In vivoinduction of human cytochrome P450 3A4 by rifabutin in chimeric mice with humanized liver

Mice Engrafted with Human Liver Cells

Rodents are commonly employed to model human liver conditions, although species differences can restrict their translational relevance. To overcome some of these limitations, researchers have long pursued human hepatocyte transplantation into rodents. More than 20 years ago, the first primary human hepatocyte transplantations into immunodeficient mice with liver injury were able to support hepatitis B and C virus infections, as these viruses cannot replicate in murine hepatocytes. Since then, hepatocyte chimeric mouse models have transitioned into mainstream preclinical research and are now employed in a diverse array of liver conditions beyond viral hepatitis, including malaria, drug metabolism, liver-targeting gene therapy, metabolic dysfunction-associated steatotic liver disease, lipoprotein and bile acid biology, and others. Concurrently, endeavors to cotransplant other cell types and humanize immune and other nonparenchymal compartments have seen growing success. Looking ahead, several challenges remain. These include enhancing immune functionality in mice doubly humanized with hepatocytes and immune systems, efficiently creating mice with genetically altered grafts and reliably humanizing chimeric mice with renewable cell sources such as patient-specific induced pluripotent stem cells. In conclusion, hepatocyte chimeric mice have evolved into vital preclinical models that address many limitations of traditional rodent models. Continued improvements may further expand their applications.

General Framework to Quantitatively Predict Pharmacokinetic Induction Drug-Drug Interactions Using In Vitro Data

INTRODUCTION

Metabolic inducers can expose people with polypharmacy to adverse health outcomes. A limited fraction of potential drug-drug interactions (DDIs) have been or can ethically be studied in clinical trials, leaving the vast majority unexplored. In the present study, an algorithm has been developed to predict the induction DDI magnitude, integrating data related to drug-metabolising enzymes.

METHODS

The area under the curve ratio (AUC_ratio) resulting from the DDI with a victim drug in the presence and absence of an inducer (rifampicin, rifabutin, efavirenz, or carbamazepine) was predicted from various in vitro parameters and then correlated with the clinical AUC_ratio (N = 319). In vitro data including fraction unbound in plasma, substrate specificity and induction potential for cytochrome P450s, phase II enzymes and uptake, and efflux transporters were integrated. To represent the interaction potential, the in vitro metabolic metric (IVMM) was generated by combining the fraction of substrate metabolised by each hepatic enzyme of interest with the corresponding in vitro fold increase in enzyme activity (E) value for the inducer.

RESULTS

Two independent variables were deemed significant and included in the algorithm: IVMM and fraction unbound in plasma. The observed and predicted magnitudes of the DDIs were categorised accordingly: no induction, mild, moderate, and strong induction. DDIs were assumed to be well classified if the predictions were in the same category as the observations, or if the ratio between these two was < 1.5-fold. This algorithm correctly classified 70.5% of the DDIs.

CONCLUSION

This research presents a rapid screening tool to identify the magnitude of potential DDIs utilising in vitro data which can be highly advantageous in early drug development.

Drug-drug interactions between moxifloxacin and rifampicin based on pharmacokinetics in vivo in rats

Moxifloxacin and rifampicin are all the first-line options for the treatment of active tuberculosis, which are often combined for the treatment of multidrug resistance pulmonary tuberculosis in clinic. However, the potential drug-drug interactions between moxifloxacin and rifampicin were unknown. The aim of this study was to investigate the drug-drug interactions between moxifloxacin and rifampicin based on their pharmacokinetics in vivo after oral administration of the single drug and both drugs, and reveal their mutual effects on their pharmacokinetics. Eighteen male Sprague-Dawley rats were randomly assigned to three groups: moxifloxacin group, rifampicin group and moxifloxacin + rifampicin group. Plasma concentrations of moxifloxacin and rifampicin were determined using LC-MS at the designated time points after drug administration, and the main pharmacokinetic parameters were calculated. In addition, effects of moxifloxacin and rifampicin on their metabolic rate and absorption were investigated using rat liver microsome incubation systems and Caco-2 cell transwell model. The main pharmacokinetic parameters of moxifloxacin including Tmax , Cmax , t1/2 and AUC(0-t) increased more in the moxifloxacin + rifampicin group than in the moxifloxacin group, but the difference was not significant (p > 0.05). However, the pharmacokinetic parameters of rifampicin, including peak concentration, area under the concentration-time curve, half-life and the area under the first moment plasma concentration-time curve, increased significantly (p < 0.05) compared with the rifampicin group, and the time to peak concentration decreased significantly (p < 0.05). The mean residence time of rifampicin also increased in moxifloxacin + rifampicin group compared with the rifampicin group, but the difference was not significant (p > 0.05). The rat liver microsome incubation experiment indicated that moxifloxacin could increase the metabolic rate of rifampicin from 23.7 to 38.7 min. However, the Caco-2 cell transwell experiment showed that moxifloxacin could not affect the absorption rate of rifampicin. These changes could enhance the drug efficacy, but they could also cause drug accumulation, which might induce adverse effect, so it was suggested that the drug dosage should be adjusted and the drug concentration in plasma should be monitored if moxifloxacin and rifampicin are co-administered. Copyright © 2016 John Wiley & Sons, Ltd.

Rifabutin: where do we stand in 2016?

Rifabutin is a spiro-piperidyl-rifamycin structurally closely related to rifampicin that shares many of its properties. We attempted to address the reasons why this drug, which was recently recognized as a WHO Essential Medicine, still had a far narrower range of indications than rifampicin, 24 years after its launch. In this comprehensive review of the classic and more recent rifabutin experimental and clinical studies, the current state of knowledge about rifabutin is depicted, relying on specific pharmacokinetics, pharmacodynamics, antimicrobial properties, resistance data and side effects compared with rifampicin. There are consistent in vitro data and clinical studies showing that rifabutin has at least equivalent activity/efficacy and acceptable tolerance compared with rifampicin in TB and non-tuberculous mycobacterial diseases. Clinical studies have emphasized the clinical benefits of low rifabutin liver induction in patients with AIDS under PIs, in solid organ transplant patients under immunosuppressive drugs or in patients presenting intolerable side effects related to rifampicin. The contribution of rifabutin for rifampicin-resistant, but rifabutin-susceptible, Mycobacterium tuberculosis isolates according to the present breakpoints has been challenged and is now controversial. Compared with rifampicin, rifabutin's lower AUC is balanced by higher intracellular penetration and lower MIC for most pathogens. Clinical studies are lacking in non-mycobacterial infections.

Generation of Human Liver Chimeric Mice for the Study of Human Hepatotropic Pathogens

Human liver chimeric mice have become valuable tools for the study of human hepatotropic pathogens and for the investigation of metabolism and pharmacokinetics of novel drugs. The evolution of the underlying mouse models has been rapid in the past years. The diverse fields of applications of those model systems and their technical challenges will be discussed in this chapter.

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.

Current status of prediction of drug disposition and toxicity in humans using chimeric mice with humanized liver

1. Human-chimeric mice with humanized liver have been constructed by transplantation of human hepatocytes into several types of mice having genetic modifications that injure endogenous liver cells. Here, we focus on liver urokinase-type plasminogen activator-transgenic severe combined immunodeficiency (uPA/SCID) mice, which are the most widely used human-chimeric mice. Studies so far indicate that drug metabolism, drug transport, pharmacological effects and toxicological action in these mice are broadly similar to those in humans. 2. Expression of various drug-metabolizing enzymes is known to be different between humans and rodents. However, the expression pattern of cytochrome P450, aldehyde oxidase and phase II enzymes in the liver of human-chimeric mice resembles that in humans, not that in the host mice. 3. Metabolism of various drugs, including S-warfarin, zaleplon, ibuprofen, naproxen, coumarin, troglitazone and midazolam, in human-chimeric mice is mediated by human drug-metabolizing enzymes, not by host mouse enzymes, and thus resembles that in humans. 4. Pharmacological and toxicological effects of various drugs in human-chimeric mice are also similar to those in humans. 5. The current consensus is that chimeric mice with humanized liver are useful to predict drug metabolism catalyzed by cytochrome P450, aldehyde oxidase and phase II enzymes in humans in vivo and in vitro. Some remaining issues are discussed in this review.

Mice with human livers

Animal models are used to study many aspects of human disease and to test therapeutic interventions. However, some very important features of human biology cannot be replicated in animals, even in nonhuman primates or transgenic rodents engineered with human genes. Most human microbial pathogens do not infect animals and the metabolism of many xenobiotics is different between human beings and animals. The advent of transgenic immune-deficient mice has made it possible to generate chimeric animals harboring human tissues and cells, including hepatocytes. The liver plays a central role in many human-specific biological processes and mice with humanized livers can be used to model human metabolism, liver injury, gene regulation, drug toxicity, and hepatotropic infections.

Downregulation of mouse hepatic CYP3A protein by 3-methylcholanthrene does not require cytochrome P450-dependent metabolism

The aryl hydrocarbon receptor (AHR)-dependent induction of cytochromes P450 (P450) such as CYP1A1 by 3-methylcholanthrene (MC) and related polycyclic aromatic hydrocarbons is well characterized. We reported previously that MC treatment triggers a pronounced downregulation, particularly at the protein level, of mouse hepatic Cyp3a11, a counterpart of the key human drug-metabolizing enzyme CYP3A4. To determine whether this effect of MC requires hepatic microsomal P450 activity, we studied liver Cpr-null (LCN) mice with hepatocyte-specific conditional deletion of the NADPH-cytochrome P450 oxidoreductase gene. In vehicle-treated animals, basal levels of CYP3A11 mRNA and CYP3A protein immunoreactivity were elevated by approximately 9-fold in LCN mice compared with wild-type (WT) mice, whereas CYP3A catalytic activity was profoundly compromised in LCN mice. MC treatment caused suppression of CYP3A11 mRNA, CYP3A protein immunoreactivity, and CYP3A catalytic activity in WT mice, and the MC effects at the mRNA and protein levels were maintained in LCN mice. Flavin-containing monooxygenase-3 (Fmo3) induction by MC was suggested previously to occur via an AHR-dependent mechanism requiring conversion of the parent compound to DNA-damaging reactive metabolites; however, hepatic FMO3 mRNA levels were dramatically increased by MC in both WT and LCN mice. MC did not function as a mechanism-based inactivator of CYP3A enzymes in hepatic microsomes prepared from untreated WT mice, under conditions in which 1-aminobenzotriazole caused marked NADPH-dependent loss of total P450 content and CYP3A catalytic activity. These results indicate that MC downregulates mouse hepatic CYP3A protein via a pretranslational mechanism that does not require hepatic microsomal P450-dependent activity.

Targeting the pregnane X receptor in liver injury

INTRODUCTION

The nuclear receptor pregnane X receptor (PXR) is a well-characterized hepatic xenobiotic sensor whose activation by chemically diverse compounds results in the induction of drug clearance pathways that rid the body of potentially toxic substances, thus conferring protection from foreign chemicals and endobiotics.

AREAS COVERED

PXR activities are implicated in drug-drug interactions and endocrine disruption. Recent evidence supports a hepatoprotective role for PXR in chronic liver injury, inhibiting liver inflammation through suppression of the NF-κB pathway. However, PXR-mediated induction of CYP3A enhances APAP-induced acute liver injury by generating toxic metabolites. While these observations implicate PXR as a therapeutic target for liver injury, they also caution against PXR activation by pharmaceutical drugs.

EXPERT OPINION

While evidence of PXR involvement in acute and chronic liver injuries identifies it as a possible therapeutic target, it raises additional concerns for all drug candidates. The in vitro and in vivo tests for human PXR activation should be incorporated into the FDA regulations for therapeutic drug approval to identify potential liver toxicities. In addition, PXR pharmacogenetic studies will facilitate the prediction of patient-specific drug reactivities and associated liver disorders.

Animal models for studying hepatitis C and alcohol effects on liver

Chronic consumption of ethanol has a dramatic effect on the clinical outcome of patients with hepatitis C virus (HCV) infection, but the mechanism linking these two pathologies is unknown. Presently, in vitro systems are limited in their ability to study the interaction between a productive wild-type HCV infection and chronic ethanol exposure. Mouse models are potentially very useful in dissecting elements of the HCV-ethanol relationship. Experiments in mice that transgenically express HCV proteins are outlined, as are experiments for the generation of mice with chimeric human livers. The latter models appear to have the most promise for accurately modeling the effects of chronic ethanol intake in HCV-infected human livers.

Rifamycins--obstacles and opportunities

With nearly one-third of the global population infected by Mycobacterium tuberculosis, TB remains a major cause of death (1.7 million in 2006). TB is particularly severe in parts of Asia and Africa where it is often present in AIDS patients. Difficulties in treatment are exacerbated by the 6-9 month treatment times and numerous side effects. There is significant concern about the multi-drug-resistant (MDR) strains of TB (0.5 million MDR-TB cases worldwide in 2006). The rifamycins, long considered a mainstay of TB treatment, were a tremendous breakthrough when they were developed in the 1960's. While the rifamycins display many admirable qualities, they still have a number of shortfalls including: rapid selection of resistant mutants, hepatotoxicity, a flu-like syndrome (especially at higher doses), potent induction of cytochromes P450 (CYP) and inhibition of hepatic transporters. This review of the state-of-the-art regarding rifamycins suggests that it is quite possible to devise improved rifamycin analogs. Studies showing the potential of shortening the duration of treatment if higher doses could be tolerated, also suggest that more potent (or less toxic) rifamycin analogs might accomplish the same end. The improved activity against rifampin-resistant strains by some analogs promises that further work in this area, especially if the information from co-crystal structures with RNA polymerase is applied, should lead to even better analogs. The extensive drug-drug interactions seen with rifampin have already been somewhat ameliorated with rifabutin and rifalazil, and the use of a CYP-induction screening assay should serve to efficiently identify even better analogs. The toxicity due to the flu-like syndrome is an issue that needs effective resolution, particularly for analogs in the rifalazil class. It would be of interest to profile rifalazil and analogs in relation to rifampin, rifapentine, and rifabutin in a variety of screens, particularly those that might relate to hypersensitivity or immunomodulatory processes.

Chimeric mice with humanized liver: tools for the study of drug metabolism, excretion, and toxicity

Recent developments in animal models have allowed the creation of mice with genetic alterations that cause hepatocyte damage that results, over time, in the loss of native hepatocytes. If donor, human hepatocytes are transplanted into these animals, they repopulate the host liver, frequently replacing over 70% of the native liver with human cells. Immunodeficient mice that overexpress urokinase-type plasminogen activator (uPA) and, alternatively, with a knockout of the fumarylacetoacetate hydrolase (Fah) genes are the two most common mouse models for these studies. These mice are called chimeric or "humanized" because the liver is now partially repopulated with human cells. In this report we will review the published work with respect to Phase I and Phase II metabolic pathways and the expression of hepatic transport proteins. While the studies are still at the descriptive stage, it is already clear that some humanized mice display high levels of repopulation with human hepatocytes, express basal and inducible human CYP450 genes, and human conjugation and hepatic transport pathways. When the strengths and weaknesses of these humanized mouse models are fully understood, they will likely be quite valuable for investigations of human liver-mediated metabolism and excretion of drugs and xenobiotics, drug-drug interactions, and for short- and long-term investigation of the toxicity of drugs or chemicals with significant human exposure.

Current industrial practices in assessing CYP450 enzyme induction: preclinical and clinical

Induction of drug metabolizing enzymes, such as the cytochromes P450 (CYP) is known to cause drug-drug interactions due to increased elimination of co-administered drugs. This increased elimination may lead to significant reduction or complete loss of efficacy of the co-administered drug. Due to the significance of such drug interactions, many pharmaceutical companies employ screening and characterization models which predict CYP enzyme induction to avoid or attenuate the potential for drug interactions with new drug candidates. The most common mechanism of CYP induction is transcriptional gene activation. Activation is mediated by nuclear receptors, such as AhR, CAR, and PXR that function as transcription factors. Early high throughput screening models utilize these nuclear hormone receptors in ligand binding or cell-based transactivation/reporter assays. In addition, immortalized hepatocyte cell lines can be used to assess enzyme induction of specific drug metabolizing enzymes. Cultured primary human hepatocytes, the best established in vitro model for predicting enzyme induction and most accepted by regulatory agencies, is the predominant assay used to evaluate induction of a wide variety of drug metabolizing enzymes. These in vitro models are able to appropriately predict enzyme induction in patients when compared to clinical drug-drug interactions. Finally, transgenic animal models and the cynomolgus monkey have also been shown to recapitulate human enzyme induction and may be appropriate in vivo animal models for predicting human drug interactions.

Chemical proteomic probes for profiling cytochrome p450 activities and drug interactions in vivo

The cytochrome P450 (P450) superfamily metabolizes many endogenous signaling molecules and drugs. P450 enzymes are regulated by posttranslational mechanisms in vivo, which hinders their functional characterization by conventional genomic or proteomic methods. Here we describe a chemical proteomic strategy to profile P450 activities directly in living systems. Derivatization of a mechanism-based inhibitor with a "clickable" handle provided an activity-based probe that labels multiple P450s both in proteomic extracts and in vivo. This probe was used to record alterations in liver P450 activities triggered by chemical agents, including inducers of P450 expression and direct P450 inhibitors. The chemical proteomic strategy described herein thus offers a versatile method to monitor P450 activities and small-molecule interactions in any biological system and, through doing so, should facilitate the functional characterization of this large and diverse enzyme class.

Chimeric mice with humanized liver

Recently, chimeric mice with humanized liver were established by transplanting human hepatocytes into an urokinase-type plasminogen activator(+/+)/severe combined immunodeficient transgenic mouse line. The replacement with human hepatocytes is more than 80-90% and is higher than any other chimeric mouse reported previously. In drug development, the liver is one of the most important organs because it is mainly involved in the pharmacokinetics of drugs and is frequently damaged by many drugs due to the accumulation of drugs and/or metabolites. The pharmacokinetics could affect the efficacy and toxicity of a drug, and thus prediction of the human pharmacokinetics is important for developing new drugs without adverse reactions and toxicity. Extrapolation from experimental animals or in vitro studies to the human in vivo pharmacokinetics is still difficult. To date, human hepatocytes and liver microsomes are recognized as better tools and are frequently used to estimate the human pharmacokinetics. We thought that chimeric mice with humanized liver could become a new tool for estimating the human toxicity and pharmacokinetics. At first, metabolism, which plays an essential role in pharmacokinetics, was investigated in the chimeric mice. In the liver of the chimeric mice, human drug metabolizing enzymes were found to be expressed and to reflect the capacities and genetic polymorphism of the donor. In an in vivo study on metabolism, human specific metabolites could be detected in the serum of the chimeric mice indicating that the chimeric mice could be used as an in vivo model to address human metabolism. These results suggested that the chimeric mice could overcome the species differences in drug metabolism and be used to evaluate drug toxicity due to genetic polymorphism. The reasons for drug interaction are often enzyme induction and inhibition. By the treatment with a typical inducer of cytochrome P450 (P450), which is the central drug-metabolizing enzyme, P450s expressed in the liver of the chimeric mice were found to possess induction potencies. After the treatment with a specific inhibitor of human P450, the area under the curve of the P450 metabolite was significantly decreased in the chimeric mice but not in the control mice. Therefore, it was indicated that the chimeric mice could be useful for assessing drug interactions in vivo. Moreover, drug excretion was determined to be humanized because cefmetazole was mainly excreted in urine both in the chimeric mice and humans but in the feces in control uPA(-/-)/SCID mice. Drug transporters expressed in the liver of the chimeric mice were also humanized. In this review, studies of the chimeric mice with humanized liver, particularly on metabolism and excretion, are summarized and the possibility of using the chimeric mice is proposed for the advanced prediction of human pharmacokinetics and toxicity.

Application of chimeric mice with humanized liver for predictive ADME

Much effort to extrapolate the in vivo pharmacokinetics of drugs in human from experimental animals or in vitro studies has been made by many researchers. A urokinase-type plasminogen activator+/+/severe combined immunodeficient transgenic mouse line, in which the liver could be replaced by more than 80% with human hepatocytes, was established recently in Japan. This chimeric mouse line is remarkable because the replacement is higher than any other chimeric mouse reported previously. Since the liver is the critical organ involved in the pharmacokinetics of drugs, human liver is essential for the development of new drugs. To predict the human drug metabolism and pharmacokinetics, human hepatocytes and liver microsomes are recognized as better tools and are frequently used. Thus, chimeric mice with humanized liver would have great advantages in studies on drug metabolism and pharmacokinetics. We have evaluated chimeric mice for studies on absorption, distribution, metabolism, and excretion (ADME). In the liver of the chimeric mice, human phase I and phase II enzymes were clarified to be expressed and to have a similar drug metabolizing capacity as the donor. Human specific metabolites could be detected in the serum, suggesting that the chimeric mice might be used as a human ADME model for both in vitro and in vivo studies. For predicting human drug interactions, enzyme induction and inhibition are serious problems. By the treatment with typical inducers, human CYP1A2 and CYP3A4 expressed in the liver of the chimeric mice had induction potencies. After the treatment with quinidine, a specific inhibitor of human CYP2D6, the area under the curve (AUC) of a CYP2D6 metabolite, 4'-hydroxydebrisoquin, was significantly decreased in the chimeric mice but not in the control mice. Therefore, it was indicated that the chimeric mice could be used for assessing the drug interactions via enzyme induction and inhibition. As well as drug metabolism, the drug excretion was demonstrated to be humanized because cefmetazole was mainly excreted in urine both in the chimeric mice and human but in feces in control uPA-/-/SCID mice. In this review, basic researches on ADME in the chimeric mice with humanized liver are summarized and the application of the chimeric mice for predictive ADME is proposed.

Evaluation of human CYP1A2 and CYP3A4 mRNA expression in hepatocytes from chimeric mice with humanized liver

We investigated and compared the expression of human CYPs mRNA in primary cultures of cryopreserved human hepatocytes and in chimeric mice constructed by transplanting hepatocytes from the same human donors. Analysis was performed by real-time reverse-transcription polymerase chain reaction. Initial expression levels for the 12 human CYPs mRNA in chimeric mouse hepatocytes were higher than those in human hepatocytes, but a low correlation coefficient was observed (r=0.690). After 24 h of culture, the correlation remained low (r=0.699). The medium was replaced with fresh medium without human epidermal growth factor, and after 48 h of culture, expression of the 12 human CYPs mRNA were very similar in human hepatocytes and chimeric mouse hepatocytes, and a higher correlation coefficient was observed (r=0.809). After 72 h of culture, the correlation remained high (r=0.873). The ratio of human CYP1A2 mRNA to beta-actin mRNA in chimeric mouse hepatocytes decreased quickly during the first 24 h of culture, and then remained constant. Expression profiles of human CYP1A2 mRNA in chimeric mouse hepatocytes were similar to those in human hepatocytes after exposure of beta-naphthoflavone. CYP3A4 mRNA expression was increased significantly by rifampicin (Rif) exposure in human hepatocytes, whereas Rif-induced increases in CYP3A4 mRNA expression in chimeric mouse hepatocytes was seen for two of the three donors. In conclusion, we demonstrated that expression and induction of human CYPs in human hepatocytes can be reproduced in chimeric mouse hepatocytes.

In vivo drug metabolism model for human cytochrome P450 enzyme using chimeric mice with humanized liver

We previously clarified that major human drug metabolizing enzymes were expressed in a chimeric urokinase-type plasminogen activator (uPA)+/+/severe combined immunodeficient (SCID) mouse line established recently, in which the liver could be replaced by more than 80% with human hepatocytes. In the present study, we investigated the in vivo drug metabolism of a CYP2D6 substrate, debrisoquin (DB), in chimeric mice with high (High) or low (Low) human albumin (hAlb) concentrations and in control uPA-/-/SCID mice. The hAlb in the mouse blood is one of the indices of humanized liver because the chimeric mice produce hAlb. After oral administration of DB at 2.0 mg/kg, the AUC0-8 value of a major CYP2D6 metabolite of DB, 4'-hydroxydebrisoquin (4-OH DB), in High was 3.6-fold higher than those of Low and uPA-/-/SCID mice. By pre-treatment with a typical CYP2D6 inhibitor, quinidine, the AUC0-8 value of 4-OH DB in High was decreased although such values in Low and uPA-/-/SCID mice did not change. The in vitro kinetic analyses and the Ki values of quinidine on the DB 4'-hydroxylase activity in liver microsomes also supported the humanization of the chimeric mice. In conclusion, the chimeric mice exhibited a humanized profile of drug metabolism and the inhibition of P450.