Polymorphisms in drug-metabolizing enzymes and steady-state exemestane concentration in postmenopausal patients with breast cancer.

Discovery of clinical and genetic predictors of exemestane pharmacokinetics was attempted in 246 postmenopausal patients with breast cancer enrolled on a prospective clinical study. A sample was collected 2 h after exemestane dosing at a 1- or 3-month study visit to measure drug concentration. The primary hypothesis was that patients carrying the low-activity CYP3A4*22 (rs35599367) single-nucleotide polymorphism (SNP) would have greater exemestane concentration. Additional SNPs in genes relevant to exemestane metabolism (CYP1A1/2, CYP1B1, CYP3A4, CYP4A11, AKR1C3/4, AKR7A2) were screened in secondary analyses and adjusted for clinical covariates. CYP3A4*22 was associated with a 54% greater exemestane concentration (P<0.01). Concentration was greater in patients who reported White race, had elevated aminotransferases, renal insufficiency, lower body mass index and had not received chemotherapy (all P<0.05), and CYP3A4*22 maintained significance after adjustment for covariates (P<0.01). These genetic and clinical predictors of exemestane concentration may be useful for treatment individualization in patients with breast cancer.

Genetic variant predicts bevacizumab-induced hypertension in ECOG-5103 and ECOG-2100.

BACKGROUND: Bevacizumab has broad anti-tumour activity, but substantial risk of hypertension. No reliable markers are available for predicting bevacizumab-induced hypertension. METHODS: A genome-wide association study (GWAS) was performed in the phase III bevacizumab-based adjuvant breast cancer trial, ECOG-5103, to evaluate for an association between genotypes and hypertension. GWAS was conducted in those who had experienced systolic blood pressure (SBP) >160 mm Hg during therapy using binary analysis and a cumulative dose model for the total exposure of bevacizumab. Common toxicity criteria (CTC) grade 3-5 hypertension was also assessed. Candidate SNP validation was performed in the randomised phase III trial, ECOG-2100. RESULTS: When using the phenotype of SBP>160 mm Hg, the most significant association in SV2C (rs6453204) approached and met genome-wide significance in the binary model (P=6.0 × 10(-8); OR=3.3) and in the cumulative dose model (P=4.7 × 10(-8); HR=2.2), respectively. Similar associations with rs6453204 were seen for CTC grade 3-5 hypertension but did not meet genome-wide significance. Validation study from ECOG-2100 demonstrated a statistically significant association between this SNP and grade 3/4 hypertension using the binary model (P-value=0.037; OR=2.4). CONCLUSIONS: A genetic variant in SV2C predicted clinically relevant bevacizumab-induced hypertension in two independent, randomised phase III trials.

Efavirenz-mediated induction of omeprazole metabolism is CYP2C19 genotype dependent.

Efavirenz increases CYP2C19- and CYP3A-mediated omeprazole metabolism. We hypothesized that CYP2C19 and CYP2B6 genetic polymorphisms influence the extent of induction of omeprazole metabolism by efavirenz. Healthy subjects (n=57) were administered a single 20 mg oral dose of omeprazole on two occasions: with a single 600 mg efavirenz dose; and after a 17-day treatment with efavirenz (600 mg per day). DNA was genotyped for CYP2C19*2, *3 and *17 alleles and CYP2B6*6, *4 and *9 alleles using Taqman assays. Omeprazole, its enantiomers and metabolites were measured by liquid chromatography/tandem mass spectrometry. Our results showed that efavirenz increased omeprazole clearances in all CYP2C19 genotypes in non-stereoselective manner, but the magnitude of induction was genotype dependent. Metabolic ratios of 5-hydroxylation of omeprazole were reduced in extensive and intermediate metabolizers of CYP2C19 (P<0.05). No significant associations were observed between CYP2B6 genotypes and induction by efavirenz on omeprazole metabolism. Our data indicate how interplays between drug interactions and CYP2C19 genetic variations may influence systemic exposure of CYP2C19 substrates.

Pharmacoepidemiologic and in vitro evaluation of potential drug-drug interactions of sulfonylureas with fibrates and statins.

AIMS: To examine whether initiation of fibrates or statins in sulfonylurea users is associated with hypoglycaemia, and examine in vitro inhibition of cytochrome P450 (CYP) enzymes by statins, fenofibrate and glipizide. METHODS: We used healthcare data to conduct nested case-control studies of serious hypoglycaemia (i.e. resulting in hospital admission or emergency department treatment) in persons taking glipizide or glyburide, and calculated adjusted overall and time-stratified odds ratios (ORs) and 95% confidence intervals (CIs). We also characterized the in vitro inhibition of CYP enzymes by statins, fenofibrate and glipizide using fluorometric CYP450 inhibition assays, and estimated area under the concentration-time curve ratios (AUCRs) for the drug pairs. RESULTS: We found elevated adjusted overall ORs for glyburide-fenofibrate (OR 1.84, 95% CI 1.37, 2.47) and glyburide-gemfibrozil (OR 1.57, 95% CI 1.25, 1.96). The apparent risk did decline over time as might be expected from a pharmacokinetic mechanism. Fenofibrate was a potent in vitro inhibitor of CYP2C19 (IC50 = 0.2 µm) and CYP2B6 (IC50 = 0.7 µm) and a moderate inhibitor of CYP2C9 (IC50 = 9.7 µm). The predicted CYP-based AUCRs for fenofibrate-glyburide and gemfibrozil-glyburide interactions were only 1.09 and 1.04, suggesting that CYP inhibition is unlikely to explain such an interaction. CONCLUSIONS: Use of fenofibrate or gemfibrozil together with glyburide was associated with elevated overall risks of serious hypoglycaemia. CYP inhibition seems unlikely to explain this observation. We speculate that a pharmacodynamic effect of fibrates (e.g. activate peroxisome proliferator-activator receptor alpha) may contribute to these apparent interactions.

Aromatase inhibitor-induced modulation of breast density: clinical and genetic effects.

BACKGROUND: Change in breast density may predict outcome of women receiving adjuvant hormone therapy for breast cancer. We performed a prospective clinical trial to evaluate the impact of inherited variants in genes involved in oestrogen metabolism and signalling on change in mammographic percent density (MPD) with aromatase inhibitor (AI) therapy. METHODS: Postmenopausal women with breast cancer who were initiating adjuvant AI therapy were enrolled onto a multicentre, randomised clinical trial of exemestane vs letrozole, designed to identify associations between AI-induced change in MPD and single-nucleotide polymorphisms in candidate genes. Subjects underwent unilateral craniocaudal mammography before and following 24 months of treatment. RESULTS: Of the 503 enrolled subjects, 259 had both paired mammograms at baseline and following 24 months of treatment and evaluable DNA. We observed a statistically significant decrease in mean MPD from 17.1 to 15.1% (P<0.001), more pronounced in women with baseline MPD ≥20%. No AI-specific difference in change in MPD was identified. No significant associations between change in MPD and inherited genetic variants were observed. CONCLUSION: Subjects with higher baseline MPD had a greater average decrease in MPD with AI therapy. There does not appear to be a substantial effect of inherited variants in biologically selected candidate genes.

Genome-wide discovery of genetic variants affecting tamoxifen sensitivity and their clinical and functional validation.

BACKGROUND: Beyond estrogen receptor (ER), there are no validated predictors for tamoxifen (TAM) efficacy and toxicity. We utilized a genome-wide cell-based model to comprehensively evaluate genetic variants for their contribution to cellular sensitivity to TAM. DESIGN: Our discovery model incorporates multidimensional datasets, including genome-wide genotype, gene expression, and endoxifen-induced cellular growth inhibition in the International HapMap lymphoblastoid cell lines (LCLs). Genome-wide findings were further evaluated in NCI60 cancer cell lines. Gene knock-down experiments were performed in four breast cancer cell lines. Genetic variants identified in the cell-based model were examined in 245 Caucasian breast cancer patients who underwent TAM treatment. RESULTS: We identified seven novel single-nucleotide polymorphisms (SNPs) associated with endoxifen sensitivity through the expression of 10 genes using the genome-wide integrative analysis. All 10 genes identified in LCLs were associated with TAM sensitivity in NCI60 cancer cell lines, including USP7. USP7 knock-down resulted in increasing resistance to TAM in four breast cancer cell lines tested, which is consistent with the finding in LCLs and in the NCI60 cells. Furthermore, we identified SNPs that were associated with TAM-induced toxicities in breast cancer patients, after adjusting for other clinical factors. CONCLUSION: Our work demonstrates the utility of a cell-based model in genome-wide identification of pharmacogenomic markers.

Lack of association between oestrogen receptor polymorphisms and change in bone mineral density with tamoxifen therapy.

BACKGROUND: Tamoxifen, a selective oestrogen receptor (ER) modulator, increases bone mineral density (BMD) in postmenopausal women and decreases BMD in premenopausal women. We hypothesised that inherited variants in candidate genes involved in oestrogen signalling and tamoxifen metabolism might be associated with tamoxifen effects in bone. METHODS: A total of 297 women who were initiating tamoxifen therapy were enrolled in a prospective multicentre clinical trial. Lumbar spine and total hip BMD values were measured using dual-energy X-ray absorptiometry (DXA) at baseline and after 12 months of tamoxifen therapy. Single-nucleotide polymorphisms (SNPs) in ESR1, ESR2, and CYP2D6 were tested for associations in the context of menopausal status and previous chemotherapy, with a mean percentage change in BMD over 12 months. RESULTS: The percentage increase in BMD was greater in postmenopausal women and in those patients who had been treated with chemotherapy. No significant associations between tested SNPs and either baseline BMD or change in BMD with 1 year of tamoxifen therapy were detected. CONCLUSION: The evaluated SNPs in ESR and CYP2D6 do not seem to influence BMD in tamoxifen-treated subjects.

Inflammatory cytokines and aromatase inhibitor-associated musculoskeletal syndrome: a case-control study.

BACKGROUND: The aromatase inhibitor (AI)-associated musculoskeletal syndrome (AIMSS) occurs in approximately 50% of AI-treated patients. Inflammatory mediators are associated with oestrogen signalling and may change with oestrogen depletion. We hypothesised that AIMSS may be associated with changes in circulating inflammatory markers. METHODS: Patients with breast cancer were enrolled in a trial of adjuvant AI therapy. Changes in pain and function during therapy were assessed prospectively. We selected 30 cases with AIMSS and 22 controls without AIMSS, matched for demographics and prior therapy. Serum samples collected at baseline and during treatment were assayed for multiple inflammatory cytokines and lipid mediators using multiplex assays. RESULTS: Before AI therapy, mean serum concentrations of 6 of 36 assayed factors were statistically significantly lower in cases than controls (all P<0.003). No statistically significant changes during AI therapy relative to pre-treatment were observed between cases and controls for any of the inflammatory markers tested. CONCLUSION: AIMSS is probably not associated with a systemic inflammatory response. Pre-treatment cytokine levels may predict for development of AIMSS.

Cytochrome P450 2D6 activity predicts discontinuation of tamoxifen therapy in breast cancer patients.

The selective estrogen receptor modulator tamoxifen is routinely used for treatment and prevention of estrogen-receptor-positive breast cancer. Studies of tamoxifen adherence suggest that over half of patients discontinue treatment before the recommended 5 years. We hypothesized that polymorphisms in CYP2D6, the enzyme responsible for tamoxifen activation, predict for tamoxifen discontinuation. Tamoxifen-treated women (n=297) were genotyped for CYP2D6 variants and assigned a 'score' based on predicted allele activities from 0 (no activity) to 2 (high activity). Correlation between CYP2D6 score and discontinuation rates at 4 months was tested. We observed a strong nonlinear correlation between higher CYP2D6 score and increased rates of discontinuation (r(2)=0.935, P=0.018). These data suggest that presence of active CYP2D6 alleles may predict for higher likelihood of tamoxifen discontinuation. Therefore, patients who may be most likely to benefit from tamoxifen may paradoxically be most likely to discontinue treatment prematurely.

Enhancing race-based prescribing precision with pharmacogenomics.

In the world of medicine and therapeutics, race and ethnicity might reasonably be considered as biomarkers or predictors of drug effect. Recognizing that all biomarkers are imperfect, self-reported race can be viewed as a complex combination of genetic and nongenetic biomarkers that is used by prescribing physicians as a predictor of drug effect. The use of pharmacogenetic markers, such as haplotypes, patterns of candidate genes, and specific genotypes, may be used to enhance the precision of race-based prescribing and, when possible, should be combined with nongenetic predictors of responses to optimize the individualization of therapy.

The star-allele nomenclature: retooling for translational genomics.

The star-allele nomenclature is the result of efforts to standardize genetic polymorphism annotation for the cytochrome P450 genes. As clinical pharmacogenetic testing becomes widespread, it is important that this system effectively communicate a patient's genotype and predicted clinical phenotype. As genomics research expands, it is equally important that the system remain a valuable tool for the wider community of genetic researchers to exploit our ever-improving ability to catalog variability in the human genome.

The pharmacogenetics research network: from SNP discovery to clinical drug response.

The NIH Pharmacogenetics Research Network (PGRN) is a collaborative group of investigators with a wide range of research interests, but all attempting to correlate drug response with genetic variation. Several research groups concentrate on drugs used to treat specific medical disorders (asthma, depression, cardiovascular disease, addiction of nicotine, and cancer), whereas others are focused on specific groups of proteins that interact with drugs (membrane transporters and phase II drug-metabolizing enzymes). The diverse scientific information is stored and annotated in a publicly accessible knowledge base, the Pharmacogenetics and Pharmacogenomics Knowledge base (PharmGKB). This report highlights selected achievements and scientific approaches as well as hypotheses about future directions of each of the groups within the PGRN. Seven major topics are included: informatics (PharmGKB), cardiovascular, pulmonary, addiction, cancer, transport, and metabolism.

Severe hypoglycemia in users of sulfonylurea antidiabetic agents and antihyperlipidemics.

Drug-drug interactions causing severe hypoglycemia due to antidiabetic drugs is a major clinical and public health problem. We assessed whether sulfonylurea use with a statin or fibrate was associated with severe hypoglycemia. We conducted cohort studies of users of glyburide, glipizide, and glimepiride plus a statin or fibrate within a Medicaid population. The outcome was a validated, diagnosis-based algorithm for severe hypoglycemia. Among 592,872 persons newly exposed to a sulfonylurea+antihyperlipidemic, the incidence of severe hypoglycemia was 5.8/100 person-years. Adjusted hazard ratios (HRs) for sulfonylurea+statins were consistent with no association. Most overall HRs for sulfonylurea+fibrate were elevated, with sulfonylurea-specific adjusted HRs as large as 1.50 (95% confidence interval (CI): 1.24-1.81) for glyburide+gemfibrozil, 1.37 (95% CI: 1.11-1.69) for glipizide+gemfibrozil, and 1.63 (95% CI: 1.29-2.06) for glimepiride+fenofibrate. Concomitant therapy with a sulfonylurea and fibrate is associated with an often delayed increased rate of severe hypoglycemia.

Implementation of a pharmacogenomics consult service to support the INGENIOUS trial.

Hospital systems increasingly utilize pharmacogenomic testing to inform clinical prescribing. Successful implementation efforts have been modeled at many academic centers. In contrast, this report provides insights into the formation of a pharmacogenomics consultation service at a safety-net hospital, which predominantly serves low-income, uninsured, and vulnerable populations. The report describes the INdiana GENomics Implementation: an Opportunity for the UnderServed (INGENIOUS) trial and addresses concerns of adjudication, credentialing, and funding.

Investigation of the subcellular distribution of cyclic-AMP phosphodiesterase in rat hepatocytes, using a rapid immunological procedure for the isolation of plasma membrane.

The distribution of cyclic-AMP phosphodiesterase was investigated in subcellular fractions prepared from homogenates of rat liver or isolated hepatocytes. When measured at 1 mM or 1 microM substrate concentration, approx. 35% or 50%, respectively, of enzyme activity was particulate. The soluble activity appeared to be predominantly a 'high Km' form, whereas the particulate activity had both 'high Km' and 'low Km' components. The recovery of cyclic-AMP phosphodiesterase was measured using 1 microM substrate concentraiton, in plasma membrane-containing fractions prepared either by centrifugation or by the use of specific immunoadsorbents. The recovery of phosphodiesterase was lower than that of marker enzymes for plasma membrane, and comparable with the recovery of markers for intracellular membranes. It was concluded that regulation of both 'high Km' and 'low Km' phosphodiesterase could potentially make a significant contribution to the control of cyclic AMP concentration, even at microM levels, in the liver. the 'low Km' enzyme, for which activation by hormones has been previously described, appears to be located predominantly in intracellular membranes in hepatocytes. The immunological procedure for membrane isolation allowed the rapid preparation of plasma membranes in high yield. Liver cells were incubated with rabbit anti-(rat erythrocyte) serum and homogenized. The antibody-coated membrane fragments were then extracted onto an immunoadsorbent consisting of sheep anti-(rabbit IgG) immunoglobulin covalently bound to aminocellulose. Plasma membrane was obtained in approx. 40% yield within 50 min of homogenizing cells.

Identification and Mechanistic Investigation of Drug-Drug Interactions Associated With Myopathy: A Translational Approach.

Myopathy is a group of muscle diseases that can be induced or exacerbated by drug-drug interactions (DDIs). We sought to identify clinically important myopathic DDIs and elucidate their underlying mechanisms. Five DDIs were found to increase the risk of myopathy based on analysis of observational data from the Indiana Network of Patient Care. Loratadine interacted with simvastatin (relative risk 95% confidence interval [CI] = [1.39, 2.06]), alprazolam (1.50, 2.31), ropinirole (2.06, 5.00), and omeprazole (1.15, 1.38). Promethazine interacted with tegaserod (1.94, 4.64). In vitro investigation showed that these DDIs were unlikely to result from inhibition of drug metabolism by CYP450 enzymes or from inhibition of hepatic uptake via the membrane transporter OATP1B1/1B3. However, we did observe in vitro synergistic myotoxicity of simvastatin and desloratadine, suggesting a role in loratadine-simvastatin interaction. This interaction was epidemiologically confirmed (odds ratio 95% CI = [2.02, 3.65]) using the data from the US Food and Drug Administration Adverse Event Reporting System.

Effects of ethnicity on the distribution of clinically relevant endothelial nitric oxide variants.

Polymorphisms in the endothelial nitric oxide synthase (eNOS) gene have been associated inconsistently with cardiovascular diseases. A maldistribution of eNOS variants among ethnic groups may explain interethnic differences in nitric oxide (NO)-mediated vasodilation and response to drugs. To test this possibility, we examined the distribution of genetic variants of three clinically relevant eNOS polymorphisms (T-786C in the promoter, the variable number of tandem repeats (VNTR) in intron 4, and the Glu298Asp variant in exon 7) in 305 ethnically well-characterized DNA samples (100 Caucasians, 100 African-Americans, and 105 Asians). We estimated the haplotype frequency, and evaluated associations between these variants. The Asp298 variant was more common in Caucasians (34.5%) than in African-Americans (15.5%) or Asians (8.6%)(P < 0.0001). The C-786 variant was also more common in Caucasians (42.0%) than in African-Americans (17.5%) or Asians (13.8%) (P < 0.0001). The 4a variant in intron 4 was more common in African-Americans (26.5%) than in Caucasians (16.0%) or Asians (12.9%) (P < 0.0001). The most common predicted haplotype in the three groups combined only wild-type variants. Asians had the highest frequency of this haplotype (77% in Asians v. 46% in the other groups). In Caucasians, the Asp298 and C-786variants were associated, and this haplotype was predicted to have a frequency of 24%. In African-Americans, the second most common haplotype included the variant 4a and wild-type variants; the Asp298 and 4a variants were associated negatively in this group. The C-786 and 4a variants were associated in Asians (P < 0.0001). The marked interethnic differences that we found in the distribution of eNOS variants, in the estimated haplotype frequency, and in the association between variants may help us to understand how the combination of these genetic variants may influence cardiovascular diseases.

Allelic, genotypic and phenotypic distributions of S-mephenytoin 4'-hydroxylase (CYP2C19) in healthy Caucasian populations of European descent throughout the world.

Impaired S-mephenytoin 4'-hydroxylation is a well-described genetic polymorphism affecting drug metabolism in humans. The reported population prevalence of the CYP2C19 poor metabolizer phenotype in Caucasians of European descent has been described as ranging from 0.9% to 7.7%. To address the question of whether the difference in the frequency of poor metabolizers represents an ethnic genetic microheterogeneity in the structure and expression of the CYP2C19 gene in Caucasian individuals, we performed a pooled analysis of available studies. Combined data from the 22 homogeneous studies showed that the frequency of poor metabolizers in healthy unrelated Caucasians determined by phenotyping was 2.8% (110 of 3990; 95% confidence interval 2.3-3.3). Data obtained from eight homogeneous studies that determined the frequency of poor metabolizers by genotyping showed that the genotypic frequency of poor metabolizers was 2.1% (28 of 1356; 95% confidence interval 1.3-2.8), consistent with the poor metabolizer frequency determined by phenotyping. In the extensive metabolizers, 26% (471 of 1786; 95% confidence interval 24.4-28.4) were heterozygotes. The observed frequencies of the three Mendelian genotypes were 73% for wt/wt, 26% for wt/m, and 2.1% for m/m. Based on the overall phenotypic poor metabolizer frequency of 2.8%, the expected genotypic frequencies were 69% for wt/wt, 28% for wt/m and 2.8% for m/m, which are in good agreement to the observed values. However, in the 84 Caucasian phenotyped and genotyped poor metabolizers, approximately 10% of the putative poor metabolizer alleles (17 of 168) were unknown. This study provides a systematic overview of the population distribution of the CYP2C19 poor metabolizer phenotype and CYP2C19 alleles and genotypes in healthy Caucasians living in different geographical areas, and shows a similar polymorphic pattern in the structure and expression of the CYP2C19 gene in the worldwide Caucasian populations.

Effect of clarithromycin on the pharmacokinetics and pharmacodynamics of pimozide in healthy poor and extensive metabolizers of cytochrome P450 2D6 (CYP2D6).

BACKGROUND: The use of pimozide is associated with prolongation of the QT interval and fatal ventricular arrhythmia. We recently reported 2 fatal cases in patients taking pimozide and clarithromycin and we have shown that clarithromycin inhibits CYP3A-mediated metabolism of pimozide in vitro. In this study, we examined the effect of clarithromycin on pimozide pharmacokinetics and QT interval changes in a total of 12 healthy subjects (7 men and 5 women), documented as extensive metabolizers or poor metabolizers of CYP2D6. METHODS: In a randomized, double-blind placebo-controlled crossover design, subjects were given a single 6-mg oral dose of pimozide after 5 days of treatment with clarithromycin (500 mg twice a day) or a placebo pill. Blood samples were obtained before and for 96 hours after pimozide administration, and plasma pimozide and clarithromycin concentrations were measured by HPLC. Electrocardiograms for the analysis of the QTc intervals were recorded immediately before each blood sample. RESULTS: Pimozide significantly lengthened QTc interval in the first 20 hours in both the placebo-treated groups (delta QTcmax = 13.3 +/- 5.3 ms; P = .003) and clarithromycin-treated groups (delta QTcmax = 15.7 +/- 9.5 ms; P = .005) compared with baseline values. This is consistent with an effect of the parent drug. Clarithromycin caused a significant increase in the peak plasma concentration (P = .015), terminal elimination half-life (P = .003), and area under the plasma concentration-time curve (P = .024) and a decrease in the clearance (P = .029) of pimozide. Mean QTcmax observed within 20 hours of pimozide administration was significantly greater in the clarithromycin-treated group (23.8 +/- 12.2 ms; P = .0397) than in the placebo-treated group (16.8 +/- 6 ms). There was no significant effect of CYP2D6 or gender on the pharmacokinetics or pharmacodynamics of pimozide. CONCLUSIONS: A single 6-mg oral dose of pimozide resulted in measurable QT interval changes. Clarithromycin inhibited CYP3A-mediated pimozide metabolism and the resulting elevation in plasma concentrations may increase the risk of pimozide cardiotoxicity.

Ticlopidine inhibits phenytoin clearance.

Because cases of phenytoin toxicity during concomitant ticlopidine therapy have been reported, we investigated the effects of multiple doses of ticlopidine on phenytoin pharmacokinetics in six patients receiving phenytoin monotherapy. Two steady-state dosing rate and serum phenytoin minimum concentration (Cmin) pairs were obtained for each patient administered oral phenytoin alone, then phenytoin plus 250 mg ticlopidine twice daily. All patients had serum Cmin ticlopidine values of 0.06 to 0.25 microg/mL when receiving ticlopidine. Individual pharmacokinetic parameters for phenytoin were calculated. The Michaelis-Menten constant (Km) was determined as the slope and maximum velocity (Vmax; equivalent to the maximal rate of elimination or the maximum daily dose that can be metabolized) as the y-intercept of the linear Michaelis-Menten plot. Mean phenytoin Km significantly increased from 5.8 to 12.3 during ticlopidine coadministration compared with administration of phenytoin alone (P = .02). Mean phenytoin Vmax was not significantly changed by the coadministration of ticlopidine. These data indicate that ticlopidine inhibits the clearance and alters the clinical pharmacokinetics of phenytoin so that dosage adjustment of phenytoin should be considered when ticlopidine is coadministered. The results are consistent with previous human liver microsome findings that ticlopidine is a potent inhibitor of CYP2C19, a P450 isozyme that is significantly responsible for phenytoin metabolism.