Use of the nuclear receptor PXR to predict drug interactions

Effect of Gut Microbiota on the Pharmacokinetics of Nifedipine in Spontaneously Hypertensive Rats

The pharmacokinetic variability of nifedipine widely observed in the clinic cannot be fully explained by pharmacogenomics. As a new factor affecting drug metabolism, how the gut microbiota affects the pharmacokinetics of nifedipine needs to be explored. Spontaneously hypertensive rats (SHRs) have been commonly used in hypertension-related research and served as the experimental groups; Wistar rats were used as control groups. In this study, the bioavailability of nifedipine decreased by 18.62% (p < 0.05) in the SHRs compared with the Wistar rats. Changes in microbiota were associated with the difference in pharmacokinetics. The relative abundance of Bacteroides dorei was negatively correlated with AUC0-t (r = -0.881, p = 0.004) and Cmax (r = -0.714, p = 0.047). Analysis of serum bile acid (BA) profiles indicated that glycoursodeoxycholic acid (GUDCA) and glycochenodeoxycholic acid (GCDCA) were significantly increased in the SHRs. Compared with the Wistar rats, the expressions of CYP3A1 and PXR were upregulated and the enzyme activity of CYP3A1 increased in the SHRs. Spearman's rank correlation revealed that Bacteroides stercoris was negatively correlated with GUDCA (r = -0.7126, p = 0.0264) and GCDCA (r = -0.6878, p = 0.0339). Moreover, GUDCA was negatively correlated with Cmax (r = -0.556, p = 0.025). In primary rat hepatocytes, GUDCA could induce the expressions of PXR target genes CYP3A1 and Mdr1a. Furthermore, antibiotic treatments in SHRs verified the impact of microbiota on the pharmacokinetics of nifedipine. Generally, gut microbiota affects the pharmacokinetics of nifedipine through microbial biotransformation or by regulating the enzyme activity of CYP3A1.

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.

Helix 12 stabilization contributes to basal transcriptional activity of PXR

Pregnane X receptor (PXR) plays an important role in xenobiotic metabolism. While ligand binding induces PXR-dependent gene transcription, PXR shows constitutive transcriptional activity in the absence of ligands when expressed in cultured cells. This constitutive activity sometimes hampers investigation of PXR activation by compounds of interest. In this study, we investigated the molecular mechanism of PXR activation. In the reported crystal structures of unliganded PXR, helix 12 (H12), including a coactivator binding motif, was stabilized, while it is destabilized in the unliganded structures of other nuclear receptors, suggesting a role for H12 stabilization in the basal activity of PXR. Since Phe420, located in the loop between H11 and H12, is thought to interact with Leu411 and Ile414 to stabilize H12, we substituted alanine at Phe420 (PXR-F420A) and separately inserted three alanine residues directly after Phe420 (PXR-3A) and investigated their influence on PXR-mediated transcription. Reporter gene assays demonstrated that the mutants showed drastically reduced basal activity and enhanced responses to various ligands, which was further enhanced by coexpression of the coactivator peroxisome proliferator-activated receptor gamma coactivator 1α. Mutations of both Leu411 and Ile414 to alanine also suppressed basal activity. Mammalian two-hybrid assays showed that PXR-F420A and PXR-3A bound to corepressors and coactivators in the absence and presence of ligands, respectively. We conclude that the intramolecular interactions of Phe420 with Leu411 and Ile414 stabilize H12 to recruit coactivators even in the absence of ligands, contributing to the basal transcriptional activity of PXR. We propose that the generated mutants might be useful for PXR ligand screening.

Deconvolution of Cytochrome P450 Induction Mechanisms in HepaRG Nuclear Hormone Receptor Knockout Cells

Pregnane X receptor (PXR), constitutive androstane receptor (CAR), and PXR/CAR knockout (KO) HepaRG cells, as well as a PXR reporter gene assay, were used to investigate the mechanism of CYP3A4 and CYP2B6 induction by prototypical substrates and a group of compounds from the Merck KGaA oncology drug discovery pipeline. The basal and inducible gene expression of CYP3A4 and CYP2B6 of nuclear hormone receptor (NHR) KO HepaRG relative to control HepaRG was characterized. The basal expression of CYP3A4 was markedly higher in the PXR (10-fold) and CAR (11-fold) KO cell lines compared with control HepaRG, whereas inducibility was substantially lower. Inversely, basal expression of CYP3A4 in PXR/CAR double KO (dKO) was low (10-fold reduction). Basal CYP2B6 expression was high in PXR KO (9-fold) cells which showed low inducibility, whereas the basal expression remained unchanged in CAR and dKO cell lines compared with control cells. Most of the test compounds induced CYP3A4 and CYP2B6 via PXR and, to a lesser extent, via CAR. Furthermore, other non-NHR-driven induction mechanisms were implicated, either alone or in addition to NHRs. Notably, 5 of the 16 compounds (31%) that were PXR inducers in HepaRG did not activate PXR in the reporter gene assay, illustrating the limitations of this system. This study indicates that HepaRG is a highly sensitive system fit for early screening of cytochrome P450 (P450) induction in drug discovery. Furthermore, it shows the applicability of HepaRG NHR KO cells as tools to deconvolute mechanisms of P450 induction using novel compounds representative for oncology drug discovery. SIGNIFICANCE STATEMENT: This work describes the identification of induction mechanisms of CYP3A4 and CYP2B6 for an assembly of oncology drug candidates using HepaRG nuclear hormone receptor knockout and displays its advantages compared to a pregnane X receptor reporter gene assay. With this study, risk assessment of drug candidates in early drug development can be improved.

A high-throughput cell-based gaussia luciferase reporter assay for measurement of CYP1A1, CYP2B6, and CYP3A4 induction

The induction of cytochrome P450s can result in reduced drug efficacy and lead to potential drug-drug interactions. The xenoreceptors-aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR)-play key roles in CYP induction by xenobiotics. In order to be able to rapidly screen for the induction of three enzymes (CYP1A1, CYP2B6, and CYP3A4), we generated a stable AhR-responsive HepG2 cell line, a stable CAR-responsive HepG2 cell line, and a stable PXR-responsive HepG2 cell line.To validate these stable xenoreceptor-responsive HepG2 cell lines, we evaluated the induction of the different Gaussia reporter activities, as well as the mRNA and protein expression levels of endogenous CYPs in response to different inducers.The induction of luciferase activity in the stable xenoreceptor-responsive HepG2 cell lines by specific inducers occurred in a concentration dependent manner. There was a positive correlation between the induction of luciferase activities and the induction endogenous CYP mRNA expression levels. These xenoreceptor-responsive HepG2 cell lines were further validated with known CYP1A1, CYP2B6, and CYP3A4 inducers.These stable xenoreceptor-responsive HepG2 cell lines may be used in preclinical research for the rapid and sensitive detection of AhR, CAR, and PXR ligands that induce CYP450 isoforms.

Bioactivity profiling of per- and polyfluoroalkyl substances (PFAS) identifies potential toxicity pathways related to molecular structure

Per- and polyfluoroalkyl substances (PFAS) are a broad class of hundreds of fluorinated chemicals with environmental health concerns due to their widespread presence and persistence in the environment. Several of these chemicals have been comprehensively studied for experimental toxicity, environmental fate and exposure, and human epidemiology; however, most chemicals have limited or no data available. To inform methods for prioritizing these data-poor chemicals for detailed toxicity studies, we evaluated 142 PFAS using an in vitro screening platform consisting of two multiplexed transactivation assays encompassing 81 diverse transcription factor activities and tested in concentration-response format ranging from 137 nM to 300 μM. Results showed activity for various nuclear receptors, including three known PFAS targets--specifically estrogen receptor alpha and peroxisome proliferator receptors alpha and gamma. We also report activity against the retinoid X receptor beta, the key heterodimeric partner of type II, non-steroidal nuclear receptors. Additional activities were found against the pregnane X receptor, nuclear receptor related-1 protein, and nuclear factor erythroid 2-related factor 2, a sensor of oxidative stress. Using orthogonal assay approaches, we confirmed activity of representative PFAS against several of these targets. Finally, we identified key PFAS structural features associated with nuclear receptor activity that can inform future predictive models for use in prioritizing chemicals for risk assessment and in the design of new structures devoid of biological activity.

Application of Advanced Technologies in Natural Product Research: A Review with Special Emphasis on ADMET Profiling

The successful conversion of natural products (NPs) into lead compounds and novel pharmacophores has emboldened the researchers to harness the drug discovery process with a lot more enthusiasm. However, forfeit of bioactive NPs resulting from an overabundance of metabolites and their wide dynamic range have created the bottleneck in NP researches. Similarly, the existence of multidimensional challenges, including the evaluation of pharmacokinetics, pharmacodynamics, and safety parameters, has been a concerning issue. Advancement of technology has brought the evolution of traditional natural product researches into the computer-based assessment exhibiting pretentious remarks about their efficiency in drug discovery. The early attention to the quality of the NPs may reduce the attrition rate of drug candidates by parallel assessment of ADMET profiling. This article reviews the status, challenges, opportunities, and integration of advanced technologies in natural product research. Indeed, emphasis will be laid on the current and futuristic direction towards the application of newer technologies in early-stage ADMET profiling of bioactive moieties from the natural sources. It can be expected that combinatorial approaches in ADMET profiling will fortify the natural product-based drug discovery in the near future.

Transcription factors and ABC transporters: from pleiotropic drug resistance to cellular signaling in yeast

Budding yeast Saccharomyces cerevisiae survives in microenvironments utilizing networks of regulators and ATP-binding cassette (ABC) transporters to circumvent toxins and a variety of drugs. Our understanding of transcriptional regulation of ABC transporters in yeast is mainly derived from the study of multidrug resistance protein networks. Over the past two decades, this research has not only expanded the role of transcriptional regulators in pleiotropic drug resistance (PDR) but evolved to include the role that regulators play in cellular signaling and environmental adaptation. Inspection of the gene networks of the transcriptional regulators and characterization of the ABC transporters has clarified that they also contribute to environmental adaptation by controlling plasma membrane composition, toxic-metal sequestration, and oxidative stress adaptation. Additionally, ABC transporters and their regulators appear to be involved in cellular signaling for adaptation of S. cerevisiae populations to nutrient availability. In this review, we summarize the current understanding of the S. cerevisiae transcriptional regulatory networks and highlight recent work in other notable fungal organisms, underlining the expansion of the study of these gene networks across the kingdom fungi.

Computational prediction of cytochrome P450 inhibition and induction

Cytochrome P450 (CYP) enzymes play an important role in the phase I metabolism of many xenobiotics. Most drug-drug interactions (DDIs) associated with CYP are caused by either CYP inhibition or induction. The early detection of potential DDIs is highly desirable in the pharmaceutical industry because DDIs can cause serious adverse events, which can lead to poor patient health and drug development failures. Recently, many computational studies predicting CYP inhibition and induction have been reported. The current computational modeling approaches for CYP metabolism are classified as ligand- and structure-based; various techniques, such as quantitative structure-activity relationships, machine learning, docking, and molecular dynamic simulation, are involved in both the approaches. Recently, combining these two approaches have resulted in improvements in the prediction accuracy of DDIs. In this review, we present important, recent developments in the computational prediction of the inhibition of four clinically crucial CYP isoforms (CYP1A2, 2C9, 2D6, and 3A4) and three nuclear receptors (aryl hydrocarbon receptor, constitutive androstane receptor, and pregnane X receptor) involved in the induction of CYP1A2, 2B6, and 3A4, respectively.

Herb-Drug Interactions and Hepatotoxicity

BACKGROUND

In recent times, herbals or phytomedicines have become very popular due to their global acceptance as a complementary and alternative remedy. While modern drugs are commercially available only after laboratory validations, clinical trials, as well as approval from drug regulatory authorities, majority of the marketed herbal products lack such scientific evidence of efficacy and safety. This results in herb or herb-drug interaction induced unfavorable clinical outcomes without crucial documentation on their temporal relations and concomitant use.

METHODS

An online literature search for peer-reviewed articles was conducted on the PubMed, Europe PMC, Medline and Google Scholar portals, using the phrases: complementary & alternative medicine, traditional Chinese medicine, herb-drug interaction, mechanisms of herb-drug interaction, herb-induced toxicity, herbal hepatotoxicity and causality, traditional medicine, viral hepatitis, etc.

Results

The retrieved data showed that globally, patients are attracted to herbal remedies with the misconception that these are completely safe and therefore, use them simultaneously with prescription drugs. Notably, there exists a potential risk of herb-drug interactions leading to some adverse side effects, including hepatotoxicity. The toxicological effect of a drug or herb is due to the inhibition of drug metabolizing enzymes (e.g., cytochrome P450), including interactions with certain prescription drugs through various mechanisms. Several cases of hepatotoxicity due to use of herbals in viral hepatitis-related liver diseases have been recently reported. However, limited experimental data and clinical evidence on herbal pharmacokinetics hamper the evaluation and reporting of adverse reactions and the underlying mechanisms.

CONCLUSION

Herb-drug interaction related morbidity is thus an emerging serious public health issue with broad implications for clinicians, pharmaceutical industries and health authorities. Nonetheless, despite increasing recognition of herb-drug interaction, a standard system for interaction prediction and evaluation is still nonexistent. This review article discusses the herb-drug interactions related hepatotoxicity and underlying mechanisms, including drug metabolizing enzymes and their regulation.

Non-genomic effects of the Pregnane X Receptor negatively regulate platelet functions, thrombosis and haemostasis

The pregnane X receptor (PXR) is a nuclear receptor (NR), involved in the detoxification of xenobiotic compounds. Recently, its presence was reported in the human vasculature and its ligands were proposed to exhibit anti-atherosclerotic effects. Since platelets contribute towards the development of atherosclerosis and possess numerous NRs, we investigated the expression of PXR in platelets along with the ability of its ligands to modulate platelet activation. The expression of PXR in human platelets was confirmed using immunoprecipitation analysis. Treatment with PXR ligands was found to inhibit platelet functions stimulated by a range of agonists, with platelet aggregation, granule secretion, adhesion and spreading on fibrinogen all attenuated along with a reduction in thrombus formation (both in vitro and in vivo). The effects of PXR ligands were observed in a species-specific manner, and the human-specific ligand, SR12813, was observed to attenuate thrombus formation in vivo in humanised PXR transgenic mice. PXR ligand-mediated inhibition of platelet function was found to be associated with the inhibition of Src-family kinases (SFKs). This study identifies acute, non-genomic regulatory effects of PXR ligands on platelet function and thrombus formation. In combination with the emerging anti-atherosclerotic properties of PXR ligands, these anti-thrombotic effects may provide additional cardio-protective benefits.

Nutritional status modifies pregnane X receptor regulated transcriptome

Pregnane X receptor (PXR) regulates glucose and lipid metabolism, but little is known of the nutritional regulation of PXR function. We investigated the genome wide effects of the nutritional status on the PXR mediated gene regulation in the liver. Mice were treated with a PXR ligand pregnenolone 16α-carbonitrile (PCN) for 4 days and subsequently either fasted for 5 hours or after 4-hour fast treated with intragastric glucose 1 hour before sample collection. Gene expression microarray study indicated that PCN both induced and repressed much higher number of genes in the glucose fed mice and the induction of multiple well-established PXR target genes was potentiated by glucose. A subset of genes, including bile acid synthesis gene Cyp8b1, responded in an opposite direction during fasting and after glucose feeding. PXR knockout abolished these effects. In agreement with the Cyp8b1 regulation, PCN also modified the bile acid composition in the glucose fed mice. Contribution of glucose, insulin and glucagon on the observed nutritional effects was investigated in primary hepatocytes. However, only mild impact on PXR function was observed. These results show that nutritional status modifies the PXR regulated transcriptome both qualitatively and quantitatively and reveal a complex crosstalk between PXR and energy homeostasis.

Role of vitamin D receptor in the regulation of CYP3A gene expression

Vitamin D₃ (VD₃) is a multifunctional nutrient which can be either synthesized or absorbed from the diet. It plays a pivotal role in systemic calcium and phosphate homeostasis, as well as in various physiological and pathological processes. VD₃ is converted to the active form, 1α,25-dihydroxyvitamin D₃ (1,25-D3), by cytochrome P450 2R1 (CYP2R1)/CYP27A1 and CYP27B1 sequentially, and deactivated by multiple enzymes including CYP3A4. On the other hand, 1,25-D3 is capable of activating the transcription of CYP3A genes in humans, mice and rats. The vitamin D receptor (VDR)-mediated transactivation of human CYP3A4 and CYP3A5 resembles that known for pregnane X receptor (PXR). Activated VDR forms a heterodimer with retinoid X receptor α (RXRα), recruits co-activators, translocates to the cell nucleus, binds to the specific vitamin D responsive elements (VDRE), and activates the gene transcription. In mice, intestinal Cyp3a11 mRNA levels, but not those of hepatic CYP3As, were induced by in vivo administration of VDR and PXR agonists. In rats, intestinal Cyp3a1 and Cyp3a2 mRNAs were induced by 1,25-D3 or lithocholic acid (LCA), whereas hepatic Cyp3a2, but not Cyp3a1 and Cyp3a9, was modulated to 1,25-D3 treatment. In general, the VDR-mediated regulation of CYP3A presents species and organ specificity.

A Network of SLC and ABC Transporter and DME Genes Involved in Remote Sensing and Signaling in the Gut-Liver-Kidney Axis

Genes central to drug absorption, distribution, metabolism and elimination (ADME) also regulate numerous endogenous molecules. The Remote Sensing and Signaling Hypothesis argues that an ADME gene-centered network-including SLC and ABC "drug" transporters, "drug" metabolizing enzymes (DMEs), and regulatory genes-is essential for inter-organ communication via metabolites, signaling molecules, antioxidants, gut microbiome products, uremic solutes, and uremic toxins. By cross-tissue co-expression network analysis, the gut, liver, and kidney (GLK) formed highly connected tissue-specific clusters of SLC transporters, ABC transporters, and DMEs. SLC22, SLC25 and SLC35 families were network hubs, having more inter-organ and intra-organ connections than other families. Analysis of the GLK network revealed key physiological pathways (e.g., involving bile acids and uric acid). A search for additional genes interacting with the network identified HNF4α, HNF1α, and PXR. Knockout gene expression data confirmed ~60-70% of predictions of ADME gene regulation by these transcription factors. Using the GLK network and known ADME genes, we built a tentative gut-liver-kidney "remote sensing and signaling network" consisting of SLC and ABC transporters, as well as DMEs and regulatory proteins. Together with protein-protein interactions to prioritize likely functional connections, this network suggests how multi-specificity combines with oligo-specificity and mono-specificity to regulate homeostasis of numerous endogenous small molecules.

Conflicting alterations in hepatic expression of CYP3A and enzyme kinetics in rats exposed to 5-fluorouracil: relevance to pharmacokinetics of midazolam

1. 5-Fluorouracil (5-FU) is a pyrimidine derivative widely used for the treatment of cancer. In this study, we investigated the effects of 5-FU on the protein expression of hepatic CYP3A and their enzyme activity for metabolizing midazolam (MDZ), a typical substrate of CYP3A, in rat liver microsomes. We also examined the pharmacokinetic behavior of intravenously administered MDZ in rats treated with 5-FU (120 mg/kg, ip). 2. 5-FU was shown to induce hepatic CYP3A2 protein 2 days after administration without changing the expression of CYP3A1/3A23. However, affinity of 5-FU-inducible CYP3A protein to MDZ for its 4- and 1'-hydroxylation was decreased. Furthermore, the susceptibility of MDZ hydroxylation activity to a CYP3A inhibitor differed between the control and 5-FU groups. 3. Pharmacokinetic analysis of the MDZ disposition demonstrated no significant differences in the total clearance (CL_tot) and elimination rate constant (k_e) between the control and 5-FU-treated rats. Lack of alteration in the metabolic clearance of MDZ may be attributable to the induction of CYP3A protein with reduced affinity for the substrate of CYP3A enzymes. 4. Our findings provide novel information regarding the manifestation of inductive and interfering actions of 5-FU toward hepatic CYP3A to help in assessing the pharmacokinetics of CYP3A substrate drugs.

Pharmacokinetic alterations in poloxamer 407-induced hyperlipidemic rats

1. Plasma lipid profile abnormalities in hyperlipidemia can potentially alter the pharmacokinetics of a drug in a complex manner. To evaluate these pharmacokinetic alterations in hyperlipidemia and to determine the underlying mechanism(s), poloxamer 407-induced hyperlipidemic rats (HL rats), a well-established animal model of hyperlipidemia have been used. 2. In this review, we summarize findings on the pathophysiological and gene expression changes in drug-metabolizing enzymes and transporters in HL rats. We discuss pharmacokinetic changes in drugs metabolized primarily via hepatic cytochrome P450 (CYPs) in terms of alterations in hepatic intrinsic clearance (CL^'_int), free fraction in plasma (f_u) and hepatic blood flow rate (Q_H), depending on the hepatic excretion ratio, as well as drugs eliminated primarily by mechanisms other than hepatic CYPs. 3. For lipoprotein-bound drugs, increased binding to lipoproteins resulted in lower f_u values and volumes of distribution, with some exceptions. Generally, slower non-renal clearance (or total body clearance) of drugs that are substrates of hepatic CYP3A and CYP2C is well explained by the following factors: alterations in CL^'_int (due to down-regulation of hepatic CYPs), decreased f_u and/or possible decreased Q_H. 4. These consistent findings across studies in HL rats suggest more studies are needed at the clinical level for optimal pharmacotherapies for hyperlipidemia.

RNA-Seq reveals common and unique PXR- and CAR-target gene signatures in the mouse liver transcriptome

The pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are well-known xenobiotic-sensing nuclear receptors with overlapping functions. However, there lacks a quantitative characterization to distinguish between the PXR and CAR target genes and signaling pathways in the liver. The present study performed a transcriptomic comparison of the PXR- and CAR-targets using RNA-Seq in livers of adult wild-type mice that were treated with the prototypical PXR ligand PCN (200mg/kg, i.p. once daily for 4days in corn oil) or the prototypical CAR ligand TCPOBOP (3mg/kg, i.p., once daily for 4days in corn oil). At the given doses, TCPOBOP differentially regulated many more genes (2125) than PCN (212), and 147 of the same genes were differentially regulated by both chemicals. As expected, the top pathways differentially regulated by both PCN and TCPOBOP were involved in xenobiotic metabolism, and they also up-regulated genes involved in retinoid metabolism, but down-regulated genes involved in inflammation and iron homeostasis. Regarding unique pathways, PXR activation appeared to overlap with the aryl hydrocarbon receptor signaling, whereas CAR activation appeared to overlap with the farnesoid X receptor signaling, acute-phase response, and mitochondrial dysfunction. The mRNAs of differentially regulated drug-processing genes (DPGs) partitioned into three patterns, namely TCPOBOP-induced, PCN-induced, as well as TCPOBOP-suppressed gene clusters. The cumulative mRNAs of the differentially regulated DPGs, phase-I and -II enzymes, as well as efflux transporters were all up-regulated by both PCN and TCPOBOPOP, whereas the cumulative mRNAs of the uptake transporters were down-regulated only by TCPOBOP. The absolute mRNA abundance in control and receptor-activated conditions was examined in each DPG category to predict the contribution of specific DPG genes in the PXR/CAR-mediated pharmacokinetic responses. The preferable differential regulation by TCPOBOP in the entire hepatic transcriptome correlated with a marked change in the expression of many DNA and histone epigenetic modifiers. In conclusion, the present study has revealed known and novel, as well as common and unique targets of PXR and CAR in mouse liver following pharmacological activation using their prototypical ligands. Results from this study will further support the role of these receptors in regulating the homeostasis of xenobiotic and intermediary metabolism in the liver, and aid in distinguishing between PXR and CAR signaling at various physiological and pathophysiological conditions. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

BFCOD activity in fish cell lines and zebrafish embryos and its modulation by chemical ligands of human aryl hydrocarbon and nuclear receptors

Assessment of exposure and effect of fish to pharmaceuticals that contaminate aquatic environment is a current major issue in ecotoxicology and there is a need to develop specific biological marker to achieve this goal. Benzyloxy-4-trifluoromethylcoumarin-O-debenzyloxylase (BFCOD) enzymatic activity has been commonly used to monitor CYP3A activity in fish. In this study, we assessed the capacity of a panel of toxicologically relevant chemicals to modulate BFCOD activity in fish, by using in vitro and in vivo bioassays based on fish liver cell lines (PLHC-1, ZFL, RTL-W1) and zebrafish embryos, respectively. Basal BFCOD activity was detectable in all biological models and was differently modulated by chemicals. Ligands of human androgens, glucocorticoids, or pregnanes X receptors (i.e., dexamethasone, RU486, rifampicin, SR12813, T0901317, clotrimazole, ketoconazole, testosterone, and dihydrotestosterone) moderately increased or inhibited BFCOD activity, with some variations between the models. No common feature could be drawn by regards to their capacity to bind to these receptors, which contrasts with their known effect on mammalian CYP3A. In contrast, dioxins and polycyclic aromatic hydrocarbons (PAHs) strongly induced BFCOD activity (up to 30-fold) in a time- and concentration-dependent manner, both in vitro in all cell lines and in vivo in zebrafish embryos. These effects were AhR dependent as indicated by suppression of induced BFCOD by the AhR pathway inhibitors 8-methoxypsoralen and α-naphthoflavone. Altogether our result further question the relevance of using liver BFCOD activity as a biomarker of fish exposure to CYP3A-active compounds such as pharmaceuticals.

Sustained Isoprostane E2 Elevation, Inflammation and Fibrosis after Acute Ischaemia-Reperfusion Injury Are Reduced by Pregnane X Receptor Activation

Liver grafts donated after cardiac death are increasingly used to expand the donor pool but are prone to ischaemic-type biliary lesions. The anti-inflammatory effects of the activated pregnane X receptor have previously been shown to be beneficial in a number of inflammatory liver conditions. However, its role in reducing peri-portal inflammation and fibrosis following ischaemia-reperfusion injury has not been investigated. Hepatic injury and its response to pregnane X receptor activation was examined after partial hepatic ischaemia-reperfusion injury induced by surgically clamping the left and middle lobar blood vessels in rats. Molecular and pathological changes in the liver were examined over the following 28 days. Ischaemia-reperfusion injury resulted in transient cholestasis associated with microvillar changes in biliary epithelial cell membranes and hepatocellular injury which resolved within days after reperfusion. However, in contrast to chemically-induced acute liver injuries, this was followed by sustained elevation in isoprostane E2, peri-portal inflammation and fibrosis that remained unresolved in the ischaemic reperfused lobe for at least 28 days after clamping. Administration of pregnenolone-16α-carbonitrile—a rodent-specific pregnane X receptor activator—resulted in significant reductions in cholestasis, hepatic injury, ischaemic lobe isoprostane E2 levels, peri-portal inflammation and fibrosis. Hepatic ischaemia-reperfusion injury therefore results in inflammatory and fibrotic changes that persist well beyond the initial ischaemic insult. Drug-mediated activation of the pregnane X receptor reduced these adverse changes in rats, suggesting that the pregnane X receptor is a viable drug target to reduce ischaemic-type biliary lesions in recipients of liver transplants donated after cardiac death.

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.

Enhancement of CYP3A4 activity in Hep G2 cells by lentiviral transfection of hepatocyte nuclear factor-1 alpha

Human hepatoma cell lines are commonly used as alternatives to primary hepatocytes for the study of drug metabolism in vitro. However, the phase I cytochrome P450 (CYP) enzyme activities in these cell lines occur at a much lower level than their corresponding activities in primary hepatocytes, and thus these cell lines may not accurately predict drug metabolism. In the present study, we selected hepatocyte nuclear factor-1 alpha (HNF1α) from six transcriptional regulators for lentiviral transfection into Hep G2 cells to optimally increase their expression of the CYP3A4 enzyme, which is the major CYP enzyme in the human body. We subsequently found that HNF1α-transfected Hep G2 enhanced the CYP3A4 expression in a time- and dose-dependent manner and the activity was noted to increase with time and peaked 7 days. With a multiplicity of infection (MOI) of 100, CYP3A4 expression increased 19-fold and enzyme activity more than doubled at day 7. With higher MOI (1,000 to 3,000), the activity increased 8- to 10-fold; however, it was noted the higher MOI, the higher cell death rate and lower cell survival. Furthermore, the CYP3A4 activity in the HNF1α-transfected cells could be induced by CYP3A4-specific inducer, rifampicin, and metabolized nifedipine in a dose-dependent manner. With an MOI of 3,000, nifedipine-metabolizing activity was 6-fold of control and as high as 66% of primary hepatocytes. In conclusion, forceful delivery of selected transcriptional regulators into human hepatoma cells might be a valuable method to enhance the CYP activity for a more accurate determination of drug metabolism in vitro.

Tissue-specific regulation of pregnane X receptor in cancer development and therapy

As a ligand-dependent transcription factor of the nuclear hormone receptor superfamily, the pregnane X receptor (PXR) has a multitude of functions including regulating xenobiotic and cholesterol metabolism, energy homeostasis, gut mucosal defense, and cancer development. Whereas the detoxification functions of PXR have been widely studied and well established, the role of PXR in cancer has become controversial. With more than 60% of non-prescription and prescription drugs being metabolized by cytochrome P450 enzyme 3A4 (CYP3A4), a transcriptional target of PXR, insights into the regulation of PXR during systemic administration of novel treatment modalities will lead to a better understanding of PXR function in the context of human disease. Previous studies have suggested that PXR activation decreases drug sensitivity and augments chemoresistance in certain colon cancers mainly through the upregulation of CYP3A4 and multidrug resistance protein-1 (MDR1). Later studies suggest that downregulation of PXR expression may be oncogenic in hormone-dependent breast and endometrial cancers by reducing estrogen metabolism via CYP3A4; thus, higher estradiol concentrations contribute to carcinogenesis. These results suggest a differential role of PXR in tumor growth regulation dependent on tissue type and tumor microenvironment. Here, we will summarize the various mechanisms utilized by PXR to induce its diverse effects on cancerous tissues. Moreover, current approaches will be explored to evaluate the exploitation of PXR-mediated pathways as a novel mechanistic approach to cancer therapy.

Structure-activity relationship (SAR) optimization of 6-(indol-2-yl)pyridine-3-sulfonamides: identification of potent, selective, and orally bioavailable small molecules targeting hepatitis C (HCV) NS4B

A novel, potent, and orally bioavailable inhibitor of hepatitis C RNA replication targeting NS4B, compound 4t (PTC725), has been identified through chemical optimization of the 6-(indol-2-yl)pyridine-3-sulfonamide 2 to improve DMPK and safety properties. The focus of the SAR investigations has been to identify the optimal combination of substituents at the indole N-1, C-5, and C-6 positions and the sulfonamide group to limit the potential for in vivo oxidative metabolism and to achieve an acceptable pharmacokinetic profile. Compound 4t has excellent potency against the HCV 1b replicon, with an EC50 = 2 nM and a selectivity index of >5000 with respect to cellular GAPDH. Compound 4t has an overall favorable pharmacokinetic profile with oral bioavailability values of 62%, 78%, and 18% in rats, dogs, and monkeys, respectively, as well as favorable tissue distribution properties with a liver to plasma exposure ratio of 25 in rats.

Functional characterization of a full length pregnane X receptor, expression in vivo, and identification of PXR alleles, in zebrafish (Danio rerio)

The pregnane X receptor (PXR) (nuclear receptor NR1I2) is a ligand activated transcription factor, mediating responses to diverse xenobiotic and endogenous chemicals. The properties of PXR in fish are not fully understood. Here we report on cloning and characterization of full-length PXR of zebrafish, Danio rerio, and pxr expression in vivo. Initial efforts gave a cDNA encoding a 430 amino acid protein identified as zebrafish pxr by phylogenetic and synteny analysis. The sequence of the cloned Pxr DNA binding domain (DBD) was highly conserved, with 74% identity to human PXR-DBD, while the ligand-binding domain (LBD) of the cloned sequence was only 44% identical to human PXR-LBD. Sequence variation among clones in the initial effort prompted sequencing of multiple clones from a single fish. There were two prominent variants, one sequence with S183, Y218 and H383 and the other with I183, C218 and N383, which we designate as alleles pxr*1 (nr1i2*1) and pxr*2 (nr1i2*2), respectively. In COS-7 cells co-transfected with a PXR-responsive reporter gene, the full-length Pxr*1 (the more common variant) was activated by known PXR agonists clotrimazole and pregnenolone 16α-carbonitrile but to a lesser extent than the full-length human PXR. Activation of full-length Pxr*1 was only 10% of that with the Pxr*1 LBD. Quantitative real time PCR analysis showed prominent expression of pxr in liver and eye, as well as brain and intestine of adult zebrafish. The pxr was expressed in heart and kidney at levels similar to that in intestine. The expression of pxr in liver was weakly induced by ligands for mammalian PXR or constitutive androstane receptor (NR1I3). The results establish a foundation for PXR studies in this vertebrate model. PXR allelic variation and the differences between the full-length PXR and the LBD in reporter assays have implications for assessing the action of PXR ligands in zebrafish.

PXR antagonists and implication in drug metabolism

Adopted orphan nuclear receptor (NR), pregnane X receptor (PXR), plays a central role in the regulation of xeno- and endobiotic metabolism. Since the discovery of the functional role of PXR in 1998, there is evolving evidence for the role of PXR agonists in abrogating metabolic pathophysiology (e.g., cholestasis, hypercholesterolemia, and inflammation). However, more recently, it is clear that PXR is also an important mediator of adverse xeno- (e.g., enhances acetaminophen toxicity) and endobiotic (e.g., hepatic steatosis) metabolic phenotypes. Moreover, in cancer therapeutics, PXR activation can induce drug resistance, and there is growing evidence for tissue-specific enhancement of the malignant phenotype. Thus, in these instances, there may be a role for PXR antagonists. However, as opposed to the discovery efforts for PXR agonists, there are only a few antagonists described. The mode of action of these antagonists (e.g., sulforaphane) remains less clear. Our laboratory efforts have focused on this question. Since the original discovery of azoles analogs as PXR antagonists, we have preliminarily defined an important PXR antagonist pharmacophore and developed less-toxic PXR antagonists. In this review, we describe our published and unpublished findings on recent structure-function studies involving the azole chemical scaffold. Further work in the future is needed to fully define potent, more-selective PXR antagonists that may be useful in clinical application.

From gut to kidney: transporting and metabolizing calcineurin-inhibitors in solid organ transplantation

Since their introduction circa 35 years ago, calcineurin-inhibitors (CNI) have become the cornerstone of immunosuppressive therapy in solid organ transplantation. However, CNI's possess a narrow therapeutic index with potential severe consequences of drug under- or overexposure. This demands a meticulous policy of Therapeutic Drug Monitoring (TDM) to optimize outcome. In clinical practice optimal dosing is difficult to achieve due to important inter- and intraindividual variation in CNI pharmacokinetics. A complex and often interdependent set of factors appears relevant in determining drug exposure. These include recipient characteristics such as age, race, body composition, organ function, and food intake, but also graft-related characteristics such as: size, donor-age, and time after transplantation can be important. Fundamental (in vitro) and clinical studies have pointed out the intrinsic relation between the aforementioned variables and the functional capacity of enzymes and transporters involved in CNI metabolism, primarily located in intestine, liver and kidney. Commonly occurring polymorphisms in genes responsible for CNI metabolism (CYP3A4, CYP3A5, CYP3A7, PXR, POR, ABCB1 (P-gp) and possibly UGT) are able to explain an important part of interindividual variability. In particular, a highly prevalent SNP in CYP3A5 has proven to be an important determinant of CNI dose requirements and drug-dose-interactions. In addition, a discrepancy in genotype between graft and receptor has to be taken into account. Furthermore, common phenomena in solid organ transplantation such as inflammation, ischemia- reperfusion injury, graft function, co-medication, altered food intake and intestinal motility can have a differential effect on the expression enzymes and transporters involved in CNI metabolism. Notwithstanding the built-up knowledge, predicting individual CNI pharmacokinetics and dose requirements on the basis of current clinical and experimental data remains a challenge.

Pregnane-x-receptor controls hepatic glucuronidation during pregnancy and neonatal development in humanized UGT1 mice

In humanized UDP glucuronosyltransferase-1 (hUGT1) mice that express the entire UGT1 locus, the maternal hepatic UGT1A genes are dramatically induced 12-14 days after conception. Steroid induction of the UGT1A1 gene indicates that xenobiotic sensors, such as the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), may underlie the induction process. In contrast, neonatal hUGT1 mice display severe hyperbilirubinemia, with limited expression of the UGT1A genes. This study identifies PXR as both a positive and negative regulator of the UGT1A1 gene. Pregnancy hormones, in particular the glucocorticoids, target PXR as a positive regulator of human glucuronidation. Employing reverse genetics, where PXR has been genetically deleted, hUGT1/Pxr(-/-) mice show limited induction of the liver UGT1A genes during pregnancy, whereas the exact opposite occurs in newborn mice. Neonatal hUGT1 mice show delayed expression of hepatic UGT1A1 and are severely hyperbilirubinemic. However, in hUGT1/Pxr(-/-) mice, hyperbilirubinemia is greatly reduced due to induction of hepatic UGT1A1. Thus, PXR serves to repress UGT1A1 gene expression during development. Transcriptional silencing of the UGT1A1 gene was relieved in neonatal hUGT1 hepatocytes through interruption of PXR by small interfering RNA.

CONCLUSION

PXR is a key regulator of pregnancy induced glucuronidation capacity in addition to modulating the severity of neonatal jaundice.

Estimation of the effect of SLCO1B1 polymorphisms on lopinavir plasma concentration in HIV-infected adults

BACKGROUND

The organic anion transporting polypeptides (OATP)/SLCO family represents an important class of hepatic drug uptake transporters that mediate the sodium independent transport of a diverse range of amphipathic organic compounds, including the protease inhibitors. The SLCO1B1 521T>C (rs4149056) single nucleotide polymorphism (SNP) has been consistently associated with reduced transport activity in vivo, and we previously showed an association of this polymorphism with lopinavir plasma concentrations. The aim of this study was to develop a population pharmacokinetic (PK) model to quantify the impact of 521T>C.

METHODS

A population PK analysis was performed with 594 plasma samples from 375 patients receiving lopinavir/ritonavir. Non-linear mixed effects modelling was applied to explore the effects of SLCO1B1 521T>C and patient demographics. Simulations of the lopinavir concentration profile were performed with different dosing regimens considering the different alleles.

RESULTS

A one-compartment model with first-order absorption best described the data. Population clearance was 5.67 l/h with inter-patient variability of 37%. Body weight was the only demographic factor influencing clearance, which increased 0.5 l/h for every 10 kg increase. Homozygosity for the C allele was associated with a 37% lower clearance, and 14% for heterozygosity, which were statistically significant.

CONCLUSIONS

These data show an association between SLCO1B1 521T>C and lopinavir clearance. The association is likely to be mediated through reduced uptake by hepatocytes leading to higher plasma concentrations of lopinavir. Further studies are now required to confirm the association and to assess the influence of other polymorphisms in the SLCO family on lopinavir PK.

Associations of ABCB1 3435C>T and IL-10-1082G>A polymorphisms with long-term sirolimus dose requirements in renal transplant patients

BACKGROUNDS

Sirolimus (SRL) absorption and metabolism are affected by p-glycoprotein-mediated transport and CYP3A enzyme activity, which are further under the influences of cytokine concentrations. This retrospective study determined the associations of adenosine triphosphate-binding cassette, subfamily B, member 1 (ABCB1) 1236C>T, 2677 G>T/A, and 3435C>T, cytochrome P450, family 3, subfamily A, polypeptide 4 (CYP3A4) -392A>G, cytochrome P450, family 3, subfamily A, polypeptide 5 (CYP3A5) 6986A>G and 14690G>A, interleukin (IL)-10 -1082G>A, and tumor necrosis factor (TNF) -308G>A polymorphisms with SRL dose-adjusted, weight-normalized trough concentrations (C/D) at 7 days, and at 1, 3, 6, and 12 months after initiation of SRL.

METHODS

Genotypes for 86 renal transplant patients who received SRL-based maintenance immunosuppressive therapy were determined using polymerase chain reaction followed by chip-based mass spectrometry. The changes of log-transformed C/D over the days posttransplantation were analyzed using a linear mixed-effects model, with adjustments for body mass index and weight-normalized doses of tacrolimus, prednisone, clotrimazole, and statins.

RESULTS

ABCB1 3435C>T and IL-10 -1082G>A were significantly associated with log C/D (P=0.0016 and 0.0394, respectively). Mean SRL C/D was 48% higher in patients with ABCB1 3435CT/TT genotype than those with 3435CC genotype, and was 24% higher in IL-10 -1082GG compared with -1082AG/AA.

CONCLUSIONS

ABCB1 3435C>T and IL-10 -1082G>A were significantly associated with long-term SRL dose requirements. Genetics can play a significant role in SRL dosing and may be useful in therapeutic monitoring of SRL in renal transplantation. Future replication studies are needed to confirm these associations.

Differential modulation of the expression of important drug metabolising enzymes and transporters by endothelin-1 receptor antagonists ambrisentan and bosentan in vitro

The safety and effectiveness of drugs used to treat chronic diseases critically depend on their propensity to interact with co-administered drugs. Induction of enzymes and drug transporters involved in the clearance and distribution of drugs may critically reduce exposure with their substrates and thus lead to nonresponse. We therefore investigated the impact of the endothelin-1 receptor antagonists bosentan and ambrisentan on the expression of relevant human efflux and uptake transporters and on phase 1 and phase 2 enzymes. LS180 adenocarcinoma cells were treated for four days with bosentan or ambrisentan (1-50 μM), the positive control rifampicin, or medium only (negative control). For evaluation of bosentan also HuH-7 human hepatoma cells were used and treated similarly. Gene expression was quantified at the mRNA level by real-time reverse transcription polymerase chain reaction and for some genes also at the protein level by western blot analysis. Comparable to rifampicin, bosentan was a moderate to strong inductor for all cytochrome P450 isozymes and ATP-binding cassette transporters tested, and it also induced organic anion transporting polypeptides. 50 μM bosentan up-regulated e.g. CYP3A4 8.5-fold, ABCB1 5.1-fold, and ABCB11 1.9-fold at the mRNA level in LS180 cells. In HuH-7 cells induction was much less pronounced (e.g. CYP3A4 1.9-fold for bosentan). In contrast, ambrisentan only weakly induced some of the genes investigated in LS180 cells. These findings corroborate the in vivo finding that bosentan is much more prone to drug interactions than ambrisentan.

Xenobiotic metabolism, disposition, and regulation by receptors: from biochemical phenomenon to predictors of major toxicities

To commemorate the 50th anniversary of the Society of Toxicology, this special edition article reviews the history and current scope of xenobiotic metabolism and transport, with special emphasis on the discoveries and impact of selected "xenobiotic receptors." This overall research realm has witnessed dynamic development in the past 50 years, and several of the key milestone events that mark the impressive progress in these areas of toxicological sciences are highlighted. From the initial observations regarding aspects of drug metabolism dating from the mid- to late 1800's, the area of biotransformation research witnessed seminal discoveries in the mid-1900's and onward that are remarkable in retrospect, including the discovery and characterization of the phase I monooxygenases, the cytochrome P450s. Further research uncovered many aspects of the biochemistry of xenobiotic metabolism, expanding to phase II conjugation and phase III xenobiotic transport. This led to hallmark developments involving integration of genomic technologies to elucidate the basis for interindividual differences in response to xenobiotic exposures and discovery of nuclear and soluble receptor families that selectively "sense" the chemical milieu of the mammalian cell and orchestrate compensatory changes in gene expression programming to accommodate complex xenobiotic exposures. This review will briefly summarize these developments and investigate the expanding roles of xenobiotic receptor biology in the underlying basis of toxicological response to chemical agents.

Nuclear receptor PXR, transcriptional circuits and metabolic relevance

The pregnane X receptor (PXR, NR1I2) is a ligand activated transcription factor that belongs to the nuclear hormone receptor (NR) superfamily. PXR is highly expressed in the liver and intestine, but low levels of expression have also been found in many other tissues. PXR plays an integral role in xenobiotic and endobiotic metabolism by regulating the expression of drug-metabolizing enzymes and transporters, as well as genes implicated in the metabolism of endobiotics. PXR exerts its transcriptional regulation by binding to its DNA response elements as a heterodimer with the retinoid X receptor (RXR) and recruitment of a host of coactivators. The biological and physiological implications of PXR activation are broad, ranging from drug metabolism and drug-drug interactions to the homeostasis of numerous endobiotics, such as glucose, lipids, steroids, bile acids, bilirubin, retinoic acid, and bone minerals. The purpose of this article is to provide an overview on the transcriptional circuits and metabolic relevance controlled by PXR. This article is part of a Special Issue entitled: Translating Nuclear Receptors from Health to Disease.

The suitability of rat hepatoma cell line H4IIE for evaluating the potentials of compounds to induce CYP3A23 expression

To investigate the suitability of H4IIE cells for detecting cytochrome P450 (CYP) induction in vitro, we compared CYP induction by typical CYP inducers in H4IIE cells and rat primary hepatocytes by examining gene expression and enzyme activity, and by immunocytochemistry. The cells were preincubated with 0.1 μM of dexamethasone (DEX) for 24 h, followed by 48 h of exposure to 10 μM of beta-naphthoflavone (bNF), 100 μM of phenobarbital (PB) and 10 μM of DEX. Cyp1a1, Cyp2b1/2 and Cyp3a23/3a1 (Cyp3a23) expressions in H4IIE cells were up-regulated 280-, 1.5- and 65-fold relative to those in vehicle-treated cells, respectively. The fold inductions of those expressions in rat primary hepatocytes were 80-, 33- and 152-fold, respectively. Comprehensive gene expression analysis using DNA microarrays showed that Cyp3a23, Gsta2, Ugt2b12, Udpgt and Sult2a1 expressions were up-regulated in H4IIE cells exposed to 10 μM of DEX. CYP3A activity was not increased, but some H4IIE cells exposed to DEX were stained strongly with anti-CYP3A antibody. We cloned these cells and obtained cloned H4IIE (cH4IIE) cells with expression level of Cyp3a23 higher than those of vehicle-treated cells. It was confirmed that preincubation with 0.1 μM of DEX increased pregnane X receptor (Pxr) expression level and enhanced the Cyp3a23 induction effects of test compounds significantly. Retrospective examination of in vitro CYP induction assay using cH4IIE cells resulted in 80% correlation with the data from in vivo rat toxicity studies. These results suggested that cH4IIE cells are suitable for evaluating the potentials of a compound to induce CYP3A23 expression.

Regulation of drug-metabolizing enzymes by xenobiotic receptors: PXR and CAR

Drug-metabolizing enzymes (DMEs) and transporters play pivotal roles in the disposition and detoxification of numerous foreign and endogenous chemicals. To accommodate chemical challenges, the expression of many DMEs and transporters is up-regulated by a group of ligand-activated transcription factors namely nuclear receptors (NRs). The importance of NRs in xenobiotic metabolism and clearance is best exemplified by the most promiscuous xenobiotic receptors: pregnane X receptor (PXR, NR1I2) and constitutive androstane/activated receptor (CAR, NR1I3). Together, these two receptors govern the inductive expression of a largely overlapping array of target genes encoding phase I and II DMEs, and drug transporters. Moreover, PXR and CAR also represent two distinctive mechanisms of NR activation, whereby CAR demonstrates both constitutive and ligand-independent activation. In this review, recent advances in our understanding of PXR and CAR as xenosensors are discussed with emphasis placed on the differences rather than similarities of these two xenobiotic receptors in ligand recognition and target gene regulation.

Population pharmacokinetic modeling of the association between 63396C->T pregnane X receptor polymorphism and unboosted atazanavir clearance

Atazanavir (ATV) plasma concentrations are influenced by CYP3A4 and ABCB1, which are regulated by the pregnane X receptor (PXR; NR1I2). PXR expression is correlated with CYP3A4 in liver in the absence of enzyme inducers. The PXR single nucleotide polymorphism (SNP) 63396C→T (rs2472677) alters PXR expression and CYP3A4 activity in vitro, and we previously showed an association of this polymorphism with unboosted ATV plasma concentrations. The aim of this study was to develop a population pharmacokinetic analysis to quantify the impact of 63396C→T and diurnal variation on ATV clearance. A population analysis was performed with 323 plasma samples from 182 randomly selected patients receiving unboosted ATV. Two hundred fifty-nine of the blood samples were collected at random time points, and 11 patients had a full concentration-time profile at steady state. Nonlinear mixed effects modeling was applied to explore the effects of PXR 63396C→T, patient demographics, and diurnal variation. A one-compartment model with first-order absorption and lag time best described the data. Population clearance was 19.7 liters/h with interpatient variability or coefficient of variation (CV) of 21.5%. Homozygosity for the T allele for PXR 63396 was associated with a 17.0% higher clearance that was statistically significant. Evening dosing was associated with 34% higher bioavailability than morning dosing. Patient demographic factors had no effect on ATV clearance. These data show an association of PXR 63396C→T and diurnal variation on unboosted ATV clearance. The association is likely to be mediated through an effect on hepatic PXR expression and therefore expression of its target genes (e.g., CYP3A4, SLCO1B1, and ABCB1), which are known to be involved in ATV clearance.

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.

Cytochrome P450 3A4 mRNA is a more reliable marker than CYP3A4 activity for detecting pregnane X receptor-activated induction of drug-metabolizing enzymes

Induction of cytochrome P450 (P450) activity in the clinic can result in therapeutic failure such as tissue rejection in transplant patients or unwanted pregnancy, among others. CYP3A4 is by far the most abundant isoform and is responsible for the majority of P450-related metabolism of all marketed drugs. However, it is of importance to understand the significance of induction mediated through other P450 enzymes. The objective of this investigation was to evaluate several known inducers in vitro using cryopreserved human hepatocytes, with the aim of assessing the relevant induction of CYP3A4, CYP2B6, CYP2C9, CYP2C19, and CYP3A5, based on mRNA expression. CYP3A4 induction was also assessed based on enzymatic activity in three different lots to investigate whether mRNA expression data have any advantages over enzymatic activity. In general, the mRNA fold-induction data results were more sensitive compared with activity data, and more informative in cases in which the drug is also a P450 inhibitor. The induction of transcription of other drug-metabolizing enzymes including CYP2B6 and CYP2C enzymes occurred every time that CYP3A4 mRNA levels increased, but to a lesser extent, indicating that measurement of CYP3A4 mRNA is a sensitive marker for the induction of these other enzymes. This was the case even for enzymes and inducers that are known to also act via the constitutive androstane receptor pathway. Finally, the utility of in vitro induction measurements in the identification of clinically meaningful inducers was tested by using two simple binary classification approaches: 1) fold-induction versus vehicle control and 2) induction response relative to rifampin. The best classification was observed when the cutoff criteria based on fold induction relative to the vehicle control, using mRNA data are used.

A structural view of nuclear hormone receptor: endocrine disruptor interactions

Endocrine-disrupting chemicals (EDCs) represent a broad class of exogenous substances that cause adverse effects in the endocrine system by interfering with hormone biosynthesis, metabolism, or action. The molecular mechanisms of EDCs involve different pathways including interactions with nuclear hormone receptors (NHRs) which are primary targets of a large variety of environmental contaminants. Here, based on the crystal structures currently available in the Protein Data Bank, we review recent studies showing the many ways in which EDCs interact with NHRs and impact their signaling pathways. Like the estrogenic chemical diethylstilbestrol, some EDCs mimic the natural hormones through conserved protein-ligand contacts, while others, such as organotins, employ radically different binding mechanisms. Such structure-based knowledge, in addition to providing a better understanding of EDC activities, can be used to predict the endocrine-disrupting potential of environmental pollutants and may have applications in drug discovery.