A cell-based reporter gene assay for determining induction of CYP3A4 in a high-volume system

Pregnane X Receptor and the Gut-Liver Axis: A Recent Update

It is well-known that the pregnane X receptor (PXR)/Nr1i2 is a critical xenobiotic-sensing nuclear receptor enriched in liver and intestine and is responsible for drug-drug interactions, due to its versatile ligand binding domain (LBD) and target genes involved in xenobiotic biotransformation. PXR can be modulated by various xenobiotics including pharmaceuticals, nutraceuticals, dietary factors, and environmental chemicals. Microbial metabolites such as certain secondary bile acids (BAs) and the tryptophan metabolite indole-3-propionic acid (IPA) are endogenous PXR activators. Gut microbiome is increasingly recognized as an important regulator for host xenobiotic biotransformation and intermediary metabolism. PXR regulates and is regulated by the gut-liver axis. This review summarizes recent research advancements leveraging pharmaco- and toxico-metagenomic approaches that have redefined the previous understanding of PXR. Key topics covered in this review include: (1) genome-wide investigations on novel PXR-target genes, novel PXR-DNA interaction patterns, and novel PXR-targeted intestinal bacteria; (2) key PXR-modulating activators and suppressors of exogenous and endogenous sources; (3) novel bidirectional interactions between PXR and gut microbiome under physiologic, pathophysiological, pharmacological, and toxicological conditions; and (4) modifying factors of PXR-signaling including species and sex differences and time (age, critical windows of exposure, and circadian rhythm). The review also discusses critical knowledge gaps and important future research topics centering around PXR. SIGNIFICANCE STATEMENT: This review summarizes recent research advancements leveraging O'mics approaches that have redefined the previous understanding of the xenobiotic-sensing nuclear receptor pregnane X receptor (PXR). Key topics include: (1) genome-wide investigations on novel PXR-targeted host genes and intestinal bacteria as well as novel PXR-DNA interaction patterns; (2) key PXR modulators including microbial metabolites under physiological, pathophysiological, pharmacological, and toxicological conditions; and (3) modifying factors including species, sex, and time.

Effects of statins on the chemoresistance-The antagonistic drug-drug interactions versus the anti-cancer effects

There has been growing interest in the potential anti-cancer activity of statins based on evidence of their anti-proliferative, pro-apoptotic, and radiosensitizing properties, but no studies have focused on the effects of statins on the chemoresistance. In spite of their direct cytostatic/cytotoxic effects on the cancer cells, statins via drug interactions may affect therapeutic effects of the chemotherapy agents and so cause chemoresistance in cancer cells. Here, we aim to present the molecular mechanisms underlying cytotoxic effects of statins on the cancer cells against those mechanisms by which statins may lead to chemoresistance, in order to clarify whether the positive effects of the co-treatment of statins on the efficiency of chemotherapeutic agents is due to the natural anti-cancer effects of statins or it is due to increasing the cellular concentrations of chemotherapy drugs in cancer cells.

Transactivation Assays to Assess Canine and Rodent Pregnane X Receptor (PXR) and Constitutive Androstane Receptor (CAR) Activation

The pregnane X receptor (PXR/SXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are nuclear receptors (NRs) involved in the regulation of many genes including cytochrome P450 enzymes (CYPs) and transporters important in metabolism and uptake of both endogenous substrates and xenobiotics. Activation of these receptors can lead to adverse drug effects as well as drug-drug interactions. Depending on which nuclear receptor is activated will determine which adverse effect could occur, making identification important. Screening for NR activation by New Molecular Entities (NMEs) using cell-based transactivation assays is the singular high throughput method currently available for identifying the activation of a particular NR. Moreover, screening for species-specific NR activation can minimize the use of animals in drug development and toxicology studies. With this in mind, we have developed in vitro transactivation assays to identify compounds that activate canine and rat PXR and CAR3. We found differences in specificity for canine and rat PXR, with the best activator for canine PXR being 10 μM SR12813 (60.1 ± 3.1-fold) and for rat PXR, 10 μM dexamethasone (60.9 ± 8.4 fold). Of the 19 test agents examined, 10 and 9 significantly activated rat and canine PXR at varying degrees, respectively. In contrast, 5 compounds exhibited statistically significant activation of rat CAR3 and 4 activated the canine receptor. For canine CAR3, 50 μM artemisinin proved to be the best activator (7.3 ± 1.8 and 10.5 ± 2.2 fold) while clotrimazole (10 μM) was the primary activator of the rat variant (13.7 ± 0.8 and 26.9 ± 1.3 fold). Results from these studies demonstrated that cell-based transactivation assays can detect species-specific activators and revealed that PXR was activated by at least twice as many compounds as was CAR3, suggesting that there are many more agonists for PXR than CAR.

Determination of CYP3A4 Inducing Properties of Compounds Using a Laboratory-Developed Cell-Based Assay

A cell-based assay to measure cytochrome P450 3A4 (CYP3A4) induction was developed to screen for potential CYP3A4 inducers. This 96-well format assay utilizes HepG2 cells transfected with a gene construct of CYP3A4 proximal promoter linked to green fluorescence protein (GFP) gene, and the expression of the GFP is then measured quantitatively. Bergamottin at 5 to 25 µmol/L produced low induction relative to the positive control. Both curcumin and lycopene were not found to affect the expression of GFP, suggesting no induction properties toward CYP3A4. Interestingly, resveratrol produced significant induction from 25 µmol/L onward, which was similar to omeprazole and may warrant further studies. In conclusion, the present study demonstrated that this cell-based assay can be used as a tool to evaluate the potential CYP3A4 induction properties of compounds. However, molecular docking data have not provided satisfactory pointers to differentiate between CYP3A4 inducers from noninducers or from inhibitors, more comprehensive molecular screening may be indicated.

Cytochrome P450 time-dependent inhibition and induction: advances in assays, risk analysis and modelling

INTRODUCTION

It is widely accepted that current practice of polypharmacy inevitably increases the incidence of drug-drug interactions (DDIs). Serious DDIs are a major liability for new molecular entities entering the pharmaceutical market. Various strategies are employed to avoid problematic compounds for clinical development. Progress made with reversible CYP DDIs has prompted a switch to study and model time-dependent inhibition and induction interactions.

AREAS COVERED

An overview of popular experimental practices is presented with discussion of techniques and algorithms used to analyse the clinical DDI risk. Emphasis is placed on the transition from early, simple static equations, via more complex net mechanistic, static models to dynamic approaches involving multiple perpetrators and metabolites, simultaneous inhibition and induction.

EXPERT OPINION

Inclusion of the more conservative terms for parameters required for DDI evaluation may eliminate promising chemical space, encourages poor practice and hampers innovation. Breakthroughs have originated from understanding of 'outliers' from such analyses where CYP enzyme-transporter interplay may be involved. The role of key transporters in drug disposition is firmly established as the chemistry required to address new targets deviates from traditional 'drug-like' space. Attempts to model more complex interactions for substrates of both CYP enzymes and drug transporters are still in their infancy and will benefit from dynamic modelling.

Reporter Cell Lines for the Characterization of the Interactions between Human Nuclear Receptors and Endocrine Disruptors

Endocrine-disrupting chemicals (EDCs) are exogenous substances interfering with hormone biosynthesis, metabolism, or action, and consequently causing disturbances in the endocrine system. Various pathways are activated by EDCs, including interactions with nuclear receptors (NRs), which are primary targets of numerous environmental contaminants. The main NRs targeted by environmental contaminants are the estrogen (ER α, β) and the androgen (AR) receptors. ERs and AR have pleiotropic regulatory roles in a diverse range of tissues, notably in the mammary gland, the uterus, and the prostate. Thus, dysfunctional ERs and AR signaling due to inappropriate exposure to environmental pollutants may lead to hormonal cancers and infertility. The pregnane X receptor (PXR) is also recognized by many environmental molecules. PXR has a protective role of the body through its ability to regulate proteins involved in the metabolism, the conjugation, and the transport of many exogenous and endogenous compounds. However, the permanent activation of this receptor by xenobiotics may lead to premature drug metabolism, the formation, and accumulation of toxic metabolites and defects in hormones homeostasis. The activity of other NRs can also be affected by environmental molecules. Compounds capable of inhibiting or activating the estrogen related (ERRγ), the thyroid hormone (TRα, β), the retinoid X receptors (RXRα, β, γ), and peroxisome proliferator-activated (PPAR α, γ) receptors have been identified and are highly suspected to promote developmental, reproductive, neurological, or metabolic diseases in humans and wildlife. In this review, we provide an overview of reporter cell lines established to characterize the human NR activities of a large panel of EDCs including natural as well as industrial compounds such as pesticides, plasticizers, surfactants, flame retardants, and cosmetics.

Omeprazole and lansoprazole enantiomers induce CYP3A4 in human hepatocytes and cell lines via glucocorticoid receptor and pregnane X receptor axis

Benzimidazole drugs lansoprazole and omeprazole are used for treatment of various gastrointestinal pathologies. Both compounds cause drug-drug interactions because they activate aryl hydrocarbon receptor and induce CYP1A genes. In the current paper, we examined the effects of lansoprazole and omeprazole enantiomers on the expression of key drug-metabolizing enzyme CYP3A4 in human hepatocytes and human cancer cell lines. Lansoprazole enantiomers, but not omeprazole, were equipotent inducers of CYP3A4 mRNA in HepG2 cells. All forms (S-, R-, rac-) of lansoprazole and omeprazole induced CYP3A4 mRNA and protein in human hepatocytes. The quantitative profiles of CYP3A4 induction by individual forms of lansoprazole and omeprazole exerted enantiospecific patterns. Lansoprazole dose-dependently activated pregnane X receptor PXR in gene reporter assays, and slightly modulated rifampicin-inducible PXR activity, with similar potency for each enantiomer. Omeprazole dose-dependently activated PXR and inhibited rifampicin-inducible PXR activity. The effects of S-omeprazole were much stronger as compared to those of R-omeprazole. All forms of lansoprazole, but not omeprazole, slightly activated glucocorticoid receptor and augmented dexamethasone-induced GR transcriptional activity. Omeprazole and lansoprazole influenced basal and ligand inducible expression of tyrosine aminotransferase, a GR-target gene, in HepG2 cells and human hepatocytes. Overall, we demonstrate here that omeprazole and lansoprazole enantiomers induce CYP3A4 in HepG2 cells and human hepatocytes. The induction comprises differential interactions of omeprazole and lansoprazole with transcriptional regulators PXR and GR, and some of the effects were enantiospecific. The data presented here might be of toxicological and clinical importance, since the effects occurred in therapeutically relevant concentrations.

Fetal bovine serum and human constitutive androstane receptor: evidence for activation of the SV23 splice variant by artemisinin, artemether, and arteether in a serum-free cell culture system

The naturally occurring SV23 splice variant of human constitutive androstane receptor (hCAR-SV23) is activated by di-(2-ethylhexyl)phthalate (DEHP), which is detected as a contaminant in fetal bovine serum (FBS). In our initial experiment, we compared the effect of dialyzed FBS, charcoal-stripped, dextran-treated FBS (CS-FBS), and regular FBS on the basal activity and ligand-activation of hCAR-SV23 in a cell-based reporter gene assay. In transfected HepG2 cells cultured in medium supplemented with 10% FBS, basal hCAR-SV23 activity varied with the type of FBS (regular>dialyzed>CS). DEHP increased hCAR-SV23 activity when 10% CS-FBS, but not regular FBS or dialyzed FBS, was used. With increasing concentrations (1-10%) of regular FBS or CS-FBS, hCAR-SV23 basal activity increased, whereas in DEHP-treated cells, hCAR-SV23 activity remained similar (regular FBS) or slightly increased (CS-FBS). Subsequent experiments identified a serum-free culture condition to detect DEHP activation of hCAR-SV23. Under this condition, artemisinin, artemether, and arteether increased hCAR-SV23 activity, whereas they decreased it in cells cultured in medium supplemented with 10% regular FBS. By comparison, FBS increased the basal activity of the wild-type isoform of hCAR (hCAR-WT), whereas it did not affect the basal activity of the SV24 splice variant (hCAR-SV24) or ligand activation of hCAR-SV24 and hCAR-WT by 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime (CITCO). The use of serum-free culture condition was suitable for detecting CITCO activation of hCAR-WT and hCAR-SV24. In conclusion, FBS leads to erroneous classification of pharmacological ligands of hCAR-SV23 in cell-based assays, but investigations on functional ligands of hCAR isoforms can be conducted in serum-free culture condition.

Evidence for triclosan-induced activation of human and rodent xenobiotic nuclear receptors

The bacteriostat triclosan (2,4,4'-trichloro-2'-hydroxydiphenylether) (TCS) decreases rat serum thyroxine via putative nuclear receptor (NR) interaction(s) and subsequent transcriptional up-regulation of hepatic catabolism and clearance. However, due to the evolutionary divergence of the constitutive androstane and pregnane-X receptors (CAR, PXR), TCS-mediated downstream effects may be species-dependent. To test the hypothesis that TCS activates xenobiotic NRs across species, cell-based NR reporter assays were employed to assess potential activation of rat, mouse, and human PXR, and rat, mouse, and three splice variants of human CAR. TCS activated hPXR, acted as an inverse agonist of hCAR1, and as a weak agonist of hCAR3. TCS failed to activate rPXR in full-length receptor reporter assays, and instead acted as a modest inverse agonist of rCAR. Consistent with the rat data, TCS also failed to activate mPXR and was a modest inverse agonist of mCAR. These data suggest that TCS may interact with multiple NRs, including hPXR, hCAR1, hCAR3, and rCAR in order to potentially affect hepatic catabolism. Overall these data support the conclusion that TCS may interact with NRs to regulate hepatic catabolism and downstream thyroid hormone homeostasis in both rat and human models, though perhaps by divergent mechanisms.

Utility of DPX2 cells for predicting CYP3A induction-mediated drug-drug interactions and associated structure-activity relationships

The increase in cytochrome P450 (P450) enzyme activity noted upon exposure to therapeutics can elicit marked drug-drug interactions (DDIs) that may ultimately result in poor clinical outcome or adverse drug effects. As such, in vitro model systems that can rapidly and accurately determine whether potential therapeutics activate the human pregnane X receptor (PXR) and thus induce CYP3A P450 levels are highly sought after tools for drug discovery. To that end, we assessed whether DPX2 cells, a HepG2-derived cell line stably integrated with a PXR expression vector plus a luciferase reporter, could detect agents that not only cause PXR activation/CYP3A induction but also elicit clinical DDIs. All 20 clinical inducers and 9 of 15 clinical noninducers examined activated PXR in DPX2 cells (E(max) > 8-fold), although activation parameters obtained with the noninducers were not predictive of DDI. The relative induction score, calculated by combining PXR activation parameters (EC(50) and E(max)) in DPX2 cells for seven inducers plus four noninducers with their efficacious total plasma concentrations, strongly correlated (R(2) = 0.90) with the magnitude of induction of midazolam clearance. Thus, the DPX cell-based PXR activation system is not only capable of distinguishing potential inducers in a high-throughput manner but can also differentiate among compounds in predicting the magnitude of induction-mediated DDIs, providing a means for structure-activity relationship screening during discovery and development.

Human CYP2C8 is post-transcriptionally regulated by microRNAs 103 and 107 in human liver

The CYP2C genes are extensively regulated at the transcriptional stage. The present study shows for the first time that CYP2Cs are also regulated post-transcriptionally by microRNAs (miRNAs). By using online search engines, we found potential miRNA response elements (MREs) in the 3'-untranslated region (3'-UTR) of the CYP2C mRNAs. Among these were a MRE for the miRNAs miR-103 and miR-107 in the 3'-UTR of human CYP2C8. CYP2C8 protein levels (measured through immunoblot analyses) did not correlate with CYP2C8 mRNA levels (measured through quantitative polymerase chain reaction analyses) in human liver samples. The translation efficiency (protein/mRNA ratio) for CYP2C8 was inversely correlated with the expression of miR-103 and miR-107. When three copies of the putative MRE from CYP2C8 were inserted downstream from a luciferase expression reporter, transfection with precursors for miR-103 or miR-107 decreased luciferase activity in primary hepatocytes, whereas transfection with antisense oligonucleotides (AsOs) for miR-103/miR-107 increased luciferase activity. As expected, there was no effect of the precursors or AsOs when three copies of the putative MRE were inserted in the reverse orientation. When precursors for miR-103/miR-107 were transfected into primary human hepatocytes, CYP2C8 protein levels were decreased, whereas AsOs increased CYP2C8 protein levels. Neither precursors nor AsOs affected CYP2C8 mRNA levels, which indicated that the effect was post-transcriptional. Putative MRE motifs were also found in the 3'-UTRs of CYP2C9 and CYP2C19, which suggested that the same miRNAs could regulate translation of other members of the CYP2C family, although to a lesser degree than CYP2C8. These results clearly show that CYP2Cs are regulated post-transcriptionally by miR-103 and miR-107.

The statin class of HMG-CoA reductase inhibitors demonstrate differential activation of the nuclear receptors PXR, CAR and FXR, as well as their downstream target genes

The therapeutic class of HMG-CoA reductase inhibitors, the statins are central agents in the treatment of hypercholesterolaemia and the associated conditions of cardiovascular disease, obesity and metabolic syndrome. Although statin therapy is generally considered safe, a number of known adverse effects do occur, most commonly treatment-associated muscular pain. In vitro evidence also supports the potential for drug-drug interactions involving this class of agents, and to examine this a ligand-binding assay was used to determine the ability of six clinically used statins for their ability to directly activate the nuclear receptors pregnane X-receptor (PXR), farnesoid X-receptor (FXR) and constitutive androstane receptor (CAR), demonstrating a relative activation of PXR>FXR>CAR. Using reporter gene constructs, we demonstrated that this order of activation is mirrored at the transcriptional activation level, with PXR-mediated gene activation being pre-eminent. Finally, we described a novel regulatory loop, whereby activation of FXR by statins increases PXR reporter gene expression, potentially enhancing PXR-mediated responses. Delineating the molecular interactions of statins with nuclear receptors is an important step in understanding the full biological consequences of statin exposure. This demonstration of their ability to directly activate nuclear receptors, leading to nuclear receptor cross-talk, has important potential implications for their use within a polypharmacy paradigm.

Molecular mechanisms of drug transporter regulation

Interindividual differences in drug transporter expression can result in variability in drug response. This variation in gene expression is determined, in part, by the actions of nuclear hormone receptors that act as xenobiotic- and endobiotic-sensing transcription factors. Among the ligand-activated nuclear receptors, signaling through the pregnane X receptor (PXR), constitutive androstane receptor (CAR), farnesoid X receptor (FXR), and vitamin D receptor (VDR) constitute major pathways regulating drug transporter expression in tissues. Hence, these endobiotic- and xenobiotic-sensing nuclear receptors are intrinsically involved in environmental influences of drug response. Moreover, because nuclear receptor genes are polymorphic, these transcription factors are also thought to contribute to heritability of variable drug action. In this chapter, the molecular aspects of drug transporter gene regulation by ligand-activated nuclear receptors will be reviewed including their clinical relevance.

NCOA6 differentially regulates the expression of the CYP2C9 and CYP3A4 genes

CYP2Cs and CYP3A4 sub families of enzymes of the Cytochrome P450 super family metabolize clinically prescribed therapeutics. Constitutive and induced expressions of these enzymes are under the control of HNF4α and rifampicin activated PXR. In the present study, we show a mechanism for ligand dependent synergistic cross talk between PXR and HNF4α. Two-hybrid screening identified NCOA6 as a HNF4α interacting protein. NCOA6 was also found to interact with PXR through the first LXXLL motif in GST pull down and mammalian two hybrid assays. NCOA6 enhances the synergistic activation of CYP2C9 and CYP3A4 promoter activity by PXR and HNF4α in the presence of rifampicin. However silencing NCOA6 abrogated the synergistic activation and induction of CYP2C9 by PXR-HNF4α but not of CYP3A4. ChIP analysis revealed that NCOA6 could bridge HNF4α and PXR binding sites of the CYP2C9 promoter. Our results indicate that NCOA6 is responsible for the synergistic activation of CYP2C9 by HNF4α and PXR and NCOA6 differentially regulates CYP2C9 and CYP3A4 gene expression though both the genes are regulated by the same nuclear receptors.

Identification of clinically used drugs that activate pregnane X receptors

The pregnane X receptor (PXR) binds xenobiotics and regulates the expression of several drug-metabolizing enzymes and transporters. Human PXR (hPXR) activation and CYP3A4 induction can be involved in drug-drug interactions, resulting in reduced efficacy or increased toxicity. However, there are known species-specific differences with regard to PXR activation that should be taken into account when animal PXR data are extrapolated to humans. We profiled 2816 clinically used drugs from the National Institutes of Health Chemical Genomics Center Pharmaceutical Collection for their ability to activate hPXR and rat PXR (rPXR) at the cellular level, induce human CYP3A4 at the cellular level, and bind human PXR at the protein level. From 6 to 11% of drugs were identified as active across the four assays, which included assay-specific and pan-active compounds. The lowest concordance was observed between the hPXR and rPXR assays, and many compounds active in both assays nonetheless demonstrated significant potency differences between species. Analysis based on clustering potency values demonstrated the greatest activity correlation between the hPXR activation and CYP3A4 induction assays. Structure-activity relationship analysis identified chemical scaffolds that were pan-active (e.g., dihydropyridine calcium channel blockers) and others that were uniquely active in individual assays (e.g., steroids and fatty acids). These results provide important information on PXR activation by clinically used drugs, highlight the species specificity of PXR activation by xenobiotics, and provide a means of prioritizing compounds for follow-up studies and optimization efforts.

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.

The Chinese herbal medicine Sophora flavescens activates pregnane X receptor

Sophora flavescens (SF) is an herbal medicine widely used for the treatment of viral hepatitis, cancer, viral myocarditis, gastrointestinal hemorrhage, and skin diseases. It was recently reported that SF up-regulates CYP3A expression. The mechanism of SF-induced CYP3A expression is unknown. In the current study, we tested the hypothesis that SF-induced CYP3A expression is mediated by the activation of pregnane X receptor (PXR). We used two cell lines, DPX2 and HepaRG, to investigate the role of PXR in SF-induced CYP3A expression. The DPX2 cell line is derived from HepG2 cells with the stable transfection of human PXR and a luciferase reporter gene linked with a human PXR response element identified in the CYP3A4 gene promoter. In DPX2 cells, SF activated PXR in a concentration-dependent manner. We used a metabolomic approach to identify the chemical constituents in SF, which were further analyzed for their effect on PXR activation and CYP3A regulation. One chemical in SF, N-methylcytisine, was identified as an individual chemical that activated PXR. HepaRG is a highly differentiated hepatoma cell line that mimics human hepatocytes. In HepaRG cells, N-methylcytisine significantly induced CYP3A4 expression, and this induction was suppressed by the PXR antagonist sulforaphane. These results suggest that SF induces CYP3A expression via the activation of PXR.

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.

Humanized mouse lines and their application for prediction of human drug metabolism and toxicological risk assessment

Cytochrome P450s (P450s) are important enzymes involved in the metabolism of xenobiotics, particularly clinically used drugs, and are also responsible for metabolic activation of chemical carcinogens and toxins. Many xenobiotics can activate nuclear receptors that in turn induce the expression of genes encoding xenobiotic metabolizing enzymes and drug transporters. Marked species differences in the expression and regulation of cytochromes P450 and xenobiotic nuclear receptors exist. Thus, obtaining reliable rodent models to accurately reflect human drug and carcinogen metabolism is severely limited. Humanized transgenic mice were developed in an effort to create more reliable in vivo systems to study and predict human responses to xenobiotics. Human P450s or human xenobiotic-activated nuclear receptors were introduced directly or replaced the corresponding mouse gene, thus creating "humanized" transgenic mice. Mice expressing human CYP1A1/CYP1A2, CYP2E1, CYP2D6, CYP3A4, CY3A7, pregnane X receptor, and peroxisome proliferator-activated receptor alpha were generated and characterized. These humanized mouse models offer a broad utility in the evaluation and prediction of toxicological risk that may aid in the development of safer drugs.

Induction of P-glycoprotein expression and function in human intestinal epithelial cells (T84)

Intestinal induction of Pgp is known to limit the oral availability of certain drug compounds and give rise to detrimental drug-drug interactions. We have investigated the induction of P-glycoprotein (Pgp; MDR1) activity in a human intestinal epithelial cell line (T84) following pre-exposure to a panel of drug compounds, reported to be Pgp substrates, inhibitors or inducers. Human MDR1-transfected MDCKII epithelial monolayers were used to assess Pgp substrate interactions and inhibition of digoxin secretion by the selected drug compounds. The T84 cell line was used to assess induction of Pgp-mediated digoxin secretion following pre-exposure to the same compounds. Changes in gene expression (MDR1, MRP2, PXR and CAR) were determined by quantitative RT-PCR. Net transepithelial digoxin secretion was increased (1.3 fold, n=6, P<0.05) following pre-exposure to the PXR activator hyperforin (100nM, 72h), as was MDR1 mRNA expression (3.0 fold, n=4, P<0.05). A number of Pgp substrates (quinidine, amprenavir, irinotecan, topotecan, atorvastatin and erythromycin) induced net digoxin secretion, as did the non-Pgp substrate artemisinin. Various non-Pgp substrates demonstrated inhibition of digoxin secretion (verapamil, mifepristone, clotrimazole, mevastatin, diltiazem and isradipine) but did not induce Pgp-mediated digoxin secretion. Of the compounds that increased Pgp secretion, quinidine, topotecan, atorvastatin and amprenavir pre-exposure also elevated MDR1 mRNA levels, whereas erythromycin, irinotecan and artemisinin displayed no change in transcript levels. This indicates possible post-translational regulation of digoxin secretion. Finally, a strong correlation between drug modulation of MRP2 and PXR mRNA expression levels was evident.

Nuclear receptor coactivator 6 mediates the synergistic activation of human cytochrome P-450 2C9 by the constitutive androstane receptor and hepatic nuclear factor-4alpha

Nuclear receptor coactivator 6 (NCOA6) also known as PRIP/RAP250/ASC-2 anchors a steady-state complex of cofactors and function as a transcriptional coactivator for certain nuclear receptors. This is the first study to identify NCOA6 as a hepatic nuclear factor 4alpha (HNF4alpha)-interacting protein. CYP2C9 is an important enzyme that metabolizes both commonly used therapeutic drugs and important endogenous compounds. We have shown previously that constitutive androstane receptor (CAR) (a xenobiotic-sensing receptor) up-regulates the CYP2C9 promoter through binding to a distal site, whereas HNF4alpha transcriptionally up-regulates CYP2C9 via proximal sites. We demonstrate ligand-enhanced synergistic cross-talk between CAR and HNF4alpha. We identify NCOA6 as crucial to the underlying mechanism of this cross-talk. NCOA6 was identified as an HNF4alpha-interacting protein in this study using a yeast two-hybrid screen and GST pull-down assays. Furthermore, we identified NCOA6, CAR, and other coactivators as part of a mega complex of cofactors associated with HNF4alpha in HepG2 cells. Although the interaction of NCOA6 with CAR is specifically through the first LXXLL motif of NCOA6, both LXXLL motifs are involved in its interaction with HNF4alpha. Silencing of NCOA6 abrogated the synergistic activation of the CYP2C9 promoter and the synergistic induction of the CYP2C9 gene by CAR-HNF4alpha. Chromatin immunoprecipitation analysis revealed that NCOA6 can pull down both the proximal HNF4alpha and distal CAR binding sites of the CYP2C9 promoter and provides the basis for the recruitment of other cofactors. We conclude that the coactivator NCOA6 mediates the mechanism of the synergistic activation of the CYP2C9 gene by CAR and HNF4alpha.

Nuclear receptors and the regulation of drug-metabolizing enzymes and drug transporters: implications for interindividual variability in response to drugs

Erratic or unpredictable response to drugs remains a challenge of modern drug therapy. An important determinant of such interindividual differences in drug response is variability in the expression of drug-metabolizing enzymes and/or transporters at sites of absorption and/or tissue distribution. Variable drug-metabolizing enzyme and transporter expression can result in unpredictable exposure and tissue distribution of drugs and may manifest as adverse effects or therapeutic failure. In the past decade, important new insights have been made relating to the regulatory mechanisms governing the expression of drug-metabolizing enzymes and transporters by ligand-activated nuclear receptors. Specifically, there is compelling evidence to demonstrate that PXR, CAR, FXR, LXR, VDR, HNF4alpha, and AhR form a battery of nuclear receptors that regulate the expression of many important drug-metabolizing enzyme and transporters. In this review, the authors focus on clinically important drug-metabolizing enzymes such as CYP3A4, CYP2B6, CYP2C9, CYP2C19, UGT1A1, SULT2A1, and glutathione S-transferases and their regulation by nuclear receptors. They also review the nuclear receptor-mediated regulation of drug transporters such as MDR1, MRP2, MRP4, BSEP, BCRP, NTCP, OATP1B3, and OATP1A2. Finally, they outline how the drug development process has been affected by the current understanding of the involvement of nuclear receptors in the regulation of drug disposition genes.

Identification and characterization of cholest-4-en-3-one, oxime (TRO19622), a novel drug candidate for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive death of cortical and spinal motor neurons, for which there is no effective treatment. Using a cell-based assay for compounds capable of preventing motor neuron cell death in vitro, a collection of approximately 40,000 low-molecular-weight compounds was screened to identify potential small-molecule therapeutics. We report the identification of cholest-4-en-3-one, oxime (TRO19622) as a potential drug candidate for the treatment of ALS. In vitro, TRO19622 promoted motor neuron survival in the absence of trophic support in a dose-dependent manner. In vivo, TRO19622 rescued motor neurons from axotomy-induced cell death in neonatal rats and promoted nerve regeneration following sciatic nerve crush in mice. In SOD1(G93A) transgenic mice, a model of familial ALS, TRO19622 treatment improved motor performance, delayed the onset of the clinical disease, and extended survival. TRO19622 bound directly to two components of the mitochondrial permeability transition pore: the voltage-dependent anion channel and the translocator protein 18 kDa (or peripheral benzodiazepine receptor), suggesting a potential mechanism for its neuroprotective activity. TRO19622 may have therapeutic potential for ALS and other motor neuron and neurodegenerative diseases.

Cytotoxicity of chloroacetanilide herbicide alachlor in HepG2 cells independent of CYP3A4 and CYP3A7

Alachlor is cytotoxic to human hepatoblastoma HepG2s, a cell line that expresses constitutive CYP3A7 and dexamethasone (DEX)-inducible CYP3A4 and CYP3A7. CYP3A4 catalyzes alachlor N-dealkylation to 2-chloro-N-(2,6-diethylphenyl)acetamide (CDEPA), precursor of 2,6-diethylbenzoquinoneimine, putative reactive metabolite for rat nasal carcinogenicity. We hypothesized that HepG2 alachlor cytotoxicity would be mediated by CYP3A4/7 and increased with DEX. Here, we report time-dependent alachlor cytotoxicity (EC(50) approximately 500 microM and 264+/-17 microM at 6 and 24h, respectively) as assessed by lactate dehydrogenase leakage. DEX pretreatment (25 microM, 48 h) significantly increased CYP3A7-catalyzed luciferin 6' benzylether O-debenzylation, but had no effect on alachlor toxicity. Further, CYP3A4/7 inhibitor triacetyloleandomycin did not prevent, but rather potentiated, alachlor cytotoxicity. In agreement, CDEPA was less toxic than parent alachlor. HepG2 CYP3A4 activity was unaffected by 48 h DEX pretreatment; therefore, studies were done in DPX-2 cells, a HepG2 derivative engineered to overexpress pregnane-X receptor (PXR) that exhibits rifampicin (RIF)-inducible endogenous CYP3A4. Alachlor cytotoxicity in DPX-2 cells occurred over a concentration range equivalent to that in HepG2. CYP3A4 activity of DPX-2 cells treated with RIF (10 microM, 48 h) was twice that of untreated cells, but RIF did not increase alachlor toxicity. These results demonstrate that neither CYP3A4 nor CYP3A7 initiate a pathway leading to a toxic alachlor metabolite.

The PREgnane X receptor gene-humanized mouse: a model for investigating drug-drug interactions mediated by cytochromes P450 3A

The most common clinical implication for the activation of the human pregnane X receptor (PXR) is the occurrence of drug-drug interactions mediated by up-regulated cytochromes P450 3A (CYP3A) isozymes. Typical rodent models do not predict drug-drug interactions mediated by human PXR because of species differences in response to PXR ligands. In the current study, a PXR-humanized mouse model was generated by bacterial artificial chromosome (BAC) transgenesis in Pxr-null mice using a BAC clone containing the complete human PXR gene and 5'- and 3'-flanking sequences. In this PXR-humanized mouse model, PXR is selectively expressed in the liver and intestine, the same tissue expression pattern as CYP3A. Treatment of PXR-humanized mice with the PXR ligands mimicked the human response, since both hepatic and intestinal CYP3As were strongly induced by rifampicin, a human-specific PXR ligand, but not by pregnenolone 16alpha-carbonitrile, a rodent-specific PXR ligand. In rifampicin-pretreated PXR-humanized mice, an approximately 60% decrease was observed for both the maximal midazolam serum concentration (C(max)) and the area under the concentration-time curve, as a result of a 3-fold increase in midazolam 1'-hydroxylation. These results illustrate the potential utility of the PXR-humanized mice in the investigation of drug-drug interactions mediated by CYP3A and suggest that the PXR-humanized mouse model would be an appropriate in vivo tool for evaluation of the overall pharmacokinetic consequences of human PXR activation by drugs.

Relative activation of human pregnane X receptor versus constitutive androstane receptor defines distinct classes of CYP2B6 and CYP3A4 inducers

Both the human pregnane X receptor (hPXR) and constitutive androstane receptor (hCAR) are capable of regulating CYP3A4 and CYP2B6 gene expression. However, the majority of currently identified CYP3A4 and CYP2B6 inducers are confirmed activators of hPXR but not hCAR. To compare these receptors with respect to their chemical selectivities, 16 drugs known to induce CYP3A4 and/or CYP2B expression were evaluated for relative activation of hPXR versus hCAR. Because of the high basal but low chemical-induced activation of hCAR in immortalized cells, alternative methods were used to evaluate hCAR activation potential. Thirteen of the 16 compounds were classified as moderate to strong hPXR activators. In contrast, carbamazepine (CMZ), efavirenz (EFV), and nevirapine (NVP) were classified as negligible or weak hPXR activators at concentrations associated with efficacious CYP2B6 reporter or endogenous gene induction in primary human hepatocytes, suggesting potential activation of hCAR. Subsequent experiments demonstrated that these three drugs efficiently induced nuclear accumulation of in vivo-transfected enhanced yellow fluorescent protein-hCAR and significantly increased expression of a CYP2B6 reporter gene when hCAR was expressed in CAR-/- mice. In addition, using a recently identified, chemically responsive splice variant of hCAR (hCAR3), the hCAR activation profiles of the 16 compounds were evaluated. By combining results from the hPXR- and hCAR3-based reporter gene assays, these inducers were classified as hPXR, hCAR, or hPXR/hCAR dual activators. Our results demonstrate that CMZ, EFV, and NVP induce CYP2B6 and CYP3A4 preferentially through hCAR and that hCAR3 represents a sensitive tool for in vitro prediction of chemical-mediated human CAR activation.

PXR and CAR: nuclear receptors which play a pivotal role in drug disposition and chemical toxicity

Xenobiotic metabolism and detoxification is regulated by receptors (e.g., PXR, CAR) whose characterization has contributed significantly to our understanding of drug responses in humans. Technologies facilitating the screening of compounds for receptor interactions provide valuable tools applicable in drug development. Most use in vitro systems or mice humanized for receptors in vivo. In vitro assays are limited by the reporter systems and cell lines chosen and are uninformative about effects in vivo. Humanized mouse models provide novel, exciting ways of understanding the functions of these genes. This article evaluates these technologies and current knowledge on PXR/CAR-mediated regulation of gene expression.

Synthetic drugs and natural products as modulators of constitutive androstane receptor (CAR) and pregnane X receptor (PXR)

Constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are members of the nuclear receptor superfamily. These transcription factors are predominantly expressed in the liver, where they are activated by structurally diverse compounds, including many drugs and endogenous substances. CAR and PXR regulate the expression of a broad range of genes, which contribute to transcellular transport, bioactivation, and detoxification of numerous xenochemicals and endogenous substances. This article discusses the importance of these receptors for pharmacology and toxicology, emphasizing the role of individual drugs and natural products as agonists, indirect activators, inverse agonists, and antagonists of CAR and PXR.

Pregnane X receptor-mediated transcription

The pregnane X receptor (PXR, receptor NR1I2) is a ligand-activated transcription factor that is activated by structurally diverse endogenous steroids and foreign chemicals and serves as an important steroid and xenobiotic sensor. This member of the nuclear receptor superfamily is highly expressed in liver and in the gastrointestinal tract, where it regulates transcription of a large set of genes that contribute to foreign compound metabolism and to the metabolism and transcellular transport of steroid hormones, bile acids, and other endogenous substances. This chapter summarizes studies of PXR and its biological functions and describes a cell culture-based luciferase reporter gene assay for determination of PXR transcriptional activity. This assay can be used to identify novel drugs and environmental chemicals that serve as PXR ligands and thereby modulate PXR activity and may aid in the prediction of drug-drug interactions and foreign chemical-induced toxicities associated with the activation of PXR transcriptional responses.

Construction of Several Human-derived Stable Cell Lines Displaying Distinct Profiles of CYP3A4 Induction

Cell lines which stably express reporter proteins through CYP3A4 gene activation have been developed for use in predicting CYP3A4 induction. Twelve clones showing distinct profiles on chemical-induced response were isolated. Among them, two clones showing high response for CYP3A4 inducers, namely clone 3-1-10 and 3-1-20, were further evaluated for their sensitivities, reproducibilities and applicabilities to predict CYP3A4 induction in human. Clone 3-1-10 showed higher response to rifampicin than to clotrimazole, whereas clone 3-1-20 had rather higher response to clotrimazole. Optimal plating density and highly reproducible response were observed at the range of 1.65-5.0 x 10(4) cell/cm2. Clear induction responses of more than ten chemicals were observed in both cell lines. The reporter activity was further dramatically increased after an introduction of human PXR. Induction with rifampicin was, however, not much altered between the absence and presence of hPXR. The luciferase activity remained unaltered and showed little fluctuation during the culture for more than 6 months. Due to the strikingly high sensitivity and reproducibility of this system, as compared to previously published systems, these HepG2-derived cell lines showing distinct response profiles as developed in the present study will offer high advantages for chemical screening of CYP3A4 inducibility.

Evaluation of Cell-Based Assays for Steroid Nuclear Receptors Delivered by Recombinant Baculoviruses

The authors describe the use of modified baculoviruses containing mammalian expression cassettes (BacMam technology) in steroid nuclear receptor reporter assays designed for screening and profiling agonist and antagonist compounds. Baculo-viruses were constructed that express full-length human genes for mineralocorticoid receptor (MR), glucocorticoid receptor (GR), progesterone receptor A (PR-A), and progesterone receptor B (PR-B) from the cytomegalovirus immediate early promoter. A virus carrying the mouse mammary tumor virus–firefly luciferase (MMTV-Luc) cassette was generated to provide a suitable reporter construct. Feasibility studies with BacMam-MR in single-dose tests of 1000 compounds showed high correlation to the standard transfection-based assay results. Likewise, in dose-response experiments, BacMam-based assays for GR and PR-B produced potency and efficacy values similar to transfection assay results. At various receptor/reporter ratios, the BacMam assays showed good flexibility, demonstrating consistent signal-to-background (S/B) ratios and compound potencies. Increasing transduction time from 24 to 48 h provided no benefit, actually reducing overall assay performance as measured by S/B and Z′ values. The BacMam technology was applied in studies of isoforms PR-A and PR-B, which showed similar responses to a series of agonists. Taken together, the results demonstrate the utility of steroid nuclear receptor BacMam constructs for compound screening procedures with high reproducibility, reduced turnaround time, and lower cost.

Cell-based models to study hepatic drug metabolism and enzyme induction in humans

Cell-based in vitro models are invaluable tools in elucidating the pharmacokinetic profile of a drug candidate during its drug discovery and development process. As biotransformation is one of the key determinants of a drug's disposition in the body, many in vitro models to study drug metabolism have been established, and others are still being developed and validated. This review is aimed at providing the reader with a concise overview of the characteristics and optimal application of established and emerging in vitro cell-based models to study human drug metabolism and induction of drug metabolising enzymes in the liver. The strengths and weaknesses of liver-derived models, such as primary hepatocytes, either freshly isolated or cryopreserved, and from adult or fetal donors, precision-cut liver slices, and cell lines, including immortalised cells, reporter cell lines, hepatocarcinoma-derived cell lines and recombinant cell lines, are discussed. Relevant cell culture configuration aspects as well as other models such as stem cell-derived hepatocyte-like cells and humanised animal models are also reviewed. The status of model development, their acceptance by health authorities and recommendations for the most appropriate use of the models are presented.

Cell-based high-throughput bioassays to assess induction and inhibition of CYP1A enzymes

CYP1A is a subfamily of cytochrome P450 enzymes involved in the metabolism of numerous therapeutic drugs and in the bioactivation of procarcinogens to mutagens. Because of their diverse metabolic capacities, differences in expression of CYP1A enzymes may profoundly influence drug-drug interactions and drug or carcinogen activation and detoxification. Here, we demonstrate that cell-based bioassays are capable of identifying xenobiotics that either alter aryl hydrocarbon receptor (AhR)-mediated CYP1A levels or produce inhibition of enzyme activity. To assess induction, a stable cell line harboring a luciferase reporter driven by multiple dioxin response elements (DREs) was developed. Using this cell line, AhR agonists and antagonists were identified among drugs, dietary agents, and environmental compounds. Of the chemicals examined, the therapeutic agent omeprazole induced reporter gene activity 12.5+/-0.41 fold above control, whereas the phytochemical, chrysin and environmental pollutant, benzanthracene enhanced luciferase activity 3.3+/-0.03 and 28.7+/-1.7 fold above control, respectively. Several natural products, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) prevented TCDD-mediated increases in luciferase expression. For example, the botanical kava inhibited TCDD-mediated induction by 88%. Northern blot analyses of CYP1A1 in HepG2 cells treated with similar agents validated results generated in the stable cell line. The stable cells were further used to identify inhibitors of CYP1A-mediated metabolism. Resveratrol and furafylline exhibited dose-dependent decreases in CYP1A1 and CYP1A2 enzyme activities with IC50 values of 1.89 and 0.79 microM, respectively. In summary, chemicals that possess the ability to alter CYP1A expression or inhibit CYP1A enzyme activities can be rapidly identified with the cell-based bioassays described here.

A PXR reporter gene assay in a stable cell culture system: CYP3A4 and CYP2B6 induction by pesticides

A stable hepatoma cell line expressing the human pregnane X receptor (hPXR) and the cytochrome P4503A4 (CYP3A4) distal and proximal promoters plus the luciferase reporter gene was developed to assess the ability of several xenobiotic agents to induce CYP3A4 and CYP2B6. After selection for neomycin resistance, one clone, displaying high luciferase activity in response to rifampicin (RIF), was isolated and the stable expression of hPXR was confirmed by reverse transcription polymerase chain reaction (RT-PCR). Dose-response curves were generated by treating these cells with increasing concentrations of RIF, phenobarbital (PB), clotrimazole (CLOT) or 5beta-pregnane-3,20-dione (5beta-PREGN). The effective concentrations for half maximal response (EC50) were determined for each of these compounds. RIF was the most effective compound, with maximal luciferase activity induced at 10 microM. The agonist activities of PXR-specific inducers measured using our stable model were consistent with those measured in transient transfectants. The abilities of organochlorine (OC), organophosphate (OP) and pyrethroid pesticides (PY) to activate hPXR were also assessed and found to be consistent with the abilities of these compounds to induce CYP3A4 and CYP2B6 in primary culture of human hepatocytes. These results suggest that CYP3A4 and CYP2B6 regulation through PXR activation by persistent pesticides may have an impact on the metabolism of xenobiotic agents and endogenous steroid hormones. Our model provides a useful tool for studying hPXR activation and for identifying agents capable of inducing CYP3A4 and CYP2B6.

A simultaneous assessment of CYP3A4 metabolism and induction in the DPX-2 cell line

The DPX-2 cell line, a derivative of HepG2 cells, harbors human PXR and a luciferase-linked CYP3A4 promoter. These cells were used in a panel of cell-based assays for a parallel assessment of CYP3A4 induction, metabolism, and inhibition at the cellular level. CYP3A4 induction in the DPX-2 cell line by various agents was monitored in 96-well plates by a luciferase-based transcriptional activation assay. Of the prototypical CYP3A4 inducers examined, all exhibited elevated luciferase activity in DPX-2 cells. CYP3A4 enzyme activity in noninduced and rifampicin-induced DPX-2 cells was also assessed using Vivid fluorogenic substrates. Significantly elevated CYP3A4 activity levels (2.8-fold +/- 0.2-fold above DMSO-treated cells) were found in DPX-2 cells after 48 hours of exposure to rifampicin, but were undetectable in parental HepG2 cells. Rifampicin-induced activity levels were found to be suitable for assessing the inhibitory potential of new chemical entities in downstream CYP3A4 inhibition assays. The elevated CYP3A4 activity was inhibited 85% by 10 microM ketoconazole. In addition, a cytotoxicity assay to correct for possible toxic effects of compounds at the cellular level was applied. The comparative data obtained with a combination of the above assays suggests that the application of several independent in vitro technologies used in DPX-2 cells is the best possible strategy for the assessment of the complex phenomena of CYP3A4 induction and inhibition.

Preclinical in vitro screening assays for drug-like properties

Besides pharmacological effects, ADMET drug properties, namely, absorption, disposition, metabolism, elimination and toxicity, are important drug properties critical for clinical success. In vitro human-based experimental system used in combination with in vivo animal systems, using animal species relevant to humans, represent the best approach to assess these important drug-like properties before clinical trials. Selection of drug candidates with the best ADMET properties should enhance the probability of clinical success.:

High volume bioassays to assess CYP3A4-mediated drug interactions: induction and inhibition in a single cell line

Exposure to certain xenochemicals can alter the catalytic activity of the major drug-metabolizing enzyme, CYP3A4, either by enhancing expression of this cytochrome P450 or inhibiting its activity. Such alterations can result in adverse consequences stemming from drug-drug interactions. A simplified and reliable tool for detecting the ability of candidate drugs to alter CYP3A4 levels or inhibit catalytic activity was developed by stable integration of human pregnane X receptor and a luciferase vector harboring the CYP3A4 enhancers. Treatment of stable transformants, namely DPX-2, with various concentrations of inducers including rifampicin, mifepristone, troglitazone, methoxychlor, and kava produced dose-dependent increases in luciferase expression (between 2- and 40-fold above dimethyl sulfoxide-treated cells). Northern blot analyses of CYP3A4 mRNA in DPX-2 cells exhibited a good correlation to results generated with the reporter gene assay (r(2) = 0.5, p < 0.01). Induction of CYP3A4 protein was examined by measuring catalytic activity with the CYP3A4 substrate, luciferin 6' benzyl ether (luciferin BE). Metabolism of luciferin BE by DPX-2 cells was enhanced 5.2-fold above dimethyl sulfoxide-treated cells by treatment with rifampicin. Constitutive androstane receptor-mediated regulation of CYP3A4 protein was addressed by measuring catalytic activity in a separate cell line over-expressing this receptor. Phenobarbital and dexamethasone produced 1.5- and 2.0-fold increases, respectively, above control in luciferin BE metabolism. To determine the utility of DPX-2 cells for identifying inhibitors of CYP3A4 catabolism, luciferin BE activity was measured in the presence of various concentrations of ketoconazole, erythromycin, or kava. These agents exhibited dose-dependent decreases in CYP3A4 activity with IC(50) values of 0.3 microM for ketoconazole, 108 microM for erythromycin, and 15.5 microg/ml for kava. Collectively, DPX-2 cells were used to identify xenobiotics that induce or inhibit CYP3A4 in a high throughput manner, demonstrating their applicability to early-stage drug development.