CYP3A4 Inducer Rifampicin Research Articles

PBPK Modeling of Entrectinib and Its Active Metabolite to Derive Dose Adjustments in Pediatric Populations Co-Administered with CYP3A4 Inhibitors.

Physiologically based pharmacokinetic (PBPK) models of entrectinib and its equipotent metabolite, M5, were established in healthy adult subjects and extrapolated to pediatric patients to predict increases in steady-state systemic exposure on co-administration of strong and moderate CYP3A4 inhibitors (itraconazole at 5 mg/kg, erythromycin at 7.5-12.5 mg/kg and fluconazole at 3-12 mg/kg, respectively). Adult model establishment involved the optimization of fraction metabolized by CYP3A4 (0.92 for entrectinib and 0.98 for M5) using data from an itraconazole DDI study. This model captured well the exposure changes of entrectinib and M5 seen in adults co-administered with the strong CYP3A4 inducer rifampicin. In pediatrics, reasonable prediction of entrectinib and M5 pharmacokinetics in ≧2 year olds was achieved when using the default models for physiological development and enzyme ontogenies. However, a two to threefold misprediction of entrectinib and M5 exposures was seen in <2 year olds which may be due to missing mechanistic understanding of gut physiology and/or protein binding in very young children. Model predictions for ≧2 year olds showed that entrectinib AUC(0-t) was increased by approximately sevenfold and five to threefold by strong and high-moderate and low-moderate CYP3A4 inhibitors, respectively. Based on these victim DDI predictions, dose adjustments for entrectinib when given concomitantly with strong and moderate CYP3A4 inhibitors in pediatric subjects were recommended. These simulations informed the approved entrectinib label without the need for additional clinical pharmacology studies.

PBTK model-based analysis of CYP3A4 induction and the toxicokinetics of the pyrrolizidine alkaloid retrorsine in man

Cytochrome P450 (CYP)3A4 induction by drugs and pesticides plays a critical role in the enhancement of pyrrolizidine alkaloid (PA) toxicity as it leads to increased formation of hepatotoxic dehydro-PA metabolites. Addressing the need for a quantitative analysis of this interaction, we developed a physiologically-based toxicokinetic (PBTK) model. Specifically, the model describes the impact of the well-characterized CYP3A4 inducer rifampicin on the kinetics of retrorsine, which is a prototypic PA and contaminant in herbal teas. Based on consumption data, the kinetics after daily intake of retrorsine were simulated with concomitant rifampicin treatment. Strongest impact on retrorsine kinetics (plasma AUC24\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{24}$$\\end{document} and Cmax\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C_{{\ ext {max}}}$$\\end{document} reduced to 67% and 74% compared to the rifampicin-free reference) was predicted directly after withdrawal of rifampicin. At this time point, the competitive inhibitory effect of rifampicin stopped, while CYP3A4 induction was still near its maximum. Due to the impacted metabolism kinetics, the cumulative formation of intestinal retrorsine CYP3A4 metabolites increased to 254% (from 10 to 25 nmol), while the cumulative formation of hepatic CYP3A4 metabolites was not affected (57 nmol). Return to baseline PA toxicokinetics was predicted 14 days after stop of a 14-day rifampicin treatment. In conclusion, the PBTK model showed to be a promising tool to assess the dynamic interplay of enzyme induction and toxification pathways.

Physiologically based pharmacokinetic modelling of oxycodone drug-drug interactions.

Oxycodone is an opioid analgesic with several pharmacologically active metabolites and relatively narrow therapeutic index. Cytochrome P450 (CYP) 3A4 and CYP2D6 play major roles in the metabolism of oxycodone and its metabolites. Thus, inhibition and induction of these enzymes may result in substantial changes in the exposure of both oxycodone and its metabolites. In this study, a physiologically based pharmacokinetic (PBPK) model was built using GastroPlus™ software for oxycodone, two primary metabolites (noroxycodone, oxymorphone) and one secondary metabolite (noroxymorphone). The model was built based on literature and in house in vitro and in silico data. The model was refined and verified against literature clinical data after oxycodone administration in the absence of drug-drug interactions (DDI). The model was further challenged with simulations of oxycodone DDI with CYP3A4 inhibitors ketoconazole and itraconazole, CYP3A4 inducer rifampicin and CYP2D6 inhibitor quinidine. The magnitude of DDI (AUC ratio) was predicted within 1.5-fold error for oxycodone, within 1.8-fold and 1.3-4.5-fold error for the primary metabolites noroxycodone and oxymorphone, respectively, and within 1.4-4.5-fold error for the secondary metabolite noroxymorphone, when compared to the mean observed AUC ratios. This work demonstrated the capability of PBPK model to simulate DDI of the administered compounds and the formed metabolites of both DDI victim and perpetrator. However, the predictions for the formed metabolites tend to be associated with higher uncertainty than the predictions for the administered compound. The oxycodone model provides a tool for forecasting oxycodone DDI with other CYP3A4 and CYP2D6 DDI perpetrators that may be co-administered with oxycodone.

Drug-drug interaction (DDI) assessments of ruxolitinib, a dual substrate of CYP3A4 and CYP2C9, using a verified physiologically based pharmacokinetic (PBPK) model to support regulatory submissions

Ruxolitinib is mainly metabolized by cytochrome P450 (CYP) enzymes CYP3A4 and CYP2C9 followed by minor contributions of other hepatic CYP enzymes in vitro. A physiologically based pharmacokinetic (PBPK) model was established to evaluate the changes in the ruxolitinib systemic exposures with co-administration of CYP3A4 and CYP2C9 perpetrators. The fractions metabolized in the liver via oxidation by CYP enzymes (fm,CYP3A4 = 0.75, fm,CYP2C9 = 0.19, and fm,CYPothers = 0.06) for an initial ruxolitinib model based on in vitro data were optimized (0.43, 0.56, and 0.01, respectively) using the observed exposure changes of ruxolitinib (10 mg) with co-administered ketoconazole (200 mg). The reduced amount of fm,CYP3A4 was distributed to fm,CYP2C9. For the initial ruxolitinib model with co-administration of ketoconazole, the area under the curve (AUC) increase of 2.60-fold was over-estimated compared with the respective observation (1.91-fold). With the optimized fm values, the predicted AUC ratio was 1.82. The estimated AUC ratios of ruxolitinib by co-administration of the moderate CYP3A4 inhibitor erythromycin (500 mg) and the strong CYP3A4 inducer rifampicin (600 mg) were within a 20% error compared with the clinically observed values. The PBPK modeling results may provide information on the labeling, i.e. supporting a dose reduction by half for co-administration of strong CYP3A4 inhibitors. Furthermore, an AUC increase of ruxolitinib in the absence or presence of the dual CYP3A4 and CYP2C9 inhibitor fluconazole (100-400 mg) was prospectively estimated to be 1.94- to 4.31-fold. Fluconazole simulation results were used as a basis for ruxolitinib dose adjustment when co-administering perpetrator drugs. A ruxolitinib PBPK model with optimized fm,CYP3A4 and fm,CYP2C9 was established to evaluate victim DDI risks. The previous minimal PBPK model was supported by the FDA for the dose reduction strategy, halving the dose with the concomitant use of strong CYP3A4 inhibitors and dual inhibitors on CYP2C9 and CYP3A4, such as fluconazole at ≤200 mg. Fluconazole simulation results were used as supportive evidence in discussions with the FDA and EMA about ruxolitinib dose adjustment when co-administering perpetrator drugs. Thus, this study demonstrated that PBPK modeling can support characterizing DDI liabilities to inform the drug label and might help reduce the number of clinical DDI studies by simulations of untested scenarios, when a robust PBPK model is established.

Quantitative analysis of 4β- and 4α‑hydroxycholesterol in human plasma and serum by UHPLC/ESI-HR-MS

Cholesterol oxidation product 4β‑hydroxycholesterol (4β‑OHC) may possibly be used as an endogenous biomarker of CYP3A enzyme activity and as CYP3A4 is involved in the metabolism of approximately 50% of the drugs in clinical use, the monitoring of CYP3A activity by 4β‑OHC plasma or serum levels, may be of clinical significance. The plasma and serum concentrations of 4α‑hydroxycholesterol (4α‑OHC), an isomer of 4β‑OHC, increase during uncontrolled storage conditions and therefore serve as an indicator of proper handling of samples.A sensitive and simple high-throughput method for the simultaneous quantification of both 4α‑OHC and 4β‑OHC in human plasma and serum was developed utilizing ultrahigh performance liquid chromatography coupled with high resolution mass spectrometry (UHPLC/ESI-HR-MS). The chromatographic analysis was carried out on a Waters HSS T3 C18 reversed phase column with a mobile phase composed of 0,1% formic acid with 200 mg/l sodium acetate, and methanol. 4β‑OHC and 4α‑OHC and also internal standard d7‑4β‑OHC were monitored using HR-MS as sodium adducts, which could not be used as a precursor ions in conventional tandem mass spectrometry methods due to their extensive stability in collision for MS/MS. The use of HR-MS detection enabled avoiding laborious sample derivatization, which is required with triple quadrupole mass spectrometer-based methods to achieve adequate analytical sensitivity for 4β‑OHC, as the underivatized molecule is otherwise poorly ionized to other molecular ions than sodium adduct. Chromatographic separation of 4α‑OHC and 4β‑OHC was obtained and confirmed with standard samples prepared in blank surrogate matrix. The lower limits of quantitation in the assay were 0.5 ng/ml for 4β‑OHC, and 2 ng/ml for 4α‑OHC. Endogenous levels of 4β‑OHC can vary between 10 and 100 ng/ml depending on the possible induction or inhibition of CYP3A4, whereas the levels of 4α‑OHC can vary between 5 and 100 ng/ml, depending on the storage conditions of the samples. Thus, the sensitivity of the assay developed allows for the simultaneous measurement of endogenous levels of 4α‑OHC and 4β‑OHC cost-effectively and with high throughput.The method was successfully used for the determination of 4β‑OHC and 4α‑OHC concentrations in clinical plasma and serum samples collected before and after treatment with a known CYP3A4 inducer rifampicin. The endogenous levels in clinical human samples before treatment varied between 13.4 and 31.9 ng/ml for 4β‑OHC, and between 3.53 and 5.65 ng/ml for 4α‑OHC, and a three-fold increase in 4β‑OHC plasma levels was observed after the rifampicin treatment, while 4α‑OHC levels remained unaffected.

Development of a paediatric physiologically based pharmacokinetic model to assess the impact of drug-drug interactions in tuberculosis co-infected malaria subjects: A case study with artemether-lumefantrine and the CYP3A4-inducer rifampicin

The fixed dosed combination of artemether and lumefantrine (AL) is widely used for the treatment of malaria in adults and children in sub-Sahara Africa, with lumefantrine day 7 concentrations being widely used as a marker for clinical efficacy. Both are substrates for CYP3A4 and susceptible to drug-drug interactions (DDIs); indeed, knowledge of the impact of these factors is currently sparse in paediatric population groups. Confounding malaria treatment is the co-infection of patients with tuberculosis. The concomitant treatment of AL with tuberculosis chemotherapy, which includes the CYP3A4 inducer rifampicin, increases the risk of parasite recrudescence and malaria treatment failure. This study developed a population-based PBPK model for AL in adults capable of predicting the pharmacokinetics of AL under non-DDI and DDI conditions, as well as predicting AL pharmacokinetics in paediatrics of 2–12years of age. The validated model was utilised to assess the concomitant treatment of rifampicin and lumefantrine under standard body-weight based treatment regimens for 2–5year olds, and demonstrated that no subjects attained the target day 7 concentration (Cd7) of 280ng/mL, highlighting the importance of this DDI and the potential risk of malaria-TB based DDIs. An adapted 7-day treatment regimen was simulated and resulted in 63% and 74.5% of subjects attaining the target Cd7 for 1-tablet and 2-tablet regimens respectively.

Physiologically-Based Pharmacokinetic Modeling of Macitentan: Prediction of Drug-Drug Interactions.

Macitentan is a novel dual endothelin receptor antagonist for the treatment of pulmonary arterial hypertension (PAH). It is metabolized by cytochrome P450 (CYP) enzymes, mainly CYP3A4, to its active metabolite ACT-132577. A physiological-based pharmacokinetic (PBPK) model was developed by combining observations from clinical studies and physicochemical parameters as well as absorption, distribution, metabolism and excretion parameters determined in vitro. The model predicted the observed pharmacokinetics of macitentan and its active metabolite ACT-132577 after single and multiple dosing. It performed well in recovering the observed effect of the CYP3A4 inhibitors ketoconazole and cyclosporine, and the CYP3A4 inducer rifampicin, as well as in predicting interactions with S-warfarin and sildenafil. The model was robust enough to allow prospective predictions of macitentan-drug combinations not studied, including an alternative dosing regimen of ketoconazole and nine other CYP3A4-interacting drugs. Among these were the HIV drugs ritonavir and saquinavir, which were included because HIV infection is a known risk factor for the development of PAH. This example of the application of PBPK modeling to predict drug-drug interactions was used to support the labeling of macitentan (Opsumit).

Generation of cytochrome P450 3A4-overexpressing HepG2 cell clones for standardization of hepatocellular testosterone 6β-hydroxylation activity

Detoxification of xenobiotics including drugs is catalyzed by liver phase I and phase II enzymes. There are three main families of phase I cytochrome P450 (CYP450) monoxygenases that introduce polar groups on drugs. These phase I metabolites can then be further conjugated by transferases during phase II reaction. Liver biotransformation can also lead to toxic drug metabolites, the most common cause of drug failure during clinical investigation. CYP3A4 is considered to be the most important enzyme in drug metabolism. Drug development relies on the use of human liver cells in order to investigate drug metabolism and potential toxicity. With primary human liver cells as the gold standard several problems have to be solved, i.e. scarcity of functional human liver tissue, donor variation of CYP activity and rapid dedifferentiation processes during primary cell cultivation. These features make it difficult to use primary human liver cells as standard to measure CYP activity. To avoid problems with primary human liver cells, many attempts have been undertaken to establish liver carcinoma cell lines, non-transformed proliferating human liver cell systems and induced pluripotent stem cell-derived hepatocytes. Due to different problems with these surrogate systems, the one cell line that could be used as convenient standard cell system to benchmark CYP3A4 enzyme activity has not been established yet. Based on the widely used hepatocellular carcinoma cell line HepG2 and a lentiviral vector system, we generated cell clones for stable CYP3A4 overexpression. Here we present data on a new HepG2 cell clone (clone 9) showing higher than 10,000-fold overexpression of CYP3A4 compared to HepG2 parental cells. As measured by conversion of testosterone into 6 -hydroxytestosterone, we found an enzyme activity of about 600 pmol per minute per mg total cellular protein, which ranges at the upper end reported for primary human liver cells. This enzyme activity appeared to be kept stable in clone 9 cells, because there was no influence detectable when cells were treated with 5-azacytidine, a drug that interferes with epigenetic silencing processes. Prototypic CYP3A4 inducer rifampicin led to significant increase of CYP3A4 testosterone hydroxylase activity in HepG2 clone 9 cells. Altogether, HepG2 clone 9 strongly and stably overexpressed CYP3A4 leading to a physiological enzyme activity, which apparently was unaffected by epigenetic processes. Thus, HepG2 clone 9 could be a useful reference cell clone for CYP3A4 enzyme activity.

Studies on the Role of Metabolic Activation in Tyrosine Kinase Inhibitor–Dependent Hepatotoxicity: Induction of CYP3A4 Enhances the Cytotoxicity of Lapatinib in HepaRG Cells

Idiosyncratic hepatotoxicity has been associated with the oral tyrosine kinase inhibitor lapatinib, which is used in metastatic breast cancer therapy. Lapatinib is extensively metabolized by cytochrome P450 3A4/5 to yield an O-debenzylated metabolite, which can undergo further oxidation to a reactive quinone imine. A recent clinical study reported that concomitant use of lapatinib with dexamethasone increased the incidence of hepatotoxicity in metastatic breast cancer patients treated with lapatinib, and so we hypothesized that induction of CYP3A enhances the bioactivation of lapatinib to reactive intermediates that contribute to hepatotoxicity. Therefore, we examined the effect of CYP3A4 induction on the cytotoxicity and metabolism of lapatinib in the HepaRG human hepatic cell line. Differentiated HepaRG cells were pretreated with dexamethasone (100 μM) or the prototypical CYP3A4 inducer rifampicin (4 μM) for 72 hours, followed by incubation with lapatinib (0-100 μM) for 24 hours. Cell viability was monitored using WST-1 assays, and metabolites were quantified by liquid chromatography coupled to tandem mass spectrometry. Induction of CYP3A4 by dexamethasone or rifampicin enhanced lapatinib-induced cytotoxicity, compared with treatment with lapatinib alone. A direct comparison of the cytotoxicity of lapatinib versus O-debenzylated lapatinib demonstrated that the O-debenzylated metabolite was significantly more cytotoxic than lapatinib itself. Furthermore, pretreatment with 25 μM l-buthionine sulfoximine to deplete intracellular glutathione markedly enhanced lapatinib cytotoxicity. Cytotoxicity was correlated with increased formation of O-debenzylated lapatinib and cysteine adducts of the putative quinone imine intermediate. Collectively, these data suggest that CYP3A4 induction potentiates lapatinib-induced hepatotoxicity via increased reactive metabolite formation.

GC-MS–Based Quantitative Signatures of Cytochrome P450–Mediated Steroid Oxidation Induced by Rifampicin

Drug-induced cytochrome P450 (CYP) activity affects endocrine function and drug clearance rates, leading to the development of unpredictable pathologic and toxicologic risks. Urinary steroid profiling based on gas chromatography-mass spectrometry (GC-MS) was used for simultaneous quantification of CYP-mediated regioselective hydroxysteroids and their substrates, including 26 androgens, 9 estrogens, 5 progestins, and 7 corticoids. The quantitative data were visualized using a hierarchically clustered heat map to allow identification of CYP-mediated steroid signatures. Twelve healthy subjects were orally administered 600 mg of rifampicin a day for 7 days, and their CYP enzyme activity was evaluated. Using GC-MS, all 47 steroids were well separated with good peak shapes. This assay had good linearity (r > 0.994) in a dynamic range, and the interassay imprecision (% CV) and inaccuracy (% bias) were 3.0%-15.6% and 98.0%-109.2%, respectively. Administration of the CYP3A4 inducer rifampicin produced distinct differences in CYP3A4 and CYP11B1, CYP19A1, HSD11B, and HSD17B, which were indicated by their heat map-visualized steroid signatures. This CYP-mediated steroid signature profile allows simultaneous assessment of CYP1A, CYP1B, CYP2C, CYP3A, CYP11B, CYP17A, CYP19A, and CYP21A in urine samples. This method could therefore be a useful tool for assessing drug efficacy.

Sorafenib

Sorafenib (Nexavar) is an orally active multikinase inhibitor that is approved in the EU for the treatment of hepatocellular carcinoma. Monotherapy with sorafenib prolongs overall survival and delays the time to progression in patients with advanced hepatocellular carcinoma who are not candidates for potentially curative treatment or transarterial chemoembolization. Sorafenib is generally well tolerated in patients with advanced hepatocellular carcinoma. Thus, sorafenib represents an important advance in the treatment of advanced hepatocellular carcinoma and is the new standard of care for this condition. The bi-aryl urea sorafenib is an oral multikinase inhibitor that inhibits cell surface tyrosine kinase receptors (e.g. vascular endothelial growth factor receptors and platelet-derived growth factor receptor-beta) and downstream intracellular serine/threonine kinases (e.g. Raf-1, wild-type B-Raf and mutant B-Raf); these kinases are involved in tumour cell proliferation and tumour angiogenesis. In vitro, dose-dependent inhibition of cell proliferation and induction of apoptosis was seen with sorafenib in human hepatocellular carcinoma cells lines. Sorafenib demonstrated dose-dependent antitumour activity in a murine xenograft model of human hepatocellular carcinoma. Steady-state plasma concentrations were reached within 7 days in patients with advanced, refractory solid tumours who received twice-daily oral sorafenib. Metabolism of sorafenib occurs primarily in the liver and is mediated via cytochrome P450 (CYP) 3A4 and uridine diphosphate glucuronosyltransferase 1A9. In advanced hepatocellular carcinoma, differences in sorafenib pharmacokinetics between Child-Pugh A and B patients were not considered clinically significant. Sorafenib may be associated with drug interactions. For example, sorafenib exposure was reduced by an average 37% with concomitant administration of the CYP3A4 inducer rifampicin (rifampin); sorafenib concentrations may also be decreased by other CYP3A4 inducers. Monotherapy with oral sorafenib 400 mg twice daily prolonged median overall survival and delayed the median time to progression in patients with advanced hepatocellular carcinoma, according to the results of two randomized, double-blind, placebo-controlled, multicentre, phase III trials (the SHARP trial and the Asia-Pacific trial). There was no significant difference between sorafenib and placebo recipients in the median time to symptomatic progression in either trial. The vast majority of patients included in these trials were Child-Pugh A. Combination therapy with sorafenib plus doxorubicin did not delay the median time to progression to a significant extent compared with doxorubicin alone in patients with advanced hepatocellular carcinoma, according to the results of a randomized, double-blind, phase II trial. However, the median durations of overall survival and progression-free survival were significantly longer in patients receiving sorafenib plus doxorubicin than in those receiving doxorubicin alone. Combination therapy with sorafenib plus tegafur/uracil or mitomycin also showed potential in advanced hepatocellular carcinoma, according to the results of noncomparative trials. Monotherapy with oral sorafenib was generally well tolerated in patients with advanced hepatocellular carcinoma, with a manageable adverse effect profile; diarrhoea and hand-foot skin reaction were consistently the most commonly occurring drug-related adverse events in clinical trials. In the SHARP trial, drug-related adverse events of any grade occurring in significantly more sorafenib than placebo recipients included diarrhoea, hand-foot skin reaction, anorexia, alopecia, weight loss, dry skin, abdominal pain, voice changes and 'other' dermatological events. A similar tolerability profile was seen in the Asia-Pacific trial. As expected given the addition of a chemotherapy agent, the adverse event profile in patients with advanced hepatocellular carcinoma who received combination therapy with sorafenib plus doxorubicin differed somewhat to that seen with sorafenib monotherapy in the SHARP trial. In patients receiving sorafenib plus doxorubicin, the most commonly occurring all-cause adverse events (all grades) included fatigue, neutropenia, diarrhoea, elevated bilirubin levels, abdominal pain, hand-foot skin reaction, left ventricular dysfunction, hypertension and febrile neutropenia.

Effect of erythromycin and rifampicin on monoethylglycinexylidide test

Background:The dynamic liver function test based on the hepatic conversion of lidocaine to monoethylglycinexylidide (MEGX) provides a direct measure of the actual functional state of the liver. Cytochrome P450 (CYP) 3A4 has been proposed as the main CYP isoform responsible for MEGX formation. The concomitant use of either CYP3A4 inducer rifampicin or CYP3A4 inhibitor erythromycin may influence the results of MEGX test. Hence, the objective of this study was to evaluate the effect of a CYP3A4 inhibitor erythromycin and inducer rifampicin on the MEGX test.Materials and Methods:The study included 20 healthy male volunteers whose routine laboratory tests were normal. As per study protocol, MEGX test was carried out in all the participants after an overnight fast. All the participants were given 1 mg/kg lidocaine dose intravenously and MEGX concentration at 30 and 60 min after IV dose was measured using HPLC. These MEGX values served as control values. Ten subjects received 600 mg/day erythromycin orally for six days while remaining ten participants received 600 mg/day rifampicin orally for six days. On the sixth day, MEGX test was carried out two hours after the last dose.Result:Rifampicin increased the mean plasma concentration of MEGX30 from 93.94 ± 26.31 to 98.54 ± 24.94 μg/ml (P = 0.085) and MEGX60 from 134.34 ± 35.42 to 136.36 ± 33.14 μg/ml (P = 0.051). Erythromycin lowered the serum concentration of MEGX30 from 101.37 ± 39.39 to 96.67 ± 36.09 μg/ml (P = 0.128) and MEGX60 from 142.52 ± 42.65 to 138.98 ± 40.23 μg/ml (P = 0.159).Conclusion:It can be concluded from this study that the MEGX test is not affected by concomitant administration of CYP3A4 modifiers rifampicin and erythromycin.

Assessment of Human Pregnane X Receptor Involvement in Pesticide-Mediated Activation of CYP3A4 Gene

Assessment of foreign chemical inducibility on CYP3A4 is necessary to optimize drug therapies. The properties of chemicals such as pesticides, however, are not well investigated. In the present study, properties of various pesticides on human CYP3A4 induction have been tested using HepG2-derived cells stably expressing the CYP3A4 promoter/enhancer (3-1-10 cells) and the human pregnane X receptor (hPXR)-small interfering RNA (siRNA) system. Among the examined pesticides, 13 pesticides were observed to activate the CYP3A4 gene. Surprisingly, pyributicarb was found to increase the CYP3A4 reporter activity at 0.1 to 1 microM more strongly than typical CYP3A4 inducer rifampicin. Expression of hPXR-siRNA clearly diminished the pyributicarb-stimulated CYP3A4 reporter activity in 3-1-10 cells and decreased the endogenous CYP3A4 mRNA levels in HepG2 cells. Pyributicarb caused enhancement of CYP3A4-derived reporter activity in mouse livers introduced with hPXR by adenovirus. These results indicate pyributicarb as a potent activator of CYP3A4 gene, suggesting the existence of pesticides leading to CYP3A4 induction in our environment.

Pharmacokinetic Drug Interactions Involving 17??-Ethinylestradiol

17alpha-Ethinylestradiol (EE) is widely used as the estrogenic component of oral contraceptives (OC). In vitro and in vivo metabolism studies indicate that EE is extensively metabolised, primarily via intestinal sulfation and hepatic oxidation, glucuronidation and sulfation. Cytochrome P450 (CYP)3A4-mediated EE 2-hydroxylation is the major pathway of oxidative metabolism of EE. For some time it has been known that inducers of drug-metabolising enzymes (such as the CYP3A4 inducer rifampicin [rifampin]) can lead to breakthrough bleeding and contraceptive failure. Conversely, inhibitors of drug-metabolising enzymes can give rise to elevated EE plasma concentrations and increased risks of vascular disease and hypertension. In vitro studies have also shown that EE inhibits a number of human CYP enzymes, such as CYP2C19, CYP3A4 and CYP2B6. Consequently, there are numerous reports in the literature describing EE-containing OC formulations as perpetrators of pharmacokinetic drug interactions. Because EE may participate in multiple pharmacokinetic drug interactions as either a victim or perpetrator, pharmaceutical companies routinely conduct clinical drug interaction studies with EE-containing OCs when evaluating new chemical entities in development. It is therefore critical to understand the mechanisms underlying these drug interactions. Such an understanding can enable the interpretation of clinical data and lead to a greater appreciation of the profile of the drug by physicians, clinicians and regulators. This article summarises what is known of the drug-metabolising enzymes and transporters governing the metabolism, disposition and excretion of EE. An effort is made to relate this information to known clinical drug-drug interactions. The inhibition and induction of drug-metabolising enzymes by EE is also reviewed.