CYP Induction-Mediated Drug Interactions: in Vitro Assessment and Clinical Implications

Effect of aflatoxin B1 on nuclear receptors PXR, CAR, and AhR and their target cytochromes P450 mRNA expression in primary cultures of human hepatocytes

Aflatoxin B1 (AFB1), one of the most common mycotoxins found in human foods and animal feed, is principally hepatotoxic and hepatocarcinogenic. The aim of the present study was to explore the effect of AFB1 on messenger RNA (mRNA) expression of pregnane X receptor (PXR), constitutive androstane receptor (CAR), and aryl hydrocarbon receptor (AhR) and some of their target cytochromes using primary cultures of human hepatocytes. Our results showed that AFB1, at noncytotoxic increasing concentrations, caused a significant upregulation of cytochrome P 2B6 (CYP2B6), CYP3A5, and to a lesser extent CYP3A4 and CYP2C9. Pregnane X receptor and CAR mRNA expression increased in the 3 treated livers. Aflatoxin B1 was found also to induce an overexpression of CYP1A1 and CYP1A2 genes accompanied by an increase in AhR mRNA expression. These findings suggest that AFB1 could activate PXR, CAR, and AhR; however, further investigations are needed to confirm nuclear receptor activation by AFB1.

Architecture of the drug-drug interaction network

PURPOSE

Drug interaction information has been extensively compiled into large databases. The objective of the present study was to provide a systematic overview of the available drug interaction information, using a network approach.

METHODS

The drug-drug interaction information was retrieved from a comprehensive source reference that documents primary drug interaction information over an extended period of time. With careful examination of the information, we identified three continuously growing databases that consisted of 351, 636 and 966 drugs and 742, 1858 and 3351 pairs of interaction, respectively. We then constructed three drug-drug interaction networks in which the interacting drugs were treated as nodes and were connected with links that represent interactions. For each network, we determined the number of interactions that each drug in that network has, and prepared histograms to show the frequency distribution.

RESULTS

The frequency distribution or the probability that a given drug has k interactions, P(k), followed a power-law distribution, where the power law exponent was close to -1·5 and was independent of the network size. The results suggested that while the majority of the drugs in the network had few interactions (small k), highly interacting drugs (large k) were rare but contributed most of the network interactions.

CONCLUSIONS

The present study demonstrated that drug interaction information can be viewed and analysed as a connecting, growing network. As with many real-world networks, the drug interaction network was scale free, indicating that drug interaction information has been dominated by a relatively small number of highly interacting drugs.

Ochratoxin A induces CYP3A4, 2B6, 3A5, 2C9, 1A1, and CYP1A2 gene expression in primary cultured human hepatocytes: a possible activation of nuclear receptors

Ochratoxin A (OTA) is a mycotoxin produced by fungi of two genera: Penicillium and Aspergillus. OTA has been shown to be nephrotoxic, hepatotoxic, teratogenic, and immunotoxic to several species of animals and to cause kidney and liver tumors in mice and rats. Biotransformation of OTA has not been entirely elucidated. Several metabolites have been characterized in vitro and/or in vivo, whereas other metabolites remain to be characterized. At present, data available regarding OTA metabolism and cytochrome inductions concern only rodents or in vitro systems. The aim of the present study was to explore the effect of OTA on mRNA expression of some cytochromes known to be regulated by pregnane X receptor (PXR), constitutive androstane receptor (CAR), and aryl hydrocarbon receptor (AhR), using primary cultures of human hepatocytes. Our results showed that OTA reduced hepatocyte viability in a dose-dependent manner. Using quantitative real-time reverse-transcription polymerase chain reaction, our study showed that treatment of primary cultured human hepatocytes with noncytotoxic increasing concentrations of OTA for 24 hours caused a significant upregulation of CYP3A4, CYP2B6, and, to a lesser extent, CYP3A5 and CYP2C9. PXR mRNA expression increased in only 1 treated liver, whereas CAR mRNA expression was not affected. OTA was found also to induce an overexpression of CYP1A1 and CYP1A2 genes accompanied by an increase in AhR mRNA expression. These findings suggest that OTA could activate PXR and AhR; however, further investigations are needed to confirm nuclear-receptor activation by OTA.

Luciferin IPA-based higher throughput human hepatocyte screening assays for CYP3A4 inhibition and induction

The authors report here higher throughput screening (HTS) assays for the evaluation of CYP3A4 inhibition and CYP3A4 induction in human hepatocytes using a novel CYP3A4 substrate, luciferin IPA (LIPA). Using human recombinant CYP450 isoforms, LIPA was found to be metabolized extensively by CYP3A4 but not by CYP1A2, CYP2C9, CYP2C19, CYP2D6, or CYP2E1. In the 384-well plate CYP3A4 inhibition assay, the known inhibitors 1-aminobenzotriazole, erythromycin, ketoconazole, and verapamil were found to cause extensive (maximum inhibition of >80%), dose-dependent, statistically significant inhibition of LIPA metabolism. The non-CYP3A4 inhibitors diethyldithiocarbamate, quercetin, quinidine, sulfaphenazole, ticlopidine, and tranylcypromine were found to have substantially lower (maximum inhibition of <50%) or no apparent inhibitory effects in the HTS assay. In the 96-well plate induction assay, the CYP3A4 inducers rifampin, phenobarbital, carbamazepine, phenytoin, troglitazone, rosiglitazone, and pioglitazone yielded dose-dependent induction of LIPA metabolism, whereas the CYP1A2 inducers omeprazole and 3-methylcholanthrene did not display any induction in the CYP3A4 activity. The high sensitivity and specificity of the assays, the relative ease of execution, and reduced cost, time, and test material requirements suggest that the HTS assays may be applied routinely for screening a large number of chemicals in the drug discovery phase for CYP3A4 inhibitory and inducing potential.

Sensitivity of intravenous and oral alfentanil and pupillary miosis as minimal and noninvasive probes for hepatic and first-pass CYP3A induction

Systemic and oral clearances of alfentanil (ALF) are in vivo probes for hepatic and first-pass cytochrome P450 (CYP) 3A. Both ALF single-point plasma concentrations and miosis are surrogates for area under the concentration-time curve (AUC) and clearance and are minimal and noninvasive CYP3A probes. This investigation determined ALF sensitivity for detecting graded CYP3A induction and compared it with that of midazolam (MDZ). Twelve volunteers (sequential crossover) received 0, 5, 10, 25, or 75 mg oral rifampin for 5 days. MDZ and ALF were given intravenously and orally on sequential days. Dark-adapted pupil diameter was measured with blood sampling. Graded rifampin decreased plasma MDZ AUCs to 83, 76, 62, and 59% (intravenous (i.v.)) and 78, 66, 39, and 24% (oral) of control. Hepatic and first-pass CYP3A induction were detected comparably by plasma MDZ and ALF AUCs. Single ALF concentrations detected all CYP3A induction, whereas MDZ was less sensitive. ALF miosis detected induction of first-pass but not hepatic CYP3A.

Transgenic animal models in toxicology: historical perspectives and future outlook

Transgenic animal models are powerful tools for developing a more detailed understanding on the roles of specific genes in biological pathways and systems. Applications of these models have been made within the field of toxicology, most notably for the screening of mutagenic and carcinogenic potential and for the characterization of toxic mechanisms of action. It has long been a goal of research toxicologists to use the data from these models to refine hazard identification and characterization to better inform human health risk assessments. This review provides an overview on the applications of transgenic animal models in the assessment of mutagenicity and carcinogenicity, their use as reporter systems, and as tools for understanding the roles of xenobiotic-metabolizing enzymes and biological receptors in the etiology of chemical toxicity. Perspectives are also shared on the future outlook for these models in toxicology and risk assessment and how transgenic technologies are likely to be an integral tool for toxicity testing in the 21st century.

Developing predictive CSF biomarkers-a challenge critical to success in Alzheimer's disease and neuropsychiatric translational medicine

The need to develop effective treatments for Alzheimer's disease has been confounded by repeated clinical failures where promising new chemical entities that have been extensively characterized in preclinical models of Alzheimer's disease have failed to show efficacy in the human disease state. This has been attributed to: the selection of drug targets that have yet to be shown as causal to the disease as distinct from being the result of the disease process, a lack of congruence in the animal models of Alzheimer's disease, wild-type and transgenic, to the human disease, and the enrollment of patients in proof of concept clinical trials who are at too advanced a stage of the disease to respond to any therapeutic. The development of validated biomarkers that can be used for disease diagnosis and progression is anticipated to improve patient enrollment in clinical trials, to develop new animal models and to identify new disease targets for drug discovery. The present review assesses the status of current efforts in developing CSF biomarkers for Alzheimer's disease and briefly discusses the status of CSF biomarker efforts in schizophrenia, depression, Parkinson's disease and multiple sclerosis.

Pharmacokinetic-pharmacodynamic modeling of rifampicin-mediated Cyp3a11 induction in steroid and xenobiotic X receptor humanized mice

The purpose of this study was to develop a mechanistic pharmacokinetic-pharmacodynamic (PK-PD) model to describe the effects of rifampicin on hepatic Cyp3a11 RNA, enzymatic activity, and triazolam pharmacokinetics. Rifampicin was administered to steroid and xenobiotic X receptor (SXR) humanized mice at 10 mg/kg p.o. (every day for 3 days) followed by triazolam (4 mg/kg p.o.) 24 h after the last dose of rifampicin. Rifampicin and triazolam concentrations and Cyp3a11 RNA expression and activity in the liver were measured over the 4-day period. Elevations in Cyp3a11 RNA expression were observed 24 h after the first dose of rifampicin, reaching a maximum (∼10 times baseline) after the third dose and were sustained until day 4 and began declining 48 h after the last rifampicin dose. Similar changes in enzymatic activity were also observed. The triazolam serum area under the curve (AUC) was 5-fold lower in mice pretreated with rifampicin, consistent with enzyme induction. The final PK-PD model incorporated rifampicin liver concentration as the driving force for the time-delayed Cyp3a11 induction governed by in vitro potency estimates, which in turn regulated the turnover of enzyme activity. The PK-PD model was able to recapitulate the delayed induction of Cyp3a11 mRNA and enzymatic activity by rifampicin. Furthermore, the model was able to accurately anticipate the reduction in the triazolam plasma AUC by integrating a ratio of the predicted induced enzyme activity and basal activity into the equations describing triazolam pharmacokinetics. In conjunction with the SXR humanized mouse model, this mathematical approach may serve as a tool for predicting clinically relevant drug-drug interactions via pregnane X receptor-mediated enzyme induction and possibly extended to other induction pathways (e.g., constitutive androstane receptor).

Development of stably transfected human and rat hepatoma cell lines for the species-specific assessment of xenobiotic response enhancer module (XREM)-dependent induction of drug metabolism

Based on our current knowledge, PXR holds a key position in the induction of a selective battery of enzymes and transporters of drug metabolism. In order to prevent serious adverse drug effects or unpredicted drug-drug interactions (DDI), it is compulsory to investigate the possible inducing potency of drugs under development. Furthermore, analysis of the inducing potency of environmental pollutants and new or manufactured chemicals is part of toxicological risk assessment. In non-transfected human HepG2 and rat H4IIE hepatoma cells, we examined the characteristics of expression of 45 genes involved in drug metabolism. A few gene products such as CYP2B6 or CYP3A4 mRNA were prominent in HepG2 cells while their major rat counterparts were, e.g., CYP2B3 or CYP3A1/3A3. Furthermore, a number of xenobiotic receptors including PXR were expressed in both cell lines. A number of genes were regulated in a cell type and species-specific manner after incubation with the prototypical PXR agonists rifampicin or dexamethasone, respectively. Then, we established cell-based reporter gene assays for screening for PXR-dependent induction of drug metabolism. HepG2 and H4IIE cells were stably transfected with a reporter gene containing PXR responsive elements (XREMs) which mediate the induction of PXR target genes such as CYP3A enzymes. With both stable cell lines the CYP inducers clotrimazole, dexamethasone, omeprazole, phenobarbital, rifampicin, as well as the drug candidate EMD 392949 and the brominated flame retardants hexabromocylododecane (HBCD) and a pentabromodiphenyl ether (pentaBDE) mixture were screened. In the human HepG2-XREM3 and rat H4IIE-XREM3 cells, clotrimazole and HBCD were found as common activators of the human and rat PXR whereas pentaBDE was more effective with the human cell system. Omeprazole and phenobarbital did not induce the rat PXR-dependent reporter gene expression in H4IIE-XREM3 cells, while a moderate increase was found in HepG2-XREM3 cells. EMD 392949 also acted as inducer in human but not in rat cells confirming in vivo observations. In summary, the established PXR-dependent in vitro system allows the simultaneous, fast, and species-specific screening of chemicals, environmental contaminants, food ingredients and drugs for CYP3A induction in cells of human and rat origin.

Effects of rifampicin on the pharmacokinetics of roflumilast and roflumilast N-oxide in healthy subjects

AIMS

To evaluate the effect of co-administration of rifampicin, an inducer of cytochrome P450 (CYP)3A4, on the pharmacokinetics of roflumilast and roflumilast N-oxide. Roflumilast is an oral, once-daily phosphodiesterase 4 (PDE4) inhibitor, being developed for the treatment of chronic obstructive pulmonary disease. Roflumilast is metabolized by CYP3A4 and CYP1A2, with further involvement of CYP2C19 and extrahepatic CYP1A1. In vivo, roflumilast N-oxide contributes >90% to the total PDE4 inhibitory activity.

METHODS

Sixteen healthy male subjects were enrolled in an open-label, three-period, fixed-sequence study. They received a single oral dose of roflumilast 500 microg on days 1 and 12 and repeated oral doses of rifampicin 600 mg once daily on days 5-15. Plasma concentrations of roflumilast and roflumilast N-oxide were measured for up to 96 h. Test/Reference ratios and 90% confidence intervals (CIs) of geometric means for AUC and C(max) of roflumilast and roflumilast N-oxide and for oral apparent clearance (CL/F) of roflumilast were estimated.

RESULTS

During the steady-state of rifampicin, the AUC(0-infinity) of roflumilast decreased by 80% (point estimate 0.21; 90% CI 0.16, 0.27); C(max) by 68% (0.32; CI 0.26, 0.39); for roflumilast N-oxide, the AUC(0-infinity) decreased by 56% (0.44; CI 0.36, 0.55); C(max) increased by 30% (1.30; 1.15, 1.48); total PDE4 inhibitory activity decreased by 58% (0.42; 0.38, 0.48).

CONCLUSIONS

Co-administration of rifampicin and roflumilast led to a reduction in total PDE4 inhibitory activity of roflumilast by about 58%. The use of potent cytochrome P450 inducers may reduce the therapeutic effect of roflumilast.

Hepatic Drug-Metabolizing Enzyme Induction and Implications for Preclinical and Clinical Risk Assessment

Hepatic drug metabolizing enzyme (DME) induction complicates the development of new drugs owing to altered efficacy of concomitant treatments, reduction in exposure resulting from autoinduction, and potential generation of toxic metabolites. Risk assessment of DME induction during clinical evaluation is confounded by several uncertainties pertaining to hazard identification and dose response analysis. Hepatic DME induction rarely leads to clinical evidence of altered metabolism and toxicity in the patient, which typically occur only if the DME induction is relatively severe. High drug doses are associated with a greater likelihood of hepatic DME induction and downstream effects; therefore, drugs of low potency requiring higher dosing tend to lead to a greater risk of drug–drug interactions. Vigilance in clinical trials for increased or diminished drug effect and, specifically, pharmacokinetic studies in the presence of other drugs and concomitant diseases are necessary for a drug risk assessment profile.

Efforts to remove hepatic DME-inducing drugs from development can be facilitated with current in vitro and in vivo assessments and will improve with the development of newer technologies. A carefully tailored case-by-case approach will lead to the development of efficacious drugs with an acceptable risk/benefit profile available to patients.

Liverbeads: a practical and relevant in vitro model for gene induction investigations

Cryopreserved rat hepatocytes entrapped within an alginate matrix, commercially available as Liverbeads, were evaluated for their relevance as a screening tool for gene induction in vitro, using quantitative real-time reverse transcriptase-polymerase chain reaction. They were treated with the reference compounds beta-naphthoflavone (BNF), phenobarbital (PB), pregnenolone 16alpha-carbonitrile (PCN), and clofibric acid (CLO) and analyzed for mRNA levels of Cyp1a1, Cyp2b1, Cyp3a1, Cyp4a1, Ugt1a6, and Ugt2b1. In addition, for PB and PCN, the results were compared with those obtained in rat liver in vivo. For each inducer, the gene induction profiles obtained with the Liverbeads in vitro model were time- and dose-dependent. The in vitro gene expression profiles confirmed the corresponding known P450 and UGT induction by each reference compound. In particular, the most strongly induced genes were Cyp1a1 by BNF, Cyp2b1 by PB, Cyp3a1 and Ugt2b1 by PCN, and Cyp4a1 and Cyp2b1 by CLO. Other genes investigated were also induced by the reference compounds, but the expression levels were lower, and increases were seen only after prolonged treatment. In particular, Ugt1a6 and Cyp2b1 were increased by BNF, Cyp1a1, Cyp3a1, and Ugt2b1 by PB, and Cyp3a1 and Ugt2b1 by CLO. All of these results correlated well with published in vitro data and our in vivo data. In conclusion, our results suggest that Liverbeads is a relevant and useful in vitro screening tool for determining gene induction profiles of new molecules. In addition, because Liverbeads from different species are available, this tool offers the possibility to conduct interspecies comparisons.

Drug-botanical interactions: a review of the laboratory, animal, and human data for 8 common botanicals

Many Americans use complementary and alternative medicine (CAM) to prevent or alleviate common illnesses, and these medicines are commonly used by individuals with cancer.These medicines or botanicals share the same metabolic and transport proteins, including cytochrome P450 enzymes (CYP), glucuronosyltransferases (UGTs), and P-glycoprotein (Pgp), with over-the-counter and prescription medicines increasing the likelihood of drug-botanical interactions.This review provides a brief description of the different proteins, such as CYPs, UGTs, and Pgp.The potential effects of drug-botanical interactions on the pharmacokinetics and pharmacodynamics of the drug or botanical and a summary of the more common models used to study drug metabolism are described.The remaining portion of this review summarizes the data extracted from several laboratory, animal, and clinical studies that describe the metabolism, transport, and potential interactions of 8 selected botanicals. The 8 botanicals include black cohosh, Echinacea, garlic, Gingko biloba, green tea, kava, milk thistle, and St John's wort; these botanicals are among some of the more common botanicals taken by individuals with cancer.These examples are included to demonstrate how to interpret the different studies and how to use these data to predict the likelihood of a clinically significant drug-botanical interaction.

Predicting drug-target interaction networks based on functional groups and biological features

BACKGROUND

Study of drug-target interaction networks is an important topic for drug development. It is both time-consuming and costly to determine compound-protein interactions or potential drug-target interactions by experiments alone. As a complement, the in silico prediction methods can provide us with very useful information in a timely manner.

METHODS/PRINCIPAL FINDINGS

To realize this, drug compounds are encoded with functional groups and proteins encoded by biological features including biochemical and physicochemical properties. The optimal feature selection procedures are adopted by means of the mRMR (Maximum Relevance Minimum Redundancy) method. Instead of classifying the proteins as a whole family, target proteins are divided into four groups: enzymes, ion channels, G-protein- coupled receptors and nuclear receptors. Thus, four independent predictors are established using the Nearest Neighbor algorithm as their operation engine, with each to predict the interactions between drugs and one of the four protein groups. As a result, the overall success rates by the jackknife cross-validation tests achieved with the four predictors are 85.48%, 80.78%, 78.49%, and 85.66%, respectively.

CONCLUSION/SIGNIFICANCE

Our results indicate that the network prediction system thus established is quite promising and encouraging.

Effect of prototypical inducers on ligand activated nuclear receptor regulated drug disposition genes in rodent hepatic and intestinal cells

AIM

The aim of this study was to investigate the impact on expression of mRNA and protein by paradigm inducers/activators of nuclear receptors and their target genes in rat hepatic and intestinal cells. Furthermore, assess marked inter laboratory conflicting reports regarding species and tissue differences in expression to gain further insight and rationalise previously observed species differences between rodent and human based systems.

METHODS

Quantitative real time-polymerase chain reaction (QRT-PCR) and immunoblots were used to assess messenger RNA (mRNA) and protein expression for CYP2B2, CYP3A1, CYP3A2, CYP3A9, ABCB1a, ABCB1b, ABCC1, ABCC2, pregnane X receptor (PXR), farnesoid X receptor (FXR) and constituitive androstane receptor (CAR) in rat hepatoma cell line H411E, intestinal cells, Iec-6, and rat primary hepatocytes, in response to exposure for 18 h with prototypical inducers.

RESULTS

Dexamethasone (DEX) and pregnenolone 16alpha carbonitrile (PCN) significantly induced PXR, CYP3A9, ABCB1a and ABCB1b. However, when co-incubated, DEX appeared to restrict PCN-dependent induction. Chenodeoxycholic acid (CDCA) was the only ligand to induce FXR in all three cell types. Despite previously reported species differences between PCN and rifampicin (RIF), both compounds exhibited a similar profile of induction.

CONCLUSION

Data presented herein may explain some of the discrepancies previously reported with respect to species differences from different laboratories and have important implications for study design.

Drug interactions

Drugs for allergy are often taken in combination with other drugs, either to treat allergy or other conditions. In common with many pharmaceuticals, most such drugs are subject to metabolism by P450 enzymes and to transmembrane transport. This gives rise to considerable potential for drug-drug interactions, to which must be added consideration of drug-diet interactions. The potential for metabolism-based drug interactions is increasingly being taken into account during drug development, using a variety of in silico and in vitro approaches. Prediction of transporter-based interactions is not as advanced. The clinical importance of a drug interaction will depend upon a number of factors, and it is important to address concerns quantitatively, taking into account the therapeutic index of the compound.

Predicting the clinical relevance of drug interactions from pre-approval studies

Drug interactions (DIs) may result in adverse drug events that could be prevented, but in many cases the available information on potential DIs is not easily transferable to clinical practice. The majority of studies date from preclinical or premarketing phases, using animals or human-derived sources that may not accurately reflect the growing clinical complexity of high-risk populations, such as the elderly, women, children, patients with chronic disease, polytherapy and impaired organ functions. Thus, at the time of approval of a new drug the information in the summary of product characteristics refers to potential DIs, but lacks specific management recommendations and is of limited clinical utility. Therefore, we set out to review in vitro and in vivo methods to predict and quantify potential DIs, to see whether these studies could help the physician tackle daily problems of the assessment and choice of combined drug therapies, and to propose, from a clinical point of view, how premarketing studies could be improved so as to help the physician at the patient's bedside. Preclinical and premarketing study design needs to be improved to make information easily accessible and clinically transferable. Studies should also take into account appropriate sample size, duration, co-morbidity, number of coadministered drugs, within- and between-subject variability, specific at-risk populations and/or drugs with a relatively narrow therapeutic window, and clinical endpoints. After premarketing development in situations where there is potential high risk of serious adverse events, specific phase IV studies (and/or active pharmacovigilance studies) should be required to monitor and quantitatively assess their clinical impact.

Prediction of pharmacokinetic drug-drug interaction caused by changes in cytochrome P450 activity using in vivo information

The aim of the present paper was to present an overview of the current status of the methods used to predict the magnitude of pharmacokinetic drug-drug interactions (DDIs) which are caused by apparent changes in cytochrome P450 (CYP) activity with an emphasis on a method using in vivo information. In addition, more than a hundred representative CYP substrates, inhibitor and inducer drugs involved in significant pharmacokinetic DDIs were selected from the literature and are listed. Although the magnitude of DDIs has been conventionally predicted based on in vitro experiments, their predictability is restricted occasionally due to several difficulties, including a precise determination of the unbound inhibitor concentrations at the enzyme site and a reliable in vitro measurement of the inhibition constant (K(i)). Alternatively, a simple method has been recently proposed for the prediction of the magnitude of DDIs based on information fully available from in vivo clinical studies. The new in vivo-based method would be applicable to the adjustment of dose regimens in actual pharmacotherapy situations although it requires a prior clinical study for the prediction. In this review, theoretical and quantitative relationships between the in vivo- and the in vitro-based prediction methods are considered. One of the interesting outcomes of the consideration is that the K(i)-normalized dose (dose/in vitro K(i)) of larger than approximately 20L (2-200L, when variability is considered) may be a pragmatic index which predicts significant in vivo DDIs. In the last part of the article, the relevance of the inclusion of the in vivo-based method into the process of new drug development is discussed for good prediction of in vivo DDIs.

Effect of simultaneous induction and inhibition of CYP3A by St John's Wort and ritonavir on CYP3A activity

We aimed to assess the effect of coadministration and withdrawal of a potent cytochrome P450 3A (CYP3A) inhibitor (ritonavir) and a potent CYP3A inducer (St John's wort) on CYP3A enzyme activity in an open, fixed-sequence study design. We investigated the pharmacokinetics of midazolam: (i) at baseline, (ii) after a single dose of either St John's wort or ritonavir (each n = 6), (iii) after 14 days of coadministration of ritonavir (300 mg b.i.d.) and St John's wort (300 mg t.i.d.), and (iv) at 2 days after cessation of both St John's wort and ritonavir. Combined administration of inducer and inhibitor resulted in a predominance of enzyme inhibition: coadministration of St John's wort and ritonavir with intravenous administration of midazolam resulted in an increase in the area under the plasma concentration-time curve (AUC)(0-8 h) of midazolam to 180% of baseline value, whereas with orally administered midazolam, the AUC(0-6 h) increased to 412% of baseline value (P < 0.05 for each). After cessation of the coadministered drugs, the AUC(0-6 h) of orally administered midazolam decreased to 6% of the level observed during combined administration, and the AUC(0-8 h) of intravenously administered midazolam decreased to 33% of the values observed during combined administration (P < 0.001 for each). Induction may be unmasked after the withdrawal of a combination of a potent CYP3A inhibitor and a potent CYP3A inducer, leading to substantial drops in drug exposure of CYP3A substrates. This may require substantial dose adjustments, particularly of orally administered drugs.

Studies on the pharmacokinetics and metabolism of a gamma-secretase inhibitor BMS-299897, and exploratory investigation of CYP enzyme induction

BMS-299897 is a gamma-secretase inhibitor that was effective in reducing amyloid beta-peptide (A beta) in transgenic mice and guinea pigs. Therefore, pharmacokinetic and drug metabolism studies were conducted in animals to support its clinical development. The compound appeared to have low to intermediate total body clearance and was orally bioavailable (24-100%). The oral absorption of BMS-299897 from solid dosage forms appeared to be dissolution rate-limited. BMS-299897 was distributed into extravascular space (V(ss) >or= 1.3 l kg(-1)), including brain (brain-to-plasma ratio = 0.13-0.50). BMS-299897 appeared to be a P-glycoprotein (P-gp) substrate as the brain-to-plasma ratio was two-fold higher in the mdr1a knockout mouse as compared with the wild-type. Apparent autoinduction by BMS-299897 was observed in murine and rat efficacy and toxicity studies. In vitro, BMS-299897 was a weaker inducer of cytochrome P450 3A4 (CYP3A4) and a weaker transactivator of human pregnane X receptor (hPXR) as compared with rifampicin. Induction of human UGT1A and UGT2B was evaluated in primary human hepatocytes, but the results were inconclusive. A low potential for autoinduction in humans was predicted at a clinical dose of 250 mg and the prediction was consistent with the findings from a clinical multiple-dose study with BMS-299897 in probable Alzheimer's patients.

Intestinal cell-specific vitamin D receptor (VDR)-mediated transcriptional regulation of CYP3A4 gene

CYP3A4 is the most important drug-metabolizing enzyme that is involved in biotransformation of more than 50% of drugs. Pregnane X receptor (PXR) dominantly controls CYP3A4 inducibility in the liver, whereas vitamin D receptor (VDR) transactivates CYP3A4 in the intestine by secondary bile acids. Four major functional PXR-binding response elements of CYP3A4 have been discovered and their cooperation was found to be crucial for maximal up-regulation of the gene in hepatocytes. VDR and PXR recognize similar response element motifs and share DR3(XREM) and proximal ER6 (prER6) response elements of the CYP3A4 gene. In this work, we tested whether the recently discovered PXR response elements DR4(eNR3A4) in the XREM module and the distal ER6 element in the CLEM4 module (CLEM4-ER6) bind VDR/RXRalpha heterodimer, whether the elements are involved in the intestinal transactivation, and whether their cooperation with other elements is essential for maximal intestinal expression of CYP3A4. Employing a series of gene reporter plasmids with various combinations of response element mutations transiently transfected into four intestinal cell lines, electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation assay (ChIP), we found that the CLEM4-ER6 motif interacts with VDR/RXRalpha heterodimer and partially cooperates with DR3(XREM) and prER6 in both basal and VDR-mediated inducible CYP3A4 regulation in intestinal cells. In contrast, eNR3A4 is involved only in the basal transactivation in intestinal cells and in the PXR-mediated rifampicin-induced transactivation of CYP3A4 in LS174T intestinal cells. We thus describe a specific ligand-induced VDR-mediated transactivation of the CYP3A4 gene in intestinal cells that differs from PXR-mediated CYP3A4 regulation in hepatocytes.

Follow-up to the pre-validation of a harmonised protocol for assessment of CYP induction responses in freshly isolated and cryopreserved human hepatocytes with respect to culture format, treatment, positive reference inducers and incubation conditions

We have compared induction responses of human hepatocytes to known inducers of CYP1A2, CYP2B6, CYP2C and CYP3A4/5 to determine whether the culture format, treatment regimen and/or substrate incubation conditions affected the outcome. CYP induction responses to prototypical inducers were equivalent regardless of pre-culture time (24h or 48h), plate format (60mm or 24-well plates) used or whether CYP activities were measured in microsomes or whole cell monolayers. Fold-induction of CYP3A4/5 by 1000muM PB and 10microM RIF were equivalent. In contrast, the fold-induction of CYP2B6 by PB was 3-fold higher that by 10microM RIF. In addition to inducing CYP1A2, 50microM OME also induced CYP3A4/5 in 50% of the donors tested. CYP2B6 was induced in 14 out of 21 donors by BNF; however CYP3A4/5 was unaffected by BNF in these donors. In order to confirm that donor-to-donor variation was not due to inter-laboratory differences, the induction responses of 5 different batches of cryopreserved human hepatocytes were compared in two different laboratories. The induction of CYP1A2, CYP2B6 and CYP3A4 measured in our laboratory were equivalent to those obtained by the commercial companies, proving good between-laboratory reproducibility. In conclusion, there is some flexibility in the treatment and incubation protocols for classical CYP induction assays on human hepatocytes. Both RIF and PB are suitable positive control inducers of CYP3A4/5 but PB may be more appropriate for CYP2B6 induction. BNF may be more appropriate for CYP1A2 induction than OME since, in contrast to the latter, it does not induce CYP3A4. Induction responses using hepatocytes from the same donor but in different labs can be expected to be similar. The good reproducibility of induction responses between laboratories using cryopreserved hepatocytes underlines the usefulness of these cells for these types of studies.

Serum-free collagen sandwich cultures of adult rat hepatocytes maintain liver-like properties long term: a valuable model for in vitro toxicity and drug-drug interaction studies

Cultures of primary hepatocytes from various species, including human, are used in several applications during pre-clinical drug development. Their use is however limited by cell survival and conservation of liver-specific functions in vitro. The differentiation status of hepatocytes in culture strongly depends on medium formulation and the extracellular matrix environment. We incubated primary rat hepatocytes for 10 days on collagen monolayer and in collagen sandwich cultures with or without serum. Restoration of polygonal cell shape and formation of functional bile canaliculi-like structures was stable only in serum-free sandwich cultures. Variations in general cell viability, as judged by the cellular ATP content, LDH release or apoptosis, were less pronounced between alternative cultures. The intracellular glutathione content was preserved close to in vivo levels especially in serum-free sandwich cultures. Basal activities of cytochrome P450 enzymes (P450) varied strongly between cultures. There was a minor effect on CYP1A but CYP2B activity was only detectable in the serum-free sandwich culture after 3 days and beyond. CYP2C activity was slightly elevated in both sandwich cultures, whereas CYP3A showed increased levels in both serum-free cultures. Inducibility of these P450s was fully maintained over time in serum-free collagen sandwich only. Gene expression was largely constant over time in serum-free sandwich cultures that was closest to liver. This liver-like property was supported by protein profiling results. Taken together, the serum-free collagen sandwich culture of primary rat hepatocytes maintained liver-like features over 10 days and is therefore a suitable model for long-term toxicity and drug-drug interaction studies.

Identification of a metabolically stable triazolopyrimidine-based dihydroorotate dehydrogenase inhibitor with antimalarial activity in mice

Plasmodium falciparum causes 1-2 million deaths annually. Yet current drug therapies are compromised by resistance. We previously described potent and selective triazolopyrimidine-based inhibitors of P. falciparum dihydroorotate dehydrogenase (PfDHODH) that inhibited parasite growth in vitro; however, they showed no activity in vivo. Here we show that lack of efficacy against P. berghei in mice resulted from a combination of poor plasma exposure and reduced potency against P. berghei DHODH. For compounds containing naphthyl (DSM1) or anthracenyl (DSM2), plasma exposure was reduced upon repeated dosing. Phenyl-substituted triazolopyrimidines were synthesized leading to identification of analogs with low predicted metabolism in human liver microsomes and which showed prolonged exposure in mice. Compound 21 (DSM74), containing p-trifluoromethylphenyl, suppressed growth of P. berghei in mice after oral administration. This study provides the first proof of concept that DHODH inhibitors can suppress Plasmodium growth in vivo, validating DHODH as a new target for antimalarial chemotherapy.

Insights into ligand-elicited activation of human constitutive androstane receptor based on novel agonists and three-dimensional quantitative structure-activity relationship

The human constitutive androstane receptor (CAR, NR1I3) is an important regulator of xenobiotic metabolism and other physiological processes. So far, only few CAR agonists are known and no explicit mechanism has been proposed for their action. Thus, we aimed to generate a 3D QSAR model that could explain the molecular determinants of CAR agonist action. To obtain a sufficient number of agonists that cover a wide range of activity, we applied a virtual screening approach using both structure- and ligand-based methods. We identified 27 novel human CAR agonists on which a 3D QSAR model was generated. The model, complemented by coregulator recruitment and mutagenesis results, suggests a potential activation mechanism for human CAR and may serve to predict potential activation of CAR for compounds emerging from drug development projects or for chemicals undergoing toxicological risk assessment.

General framework for the prediction of oral drug interactions caused by CYP3A4 induction from in vivo information

BACKGROUND

Induction of cytochrome P450 (CYP) 3A4 potentially reduces the blood concentrations of substrate drugs to less than one-tenth, which results in ineffective pharmacotherapy. Although the prediction of drug-drug interactions (DDIs) that are mediated by induction of CYP3A4 has been performed mainly on the basis of in vitro information, such methods have met with limited success in terms of their accuracy and applicability. Therefore, a realistic method for the prediction of CYP3A4-mediated inductive DDIs is of major clinical importance.

OBJECTIVE

The objective of the present study was to construct a robust and accurate method for the prediction of CYP3A4-mediated inductive DDIs. Such a method was developed on the basis of the principle applied for prediction of inhibitory DDIs in a previous report. A unique character of this principle is that the extent of alterations in the area under the plasma concentration-time curve (AUC) is predicted on the basis of in vivo information from minimal clinical studies without using in vitro data.

METHODS

The analysis is based on 42 DDI studies in humans reported in 37 published articles over the period 1983-2007. Kinetic analysis revealed that the reduction in the AUC of a substrate of CYP3A4 produced by consecutive administration of an inducer of CYP3A4 could be approximated by the equation 1/(1 + CRCYP3A4 * ICCYP3A4), where CRCYP3A4 is the ratio of the apparent contribution of CYP3A4 to the oral clearance of a substrate and ICCYP3A4 is the apparent increase in clearance of a substrate produced by induction of CYP3A4. Using this equation, the ICCYP3A4 was calculated for seven inducers (bosentan, carbamazepine, efavirenz, phenytoin, pioglitazone, rifampicin [rifampin], and St John's wort [hypericum]) on the basis of the reduction in the AUC of a coadministered standard substrate of CYP3A4, such as simvastatin, in ten DDI studies. The CRCYP3A4 was calculated for 22 substrates on the basis of the previously reported method from inhibitory DDI studies using a potent CYP3A4 inhibitor such as itraconazole or ketoconazole.

RESULTS

The proposed method enabled the prediction of AUC reduction by CYP3A4 induction with any combination of these substrates and inducers (total 154 matches). To assess the accuracy of the prediction, the AUC reductions in 32 studies were analysed. We found that the magnitude of the deviation between the mean values of the observed and predicted AUCs of all substrate drugs was <20% of the AUCs of the respective substrate drugs before administration of the inducers. In addition, rifampicin was found to be the most potent inducer among the compounds analysed in the present study, with an ICCYP3A4 value of 7.7, followed by phenytoin and carbamazepine, with values of 4.7 and 3.0, respectively. The ICCYP3A4 values of the other CYP3A4 inducers analysed in the present study were approximately 1 or less, which suggests that the AUCs of coadministered drugs may not be reduced to less than approximately half, even if the drug is metabolized solely by CYP3A4.

CONCLUSION

By using the method reported in the present study, the susceptibilities of a substrate drug of CYP3A4 to inductive DDIs can be predicted quantitatively. It was indicated that coadministration of rifampicin, phenytoin and carbamazepine may reduce plasma AUCs to less than half for a broad range of CYP3A4 substrate drugs, with CRCYP3A4 values greater than 0.13, 0.21 and 0.33, respectively.

Identification of pregnane-X receptor target genes and coactivator and corepressor binding to promoter elements in human hepatocytes

Chromatin immunoprecipitation (ChIP) studies were conducted in human hepatocytes treated with rifampicin in order to identify new pregnane-X receptor (PXR) target genes. Genes, both previously known to be involved and not known to be involved in drug disposition, with PXR response elements (PXREs) located upstream, within or downstream from their potentially associated genes, were identified. Validation experiments identified several new drug disposition genes with PXR binding sites. Of these, only CYP4F12 demonstrated increased binding in the presence of rifampicin. The role of PXR in the basal and inductive response of CYP4F12 was confirmed in hepatocytes in which PXR was silenced. We also assessed the association of PXR-coactivators and -corepressors with known and newly identified PXREs. Both PXR and the steroid receptor coactivator (SRC-1) were found to bind to PXREs in the absence of rifampicin, although binding was stronger after rifampicin treatment. We observed promoter-dependent patterns with respect to the binding of various coactivators and corepressors involved in the regulation of CYP4F12, CYP3A4, CYP2B6, UGT1A1 and P-glycoprotein. In conclusion, our findings indicate that PXR is involved in the regulation of CYP4F12 and that PXR along with SRC1 binds to a broad range of promoters but that many of these are not inducible by rifampicin.

Global gene expression changes including drug metabolism and disposition induced by three-dimensional culture of HepG2 cells-Involvement of microtubules

Constitutive upregulation and a higher degree of induction of drug metabolism and disposition-related genes were found in a three-dimensional HepG2 culture. The upregulated genes are believed to be regulated by different regulatory factors. Global gene expression analysis using the Affymetrix GeneChip indicated that altered expression of microtubule-related genes may change the expressed levels of drug metabolizing and disposition genes. Stabilization of microtubule molecules with docetaxel, a tubulin-stabilizing agent, in the two-dimensional culture showed gene expression patterns similar to those found in the three-dimensional culture, indicating that the culture environment affects drug metabolism functions in HepG2 cells.