studies have investigated the effects of cruciferous vegetables (Cruciferae) or their constituents on drug-metabolizing activity, as these vegetables form an important part of many peoples' diets. In general, the ingestion of cruciferous vegetables is associated with induction of cytochrome P450 (CYP) 1A2 activity in vivo; however, there is contention between reports, and the clinical significance of potential diet-drug interactions remains unclear. This study reports a systematic review, critical appraisal, and meta-analysis of the published literature in this area, and discusses the clinical significance of Cruciferae-enriched diets in the context of diet-drug interactions. Twenty-three dietary intervention trials with drug metabolism end points were identified across Embase, Medline, and the Cochrane Controlled Register of Trials (CENTRAL). Cruciferous vegetables represented in the literature included broccoli, Brussels sprout, cabbage, cauliflower, radish, and watercress. A range of phase I and II drug-metabolizing enzymes and phenotyping metrics were represented in the literature. The meta-analyses performed demonstrated a significant effect on CYP1A2 and glutathione S-transferase-alpha (GST-α), with consumption of Cruciferae increasing the activities of these enzymes by 20-40% and 15-35%, respectively. The results herein suggest that patients undergoing pharmacotherapy with CYP1A2 or GST-α substrates could have altered drug exposure profiles if they regularly eat large or variable amounts of cruciferous vegetables. Recommendations regarding the design of future randomized, controlled trials to test hypotheses in this area are included.

In vitro studies on the toxicity of isoniazid in different cell lines

The aim of the present study was to investigate in vitrothe mechanism of toxicity of isoniazid (-INH), the drug most widely used for treatment of tuberculosis. The human hepatoma line HepG2, the human lymphoblastoid line AHH-1 and the murine lymphoma cells YAC-1 were used as test systems. Active cell death (-apoptosis) and necrosis were detected by different flow cytometric methods: the binding of annexin V to the cell membrane and staining with propidium iodide (PI), the TUNEL assay for detection of DNA fragmentation and the occurrence of a sub G1 peak in cell cycle histograms. Mitochondrial membrane potential was analysed with the fluorescent probe JC-1. In addition to cytotoxicity, effects of INH on cell cycle were studied in HepG2 cells. The data of the present investigations indicate that INH induces cytotoxicity via apoptosis both in hepatoma and lymphoma cells. Twenty-four hours of application of INH in concentrations -26 mM led to a remarkable number of apoptotic cells positive for Annexin V. The induction of apoptosis was accompanied by a break down of the mitochondrial membrane potential and the occurrence of DNA strand breaks. At incubation times from 36 to 48 hours, a sub-G1 peak of late apoptotic cells was detected in cell cycle analysis. Furthermore, cell cycle studies showed a disruption of the cycle at low concentrations of INH which are only mildly cytotoxic. Thus the present study unequivocally demonstrated that INH induces cytotoxicity via apoptosis and can lead to a significant disturbance of the cell cycle in mammalian cells.

Importance of multi-p450 inhibition in drug-drug interactions: evaluation of incidence, inhibition magnitude, and prediction from in vitro data

Drugs that are mainly cleared by a single enzyme are considered more sensitive to drug-drug interactions (DDIs) than drugs cleared by multiple pathways. However, whether this is true when a drug cleared by multiple pathways is coadministered with an inhibitor of multiple P450 enzymes (multi-P450 inhibition) is not known. Mathematically, simultaneous equipotent inhibition of two elimination pathways that each contribute half of the drug clearance is equal to equipotent inhibition of a single pathway that clears the drug. However, simultaneous strong or moderate inhibition of two pathways by a single inhibitor is perceived as an unlikely scenario. The aim of this study was (i) to identify P450 inhibitors currently in clinical use that can inhibit more than one clearance pathway of an object drug in vivo and (ii) to evaluate the magnitude and predictability of DDIs caused by these multi-P450 inhibitors. Multi-P450 inhibitors were identified using the Metabolism and Transport Drug Interaction Database. A total of 38 multi-P450 inhibitors, defined as inhibitors that increased the AUC or decreased the clearance of probes of two or more P450s, were identified. Seventeen (45%) multi-P450 inhibitors were strong inhibitors of at least one P450, and an additional 12 (32%) were moderate inhibitors of one or more P450s. Only one inhibitor (fluvoxamine) was a strong inhibitor of more than one enzyme. Fifteen of the multi-P450 inhibitors also inhibit drug transporters in vivo, but such data are lacking on many of the inhibitors. Inhibition of multiple P450 enzymes by a single inhibitor resulted in significant (>2-fold) clinical DDIs with drugs that are cleared by multiple pathways such as imipramine and diazepam, while strong P450 inhibitors resulted in only weak DDIs with these object drugs. The magnitude of the DDIs between multi-P450 inhibitors and diazepam, imipramine, and omeprazole could be predicted using in vitro data with similar accuracy as probe substrate studies with the same inhibitors. The results of this study suggest that inhibition of multiple clearance pathways in vivo is clinically relevant, and the risk of DDIs with object drugs may be best evaluated in studies using multi-P450 inhibitors.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

An official ATS statement: hepatotoxicity of antituberculosis therapy

Drug-induced liver injury (DILI) is a problem of increasing significance, but has been a long-standing concern in the treatment of tuberculosis (TB) infection. The liver has a central role in drug metabolism and detoxification, and is consequently vulnerable to injury. The pathogenesis and types of DILI are presented, ranging from hepatic adaptation to hepatocellular injury. Knowledge of the metabolism of anti-TB medications and of the mechanisms of TB DILI is incomplete. Understanding of TB DILI has been hampered by differences in study populations, definitions of hepatotoxicity, and monitoring and reporting practices. Available data regarding the incidence and severity of TB DILI overall, in selected demographic groups, and in those coinfected with HIV or hepatitis B or C virus are presented. Systematic steps for prevention and management of TB DILI are recommended. These include patient and regimen selection to optimize benefits over risks, effective staff and patient education, ready access to care for patients, good communication among providers, and judicious use of clinical and biochemical monitoring. During treatment of latent TB infection (LTBI) alanine aminotransferase (ALT) monitoring is recommended for those who chronically consume alcohol, take concomitant hepatotoxic drugs, have viral hepatitis or other preexisting liver disease or abnormal baseline ALT, have experienced prior isoniazid hepatitis, are pregnant or are within 3 months postpartum. During treatment of TB disease, in addition to these individuals, patients with HIV infection should have ALT monitoring. Some experts recommend biochemical monitoring for those older than 35 years. Treatment should be interrupted and, generally, a modified or alternative regimen used for those with ALT elevation more than three times the upper limit of normal (ULN) in the presence of hepatitis symptoms and/or jaundice, or five times the ULN in the absence of symptoms. Priorities for future studies to develop safer treatments for LTBI and for TB disease are presented.

Human variability for metabolic pathways with limited data (CYP2A6, CYP2C9, CYP2E1, ADH, esterases, glycine and sulphate conjugation)

Human variability in the kinetics of a number of phase I (CYP2A6, CYP2C9, CYP2E1, alcohol dehydrogenase and hydrolysis) and phase II enzymes (glycine and sulphate conjugation) was analysed using probe substrates metabolised extensively (>60%) by these routes. Published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and available data on subgroups of the population (effects of ethnicity, age and disease) were abstracted using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration time-curve (AUC)] and acute exposure (C(max)). Interindividual differences in kinetics for all these pathways were low in healthy adults ranging from 21 to 34%. Pathway-related uncertainty factors to cover the 95th, 97.5th and 99th centiles of healthy adults were derived for each metabolic route and were all below the 3.16 kinetic default uncertainty factor in healthy adults, with the possible exception of CYP2C9*3/*3 poor metabolisers (based on a very limited number of subjects). Previous analyses of other pathways have shown that neonates represent the most susceptible subgroup and this was true also for glycine conjugation for which an uncertainty factor of 29 would be required to cover 99% of this subgroup. Neonatal data were not available for any other pathway analysed.

Inhibitory Effect of Antituberculosis Drugs on Human Cytochrome P450-Mediated Activities

The potential for drug-drug interactions mediated by the inhibition of cytochrome P-450 (CYP) were concerned during antituberculosis therapy. However, the information regarding human CYP inhibition by antituberculosis drugs is limited to isoniazid. In the current study, we examined the inhibitory effects of pyrazinamide and ethionamide, both of which are chemically related to isoniazid, on the CYP-mediated activities in human liver microsomes and compared them to that of isoniazid. No remarkable effects on any CYP activities were observed by pyrazinamide and ethionamide. In contrast, in addition to the reported inhibitory effect of isoniazid on CYP1A2, CYP2A6, CYP2C19, and CYP3A activities, our results newly showed its effect on CYP2C9 and CYP2E1 activities. Isoniazid showed potent direct inhibitory effect on S-warfarin 7-hydroxylation, while a preincubation step in the presence of NADPH was needed to inhibit chlorzoxazone 6-hydroxylation. Furthermore, irreversible inhibition of CYP2C19 activity by isoniazid was also observed in the dilution study. These results suggested that pyrazinamide and ethionamide did not seem to cause drug interactions mediated by the inhibition of CYP. In contrast, isoniazid might contribute to the severe drug interactions by a different inhibitory mechanism depending on each of the CYP isozymes, in addition to the reported observations.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Inhibition of cytochrome P450 (CYP450) isoforms by isoniazid: potent inhibition of CYP2C19 and CYP3A

Isoniazid (INH) remains the most safe and cost-effective drug for the treatment and prophylaxis of tuberculosis. The use of INH has increased over the past years, largely as a result of the coepidemic of human immunodeficiency virus infection. It is frequently given chronically to critically ill patients who are coprescribed multiple medications. The ability of INH to elevate the concentrations in plasma and/or toxicity of coadministered drugs, including those of narrow therapeutic range (e.g., phenytoin), has been documented in humans, but the mechanisms involved are not well understood. Using human liver microsomes (HLMs), we tested the inhibitory effect of INH on the activity of common drug-metabolizing human cytochrome P450 (CYP450) isoforms using isoform-specific substrate probe reactions. Incubation experiments were performed at a single concentration of each substrate probe at its K(m) value with a range of INH concentrations. CYP2C19 and CYP3A were inhibited potently by INH in a concentration-dependent manner. At 50 microM INH (approximately 6.86 microg/ml), the activities of these isoforms decreased by approximately 40%. INH did not show significant inhibition (<10% at 50 microM) of other isoforms (CYP2C9, CYP1A2, and CYP2D6). To accurately estimate the inhibition constants (K(i) values) for each isoform, four concentrations of INH were incubated across a range of five concentrations of specific substrate probes. The mean K(i) values (+/- standard deviation) for the inhibition of CYP2C19 by INH in HLMs and recombinant human CYP2C19 were 25.4 +/- 6.2 and 13 +/- 2.4 microM, respectively. INH showed potent noncompetitive inhibition of CYP3A (K(i) = 51.8 +/- 2.5 to 75.9 +/- 7.8 microM, depending on the substrate used). INH was a weak noncompetitive inhibitor of CYP2E1 (K(i) = 110 +/- 33 microM) and a competitive inhibitor of CYP2D6 (K(i) = 126 +/- 23 microM), but the mean K(i) values for the inhibition of CYP2C9 and CYP1A2 were above 500 microM. Inhibition of one or both CYP2C19 and CYP3A isoforms is the likely mechanism by which INH slows the elimination of coadministered drugs, including phenytoin, carbamazepine, diazepam, triazolam, and primidone. Slow acetylators of INH may be at greater risk for adverse drug interactions, as the degree of inhibition was concentration dependent. These data provide a rational basis for understanding drug interaction with INH and predict that other drugs metabolized by these two enzymes may also interact.

The disposition of theophylline in camels after intravenous administration

The pharmacokinetics of theophylline were determined after an intravenous (i.v.) dose of 2.36 mg/kg in six camels and 4.72 mg/kg body weight in three camels. The data obtained (median and range) for the low and high dose, respectively, were as follows: the distribution half-lives (t1/2 alpha) were 1.37 (0.64-3.25) and 2.66 (0.83-3.5) h, the elimination half-lives (t1/2 beta) were 11.8 (8.25-14.9) and 10.4 (10.0-13.5) h, the steady state volumes of distribution (Vss) were 0.88 (0.62-1.54) and 0.76 (0.63-0.76) L/kg, volumes of the central compartment (Vc) were 0.41 (0.35-0.63) and 0.51 (0.36-0.52) L/kg, total body clearances (Clt) were 62.3 (39.4-97.0) and 50.2 (47.7-67.4) mL/h.kg body weight and renal clearance (Vr) for the low dose was 0.6 (0.42-0.96) mL/h.kg body weight. There was no significant difference in the pharmacokinetic parameters between the two doses. Theophylline protein binding at a concentration of 5 micrograms/mL was 32.2 +/- 3.3%. Caffeine was identified as a theophylline metabolite but its concentration in serum and urine was small. Based on the pharmacokinetic values obtained in this study, a dosage of 7.5 mg/kg body weight administered by i.v. injection at 12 h intervals can be recommended. This dosing regimen should achieve an average steady state serum concentration of 10 micrograms/mL with peak serum concentration not exceeding 15 micrograms/mL.

Isoniazid drug and food interactions

Isoniazid inhibits the metabolism of several drugs, resulting in clinically significant interactions in some patients. Clinical trials and case reports have documented that isoniazid can cause increased phenytoin and carbamazepine serum concentrations and toxicity. In relatively high doses, isoniazid can also cause increased effect of theophylline and warfarin. Isoniazid inhibits metabolism of selected benzodiazepines and vitamin D. Inhibition of monoamine oxidase and histaminase by isoniazid can cause significant drug-food interactions. Food greatly decreases isoniazid bioavailability. Although probably best recognized as an inhibitor of drug metabolism, isoniazid has a biphasic effect of inhibition-induction on one cytochrome P450 isozyme, CYP2E1, which partially explains the interaction with acetaminophen and increased risk of hepatotoxicity. Continued investigations will likely result in discovery of new isoniazid interactions.

Inhibition of chlorzoxazone metabolism, a clinical probe for CYP2E1, by a single ingestion of watercress

To investigate the effect of watercress on the metabolism of chlorzoxazone, an in vivo probe for CYP2E1, the oral pharmacokinetics of chlorzoxazone was studied in 10 healthy volunteers before and after a single ingestion of a watercress homogenate (50 gm). A third chlorzoxazone pharmacokinetic study was performed after a 1-week treatment with isoniazid (300 mg/day), a well-known CYP2E1 inhibitor. Ingestion of watercress or isoniazid did not affect the oral absorption of chlorzoxazone. The area under the chlorzoxazone plasma concentration-time curve was significantly increased by 56% (p < 0.05) after watercress ingestion and by 135% (p < 0.001) with isoniazid treatment. Similarly, chlorzoxazone elimination half-life was prolonged after watercress (53%; p < 0.05) and isoniazid (104%; p < 0.01) administration. These results show that a single ingestion of watercress inhibits the hydroxylation of chlorzoxazone, an in vivo probe for CYP2E1.

The relationship between cytochrome P4502E1 activity and plasma fluoride levels after sevoflurane anesthesia in humans

We determined whether the perianesthetic plasma fluoride levels after sevoflurane anesthesia in humans were correlated with the metabolic ratio (MR) of 6-hydroxychlorzoxazone to chlorzoxazone, an in vivo probe for cytochrome P4502E1 (CYP2E1) activity. Thirty ASA physical status I or II patients scheduled for extraabdominal surgery were randomized to a chlorzoxazone (n = 20) or a control group (n = 10). Patients in the chlorzoxazone group received 500 mg chlorzoxazone orally on the morning of the day of surgery. Chlorzoxazone and its 6-hydroxymetabolite concentrations were measured in plasma 2 h after drug administration. Anesthesia was induced with propofol, fentanyl, and atracurium intravenously and maintained with sevoflurane (inspired concentration 1-3 vol%). Plasma fluoride concentrations were determined before the induction of anesthesia, at the cessation of sevoflurane, and 2, 4, 6, 10, and 24 h thereafter. The area under the plasma fluoride concentration-time curve (AUC) was calculated up to 24 h after sevoflurane cessation. MR correlated significantly with the plasma fluoride AUC (r2 = 0.28, P < 0.025), the elimination constant calculated for the postanesthetic 10- to 24-h period (r2 = 0.30, P < 0.025), and the plasma fluoride levels 24 h after the cessation of sevoflurane (r2 = 0.48, P < 0.05). A comparison between groups indicated that the administration of chlorzoxazone itself did not alter the postanesthetic fluoride kinetics. Thus, the interindividual variability in perianesthetic plasma fluoride levels after sevoflurane anesthesia is reflected by differences in the MR of chlorzoxazone and hence is related to the interindividual variability in CYP2E1 activity. We conclude that although the predictive value is limited, this study provides a reasonable basis for examining renal function after sevoflurane anesthesia in a subgroup of patients with a high preoperative metabolic ratio of chlorzoxazone.

IMPLICATIONS

CYP2E1 metabolizes sevoflurane as measured by the metabolic ratio of chlorzoxazone. Patients with a high ratio may be used to justify examining renal function in patients receiving sevoflurane.

Influence of polymorphic N-acetyltransferase phenotype on the inhibition and induction of acetaminophen bioactivation with long-term isoniazid

OBJECTIVE

To determine in patients receiving isoniazid prophylaxis whether an increase in the CYP2E1 dependent formation clearance of acetaminophen (paracetamol) to N-acetyl-p-benzoquinone imine (NAPQI) occurs during a normal 24-hour isoniazid dose interval and whether the interaction is dependent on acetylation status.

METHODS

Acetaminophen elimination kinetics were determined on four different occasions. Ten subjects were assigned to receive acetaminophen either simultaneously with the 8 am dose of isoniazid or 12 hours after the isoniazid dose. One week later, on the last day of isoniazid therapy, subjects received acetaminophen at the alternate time of day. The control phase acetaminophen administrations were repeated 1 and 2 weeks later, following the initial randomization. Isoniazid acetylation (NAT2) genotype was determined by analysis of genomic DNA obtained from peripheral blood leukocytes.

RESULTS

The mean NAPQI formation clearance was inhibited 57% when acetaminophen and isoniazid were coadministered but was unchanged compared with time-matched control when acetaminophen was given 12 hours after the isoniazid dose. However, when data from subjects was segregated according to isoniazid (INH) acetylation phenotype, the mean ratio of NAPQI formation clearances (+INH/-INH) with 8 PM acetaminophen was significantly higher for fast acetylators compared with slow acetylators (1.36 versus 0.68; p = 0.006).

CONCLUSIONS

Fast metabolizers of isoniazid appeared to clear the inducer or inhibitor from the active site of CYP2E1 more rapidly, which resulted in an increased formation of NAPQI 12 hours after the isoniazid dose. In contrast, formation of NAPQI for slow isoniazid metabolizers remained inhibited.