Quantitative prediction of human pregnane X receptor and cytochrome P450 3A4 mediated drug-drug interaction in a novel multiple humanized mouse line

Acceleration of infectious disease drug discovery and development using a humanized model of drug metabolism

A key step in drug discovery, common to many disease areas, is preclinical demonstration of efficacy in a mouse model of disease. However, this demonstration and its translation to the clinic can be impeded by mouse-specific pathways of drug metabolism. Here, we show that a mouse line extensively humanized for the cytochrome P450 gene superfamily ("8HUM") can circumvent these problems. The pharmacokinetics, metabolite profiles, and magnitude of drug-drug interactions of a test set of approved medicines were in much closer alignment with clinical observations than in wild-type mice. Infection with Mycobacterium tuberculosis, Leishmania donovani, and Trypanosoma cruzi was well tolerated in 8HUM, permitting efficacy assessment. During such assessments, mouse-specific metabolic liabilities were bypassed while the impact of clinically relevant active metabolites and DDI on efficacy were well captured. Removal of species differences in metabolism by replacement of wild-type mice with 8HUM therefore reduces compound attrition while improving clinical translation, accelerating drug discovery.

Predictability of human exposure by human-CYP3A4-transgenic mouse models: A meta-analysis

First-in-human dose predictions are primarily based on no-observed-adverse-effect levels in animal studies. Predictions from these animal models are only as effective as their ability to predict human results. To narrow the gap between human and animals, researchers have, among other things, focused on the replacement of animal cytochrome P450 (CYP) enzymes with their human counterparts (called humanization), especially in mice. Whereas research in humanized mice is extensive, the emphasis has been particularly on qualitative rather than quantitative predictions. Because the CYP3A4 enzyme is most involved in the metabolism of clinically used drugs, most benefit was expected from CYP3A4 models. There are several applications of these mouse models regarding in vivo CYP3A4 functionality, one of which might be their capacity to help improve first-in-human (FIH) dose predictions for CYP3A4-metabolized drugs. To evaluate whether human-CYP3A4-transgenic mouse models are better predictors of human exposure compared to the wild-type mouse model, we performed a meta-analysis comparing both mouse models in their ability to accurately predict human exposure of small-molecule drugs metabolized by CYP3A4. Results showed that, in general, the human-CYP3A4-transgenic mouse model had similar accuracy in the prediction of human exposure compared to the wild-type mouse model, suggesting that there is limited added value in humanization of the mouse Cyp3a enzymes if the primary aim is to acquire more accurate FIH dose predictions. Despite the results of this meta-analysis, corrections for interspecies differences through extension of human-CYP3A4-transgenic mouse models with pharmacokinetic modeling approaches seems a promising contribution to more accurate quantitative predictions of human pharmacokinetics.

Analysis of the Bile Acid Composition in a Fibroblast Growth Factor 19-Expressing Liver-Humanized Mouse Model and Its Use for CYP3A4-Mediated Drug-Drug Interaction Studies

Numerous biomedical applications have been described for liver-humanized mouse models, such as in drug metabolism or drug-drug interaction (DDI) studies. However, the strong enlargement of the bile acid (BA) pool due to lack of recognition of murine intestine-derived fibroblast growth factor-15 by human hepatocytes and a resulting upregulation in the rate-controlling enzyme for BA synthesis, cytochrome P450 (CYP) 7A1, may pose a challenge in interpreting the results obtained from such mice. To address this challenge, the human fibroblast growth factor-19 (FGF19) gene was inserted into the Fah-/- , Rag2-/- , Il2rg-/- NOD (FRGN) mouse model, allowing repopulation with human hepatocytes capable of responding to FGF19. While a decrease in CYP7A1 expression in human hepatocytes from humanized FRGN19 mice (huFRGN19) and a concomitant reduction in BA production was previously shown, a detailed analysis of the BA pool in these animals has not been elucidated. Furthermore, there are sparse data on the use of this model to assess potential clinical DDI. In the present work, the change in BA composition in huFRGN19 compared with huFRGN control animals was systematically evaluated, and the ability of the model to recapitulate a clinically described CYP3A4-mediated DDI was assessed. In addition to a massive reduction in the total amount of BA, FGF19 expression in huFRGN19 mice resulted in significant changes in the profile of various primary, secondary, and sulfated BAs in serum and feces. Moreover, as observed clinically, administration of the pregnane X receptor agonist rifampicin reduced the oral exposure of the CYP3A4 substrate triazolam. SIGNIFICANCE STATEMENT: Transgenic expression of FGF19 normalizes the unphysiologically high level of bile acids in a chimeric liver-humanized mouse model and leads to massive changes in bile acid composition. These adaptations could overcome one of the potential impediments in the use of these mouse models for drug-drug interaction studies.

Assessing cytochrome P450 function using genetically engineered mouse models

The ability to knock out and/or humanize different genes in experimental animals, globally or in cell- and tissue-specific patterns, has revolutionized scientific research in many areas. Genetically engineered mouse models, including knockout models, transgenic models, and humanized models, have played important roles in revealing the in vivo functions of various cytochrome P450 (CYP) enzymes. These functions are very diverse, ranging from the biotransformation of drugs and other xenobiotics, events that often dictate their pharmacokinetic or toxicokinetic properties and the associated therapeutic or adverse actions, to the metabolism of endogenous compounds, such as steroid hormones and other bioactive substances, that may determine susceptibility to many diseases, such as cancer and metabolic diseases. In this review, we provide a comprehensive list of Cyp-knockout, human CYP-transgenic, and CYP-humanized mouse models that target genes in the CYP1-4 gene families, and highlight their utility in assessing the in vivo metabolism, bioactivation, and toxicity of various xenobiotic compounds, including therapeutic agents and chemical carcinogens. We aim to showcase the advantages of utilizing these mouse models for in vivo drug metabolism and toxicology studies, and to encourage and facilitate greater utility of engineered mouse models to further improve our knowledge of the in vivo functions of various P450 enzymes, which is integral to our ability to develop safer and more effective therapeutics and to identify individuals predisposed to adverse drug reactions or environmental diseases.

Cytochrome P450 Enzymes and Drug Metabolism in Humans

Human cytochrome P450 (CYP) enzymes, as membrane-bound hemoproteins, play important roles in the detoxification of drugs, cellular metabolism, and homeostasis. In humans, almost 80% of oxidative metabolism and approximately 50% of the overall elimination of common clinical drugs can be attributed to one or more of the various CYPs, from the CYP families 1–3. In addition to the basic metabolic effects for elimination, CYPs are also capable of affecting drug responses by influencing drug action, safety, bioavailability, and drug resistance through metabolism, in both metabolic organs and local sites of action. Structures of CYPs have recently provided new insights into both understanding the mechanisms of drug metabolism and exploiting CYPs as drug targets. Genetic polymorphisms and epigenetic changes in CYP genes and environmental factors may be responsible for interethnic and interindividual variations in the therapeutic efficacy of drugs. In this review, we summarize and highlight the structural knowledge about CYPs and the major CYPs in drug metabolism. Additionally, genetic and epigenetic factors, as well as several intrinsic and extrinsic factors that contribute to interindividual variation in drug response are also reviewed, to reveal the multifarious and important roles of CYP-mediated metabolism and elimination in drug therapy.

Designing Out PXR Activity on Drug Discovery Projects: A Review of Structure-Based Methods, Empirical and Computational Approaches

This perspective discusses the role of pregnane xenobiotic receptor (PXR) in drug discovery and the impact of its activation on CYP3A4 induction. The use of structural biology to reduce PXR activity on drug discovery projects has become more common in recent years. Analysis of this work highlights several important molecular interactions, and the resultant structural modifications to reduce PXR activity are summarized. The computational approaches undertaken to support the design of new drugs devoid of PXR activation potential are also discussed. Finally, the SAR of empirical design strategies to reduce PXR activity is reviewed, and the key SAR transformations are discussed and summarized. In conclusion, this perspective demonstrates that PXR activity can be greatly diminished or negated on active drug discovery projects with the knowledge now available. This perspective should be useful to anyone who seeks to reduce PXR activity on a drug discovery project.

Centromere identity and function put to use: construction and transfer of mammalian artificial chromosomes to animal models

Mammalian artificial chromosomes (MACs) are widely used as gene expression vectors and have various advantages over conventional expression vectors. We review and discuss breakthroughs in MAC construction, initiation of functional centromeres allowing their faithful inheritance, and transfer from cell culture to animal model systems. These advances have contributed to advancements in synthetic biology, biomedical research, and applications in industry and in the clinic.

Diazepam Promotes Translocation of Human Constitutive Androstane Receptor (CAR) via Direct Interaction with the Ligand-Binding Domain

The constitutive androstane receptor (CAR) is the essential regulator of genes involved both in xenobiotic and endobiotic metabolism. Diazepam has been shown as a potent stimulator of CAR nuclear translocation and is assumed as an indirect CAR activator not interacting with the CAR cavity. In this study, we sought to determine if diazepam is a ligand directly interacting with the CAR ligand binding domain (LBD) and if it regulates its target genes in a therapeutically relevant concentration. We used different CAR constructs in translocation and luciferase reporter assays, recombinant CAR-LBD in a TR-FRET assay, and target genes induction studied in primary human hepatocytes (PHHs), HepaRG cells, and in CAR humanized mice. We also used in silico docking and CAR-LBD mutants to characterize the interaction of diazepam and its metabolites with the CAR cavity. Diazepam and its metabolites such as nordazepam, temazepam, and oxazepam are activators of CAR+Ala in translocation and two-hybrid assays and fit the CAR cavity in docking experiments. In gene reporter assays with CAR3 and in the TR-FRET assay, only diazepam significantly interacts with CAR-LBD. Diazepam also promotes up-regulation of CYP2B6 in PHHs and in HepaRG cells. However, in humanized CAR mice, diazepam significantly induces neither CYP2B6 nor Cyp2b10 genes nor does it regulate critical genes involved in glucose and lipids metabolism and liver proliferation. Thus, we demonstrate that diazepam interacts with human CAR-LBD as a weak ligand, but it does not significantly affect expression of tested CAR target genes in CAR humanized mice.

Potential role of drug metabolizing enzymes in chemotherapy-induced gastrointestinal toxicity and hepatotoxicity

INTRODUCTION

Toxicity of chemotherapy drugs is the leading cause of poor therapeutic outcome in many cancer patients. Gastrointestinal (GI) toxicity and hepatotoxicity are among the most common side effects of current chemotherapies. Emerging studies indicate that many chemotherapy-induced toxicities are driven by drug metabolism, but very few reviews summarize the role of drug metabolism in chemotherapy-induced GI toxicity and hepatotoxicity. In this review, we highlighted the importance of drug metabolizing enzymes (DMEs) in chemotherapy toxicity.

AREAS COVERED

Our review demonstrated that altered activity of DMEs play important role in chemotherapy-induced GI toxicity and hepatotoxicity. Besides direct changes in catalytic activities, the transcription of DMEs is also affected by inflammation, cell-signaling pathways, and/or by drugs in cancer patients due to the disease etiology.

EXPERT OPINION

More studies should focus on how DMEs are altered during chemotherapy treatment, and how such changes affect the metabolism of chemotherapy drug itself. This mutual interaction between chemotherapies and DMEs can lead to excessive exposure of parent drug or toxic metabolites which ultimately cause GI adverse effect.

Role of CYP3A4 in bone marrow microenvironment-mediated protection of FLT3/ITD AML from tyrosine kinase inhibitors

An intriguing aspect of the clinical activity of FMS-like tyrosine kinase 3 inhibitors (FLT3 TKIs) is their apparent higher activity against peripheral blasts from FLT3/internal tandem duplication (ITD) acute myeloid leukemia than marrow disease in the same patients. Accordingly, studies showed that the bone marrow microenvironment plays a role in FLT3 TKI resistance, although the underlying mechanisms are unclear. We recently identified a previously undescribed mechanism by which the bone marrow microenvironment can contribute to drug resistance: expression of cytochrome P450 enzymes (CYPs). In fact, bone marrow stromal cells (BMSCs) expressed most CYPs, including CYP3A4. Because hepatic CYP3A4 plays a role in the inactivation of several FLT3 TKIs, we explored the potential role of CYP3A4 in bone marrow microenvironment-mediated FLT3 TKI resistance. We found that CYP3A4 plays a major role in BMSC-mediated inhibition in the activity of 3 different FLT3 TKIs (sorafenib, quizartinib, and gilteritinib) against FLT3/ITD acute myeloid leukemia (AML). Furthermore, clarithromycin, a clinically active CYP3A4 inhibitor, significantly reversed the protective effects of BMSCs. We show, for the first time, that bone marrow stromal CYP3A4 contributes to FLT3 TKI resistance in the bone marrow. These results suggest that combining FLT3 TKIs with CYP3A4 inhibitors could be a promising strategy toward improving the activity of FLT3 TKIs.

Current trends in drug metabolism and pharmacokinetics

Pharmacokinetics (PK) is the study of the absorption, distribution, metabolism, and excretion (ADME) processes of a drug. Understanding PK properties is essential for drug development and precision medication. In this review we provided an overview of recent research on PK with focus on the following aspects: (1) an update on drug-metabolizing enzymes and transporters in the determination of PK, as well as advances in xenobiotic receptors and noncoding RNAs (ncRNAs) in the modulation of PK, providing new understanding of the transcriptional and posttranscriptional regulatory mechanisms that result in inter-individual variations in pharmacotherapy; (2) current status and trends in assessing drug-drug interactions, especially interactions between drugs and herbs, between drugs and therapeutic biologics, and microbiota-mediated interactions; (3) advances in understanding the effects of diseases on PK, particularly changes in metabolizing enzymes and transporters with disease progression; (4) trends in mathematical modeling including physiologically-based PK modeling and novel animal models such as CRISPR/Cas9-based animal models for DMPK studies; (5) emerging non-classical xenobiotic metabolic pathways and the involvement of novel metabolic enzymes, especially non-P450s. Existing challenges and perspectives on future directions are discussed, and may stimulate the development of new research models, technologies, and strategies towards the development of better drugs and improved clinical practice.

An Extensively Humanized Mouse Model to Predict Pathways of Drug Disposition and Drug/Drug Interactions, and to Facilitate Design of Clinical Trials

Species differences in drug metabolism and disposition can confound the extrapolation of in vivo PK data to man and also profoundly compromise drug efficacy studies owing to differences in pharmacokinetics, in metabolites produced (which are often pharmacologically active), and in differential activation of the transcription factors constitutive androstane receptor (CAR) and pregnane X receptor (PXR), which regulate the expression of such enzymes as P450s and drug transporters. These differences have gained additional importance as a consequence of the use of genetically modified mouse models for drug-efficacy testing and also patient-derived xenografts to predict individual patient responses to anticancer drugs. A number of humanized mouse models for cytochrome P450s, CAR, and PXR have been reported. However, the utility of these models has been compromised by the redundancy in P450 reactions across gene families, whereby the remaining murine P450s can metabolize the compounds being tested. To remove this confounding factor and create a mouse model that more closely reflects human pathways of drug disposition, we substituted 33 murine P450s from the major gene families involved in drug disposition, together with Car and Pxr, for human CAR, PXR, CYP1A1, CYP1A2, CYP2C9, CYP2D6, CYP3A4, and CYP3A7. We also created a mouse line in which 34 P450s were deleted from the mouse genome. Using model compounds and anticancer drugs, we demonstrated how these mouse lines can be applied to predict drug-drug interactions in patients and discuss here their potential application in the more informed design of clinical trials and the personalized treatment of cancer.

Activation of Pregnane X Receptor-Cytochrome P450s Axis: A Possible Reason for the Enhanced Accelerated Blood Clearance Phenomenon of PEGylated Liposomes In Vivo

Recently, we reported that repeated injection of PEGylated liposomes (PEG-L) at certain intervals to the same rat lead to the disappearance of their long-circulation properties, referred to as the "accelerated blood clearance (ABC) phenomenon". Evidence from our recent studies suggested that cytochrome P450s (P450s) contribute to induction of the ABC phenomenon, a possibility that had been previously ignored. However, few details are known about the mechanism for induction of P450s. The present study was undertaken to investigate the roles in the ABC phenomenon of pregnane X receptor (PXR) and constitutive androstane receptor (CAR), the major upstream transcriptional regulators of the P450 genes, including CYP3A1, CYP2C6, and CYP1A2. The results demonstrated that expression of rat PXR and CAR was significantly increased in the ABC phenomenon and was accompanied by elevated CYP3A1, CYP2C6, and CYP1A2 levels. Further findings revealed that PXR but not CAR protein was substantially upregulated in the hepatocyte nucleus, together with marked nuclear colocalization of the PXR-retinoid X receptor alpha (RXRα) transcriptionally active heterodimer, indicating that nuclear translocation of PXR was induced in the ABC phenomenon, whereas nuclear translocation of CAR was not observed. Notably, pretreatment with the specific PXR inducer dexamethasone significantly induced accelerated systemic clearance of the subsequent injection of PEG-L, associating with increased nuclear colocalization of PXR-RXRα These results revealed that the induction of P450s in the ABC phenomenon may be attributable largely to the activation of PXR induced by sequential injections of PEG-L, thus confirming the crucial involvement of the PXR-P450s axis in promoting the ABC phenomenon. SIGNIFICANCE STATEMENT: The results of this study revealed that the induction of P450s in the ABC phenomenon may be largely attributable to the activation of PXR induced by sequential injections of PEG-L, thus confirming the crucial involvement of the PXR-P450s axis in promoting the ABC phenomenon. The data may help to extend our insights into 1) the role of P450s, which are regulated by the liver-enriched nuclear receptor PXR, in the ABC phenomenon, and 2) the therapeutic potential of targeting the PXR-P450 axis for reducing the magnitude of the ABC phenomenon in clinical practice.

Humanising the mouse genome piece by piece

To better understand human health and disease, researchers create a wide variety of mouse models that carry human DNA. With recent advances in genome engineering, the targeted replacement of mouse genomic regions with orthologous human sequences has become increasingly viable, ranging from finely tuned humanisation of individual nucleotides and amino acids to the incorporation of many megabases of human DNA. Here, we examine emerging technologies for targeted genomic humanisation, we review the spectrum of existing genomically humanised mouse models and the insights such models have provided, and consider the lessons learned for designing such models in the future.

Generation of transgenic mice has become routine in studying gene function and disease mechanisms, but often this is not enough to fully understand human biology. Here, the authors review the current state of the art of targeted genomic humanisation strategies and their advantages over classic approaches.

Plexiform Arteriopathy in Rodent Models of Pulmonary Arterial Hypertension

As time progresses, our understanding of disease pathology is propelled forward by technological advancements. Much of the advancements that aid in understanding disease mechanics are based on animal studies. Unfortunately, animal models often fail to recapitulate the entirety of the human disease. This is especially true with animal models used to study pulmonary arterial hypertension (PAH), a disease with two distinct phases. The first phase is defined by nonspecific medial and adventitial thickening of the pulmonary artery and is commonly reproduced in animal models, including the classic models (ie, hypoxia-induced pulmonary hypertension and monocrotaline lung injury model). However, many animal models, including the classic models, fail to capture the progressive, or second, phase of PAH. This is a stage defined by plexogenic arteriopathy, resulting in obliteration and occlusion of the small- to mid-sized pulmonary vessels. Each of these two phases results in severe pulmonary hypertension that directly leads to right ventricular hypertrophy, decompensated right-sided heart failure, and death. Fortunately, newly developed animal models have begun to address the second, more severe, side of PAH and aid in our ability to develop new therapeutics. Moreover, p38 mitogen-activated protein kinase activation emerges as a central molecular mediator of plexiform lesions in both experimental models and human disease. Therefore, this review will focus on plexiform arteriopathy in experimental animal models of PAH.

Suppression of Hepatic CYP3A4 Expression and Activity by 3-Methylcholanthrene in Humanized PXR-CAR-CYP3A4/3A7 Mice

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants that activate the aryl hydrocarbon receptor, thereby triggering a range of biologic responses, exemplified by the induction of CYP1A1 PAHs can also regulate the expression of members of the CYP3A subfamily, with reports of mainly suppressive effects on mouse hepatic Cyp3a11 expression, but paradoxically both inductive and suppressive effects on human hepatic CYP3A4 expression. Understanding the regulation of CYP3A4 expression by PAHs is important because of the widespread exposure of humans to these chemicals and the central role of the CYP3A4 enzyme in the metabolism of clinically important drugs and endogenous substances. The present study used 3-methylcholanthrene (MC) as a model PAH to characterize the in vivo regulation of CYP3A4 expression and activity in humanized pregnane X receptor-constitutive androstane receptor-CYP3A4/3A7 mice. Adult mice were treated by intraperitoneal injection with MC (80 mg/kg), or corn oil vehicle, and euthanized 24 or 72 hours later. As a positive control response, pronounced induction of hepatic Cyp1a1 by MC was confirmed at both time points in males and females at the mRNA, protein, and catalytic activity levels. Basal hepatic CYP3A4 expression and activity were significantly higher in female versus male mice. MC treatment suppressed hepatic CYP3A4 in female mice at 72 hours postdosing at the mRNA, protein, and catalytic activity levels. A similar response was observed in male mice, although the suppression of CYP3A4 protein levels did not achieve statistical significance. This mouse model will facilitate further studies of the mechanisms and consequences of CYP3A4 suppression by PAHs.

CRISPR-Cas9: A New Addition to the Drug Metabolism and Disposition Tool Box

Clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR associated protein 9 (Cas9), i.e., CRISPR-Cas9, has been extensively used as a gene-editing technology during recent years. Unlike earlier technologies for gene editing or gene knockdown, such as zinc finger nucleases and RNA interference, CRISPR-Cas9 is comparably easy to use, affordable, and versatile. Recently, CRISPR-Cas9 has been applied in studies of drug absorption, distribution, metabolism, and excretion (ADME) and for ADME model generation. To date, about 50 papers have been published describing in vitro or in vivo CRISPR-Cas9 gene editing of ADME and ADME-related genes. Twenty of these papers describe gene editing of clinically relevant genes, such as ATP-binding cassette drug transporters and cytochrome P450 drug-metabolizing enzymes. With CRISPR-Cas9, the ADME tool box has been substantially expanded. This new technology allows us to develop better and more predictive in vitro and in vivo ADME models and map previously underexplored ADME genes and gene families. In this mini-review, we give an overview of the CRISPR-Cas9 technology and summarize recent applications of CRISPR-Cas9 within the ADME field. We also speculate about future applications of CRISPR-Cas9 in ADME research.

P450-Humanized and Human Liver Chimeric Mouse Models for Studying Xenobiotic Metabolism and Toxicity

Preclinical evaluation of drug candidates in experimental animal models is an essential step in drug development. Humanized mouse models have emerged as a promising alternative to traditional animal models. The purpose of this mini-review is to provide a brief survey of currently available mouse models for studying human xenobiotic metabolism. Here, we describe both genetic humanization and human liver chimeric mouse models, focusing on the advantages and limitations while outlining their key features and applications. Although this field of biomedical science is relatively young, these humanized mouse models have the potential to transform preclinical drug testing and eventually lead to a more cost-effective and rapid development of new therapies.

High fat diet induced obesity is mitigated in Cyp3a-null female mice

Recent studies indicate a role for the constitutive androstane receptor (CAR), pregnane X-receptor (PXR), and hepatic xenobiotic detoxifying CYPs in fatty liver disease or obesity. Therefore, we examined whether Cyp3a-null mice show increased obesity and fatty liver disease following 8-weeks of exposure to a 60% high-fat diet (HFD). Surprisingly, HFD-fed Cyp3a-null females fed a HFD gained 50% less weight than wild-type (WT; B6) females fed a HFD. In contrast, Cyp3a-null males gained more weight than WT males, primarily during the first few weeks of HFD-treatment. Cyp3a-null females also recovered faster than WT females from a glucose tolerance test; males showed no difference in glucose tolerance between the groups. Serum concentrations of the anti-obesity hormone, adiponectin are 60% higher and β-hydroxybutyrate levels are nearly 50% lower in Cyp3a-null females than WT females, in agreement with reduced weight gain, faster glucose response, and reduced ketogenesis. In contrast, Cyp3a-null males have higher liver triglyceride concentrations and lipidomic analysis indicates an increase in phosphatidylinositol, phosphatidylserine and sphingomyelin. None of these changes were observed in females. Last, Pxr, Cyp2b, and IL-6 expression increased in Cyp3a-null females following HFD-treatment. Cyp2b and Fatp1 increased, while Pxr, Cpt1a, Srebp1 and Fasn decreased in Cyp3a-null males following a HFD, indicating compensatory biochemical responses in male (and to a lesser extent) female mice fed a HFD. In conclusion, lack of Cyp3a has a positive effect on acclimation to a HFD in females as it improves weight gain, glucose response and ketosis.

Identification of Novel Pathways of Osimertinib Disposition and Potential Implications for the Outcome of Lung Cancer Therapy

Purpose: Osimertinib is a third-generation inhibitor of the epidermal growth factor receptor used in treatment of non-small cell lung cancer. A full understanding of its disposition and capacity for interaction with other medications will facilitate its effective use as a single agent and in combination therapy.Experimental Design: Recombinant cytochrome P450s and liver microsomal preparations were used to identify novel pathways of osimertinib metabolism in vitro A panel of knockout and mouse lines humanized for pathways of drug metabolism were used to establish the relevance of these pathways in vivoResults: Although some osimertinib metabolites were similar in mouse and human liver samples there were several significant differences, in particular a marked species difference in the P450s involved. The murine Cyp2d gene cluster played a predominant role in mouse, whereas CYP3A4 was the major human enzyme responsible for osimertinib metabolism. Induction of this enzyme in CYP3A4 humanized mice substantially decreased circulating osimertinib exposure. Importantly, we discovered a further novel pathway of osimertinib disposition involving CPY1A1. Modulation of CYP1A1/CYP1A2 levels markedly reduced parent drug concentrations, significantly altering metabolite pharmacokinetics (PK) in humanized mice in vivoConclusions: We demonstrate that a P450 enzyme expressed in smokers' lungs and lung tumors has the capacity to metabolise osimertinib. This could be a significant factor in defining the outcome of osimertinib treatment. This work also illustrates how P450-humanized mice can be used to identify and mitigate species differences in drug metabolism and thereby model the in vivo effect of critical metabolic pathways on anti-tumor response. Clin Cancer Res; 24(9); 2138-47. ©2018 AACR.

Antiretroviral Drug Metabolism in Humanized PXR-CAR-CYP3A-NOG Mice

Antiretroviral drug (ARV) metabolism is linked largely to hepatic cytochrome P450 activity. One ARV drug class known to be metabolized by intestinal and hepatic CYP3A are the protease inhibitors (PIs). Plasma drug concentrations are boosted by CYP3A inhibitors such as cobisistat and ritonavir (RTV). Studies of such drug-drug interactions are limited since the enzyme pathways are human specific. While immune-deficient mice reconstituted with human cells are an excellent model to study ARVs during human immunodeficiency virus type 1 (HIV-1) infection, they cannot reflect human drug metabolism. Thus, we created a mouse strain with the human pregnane X receptor, constitutive androstane receptor, and CYP3A4/7 genes on a NOD.Cg-Prkdc^scid Il2rg^tm1Sug/JicTac background (hCYP3A-NOG) and used them to evaluate the impact of human CYP3A metabolism on ARV pharmacokinetics. In proof-of-concept studies we used nanoformulated atazanavir (nanoATV) with or without RTV. NOG and hCYP3A-NOG mice were treated weekly with 50 mg/kg nanoATV alone or boosted with nanoformulated ritonavir (nanoATV/r). Plasma was collected weekly and liver was collected at 28 days post-treatment. Plasma and liver atazanavir (ATV) concentrations in nanoATV/r-treated hCYP3A-NOG mice were 2- to 4-fold higher than in replicate NOG mice. RTV enhanced plasma and liver ATV concentrations 3-fold in hCYP3A-NOG mice and 1.7-fold in NOG mice. The results indicate that human CYP3A-mediated drug metabolism is reduced compared with mouse and that RTV differentially affects human gene activity. These differences can affect responses to PIs in humanized mouse models of HIV-1 infection. Importantly, hCYP3A-NOG mice reconstituted with human immune cells can be used for bench-to-bedside translation.

Compensatory changes in CYP expression in three different toxicology mouse models: CAR-null, Cyp3a-null, and Cyp2b9/10/13-null mice

Targeted mutant models are common in mechanistic toxicology experiments investigating the absorption, metabolism, distribution, or elimination (ADME) of chemicals from individuals. Key models include those for xenosensing transcription factors and cytochrome P450s (CYP). Here we investigated changes in transcript levels, protein expression, and steroid hydroxylation of several xenobiotic detoxifying CYPs in constitutive androstane receptor (CAR)-null and two CYP-null mouse models that have subfamily members regulated by CAR; the Cyp3a-null and a newly described Cyp2b9/10/13-null mouse model. Compensatory changes in CYP expression that occur in these models may also occur in polymorphic humans, or may complicate interpretation of ADME studies performed using these models. The loss of CAR causes significant changes in several CYPs probably due to loss of CAR-mediated constitutive regulation of these CYPs. Expression and activity changes include significant repression of Cyp2a and Cyp2b members with corresponding drops in 6α- and 16β-testosterone hydroxylase activity. Further, the ratio of 6α-/15α-hydroxylase activity, a biomarker of sexual dimorphism in the liver, indicates masculinization of female CAR-null mice, suggesting a role for CAR in the regulation of sexually dimorphic liver CYP profiles. The loss of Cyp3a causes fewer changes than CAR. Nevertheless, there are compensatory changes including gender-specific increases in Cyp2a and Cyp2b. Cyp2a and Cyp2b were down-regulated in CAR-null mice, suggesting activation of CAR and potentially PXR following loss of the Cyp3a members. However, the loss of Cyp2b causes few changes in hepatic CYP transcript levels and almost no significant compensatory changes in protein expression or activity with the possible exception of 6α-hydroxylase activity. This lack of a compensatory response in the Cyp2b9/10/13-null mice is probably due to low CYP2B hepatic expression, especially in male mice. Overall, compensatory and regulatory CYP changes followed the order CAR-null > Cyp3a-null > Cyp2b-null mice.

Overexpression of pregnane X and glucocorticoid receptors and the regulation of cytochrome P450 in human epileptic brain endothelial cells

Objective

Recent evidence suggests a metabolic contribution of cytochrome P450 enzymes (CYPs) to the drug‐resistant phenotype in human epilepsy. However, the upstream molecular regulators of CYP in the epileptic brain remain understudied. We therefore investigated the expression and function of pregnane xenobiotic (PXR) and glucocorticoid (GR) nuclear receptors in endothelial cells established from post‐epilepsy surgery brain samples.

Methods

PXR/GR localization was evaluated by immunohistochemistry in specimens from subjects who underwent temporal lobe resections to relieve drug‐resistant seizures. We used primary cultures of endothelial cells obtained from epileptic brain tissues (EPI‐ECs; n = 8), commercially available human brain microvascular endothelial cells (HBMECs; n = 8), and human hepatocytes (n = 3). PXR/GR messenger RNA (mRNA) levels in brain ECs was initially determined by complementary DNA (cDNA) microarrays. The expression of PXR/GR proteins was quantified by Western blot. PXR and GR silencing was performed in EPI‐ECs (n = 4), and the impact on downstream CYP expression was determined.

Results

PXR/GR expression was detected by immunofluorescence in ECs and neurons in the human temporal lobe samples analyzed. Elevated mRNA and protein levels of PXR and GR were found in EPI‐ECs versus control HBMECs. Hepatocytes, used as a positive control, displayed the highest levels of PXR/GR expression. We confirmed expression of PXR/GR in cytoplasmic‐nuclear subcellular fractions, with a significant increase of PXR/GR in EPI‐ECs versus controls. CYP3A4, CYP2C9, and CYP2E1 were overexpressed in EPI‐ECs versus control, whereas CYP2D6 and CYP2C19 were downregulated or absent in EPI‐ECs. GR silencing in EPI‐ECs led to decreased CYP3A4, CYP2C9, and PXR expression. PXR silencing in EPI‐ECs resulted in the specific downregulation of CYP3A4 expression.

Significance

Our results indicate increased PXR and GR in primary ECs derived from human epileptic brains. PXR or GR may be responsible for a local drug brain metabolism sustained by abnormal CYP regulation.

Application of Mice Humanized for CYP2D6 to the Study of Tamoxifen Metabolism and Drug-Drug Interaction with Antidepressants

Tamoxifen is an estrogen receptor antagonist used in the treatment of breast cancer. It is a prodrug that is converted by several cytochrome P450 enzymes to a primary metabolite, N-desmethyltamoxifen (NDT), which is then further modified by CYP2D6 to a pharmacologically potent secondary metabolite, 4-hydroxy-N-desmethyltamoxifen (endoxifen). Antidepressants (ADs), which are often coprescribed to patients receiving tamoxifen, are also metabolized by CYP2D6 and evidence suggests that a drug-drug interaction between these agents adversely affects the outcome of tamoxifen therapy by inhibiting endoxifen formation. We evaluated this potentially important drug-drug interaction in vivo in mice humanized for CYP2D6 (hCYP2D6). The rate of conversion of NDT to endoxifen by hCYP2D6 mouse liver microsomes (MLMs) in vitro was similar to that of the most active members of a panel of 13 individual human liver microsomes. Coincubation with quinidine, a CYP2D6 inhibitor, ablated endoxifen generation by hCYP2D6 MLMs. The NDT-hydroxylation activity of wild-type MLMs was 7.4 times higher than that of hCYP2D6, whereas MLMs from Cyp2d knockout animals were inactive. Hydroxylation of NDT correlated with that of bufuralol, a CYP2D6 probe substrate, in the human liver microsome panel. In vitro, ADs of the selective serotonin reuptake inhibitor class were, by an order of magnitude, more potent inhibitors of NDT hydroxylation by hCYP2D6 MLMs than were compounds of the tricyclic class. At a clinically relevant dose, paroxetine pretreatment inhibited the generation of endoxifen from NDT in hCYP2D6 mice in vivo. These data demonstrate the potential of ADs to affect endoxifen generation and, thereby, the outcome of tamoxifen therapy.

Clinical Exposure Boost Predictions by Integrating Cytochrome P450 3A4-Humanized Mouse Studies With PBPK Modeling

NVS123 is a poorly water-soluble protease 56 inhibitor in clinical development. Data from in vitro hepatocyte studies suggested that NVS123 is mainly metabolized by CYP3A4. As a consequence of limited solubility, NVS123 therapeutic plasma exposures could not be achieved even with high doses and optimized formulations. One approach to overcome NVS123 developability issues was to increase plasma exposure by coadministrating it with an inhibitor of CYP3A4 such as ritonavir. A clinical boost effect was predicted by using physiologically based pharmacokinetic (PBPK) modeling. However, initial boost predictions lacked sufficient confidence because a key parameter, fraction of drug metabolized by CYP3A4 (fmCYP3A4), could not be estimated with accuracy on account of disconnects between in vitro and in vivo preclinical data. To accurately estimate fmCYP3A4 in human, an in vivo boost effect study was conducted using CYP3A4-humanized mouse model which showed a 33- to 56-fold exposure boost effect. Using a top-down approach, human fmCYP3A4 for NVS123 was estimated to be very high and included in the human PBPK modeling to support subsequent clinical study design. The combined use of the in vivo boost study in CYP3A4-humanized mouse model mice along with PBPK modeling accurately predicted the clinical outcome and identified a significant NVS123 exposure boost (∼42-fold increase) with ritonavir.

Cyp2c70 is responsible for the species difference in bile acid metabolism between mice and humans

Bile acids are synthesized from cholesterol in the liver and subjected to multiple metabolic biotransformations in hepatocytes, including oxidation by cytochromes P450 (CYPs) and conjugation with taurine, glycine, glucuronic acid, and sulfate. Mice and rats can hydroxylate chenodeoxycholic acid (CDCA) at the 6β-position to form α-muricholic acid (MCA) and ursodeoxycholic acid (UDCA) to form β-MCA. However, MCA is not formed in humans to any appreciable degree and the mechanism for this species difference is not known. Comparison of several Cyp-null mouse lines revealed that α-MCA and β-MCA were not detected in the liver samples from Cyp2c-cluster null (Cyp2c-null) mice. Global bile acid analysis further revealed the absence of MCAs and their conjugated derivatives, and high concentrations of CDCA and UDCA in Cyp2c-null mouse cecum and feces. Analysis of recombinant CYPs revealed that α-MCA and β-MCA were produced by oxidation of CDCA and UDCA by Cyp2c70, respectively. CYP2C9-humanized mice have similar bile acid metabolites as the Cyp2c-null mice, indicating that human CYP2C9 does not oxidize CDCA and UDCA, thus explaining the species differences in production of MCA. Because humans do not produce MCA, they lack tauro-β-MCA, a farnesoid X receptor antagonist in mouse that modulates obesity, insulin resistance, and hepatosteatosis.

Rifampin-Mediated Induction of Tamoxifen Metabolism in a Humanized PXR-CAR-CYP3A4/3A7-CYP2D6 Mouse Model

Animals are not commonly used to assess drug-drug interactions due to poor clinical translatability arising from species differences that may exist in drug-metabolizing enzymes and transporters, and their regulation pathways. In this study, a transgenic mouse model expressing human pregnane X receptor (PXR), constitutive androstane receptor (CAR), CYP3A4/CYP3A7, and CYP2D6 (Tg-composite) was used to investigate the effect of induction mediated by rifampin on the pharmacokinetics of tamoxifen and its metabolites. In humans, tamoxifen is metabolized primarily by CYP3A4 and CYP2D6, and multiple-day treatment with rifampin decreased tamoxifen exposure by 6.2-fold. Interestingly, exposure of tamoxifen metabolites 4-hydroxytamoxifen (4OHT), N-desmethyltamoxifen (NDM), and endoxifen also decreased. In the Tg-composite model, pretreatment with rifampin decreased tamoxifen area under the time-concentration curve between 0 and 8 hours (AUC_0-8) from 0.82 to 0.20 µM*h, whereas AUC_0-8 of 4OHT, NDM, and endoxifen decreased by 3.4-, 4.7-, and 1.3-fold, respectively, mirroring the clinic observations. In the humanized PXR-CAR (hPXR-CAR) model, rifampin decreased AUC_0-8 of tamoxifen and its metabolites by approximately 2-fold. In contrast, no significant modulation by rifampin was observed in the nonhumanized C57BL/6 (wild-type) animals. In vitro kinetics determined in microsomes prepared from livers of the Tg-composite animals showed that, although K_m values were not different between vehicle- and rifampin-treated groups, rifampin increased the V_max for the CYP3A4-mediated pathways. These data demonstrate that, although the hPXR-CAR model is responsive to rifampin, the extent of the clinical rifampin-tamoxifen interaction is better represented by the Tg-composite model. Consequently, the Tg-composite model may be a suitable tool to examine the extent of rifampin-mediated induction for other compounds whose metabolism is mediated by CYP3A4 and/or CYP2D6.

Humanizing the zebrafish liver shifts drug metabolic profiles and improves pharmacokinetics of CYP3A4 substrates

Understanding and predicting whether new drug candidates will be safe in the clinic is a critical hurdle in pharmaceutical development, that relies in part on absorption, distribution, metabolism, excretion and toxicology studies in vivo. Zebrafish is a relatively new model system for drug metabolism and toxicity studies, offering whole organism screening coupled with small size and potential for high-throughput screening. Through toxicity and absorption analyses of a number of drugs, we find that zebrafish is generally predictive of drug toxicity, although assay outcomes are influenced by drug lipophilicity which alters drug uptake. In addition, liver microsome assays reveal specific differences in metabolism of compounds between human and zebrafish livers, likely resulting from the divergence of the cytochrome P450 superfamily between species. To reflect human metabolism more accurately, we generated a transgenic "humanized" zebrafish line that expresses the major human phase I detoxifying enzyme, CYP3A4, in the liver. Here, we show that this humanized line shows an elevated metabolism of CYP3A4-specific substrates compared to wild-type zebrafish. The generation of this first described humanized zebrafish liver suggests such approaches can enhance the accuracy of the zebrafish model for toxicity prediction.

The Role of Protein-Protein and Protein-Membrane Interactions on P450 Function

This symposium summary, sponsored by the ASPET, was held at Experimental Biology 2015 on March 29, 2015, in Boston, Massachusetts. The symposium focused on: 1) the interactions of cytochrome P450s (P450s) with their redox partners; and 2) the role of the lipid membrane in their orientation and stabilization. Two presentations discussed the interactions of P450s with NADPH-P450 reductase (CPR) and cytochrome b5. First, solution nuclear magnetic resonance was used to compare the protein interactions that facilitated either the hydroxylase or lyase activities of CYP17A1. The lyase interaction was stimulated by the presence of b5 and 17α-hydroxypregnenolone, whereas the hydroxylase reaction was predominant in the absence of b5. The role of b5 was also shown in vivo by selective hepatic knockout of b5 from mice expressing CYP3A4 and CYP2D6; the lack of b5 caused a decrease in the clearance of several substrates. The role of the membrane on P450 orientation was examined using computational methods, showing that the proximal region of the P450 molecule faced the aqueous phase. The distal region, containing the substrate-access channel, was associated with the membrane. The interaction of NADPH-P450 reductase (CPR) with the membrane was also described, showing the ability of CPR to "helicopter" above the membrane. Finally, the endoplasmic reticulum (ER) was shown to be heterogeneous, having ordered membrane regions containing cholesterol and more disordered regions. Interestingly, two closely related P450s, CYP1A1 and CYP1A2, resided in different regions of the ER. The structural characteristics of their localization were examined. These studies emphasize the importance of P450 protein organization to their function.

Dynamics of Cytosine Methylation in the Proximal Promoters of CYP3A4 and CYP3A7 in Pediatric and Prenatal Livers

Members of the human CYP3A family of metabolizing enzymes exhibit developmental changes in expression whereby CYP3A7 is expressed in fetal tissues, followed by a transition to expression of CYP3A4 in the first months of life. Despite knowledge about the general pattern of CYP3A activity in human development, the mechanisms that regulate developmental expression remain poorly understood. Epigenetic changes, including cytosine methylation, have been suggested to play a role in the regulation of CYP3A expression. The objective of this study was to investigate changes in cytosine methylation of the CYP3A4 and CYP3A7 genes in human pediatric and prenatal livers. The methylation status of cytosine-phospho-guanine dinucleotides was determined in 16 pediatric liver samples using methyl-seq and confirmed by bisulfite sequencing of 48 pediatric and 34 prenatal liver samples. Samples were separated by age into five groups (prenatal, < 1 year of age, 1.8-6 years, 7-11 years, and 12-17 years). Methyl-seq anaylsis revealed that cytosines in the proximal promoter of CYP3A7 are hypomethylated in neonates compared with adolescents (P < 0.001). In contrast, a cytosine 383 base pair upstream of CYP3A4 is hypermethylated in liver samples from neonates compared with adolescents (P = 0.00001). Developmental changes in methylation of cytosines in the proximal promoters of CYP3A4 and CYP3A7 in pediatric livers were confirmed by bisulfite sequencing. In addition, the methylation status of cytosine in the CYP3A4 and CYP3A7 proximal promoters correlated with changes in developmental expression of mRNA for the two enzymes.

Defining Human Pathways of Drug Metabolism In Vivo through the Development of a Multiple Humanized Mouse Model

Variability in drug pharmacokinetics is a major factor in defining drug efficacy and side effects. There remains an urgent need, particularly with the growing use of polypharmacy, to obtain more informative experimental data predicting clinical outcomes. Major species differences in multiplicity, substrate specificity, and regulation of enzymes from the cytochrome P450-dependent mono-oxygenase system play a critical role in drug metabolism. To develop an in vivo model for predicting human responses to drugs, we generated a mouse, where 31 P450 genes from the Cyp2c, Cyp2d, and Cyp3a gene families were exchanged for their relevant human counterparts. The model has been improved through additional humanization for the nuclear receptors constitutive androgen receptor and pregnane X receptor that control the expression of key drug metabolizing enzymes and transporters. In this most complex humanized mouse model reported to date, the cytochromes P450 function as predicted and we illustrate how these mice can be applied to predict drug-drug interactions in humans.

Activation status of the pregnane X receptor influences vemurafenib availability in humanized mouse models

Vemurafenib is a revolutionary treatment for melanoma, but the magnitude of therapeutic response is highly variable, and the rapid acquisition of resistance is frequent. Here, we examine how vemurafenib disposition, particularly through cytochrome P450-mediated oxidation pathways, could potentially influence these outcomes using a panel of knockout and transgenic humanized mouse models. We identified CYP3A4 as the major enzyme involved in the metabolism of vemurafenib in in vitro assays with human liver microsomes. However, mice expressing human CYP3A4 did not process vemurafenib to a greater extent than CYP3A4-null animals, suggesting that other pregnane X receptor (PXR)-regulated pathways may contribute more significantly to vemurafenib metabolism in vivo. Activation of PXR, but not of the closely related constitutive androstane receptor, profoundly reduced circulating levels of vemurafenib in humanized mice. This effect was independent of CYP3A4 and was negated by cotreatment with the drug efflux transporter inhibitor elacridar. Finally, vemurafenib strongly induced PXR activity in vitro, but only weakly induced PXR in vivo. Taken together, our findings demonstrate that vemurafenib is unlikely to exhibit a clinically significant interaction with CYP3A4, but that modulation of bioavailability through PXR-mediated regulation of drug transporters (e.g., by other drugs) has the potential to markedly influence systemic exposure and thereby therapeutic outcomes.

A comparison between genetically humanized and chimeric liver humanized mouse models for studies in drug metabolism and toxicity

Mice that have been genetically humanized for proteins involved in drug metabolism and toxicity and mice engrafted with human hepatocytes are emerging and promising in vivo models for an improved prediction of the pharmacokinetic, drug-drug interaction and safety characteristics of compounds in humans. The specific advantages and disadvantages of these models should be carefully considered when using them for studies in drug discovery and development. Here, an overview on the corresponding genetically humanized and chimeric liver humanized mouse models described to date is provided and illustrated with examples of their utility in drug metabolism and toxicity studies. We compare the strength and weaknesses of the two different approaches, give guidance for the selection of the appropriate model for various applications and discuss future trends and perspectives.

Opioid Analgesia in P450 Gene Cluster Knockout Mice: A Search for Analgesia-Relevant Isoforms

Cytochrome P450 monooxygenases (P450s), which are well-known drug-metabolizing enzymes, are thought to play a signal transduction role in µ opioid analgesia and may serve as high-affinity (3)H-cimetidine ((3)HCIM) binding sites in the brain. (3)HCIM binding sites may also be related to opioid or nonopioid analgesia. However, of the more than 100 murine P450 enzymes, the specific isoform(s) responsible for either function have not been identified. Presently, three lines of constitutive P450 gene cluster knockout (KO) mice with full-length deletions of 14 Cyp2c, 9 Cyp2d, and 7 Cyp3a genes were studied for deficiencies in (3)HCIM binding and for opioid analgesia. Liver and brain homogenates from all three genotypes showed normal (3)HCIM binding values, indicating that gene products of Cyp2d, Cyp3a, and Cyp2c are not (3)HCIM-binding proteins. Cyp2d KO and Cyp3a KO mice showed normal antinociceptive responses to a moderate systemic dose of morphine (20 mg/kg, s.c.), thereby excluding 16 P450 isoforms as mediators of opioid analgesia. In contrast, Cyp2c KO mice showed a 41% reduction in analgesic responses following systemically (s.c.) administered morphine. However, the significance of brain Cyp2c gene products in opioid analgesia is uncertain because little or no analgesic deficits were noted in Cyp2c KO mice following intracerebroventricular or intrathecalmorphine administration, respectively. These results show that the gene products of Cyp2d and Cyp3a do not contribute to µ opioid analgesia in the central nervous system. A possible role for Cyp2c gene products in opioid analgesia requires further consideration.

Cytochrome b5 is a major determinant of human cytochrome P450 CYP2D6 and CYP3A4 activity in vivo

The cytochrome P450-dependent mono-oxygenase system is responsible for the metabolism and disposition of chemopreventive agents, chemical toxins and carcinogens, and >80% of therapeutic drugs. Cytochrome P450 (P450) activity is regulated transcriptionally and by the rate of electron transfer from P450 reductase. In vitro studies have demonstrated that cytochrome b5 (Cyb5) also modulates P450 function. We recently showed that hepatic deletion of Cyb5 in the mouse (HBN) markedly alters in vivo drug pharmacokinetics; a key outstanding question is whether Cyb5 modulates the activity of the major human P450s in drug disposition in vivo. To address this, we crossed mice humanized for CYP2D6 or CYP3A4 with mice carrying a hepatic Cyb5 deletion. In vitro triazolam 4-hydroxylation (probe reaction for CYP3A4) was reduced by >50% in hepatic microsomes from CYP3A4-HBN mice compared with controls. Similar reductions in debrisoquine 4-hydroxylation and metoprolol α-hydroxylation were observed using CYP2D6-HBN microsomes, indicating a significant role for Cyb5 in the activity of both enzymes. This effect was confirmed by the concentration-dependent restoration of CYP3A4-mediated triazolam turnover and CYP2D6-mediated bufuralol and debrisoquine turnover on addition of Escherichia coli membranes containing recombinant Cyb5. In vivo, the peak plasma concentration and area under the concentration time curve from 0 to 8 hours (AUC0-8 h) of triazolam were increased 4- and 5.7-fold, respectively, in CYP3A4-HBN mice. Similarly, the pharmacokinetics of bufuralol and debrisoquine were significantly altered in CYP2D6-HBN mice, the AUC0-8 h being increased ∼1.5-fold and clearance decreased by 40-60%. These data demonstrate that Cyb5 can be a major determinant of CYP3A4 and CYP2D6 activity in vivo, with a potential impact on the metabolism, efficacy, and side effects of numerous therapeutic drugs.

Role of pregnane X receptor and aryl hydrocarbon receptor in transcriptional regulation of pxr, CYP2, and CYP3 genes in developing zebrafish

Ligand-activated receptors regulate numerous genes, and mediate effects of a broad set of endogenous and exogenous chemicals in vertebrates. Understanding the roles of these transcription factors in zebrafish (Danio rerio) is important to the use of this non-mammalian model in toxicological, pharmacological, and carcinogenesis research. Response to a potential agonist for the pregnane X receptor (Pxr) [pregnenolone (PN)] was examined in developing zebrafish, to assess involvement of Pxr in regulation of selected genes, including genes in cytochrome P450 subfamilies CYP2 and CYP3. We also examined interaction of Pxr and the aryl hydrocarbon receptor (Ahr) signaling pathways. Pregnenolone caused a dose-dependent increase in mRNA levels of pxr, ahr2, CYP1A, CYP2AA1, CYP2AA12, CYP3A65, and CYP3C1, most of which peaked at 3 µM PN. The well-known Ahr agonist 3,3',4,4',5-pentachlorobiphenyl (PCB126) also upregulated expression of pxr, ahr2, CYP1A, CYP2AA12, CYP3A65, and CYP3C1 in a dose-dependent manner. Inhibition of pxr translation by morpholino antisense oligonucleotides (MO) suppressed PN-induced expression of pxr, ahr2, CYP3A65, and CYP3C1 genes. Levels of CYP2AA1 and CYP2AA12 mRNA were increased in the control-MO group exposed to PN; this was prevented by knocking down Pxr. Similarly, Ahr2-MO treatment blocked PCB126-induced mRNA expression of pxr, CYP1A, CYP2AA12, CYP3A65, and CYP3C1. The present study shows self-regulation of pxr by PN in developing zebrafish. Selected zebrafish CYP1, CYP2 (including several CYP2AAs) and CYP3 genes appear to be under the regulation of both Pxr and Ahr2.

An enhanced in vivo stable isotope labeling by amino acids in cell culture (SILAC) model for quantification of drug metabolism enzymes

Many of the enzymes involved in xenobiotic metabolism are maintained at a low basal level and are only synthesized in response to activation of upstream sensor/effector proteins. This induction can have implications in a variety of contexts, particularly during the study of the pharmacokinetics, pharmacodynamics, and drug–drug interaction profile of a candidate therapeutic compound. Previously, we combined in vivo SILAC material with a targeted high resolution single ion monitoring (tHR/SIM) LC-MS/MS approach for quantification of 197 peptide pairs, representing 51 drug metabolism enzymes (DME), in mouse liver. However, as important enzymes (for example, cytochromes P450 (Cyp) of the 1a and 2b subfamilies) are maintained at low or undetectable levels in the liver of unstimulated metabolically labeled mice, quantification of these proteins was unreliable. In the present study, we induced DME expression in labeled mice through synchronous ligand-mediated activation of multiple upstream nuclear receptors, thereby enhancing signals for proteins including Cyps 1a, 2a, 2b, 2c, and 3a. With this enhancement, 115 unique, lysine-containing, Cyp-derived peptides were detected in the liver of a single animal, as opposed to 56 in a pooled sample from three uninduced animals. A total of 386 peptide pairs were quantified by tHR/SIM, representing 68 Phase I, 30 Phase II, and eight control proteins. This method was employed to quantify changes in DME expression in the hepatic cytochrome P450 reductase null (HRN) mouse. We observed compensatory induction of several enzymes, including Cyps 2b10, 2c29, 2c37, 2c54, 2c55, 2e1, 3a11, and 3a13, carboxylesterase (Ces) 2a, and glutathione S-transferases (Gst) m2 and m3, along with down-regulation of hydroxysteroid dehydrogenases (Hsd) 11b1 and 17b6. Using DME-enhanced in vivo SILAC material with tHR/SIM, therefore, permits the robust analysis of multiple DME of importance to xenobiotic metabolism, with improved utility for the study of drug pharmacokinetics, pharmacodynamics, and of chemically treated and genetically modified mouse models.

Using improved serial blood sampling method of mice to study pharmacokinetics and drug-drug interaction

In pharmacokinetic evaluation of mice, using serial sampling methods rather than a terminal blood sampling method could reduce the number of animals needed and lead to more reliable data by excluding individual differences. In addition, using serial sampling methods can be valuable for evaluation of the drug-drug interaction (DDI) potential of drug candidates. In this study, we established an improved method for serially sampling the blood from one mouse by only one incision of the lateral tail vein, and investigated whether our method could be adapted to pharmacokinetic and DDI studies. After intravenous and oral administration of ibuprofen and fexofenadine (BCS class II and III), the plasma concentration and pharmacokinetic parameters were evaluated by our method and a terminal blood sampling method, with the result that both methods gave comparable results (ibuprofen: 63.8 ± 4.0% and 64.4%, fexofenadine: 6.5 ± 0.7% and 7.9%, respectively, in bioavailability). In addition, our method could be adapted to DDI study for cytochrome P450 and organic anion transporting polypeptide inhibition. These results demonstrate that our method can be useful for pharmacokinetic evaluation from the perspective of reliable data acquisition as well as easy handling and low stress to mice and improve the quality of pharmacokinetic and DDI studies.

Novel pre-clinical methodologies for pharmacokinetic drug-drug interaction studies: spotlight on "humanized" animal models

Poly-therapy is common due to co-occurrence of several ailments in patients, leading to the elevated possibility of drug-drug interactions (DDI). Pharmacokinetic DDI often accounts for severe adverse drug reactions in patients resulting in withdrawal of drug from the market. Hence, the prediction of DDI is necessary at pre-clinical stage of drug development. Several human tissue and cell line-based in vitro systems are routinely used for screening metabolic and transporter pathways of investigational drugs and for predicting their clinical DDI potentials. However, ample constraints are associated with the in vitro systems and sometimes in vitro-in vivo extrapolation (IVIVE) fail to assess the risk of DDI in clinic. In vitro-in vivo correlation model in animals combined with human in vitro studies may be helpful in better prediction of clinical outcome. Native animal models vary remarkably from humans in drug metabolizing enzymes and transporters, hence, the interpretation of results from animal DDI studies is difficult. With the advent of modern molecular biology and engineering tools, novel pre-clinical animal models, namely, knockout rat/mouse, transgenic rat/mouse with humanized drug metabolizing enzymes and/or transporters and chimeric rat/mouse with humanized liver are developed. These models nearly simulate human-like drug metabolism and help to validate the in vivo relevance of the in vitro human DDI data. This review briefly discusses the application of such novel pre-clinical models for screening various type of DDI along with their advantages and limitations.

Deletion of 30 murine cytochrome p450 genes results in viable mice with compromised drug metabolism

In humans, 75% of all drugs are metabolized by the cytochrome P450-dependent monooxygenase system. Enzymes encoded by the CYP2C, CYP2D, and CYP3A gene clusters account for ∼80% of this activity. There are profound species differences in the multiplicity of cytochrome P450 enzymes, and the use of mouse models to predict pathways of drug metabolism is further complicated by overlapping substrate specificity between enzymes from different gene families. To establish the role of the hepatic and extrahepatic P450 system in drug and foreign chemical disposition, drug efficacy, and toxicity, we created a unique mouse model in which 30 cytochrome P450 genes from the Cyp2c, Cyp2d, and Cyp3a gene clusters have been deleted. Remarkably, despite a wide range of putative important endogenous functions, Cyp2c/2d/3a KO mice were viable and fertile, demonstrating that these genes have evolved primarily as detoxification enzymes. Although there was no overt phenotype, detailed examination showed Cyp2c/2d/3a KO mice had a smaller body size (15%) and larger livers (20%). Changes in hepatic morphology and a decreased blood glucose (30%) were also noted. A five-drug cocktail of cytochrome P450 isozyme probe substrates were used to evaluate changes in drug pharmacokinetics; marked changes were observed in either the pharmacokinetics or metabolites formed from Cyp2c, Cyp2d, and Cyp3a substrates, whereas the metabolism of the Cyp1a substrate caffeine was unchanged. Thus, Cyp2c/2d/3a KO mice provide a powerful model to study the in vivo role of the P450 system in drug metabolism and efficacy, as well as in chemical toxicity.

Development of mice exhibiting hepatic microsomal activity of human CYP3A4 comparable to that in human liver microsomes by intravenous administration of an adenovirus vector expressing human CYP3A4

Cytochrome P450 3A4 (CYP3A4) plays a crucial role in the pharmacokinetic and safety profiles of drugs. However, it is difficult to properly predict the pharmacokinetics and hepatotoxicity of drugs in humans using data from experimental animals, because the catalytic activities of CYP3A4 and other drug-metabolizing enzymes differ between human and animal organs. In order to easily generate an animal model for proper evaluation of human CYP3A4-mediated drug metabolism, we developed a human CYP3A4-expressing adenovirus (Ad) vector based on our novel Ad vector exhibiting significantly lower hepatotoxicity (Ad-E4-122aT-hCYP3A4). Intravenous administration of Ad-E4-122aT-hCYP3A4 at a dose of 2 × 10(11) virus particles/mouse produced a mouse exhibiting human CYP3A4 activity at a level similar to that in the human liver, as shown in the dexamethasone metabolic experiment using liver microsomes. The area under the curve (AUC) of 6βOHD was 2.7-fold higher in the Ad-E4-122aT-hCYP3A4-administered mice, compared with the mice receiving a control Ad vector. This Ad vector-expressing human CYP3A4 would thus be a powerful tool for evaluating human CYP3A4-mediated drug metabolism in the livers of experimental animals.

A role for cytochrome b5 in the In vivo disposition of anticancer and cytochrome P450 probe drugs in mice

The role of microsomal cytochrome b5 (Cyb5) in defining the rate of drug metabolism and disposition has been intensely debated for several decades. Recently we described mouse models involving the hepatic or global deletion of Cyb5, demonstrating its central role in in vivo drug disposition. We have now used the cytochrome b5 complete null (BCN) model to determine the role of Cyb5 in the metabolism of ten pharmaceuticals metabolized by a range of cytochrome P450s, including five anticancer drugs, in vivo and in vitro. The extent to which metabolism was significantly affected by the absence of Cyb5 was substrate-dependent; AUC increased (75-245%) and clearance decreased (35-72%) for phenacetin, metoprolol, and chlorzoxazone. Tolbutamide disposition was not significantly altered by Cyb5 deletion, while for midazolam clearance was decreased by 66%. The absence of Cyb5 had no effect on gefitinib and paclitaxel disposition, while significant changes in the in vivo pharmacokinetics were measured for: cyclophosphamide [maximum plasma concentration (Cmax) and terminal half-life increased 55% and 40%, respectively], tamoxifen (AUClast and Cmax increased 370% and 233%, respectively), and anastrozole (AUC and terminal half-life increased 125% and 62%, respectively; clearance down 80%). These data provide strong evidence that both hepatic and extrahepatic Cyb5 levels are an important determinant of in vivo drug disposition catalyzed by a range of cytochrome P450s, including currently prescribed anticancer agents, and that individuality in Cyb5 expression could be a significant determinant in rates of drug disposition in man.

Targeted mutagenesis tools for modelling psychiatric disorders

In the 1980s, the basic principles of gene targeting were discovered and forged into sharp tools for efficient and precise engineering of the mouse genome. Since then, genetic mouse models have substantially contributed to our understanding of major neurobiological concepts and are of utmost importance for our comprehension of neuropsychiatric disorders. The "domestication" of site-specific recombinases and the continuous creative technological developments involving the implementation of previously identified biological principles such as transcriptional and posttranslational control now enable conditional mutagenesis with high spatial and temporal resolution. The initiation and successful accomplishment of large-scale efforts to annotate functionally the entire mouse genome and to build strategic resources for the research community have significantly accelerated the rapid proliferation and broad propagation of mouse genetic tools. Addressing neurobiological processes with the assistance of genetic mouse models is a routine procedure in psychiatric research and will be further extended in order to improve our understanding of disease mechanisms. In light of the highly complex nature of psychiatric disorders and the current lack of strong causal genetic variants, a major future challenge is to model of psychiatric disorders more appropriately. Humanized mice, and the recently developed toolbox of site-specific nucleases for more efficient and simplified tailoring of the genome, offer the perspective of significantly improved models. Ultimately, these tools will push the limits of gene targeting beyond the mouse to allow genome engineering in any model organism of interest.