Physiologically based pharmacokinetic model analysis of the inhibitory effect of vonoprazan on the metabolic activation of proguanil

We previously reported that repeated oral administration of vonoprazan (VPZ) followed by oral administration of proguanil (PG) in healthy adults increased blood concentration of PG and decreased blood concentration of its metabolite cycloguanil (CG) compared with administration of PG alone. In this study, we investigated whether this interaction can be quantitatively explained by VPZ inhibition of PG metabolism. In an in vitro study using human liver microsomes, VPZ inhibited CG formation from PG in a concentration-dependent manner, and the inhibition was enhanced depending on preincubation time. Then, a physiologically based pharmacokinetic (PBPK) model analysis was performed incorporating the obtained inhibition parameters. By fitting the blood concentration profiles of VPZ and PG/CG after VPZ and PG were orally administered alone to our PBPK model, parameters were obtained which can reproduce their concentration profiles. In contrast, when the VPZ inhibition parameters for CG formation from the in vitro study were incorporated, the predicted blood PG and CG concentrations were unchanged; the apparent dissociation constant had to be set to about 1/23 of the obtained in vitro value to reproduce the observed interaction. Further comprehensive evaluation is required, including the possibility that mechanisms other than metabolic inhibition may be involved.

Evaluation of the non-linearity of NA808 in liver not reflected in plasma using a rat pharmacokinetic study and PBPK modelling

When NA808, a potent HCV replication inhibitor, was intravenously administered to rats, it was distributed to the liver. The AUC ratio in the liver of 20 mg/kg to 2 mg/kg was greater than the dose ratio, whereas exposure in plasma was increased in a dose-proportional manner. Saturation of biliary excretion was also shown at 20 mg/kg.NA808 was revealed to be a substrate for both OATP1B and MRP2 transporters by an in vitro study using OATP1B1-MRP2 expressing cells. [14C]NA808 was taken up into the cells by OATP1B1 and excreted from cells by MRP2 efficiently (Papp ratio: 24.2-70.2). The Papp ratio decreased with increasing NA808 concentration.PBPK modelling was constructed to display the blood and liver concentration time profile and biliary excretion of NA808. This model analysis was able to reproduce the pharmacokinetics in rats; the degree of increase in the liver exposure from 2 to 20 mg/kg was more than dose-proportional and was greater than the increase in the blood exposure due to saturation of efflux transporters.In drug development, to avoid unexpected toxicity in tissues, it is important to consider the potential for tissue non-linearity with linear plasma exposure based on pre-clinical data and PBPK modelling.

Quantitative Consideration of Clinical Increases in Serum Creatinine Caused by Renal Transporter Inhibition

Creatinine is a common biomarker of renal function and is secreted in the renal tubular cells via drug transporters, such as organic cation transporter 2 and multidrug and toxin extrusion (MATE) 1/2-K. To differentiate between drug-induced acute kidney injury (AKI) and drug interactions through the renal transporter, it has been examined whether these transporter inhibitions quantitatively explained increases in serum creatinine (SCr) at their clinically relevant concentrations using drugs without any changes in renal function. For such renal transporter inhibitors and recently approved tyrosine kinase inhibitors (TKIs), this mini-review describes clinical increases in SCr and inhibitory potentials against the renal transporters. Most cases of SCr elevations can be explained by considering the renal transporter inhibitions based on unbound maximum plasma concentrations, except for drugs associated with obvious changes in renal function. SCr increases for cobicistat, dolutegravir, and dronedarone, and some TKIs were significantly underestimated, and these underestimations were suggested to be associated with low plasma unbound fractions. Sensitivity analysis of SCr elevations regarding inhibitory potentials of MATE1/2-K demonstrated that typical inhibitors such as cimetidine, DX-619, pyrimethamine, and trimethoprim could give false interpretations of AKI according to the criteria based on relative or absolute levels of SCr elevations. Recent progress and current challenges of physiologically-based pharmacokinetics modeling for creatinine disposition were also summarized. Although it should be noted for the potential impact of in vitro assay designs on clinical translatability of transporter inhibitions data, mechanistic approaches could support decision-making in clinical development to differentiate between AKI and creatinine-drug interactions. SIGNIFICANCE STATEMENT: Serum creatinine (SCr) is widely used as an indicator of kidney function, but it increases due to inhibitions of renal transporters, such as multidrug and toxin extrusion protein 1/2-K despite no functional changes in the kidney. Such SCr elevations were quantitatively explained by renal transporter inhibitions except for some drugs with high protein binding. The present analysis demonstrated that clinically relevant inhibitors of the renal transporters could cause SCr elevations above levels corresponding to acute kidney injury criteria.

Caffeine consumption as a risk factor for childhood and adolescence migraine

BACKGROUND

Caffeine consumption is a risk factor for chronic daily headache but few studies have addressed relationships between pediatric patient caffeine levels and headache severity. We examined associations between serum and urine caffeine levels and headache severity in childhood and adolescent migraine cases.

METHODS

Levels of caffeine and caffeine metabolites in serum and urine samples were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The Wilcoxon rank-sum test was used for comparisons of age, sleep time, headache severity, caffeine consumption, and caffeine detection. Spearman's rank correlation coefficient (ρ) was calculated for associations. Correlations where ρ ≥ 0.3 and differences where p < 0.05 were considered statistically significant.

RESULTS

Of the 40 patients studied, 34 declared caffeine consumption and six declared no caffeine consumption. These two groups did not differ significantly in any of the above clinical parameters. Liquid chromatography-tandem mass spectrometry analysis of both serum and urine samples revealed nine caffeine-negative (level <0.0625 μM) and 31 caffeine-positive cases. The Headache Impact Test-6 (HIT-6) score was higher (p = 0.033) for the caffeine-positive group versus the caffeine-negative group. Caffeine was detected by LC-MS/MS in the serum and/or urine of three of the six patients who declared no caffeine consumption. No significant correlations were observed among age, sleep times, headache severity score, or levels of caffeine and caffeine metabolites.

CONCLUSION

Thirty one of 40 (77.5%) cases of childhood/ adolescence migraine showed serum and urine caffeine positivity based on LC-MS/MS. The HIT-6 score, a measure of headache severity, was significantly higher for caffeine-positive versus caffeine-negative cases. Symptoms of childhood/adolescence migraine were exacerbated by caffeine consumption.

Pharmacokinetics and safety of rilpivirine in healthy Japanese subjects and exploration of ethnic sensitivity of rilpivirine pharmacokinetics with physiologically based pharmacokinetic model approach

Rilpivirine is a non-nucleoside reverse transcriptase inhibitor, used for the treatment of human immunodeficiency virus type-1 infection. An open label study was conducted to investigate the pharmacokinetics (PK) and safety of a single oral dose of rilpivirine 25 mg in Japanese healthy adult subjects. No adverse events were reported. The mean C_max (144.3 ng/mL) and AUC_inf (4542 ng h/mL) in Japanese subjects were approximately 30 % higher than those reported from a similar study in Caucasian healthy subjects, whereas the median t_max and mean t_1/2 values were comparable between studies. A simple physiologically based PK model was developed to characterize the rilpivirine PK profile. The model adequately described rilpivirine PK profiles, and well-predicted drug-drug interactions. With exploration using the model, body size and CYP3A4 abundance were identified as factors which explained the observed inter-ethnic difference in rilpivirine exposure. The inter-ethnic difference in rilpivirine exposure was however considered not clinically relevant, since inter-individual variabilities of those intrinsic factors are larger than inter-ethnic ones; and the observed AUC_inf in Japanese subjects was within the range of AUC_tau associated with efficacy and safety in Phase 3 studies. This study results support the use of rilpivirine without dose modification specific to Japanese patients.

Collaborative Study of Thresholds for Mutagens: Hormetic Responses in Cell Proliferation Tests Using Human and Murine Lymphoid Cells

Background:

We previously showed that hormetic responses can be established in cell activity tests using human and murine adherent cells. This time, we examined whether hormetic responses can be established in cell proliferation tests using suspended human and murine lymphoid cells.

Methods:

Human lymphoblastoid cells (TK6) and mouse lymphoma cells (L5178Y) were cultured in multi-well culture plates and treated with mitomycin C, ethyl methansulfonate, hygromycin B, aclarubicin or colchicine at various dose levels and the number of cells was measured at varied times using a flow cytometer.

Results:

When the ratio of the number of cells treated with a test chemical to those in the negative control was plotted, the dose-response relationship typically showed a reverse U-shaped curve, indicating the occurrence of hormesis and existence of thresholds in cell toxicity. The hormetic responses depended largely on the test chemical, dose level and exposure time. When examining responses over the course of time, a J-shaped or fallen S-shaped curve was also observed.

Conclusions:

The dose-response relationship showed a reverse U-shaped curve, a hallmark of hormesis, at least some time points for all chemicals tested here, indicating that chemical hormesis can be established in in vitro cell proliferation tests.

In vitro-in vivo extrapolation of metabolic clearance using human liver microsomes: factors showing variability and their normalization

In vitro-in vivo extrapolation (IVIVE) using human liver microsomes has been widely used to predict metabolic clearance, but some of the factors used in the process of prediction show variability for the same compound: notably, microsomal intrinsic clearance values corrected by the unbound fraction (CL_{int, u}), physiological parameters used for scale-up, and the source of in vivo clearance data.The purpose of this study was to assess the correlation between in vitro and in vivo CL_int with a focus on factors showing variability using four cytochrome P450 (CYP)3A substrates.We surveyed in vivo clearance values in literature and also determined the microsomal CL_{int, u} values. A scaling factor (SF_direct) was defined as in vivo CL_int divided by the microsomal CL_{int, u}, which ranged from 1190 to 2310 (mg protein per kg body weight). The application of a mean SF_direct of 1600 (mg protein per kg body weight) and further normalization by the microsomal CL_{int, u} values of midazolam, the most commonly used substrate, resulted in improved prediction accuracy for CL_{int, u} values from various microsomal batches.The results suggest the normalization of variability might be useful for predicting the in vivo CL_int.

Effects of proton pump inhibitors, esomeprazole and vonoprazan, on the disposition of proguanil, a CYP2C19 substrate, in healthy volunteers

AIMS

Vonoprazan, a new class of potassium-competitive proton pump inhibitors has been found to attenuate the antiplatelet function of clopidogrel in a recent clinical study, despite weak in vitro activity against CYP2C19. To elucidate the mechanism of this interaction, the present study investigated the effects of esomeprazole and vonoprazan on the pharmacokinetics of proguanil, a CYP2C19 substrate.

METHODS

Seven healthy male volunteers (CYP2C19 extensive metabolizers) received a single oral administration of 100 mg proguanil/250 mg atovaquone (control phase), oral esomeprazole (20 mg) for 5 days followed by proguanil/atovaquone (esomeprazole phase) and oral vonoprazan (20 mg) for 5 days followed by proguanil/atovaquone (vonoprazan phase). Concentrations of proguanil and its metabolite, cycloguanil, in plasma and urine in each phase were determined using liquid chromatography-tandem mass spectrometry.

RESULTS

Coadministration with proton pump inhibitors resulted in increase and decrease in the area under the plasma concentration-time curve (AUC) of proguanil and cycloguanil, respectively, significantly reducing their AUC ratio (cycloguanil/proguanil) to 0.317-fold (95% confidence interval [CI] 0.256-0.379) and 0.507-fold (95% CI 0.409-0.605) in esomeprazole phase and vonoprazan phase, respectively. Esomeprazole and vonoprazan also significantly reduced the apparent formation clearance (cumulative amount of cycloguanil in urine divided by AUC of proguanil) to 0.324-fold (95% CI 0.212-0.436) and 0.433-fold (95% CI 0.355-0.511), respectively, without significant changes in renal clearance of proguanil and cycloguanil.

CONCLUSIONS

Although further studies are needed, both esomeprazole and vonoprazan potentially inhibit CYP2C19 at clinical doses, suggesting caution in the coadministration of these drugs with CYP2C19 substrates.

Estimation of changes in serum creatinine and creatinine clearance caused by renal transporter inhibition in healthy subjects

Creatinine is excreted into urine by glomerular filtration and renal tubular secretion through drug transporters such as organic anion transporter 2 (OAT2), organic cation transporter 2 (OCT2), OCT3, multidrug and toxin extrusion protein 1 (MATE1), and MATE2-K. We aimed to investigate whether our method for estimating percentage changes in serum creatinine concentration (SCr) and creatinine clearance (CL_cre) from the baseline is applicable for studying renal transporter inhibitors. We tested 14 compounds (cimetidine, cobicistat, dolutegravir, dronedarone, DX-619, famotidine, INCB039110, nizatidine, ondansetron, pyrimethamine, rabeprazole, ranolazine, trimethoprim, and vandetanib), which were reported to cause reversible changes in SCr and/or CL_cre in healthy subjects excluding elderly. Percentage changes were estimated from the relative contributions of the forementioned transporters to CL_cre and competitive inhibition by these compounds at their maximum plasma unbound concentrations. For 7 and 9 out of these compounds, changes in SCr and/or CL_cre were estimated within 2- and 3-fold of observed values, respectively. Less than 10% changes in SCr and/or CL_cre caused by cobicistat, dolutegravir, and rabeprazole were reproduced as such by our method. These findings suggest that our method can be used to estimate changes in SCr and CL_cre caused by competitive inhibitions of renal drug transporters.

Changes in prolactin receptor homodimer availability may cause late feathering in chickens

Chicken early (EF) and late feathering (LF) are sex-linked phenotypes conferred by wild-type k⁺ and dominant K alleles on chromosome Z, respectively. Besides prolactin (PRL) receptor (PRLR) and sperm flagellar 2 (SPEF2) genes, the K allele contains a fusion gene in which partially duplicated PRLR (dPRLR) and SPEF2 (dSPEF2) genes are linked in a tail-to-tail manner. The causative dPRLR gene encodes a C-terminal truncated receptor. LF chickens have short or no primaries at hatching; however, their feather growth rate is higher than that of EF chickens. This study aimed to elucidate the molecular basis of the K allele's biphasic effect on feather development. By 3'RACE and RT-PCR analyses, we demonstrated that dSPEF2 gene transcription occurred beyond all coding exons of the dPRLR gene on the opposite strand and that dPRLR mRNA was less abundant than PRLR mRNA. In addition, a 5'UTR splice variant (SPV) of PRL receptor mRNAs was increased in LF chickens. In vitro expression analysis of 5'UTR linked to the luciferase reporter gene revealed higher translation efficiency of SPV. RT-qPCR showed that the dPRLR mRNA level was higher in embryos; conversely, SPV was higher in hatched chickens, as was dSPEF2 mRNA. These findings suggest that the K allele inhibits feather development at the fetal stage by expressing dPRLR to attenuate PRLR function and promotes feather growth after hatching by increasing PRLR through dSPEF2 mRNA expression. Increased SPV may cause greater feather growth than that in EF chickens by increasing the availability of PRLR homodimers and enhancing PRL signaling.

Collaborative study of thresholds for mutagens: proposal of a typical protocol for detection of hormetic responses in cytotoxicity tests

BACKGROUND

According to the linear no-threshold model (LNT), even the smallest amount of radiation is hazardous. Although the LNT is not based on solid data, this hypothesis has been applied to mutagens and carcinogens. As a result, it has been postulated that there are no thresholds for these chemicals. To demonstrate negativity by experiments is practically impossible, because negative data may leave behind the possibility that additional data might make the resolution power high enough to change negativity to positivity. Furthermore, additional data collection may be endless and we may be trapped in agnosticism. When hormesis is established, in which biological responses are higher at low-doses and lower at high-doses than the control, thresholds could be established between the low- and high-doses. Before examination of thresholds in chemical mutagenesis, hormetic responses in cytotoxicity were tested using cultured mammalian cells.

METHOD

Human cells (HeLa S3 and TK6) or Chinese hamster cells (CHL/IU) were cultured in 96-well plates and treated with mitomycin C (MMC) or ethyl methanesulfonate (EMS) at various dose levels and optical density was measured after addition of a reagent to detect cellular activity. In hormetic responses, data might fluctuate to and fro; therefore, experimental conditions were examined from various aspects to eliminate confounding factors including cell numbers, detection time, the edge effect of 96-well plates, and measurement time after addition of the reagent for detection.

RESULTS

The dose response relationship was never linear. Cellular activities after treatment with MMC or EMS were generally higher at lower doses levels and lower at higher doses than the control, showing hormesis and allowing the establishment of thresholds. Dose response curves sometimes showed two or three peaks, probably reflecting different cellular responses.

CONCLUSION

Hormetic responses in cytotoxicity tests were observed and thresholds could be established. Based on the results of this investigation, we put forward a tentative protocol to detect chemical hormesis in cytotoxicity tests, i.e., inoculate 2000 cells per well, add various doses of a test chemical 48 h after inoculation, add a detection dye 10 h after treatment, and measure optical density 2 h after addition of the reagent for detection.

Clinical features and outcomes of Merkel cell carcinoma in 20 cats

The biological behaviour and prognostic factors of Merkel cell carcinoma (MCC) in 20 cats were studied. The tumours were surgically removed and histopathologically examined. The animals were 8 to 20 years old (median age: 14 years), and the tumours were predominantly located in the neck and head. Follow-up data were available in 17 cases, and 12 cats died within a year of surgery. The overall median survival time after resection was 243 days (range 16-360 days). Recurrence occurred in 11 cases, although 6 of them (55%) were found to be margin-negative. Possible metastasis occurred after the surgery in 10 cases, although 6 of them (60%) were found to be margin-negative. The histopathological features of MCC included tumour necrosis in 16 cases (80%), vascular invasion in 6 cases (38%) and high mitotic counts (median: 28.5 per high-power field). Irregular acanthosis was noted adjacent to the tumours in 9 cases (60%). Immunohistochemically, the tumour cells were positive for cytokeratin (CK) 20 and p63 in all cases, synaptophysin in 19 (95%) cases, and CK18 in 16 cases (80%). The study shows that feline MCC is associated with a poor prognosis and exhibited a strong tendency towards local recurrence, regional lymph node metastasis and distant spread.

Quantitative analysis of elevation of serum creatinine via renal transporter inhibition by trimethoprim in healthy subjects using physiologically-based pharmacokinetic model

Serum creatinine (SCr) levels rise during trimethoprim therapy for infectious diseases. This study aimed to investigate whether the elevation of SCr can be quantitatively explained using a physiologically-based pharmacokinetic (PBPK) model incorporating inhibition by trimethoprim on tubular secretion of creatinine via renal transporters such as organic cation transporter 2 (OCT2), OCT3, multidrug and toxin extrusion protein 1 (MATE1), and MATE2-K. Firstly, pharmacokinetic parameters in the PBPK model of trimethoprim were determined to reproduce the blood concentration profile after a single intravenous and oral administration of trimethoprim in healthy subjects. The model was verified with datasets of both cumulative urinary excretions after a single administration and the blood concentration profile after repeated oral administration. The pharmacokinetic model of creatinine consisted of the creatinine synthesis rate, distribution volume, and creatinine clearance (CL_cre), including tubular secretion via each transporter. When combining the models for trimethoprim and creatinine, the predicted increments in SCr from baseline were 29.0%, 39.5%, and 25.8% at trimethoprim dosages of 5 mg/kg (b.i.d.), 5 mg/kg (q.i.d.), and 200 mg (b.i.d.), respectively, which were comparable with the observed values. The present model analysis enabled us to quantitatively explain increments in SCr during trimethoprim treatment by its inhibition of renal transporters.

Evaluation of the safety and efficacy of Glycyrrhiza uralensis root extracts produced using artificial hydroponic and artificial hydroponic-field hybrid cultivation systems

Glycyrrhiza uralensis roots used in this study were produced using novel cultivation systems, including artificial hydroponics and artificial hydroponic-field hybrid cultivation. The equivalency between G. uralensis root extracts produced by hydroponics and/or hybrid cultivation and a commercial Glycyrrhiza crude drug were evaluated for both safety and efficacy, and there were no significant differences in terms of mutagenicity on the Ames tests. The levels of cadmium and mercury in both hydroponic roots and crude drugs were less than the limit of quantitation. Arsenic levels were lower in all hydroponic roots than in the crude drug, whereas mean lead levels in the crude drug were not significantly different from those in the hydroponically cultivated G. uralensis roots. Both hydroponic and hybrid-cultivated root extracts showed antiallergic activities against contact hypersensitivity that were similar to those of the crude drug extracts. These study results suggest that hydroponic and hybrid-cultivated roots are equivalent in safety and efficacy to those of commercial crude drugs. Further studies are necessary before the roots are applicable as replacements for the currently available commercial crude drugs produced from wild plant resources.

A Comparison of the Regional Circulation in the Feet between Dialysis and Non-Dialysis Patients using Indocyanine Green Angiography

BACKGROUND AND AIMS

Peripheral artery disease in dialysis cases is more prone to critical limb ischemia compared to non-dialysis cases, with a significantly high rate of major amputation of the lower limbs. Lesions are distributed on the more distal side in dialysis critical limb ischemia cases. The aim of this study was to investigate the usefulness of indocyanine green angiography to determine differences in the regional circulation in the foot between dialysis and non-dialysis patients.

MATERIALS AND METHODS

The subjects included 62 cases, among which 20 were dialysis patients and 42 were non-dialysis patients. We compared the indocyanine green angiography parameters for regions of interest in the dialysis and non-dialysis groups, which included the magnitude of intensity from indocyanine green onset to maximum intensity (Imax), the time from indocyanine green onset to maximum intensity (Tmax), the time elapsed from the fluorescence onset to half the maximum intensity (T1/2), and the time from maximum intensity to declining to 90% of the maximum intensity (Td90%). These indocyanine green angiography parameters were measured at region of interest 1 (the Chopart joint), region of interest 2 (the Lisfranc joint), and region of interest 3 (the distal region of the first metatarsal bone).

RESULTS

In the comparison between the dialysis and non-dialysis groups, a significant difference was observed regarding Tmax, T1/2, and Td90%, especially in region of interest 3.

CONCLUSION

In this study, we show that regional tissue perfusion is more deteriorated in dialysis patients compared with non-dialysis patients using indocyanine green angiography. Tmax, T1/2, and Td90% could be useful clinical parameters to compare ischemic severity of the lower limb between dialysis and non-dialysis patients.

The effect of aprepitant and dexamethasone combination on paclitaxel-induced hypersensitivity reaction

PURPOSE OF INVESTIGATION

Dexamethasone (DEX) is often administered to prevent paclitaxel (PTX)-induced hypersensitivity reactions (HSR). The DEX dose is reduced when administered in combination with aprepitant (APR). However, the influence of that dose reduction on PTX-induced HSR has not been thoroughly studied. The present authors aimed to investigate the effects of the combined administration of APR and DEX on PTX-induced HSR.

MATERIALS AND METHODS

Fifty-one patients who received a three-week PTX regimen in combination with APR and DEX were retrospectively analysed. The authors compared the dose of DEX with the incidence of HSR and other toxicities.

RESULTS

Patients were stratified into two groups depending on the DEX dose, > 20 mg (group D, 33 patients), and < 12 mg (group reD, 26 patients). The incidence of HSR in Groups D and reD were 51.5% (17/33) and 53.8% (14/26), respectively. The frequencies of other toxicities between the groups were comparable.

CONCLUSION

The findings suggest that although a reduction in DEX dose is possible when APR is co-administered, this does not affect the PTX-induced HSR. However, adverse effect should be closely monitored.

Enhancement of host immune response to cell surface antigens by a preparation of Streptococcus hemolyticus

A preparation (OK-432) of a low virulence strain, Su, of Streptococcus hemolyticus (Group A) has been found to possess 2 different effects on tumor cells: a) A direct pharmacological effect like other antitumor drugs, and b) an indirect effect through the enhancement of the host immune response. In order to examine the second effect, 2 different antigen systems were used, Gross (G) cell surface antigens (GCSAs) and the cell surface differentiation antigen PCl. At certain ages, untreated C57BL/6 and BALB/c mice occasionally produce natural antibodies to GCSAs and PCl, respectively. When the mice were injected intraperitoneally (i.p.) with OK-432 at 2 months of age, they produced natural antibodies to GCSAs or to PCl antigen, unlike untreated mice of the same strains at the same ages (negative controls), which produced no antibodies. The specificity of the antibodies was carefully examined by cytotoxicity tests. In addition, the activity of the natural antibodies was not neutralized with OK-432 itself. We have proven that OK-432 enhances the production of natural antibodies to specific cell-surface antigens.

Metronidazole reduces the expression of cytochrome P450 enzymes in HepaRG cells and cryopreserved human hepatocytes

1. Blood levels of S-warfarin have been reported to be increased by concomitant administration of metronidazole (MTZ), an antiprotozoal imidazole derivative. 2. To elucidate the mechanism of this interaction and to identify other possible drug-drug interactions, we conducted an in vitro study with the human hepatoma HepaRG cells and cryopreserved human hepatocytes on the ability of MTZ to reduce the expression of cytochrome P450 (CYP) as well as nuclear receptors that regulate the expression of these enzymes. 3. HepaRG cells and cryopreserved human hepatocytes were treated with MTZ (20 to 500 µM) and were then analyzed by real-time RT-PCR to determine mRNA levels of drug-metabolizing enzymes and nuclear receptors. 4. In both cells, the expressions of CYP2C8, CYP2C9, CYP3A4 and constitutive androstane receptor (CAR) were decreased by MTZ treatment. Particularly, in HepaRG cells, their mRNA levels were decreased by MTZ treatment in a concentration-dependent manner. 5. Our findings suggest that the interaction between MTZ and S-warfarin may be due to the MTZ-induced down-regulation of CYP2C9, the primary enzyme responsible for S-warfarin hydroxylation, and CAR, which regulates CYP2C9 expression. We also found that MTZ use may alter the disposition of drugs metabolized by the CYP isozymes investigated.