Midazolam metabolism in cytochrome P450 3A knockout mice can be attributed to up-regulated CYP2C enzymes

Application of a novel regulatable Cre recombinase system to define the role of liver and gut metabolism in drug oral bioavailability

The relative contribution of hepatic compared with intestinal oxidative metabolism is a crucial factor in drug oral bioavailability and therapeutic efficacy. Oxidative metabolism is mediated by the cytochrome P450 mono-oxygenase system to which cytochrome P450 reductase (POR) is the essential electron donor. In order to study the relative importance of these pathways in drug disposition, we have generated a novel mouse line where Cre recombinase is driven off the endogenous Cyp1a1 gene promoter; this line was then crossed on to a floxed POR mouse. A 40 mg/kg dose of the Cyp1a1 inducer 3-methylcholanthrene (3MC) eliminated POR expression in both liver and small intestine, whereas treatment at 4 mg/kg led to a more targeted deletion in the liver. Using this approach, we have studied the pharmacokinetics of three probe drugs--paroxetine, midazolam, nelfinavir--and show that intestinal metabolism is a determinant of oral bioavailability for the two latter compounds. The Endogenous Reductase Locus (ERL) mouse represents a significant advance on previous POR deletion models as it allows direct comparison of hepatic and intestinal effects on drug and xenobiotic clearance using lower doses of a single Cre inducing agent, and in addition minimizes any cytotoxic effects, which may compromise interpretation of the experimental data.

Repression of multiple CYP2D genes in mouse primary hepatocytes with a single siRNA construct

The Cyp2d subfamily is the second most abun-dant subfamily of hepatic drug-metabolizing CYPs. In mice, there are nine Cyp2d members that are believed to have redundant catalytic activity. We are testing and optimizing the ability of one short interfering RNA (siRNA) construct to knockdown the expression of multiple mouse Cyp2ds in primary hepatocytes. Expression of Cyp2d10, Cyp2d11, Cyp2d22, and Cyp2d26 was observed in the primary male mouse hepatocytes. Cyp2d9, which is male-specific and growth hormone-dependent, was not expressed in male primary hepatocytes, potentially because of its dependence on pulsatile growth hormone release from the anterior pituitary. Several different siRNAs at different concentrations and with different reagents were used to knockdown Cyp2d expression. siRNA constructs designed to repress only one construct often mildly repressed several Cyp2d isoforms. A construct designed to knockdown every Cyp2d isoform provided the best results, especially when incubated with transfection reagents designed specifically for primary cell culture. Interestingly, a construct designed to knockdown all Cyp2d isoforms, except Cyp2d10, caused a 2.5× increase in Cyp2d10 expression, presumably because of a compensatory response. However, while RNA expression is repressed 24 h after siRNA treatment, associated changes in Cyp2d-mediated metabolism are tenuous. Overall, this study provides data on the expression of murine Cyp2ds in primary cell lines, valuable information on designing siRNAs for silencing multiple murine CYPs, and potential pros and cons of using siRNA as a tool for repressing Cyp2d and estimating Cyp2d's role in murine xenobiotic metabolism.

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.

Using combined bio-omics methods to evaluate the complicated toxic effects of mixed chemical wastewater and its treated effluent

Mixed chemical wastewaters (MCWW) from industrial park contain complex mixtures of trace contaminants, which cannot be effectively removed by wastewater treatment plants (WWTP) and have become an unignored threat to ambient environment. However, limited information is available to evaluate the complicated toxic effects of MCWW and its effluent from wastewater treatment plant (WTPE) from the perspective of bio-omics. In this study, mice were exposed to the MCWW and WTPE for 90 days and distinct differences in the hepatic transcriptome and serum metabolome were analyzed by digital gene expression (DGE) and proton nuclear magnetic resonance ((1)H-NMR) spectra, respectively. Our results indicated that disruption of lipid metabolism in liver and hepatotoxicity were induced by both MCWW and WTPE exposure. WTPE is still a health risk to the environment, which is in need of more attention. Furthermore, we demonstrated the potential ability of bio-omics approaches for evaluating toxic effects of MCWW and WTPE.

In vitro ketamine CYP3A-mediated metabolism study using mammalian liver S9 fractions, cDNA expressed enzymes and liquid chromatography tandem mass spectrometry

Ketamine is widely used in medicine in combination with several benzodiazepines, including midazolam. The objectives of this study were to develop a novel HPLC-MS/selected reaction monitoring (SRM) method capable of quantifying ketamine and norketamine using an isotopic dilution strategy in biological matrices and study the formation of norketamine, the principal metabolite of ketamine with and without the presence of midazolam, a well-known CYP3A substrate. The chromatographic separation was achieved using a Thermo Betasil Phenyl 100 × 2 mm column combined with an isocratic mobile phase composed of acetonitrile, methanol, water and formic acid (60:20:20:0.4) at a flow rate of 300 μL/min. The mass spectrometer was operating in selected reaction monitoring mode and the analytical range was set at 0.05-50 μm. The precision (CV) and accuracy (NOM) observed were 3.9-7.8 and 95.9-111.1% respectively. The initial rate of formation of norketamine was determined using various ketamine concentrations and Km values of 18.4, 13.8 and 30.8 μm for rat, dog and human liver S9 fractions were observed, respectively. The metabolic stability of ketamine on liver S9 fractions was significantly higher in human (T1/2  = 159.4 min) compared with rat (T1/2  = 12.6 min) and dog (T1/2  = 7.3 min) liver S9 fractions. Moreover significantly lower IC50 and Ki values observed in human compared with rat and dog liver S9 fractions. Experiments with cDNA expressed CYP3A enzymes showed that the formation of norketamine is mediated by CYP3A but results suggest an important contribution from other isoenzymes, most likely CYP2C particularly in rat.

Bcrp1;Mdr1a/b;Mrp2 combination knockout mice: altered disposition of the dietary carcinogen PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) and its genotoxic metabolites

The multidrug transporters breast cancer resistance protein (BCRP), multidrug-resistance protein 1 (MDR1), and multidrug-resistance-associated protein (MRP) 2 and 3 eliminate toxic compounds from tissues and the body and affect the pharmacokinetics of many drugs and other potentially toxic compounds. The food-derived carcinogen PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) is transported by BCRP, MDR1, and MRP2. To investigate the overlapping functions of Bcrp1, Mdr1a/b, and Mrp2 in vivo, we generated Bcrp1;Mdr1a/b;Mrp2(-/-) mice, which are viable and fertile. These mice, together with Bcrp1;Mrp2;Mrp3(-/-) mice, were used to study the effects of the multidrug transporters on the pharmacokinetics of PhIP and its metabolites. Thirty minutes after oral or intravenous administration of PhIP (1 mg/kg), the PhIP levels in the small intestine were reduced 4- to 6-fold in Bcrp1;Mdr1a/b;Mrp2(-/) (-) and Bcrp1;Mrp2;Mrp3(-/-) mice compared with wild-type mice. Fecal excretion of PhIP was reduced 8- to 20-fold in knockouts. Biliary PhIP excretion was reduced 41-fold in Bcrp1;Mdr1a/b;Mrp2(-/-) mice. Biliary and small intestine levels of PhIP metabolites were reduced in Bcrp1;Mrp2-deficient mice. Furthermore, in both knockout strains, kidney levels and urinary excretion of genotoxic PhIP-metabolites were significantly increased, suggesting that reduced biliary excretion of PhIP and PhIP metabolites leads to increased urinary excretion of these metabolites and increased systemic exposure. Bcrp1 and Mdr1a limited PhIP brain accumulation. In Bcrp1;Mrp2;Mrp3(-/-), but not Bcrp1;Mdr1a/b;Mrp(-/-) mice, the carcinogenic metabolites N2-OH-PhIP (2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine) and PhIP-5-sulfate (a genotoxicity marker) accumulated in liver tissue, indicating that Mrp3 is involved in the sinusoidal secretion of these compounds. We conclude that Bcrp1, Mdr1a/b, Mrp2, and Mrp3 significantly affect tissue disposition and biliary and fecal elimination of PhIP and its carcinogenic metabolites and may affect PhIP-induced carcinogenesis as a result.

OATP1A/1B transporters affect irinotecan and SN-38 pharmacokinetics and carboxylesterase expression in knockout and humanized transgenic mice

Organic anion-transporting polypeptides (OATP) mediate the hepatic uptake of many drugs, thus codetermining their clearance. Impaired hepatic clearance due to low-activity polymorphisms in human OATP1B1 may increase systemic exposure to SN-38, the active and toxic metabolite of the anticancer prodrug irinotecan. We investigated the pharmacokinetics and toxicity of irinotecan and SN-38 in Oatp1a/1b-null mice: Plasma exposure of irinotecan and SN-38 was increased 2 to 3-fold after irinotecan dosing (10 mg/kg, i.v.) compared with wild-type mice. Also, liver-to-plasma ratios were significantly reduced, suggesting impaired hepatic uptake of both compounds. After 6 daily doses of irinotecan, Oatp1a/1b-null mice suffered from increased toxicity. However, Oatp1a/1b-null mice had increased levels of carboxylesterase (Ces) enzymes, which caused higher conversion of irinotecan to SN-38 in plasma, potentially complicating pharmacokinetic analyses. Ces inhibitors blocked this increased conversion. Interestingly, liver-specific humanized OATP1B1 and OATP1B3 transgenic mice had normalized hepatic expression of Ces1 genes. While irinotecan liver-to-plasma ratios in these humanized mice were similar to those in Oatp1a/1b-null mice, SN-38 liver-to-plasma ratios returned to wild-type levels, suggesting that human OATP1B proteins mediate SN-38, but not irinotecan uptake in vivo. Upon direct administration of SN-38 (1 mg/kg, i.v.), Oatp1a/1b-null mice had increased SN-38 plasma levels, lower liver concentrations, and decreased cumulative biliary excretion of SN-38. Mouse Oatp1a/1b transporters have a role in the plasma clearance of irinotecan and SN-38, whereas human OATP1B transporters may only affect SN-38 disposition. Oatp1a/1b-null mice have increased expression and activity of Ces1 enzymes, whereas humanized mice provide a rescue of this phenotype.

Generation and characterization of a Cyp4b1 null mouse and the role of CYP4B1 in the activation and toxicity of Ipomeanol

4-Ipomeanol (IPO) is a prototypical pulmonary toxin that requires P450-mediated metabolic activation to reactive intermediates in order to elicit its toxic effects. CYP4B1 is a pulmonary enzyme that has been shown, in vitro, to have a high capacity for bioactivating IPO. In order to determine, unambiguously, the role of CYP4B1 in IPO bioactivation in vivo, we generated Cyp4b1 null mice following targeted disruption of the gene downstream of exon 1. Cyp4b1 (-/-) mice are viable and healthy, with no overt phenotype, and no evidence of compensatory upregulation of other P450 isoforms in any of the tissues examined. Pulmonary and renal microsomes prepared from male Cyp4b1 (-/-) mice exhibited no detectable expression of the protein and catalyzed the in vitro bioactivation of IPO at < 10% of the rates observed in tissue microsomes from Cyp4b1 (+/+) animals. Administration of IPO (20mg/kg) to Cyp4b1 (+/+) mice resulted in characteristic lesions in the lung, and to a lesser extent in the kidney, which were completely absent in Cyp4b1 (-/-) mice. We conclude that CYP4B1 is a critical enzyme for the bioactivation of IPO in vivo and that the Cyp4b1 (-/-) mouse is a useful model for studying CYP4B1-dependent metabolism and toxicity.

Drug-drug interactions and metabolism in cytochrome P450 2C knockout mice: application to troleandomycin and midazolam

Drug-drug interactions (DDIs) may cause serious drug toxicity and delay development of candidate drugs. Screening using human liver microsomes and hepatocytes can help predict DDIs but do not always provide the degree of certainty required for confident progression of a candidate drug. Thus a suitable in vivo test system could be of great value. Here a Cyp2c knockout (KO) mouse was investigated for studying DDIs using midazolam (MDZ) a standard human CYP3A4 substrate and troleandomycin (TAO) a potent human CYP3A4 inhibitor. Pharmacokinetics (PK) and biotransformation of MDZ were investigated following dosing to Cyp2c KO and wild type mice before and after TAO treatment. The noteworthy differences in the metabolism of MDZ in Cyp2c KO compared to wild type mice confirms the important role that Cyp2c enzymes play in the murine metabolism of MDZ in vivo. The impact of Cyp3a inhibition produced a further increase in circulating MDZ concentrations in all individuals from both strains of mice though the impact of the elimination of the Cyp2c pathway in the KO mice on the AUC was less than perhaps expected. We have shown that TAO produces an increase in the MDZ concentration and a reduction in the 1'hydroxymidazolam/midazolam formation ratio but the expected difference in the magnitude of this effect between the wild type and the Cyp2c KO mice was not seen. The magnitude of the TAO effect was also smaller than is reported in humans. Hence further work is required before this animal model could be used to predict clinical interactions.

Development and validation of a LC-MS/MS method for the in vitro analysis of 1-hydroxymidazolam in human liver microsomes: application for determining CYP3A4 inhibition in complex matrix mixtures

Complementary and alternative medicines (CAM) can affect the pharmacokinetics of anticancer drugs by interacting with the metabolizing enzyme cytochrome P450 (CYP) 3A4. To evaluate changes in the activity of CYP3A4 in patients, levels of 1-hydroxymidazolam in plasma are often determined with liquid chromatography-quadrupole mass spectrometry (LC-MS/MS). However, validated LC-MS/MS methods to determine in vitro CYP3A4 inhibition in human liver microsomes are scarce and not optimized for evaluating CYP3A4 inhibition by CAM. The latter is necessary because CAM are often complex mixtures of numerous compounds that can interfere with the selective measurement of 1-hydroxymidazolam. Therefore, the aim was to validate and optimize an LC-MS/MS method for the adequate determination of CYP3A4 inhibition by CAM in human liver microsomes. After incubation of human liver microsomes with midazolam, liquid-liquid extraction with tert-butyl methyl ether was applied and dried samples were reconstituted in 50% methanol. These samples were injected onto a reversed-phase chromatography consisting of a Zorbax Extend-C18 column (2.1 × 150 mm, 5.0 µm particle size), connected to a triple quadrupole mass spectrometer with electrospray ionization. The described LC-MS/MS method was validated over linear range of 1.0-500 nm for 1-hydroxymidazolam. The results revealed good inter-assay accuracy (≥85% and ≤115%) and within-day and between-day precisions (coefficient of variation ≤ 4.43%). Furthermore, the applicability of this assay for the determination of CYP3A4 inhibition in complex matrix mixtures was successfully demonstrated in an in vitro experiment in which CYP3A4 inhibition by known CAM (β-carotene, green tea, milk thistle and St. John's wort) was determined.

Murine Oatp1a/1b uptake transporters control rosuvastatin systemic exposure without affecting its apparent liver exposure

Organic anion-transporting polypeptides (OATPs) mediate the liver uptake and hence plasma clearance of a broad range of drugs. For rosuvastatin, a cholesterol-lowering drug and OATP1A/1B substrate, the liver represents both its main therapeutic target and its primary clearance organ. Here we studied the impact of Oatp1a/1b uptake transporters on the pharmacokinetics of rosuvastatin using wild-type and Oatp1a/1b-null mice. After oral administration (15 mg/kg), intestinal absorption of rosuvastatin was not impaired in Oatp1a/1b-null mice, but systemic exposure (area under the curve) was 8-fold higher in these mice compared with wild-type. Although liver exposure was comparable between the two mouse strains (despite the increased blood exposure), the liver-to-blood ratios were markedly decreased (>10-fold) in the absence of Oatp1a/1b transporters. After intravenous administration (5 mg/kg), systemic exposure was 3-fold higher in Oatp1a/1b-null mice than in the wild-type mice. Liver, small intestinal, and kidney exposure were slightly, but not significantly, increased in Oatp1a/1b-null mice. The biliary excretion of rosuvastatin was very fast, with 60% of the dose eliminated within 15 minutes after intravenous administration, and also not significantly altered in Oatp1a/1b-null mice. Rosuvastatin renal clearance, although still minor, was increased ∼15-fold in Oatp1a/1b-null males, suggesting a role of Oatp1a1 in the renal reabsorption of rosuvastatin. Absence of Oatp1a/1b uptake transporters increases the systemic exposure of rosuvastatin by reducing its hepatic extraction ratio. However, liver concentrations are not significantly affected, most likely due to the compensatory activity of high-capacity, low-affinity alternative uptake transporters at higher systemic rosuvastatin levels and the absence of efficient alternative rosuvastatin clearance mechanisms.

Generation and characterization of novel cytochrome P450 Cyp2c gene cluster knockout and CYP2C9 humanized mouse lines

Compared with rodents and many other animal species, the human cytochrome P450 (P450) Cyp2c gene cluster varies significantly in the multiplicity of functional genes and in the substrate specificity of its enzymes. As a consequence, the use of wild-type animal models to predict the role of human CYP2C enzymes in drug metabolism and drug-drug interactions is limited. Within the human CYP2C cluster CYP2C9 is of particular importance, because it is one of the most abundant P450 enzymes in human liver, and it is involved in the metabolism of a wide variety of important drugs and environmental chemicals. To investigate the in vivo functions of cytochrome P450 Cyp2c genes and to establish a model for studying the functions of CYP2C9 in vivo, we have generated a mouse model with a deletion of the murine Cyp2c gene cluster and a corresponding humanized model expressing CYP2C9 specifically in the liver. Despite the high number of functional genes in the mouse Cyp2c cluster and the reported roles of some of these proteins in different biological processes, mice deleted for Cyp2c genes were viable and fertile but showed certain phenotypic alterations in the liver. The expression of CYP2C9 in the liver also resulted in viable animals active in the metabolism and disposition of a number of CYP2C9 substrates. These mouse lines provide a powerful tool for studying the role of Cyp2c genes and of CYP2C9 in particular in drug disposition and as a factor in drug-drug interaction.

CYP3A-dependent drug metabolism is reduced in bacterial inflammation in mice

BACKGROUND AND PURPOSE

Gene expression of Cyp3a11 is reduced by activation of Toll-like receptors (TLRs) by Gram-negative or Gram-positive bacterial components, LPS or lipoteichoic acid (LTA) respectively. The primary adaptor protein in the TLR signalling pathway, TIRAP, plays differential roles in LPS- and LTA-mediated down-regulations of Cyp3a11 mRNA. Here, we have determined the functional relevance of these findings by pharmacokinetic/pharmacodynamic (PK/PD) analysis of the Cyp3a substrate midazolam in mice. Midazolam is also metabolized by Cyp2c in mice.

EXPERIMENTAL APPROACH

Adult male C57BL/6, TIRAP+/+ and TIRAP-/- mice were pretreated with saline, LPS (2 mg·kg⁻¹) or LTA (6 mg·kg⁻¹). Cyp3a11 protein expression, activity and PK/PD studies using midazolam were performed.

KEY RESULTS

Cyp3a11 protein expression in LPS- or LTA-treated mice was reduced by 95% and 60% compared with saline-treated mice. Cyp3a11 activity was reduced by 70% in LPS- or LTA-treated mice. Plasma AUC of midazolam was increased two- to threefold in LPS- and LTA-treated mice. Plasma levels of 1'-OHMDZ decreased significantly only in LTA-treated mice. Both LPS and LTA decreased AUC of 1'-OHMDZ-glucuronide. In the PD study, sleep time was increased by ∼2-fold in LPS- and LTA-treated mice. LTA-mediated decrease in Cyp3a11 protein expression and activity was dependent on TIRAP. In PK/PD correlation, AUC of midazolam was increased only in LPS-treated mice compared with saline-treated mice.

CONCLUSIONS AND IMPLICATIONS

LPS or LTA altered PK/PD of midazolam. This is the first study to demonstrate mechanistic differences in regulation of metabolite formation of a clinically relevant drug by Gram-negative or Gram-positive bacterial endotoxins.

P-glycoprotein, multidrug-resistance associated protein 2, Cyp3a, and carboxylesterase affect the oral availability and metabolism of vinorelbine

We investigated the interactions of the anticancer drug vinorelbine with drug efflux transporters and cytochrome P450 3A drug-metabolizing enzymes. Vinorelbine was transported by human multidrug-resistance associated protein (MRP) 2, and Mrp2 knockout mice displayed increased vinorelbine plasma exposure after oral administration, suggesting that Mrp2 limits the intestinal uptake of vinorelbine. Using P-glycoprotein (P-gp), Cyp3a-, and P-gp/Cyp3a knockout mice, we found that the absence of P-gp or Cyp3a resulted in increased vinorelbine plasma exposure, both after oral and intravenous administration. Surprisingly, P-gp/Cyp3a knockout mice displayed markedly lower vinorelbine plasma concentrations than wild-type mice upon intravenous administration but higher concentrations upon oral administration. This could be explained by highly increased formation of 4'-O-deacetylvinorelbine, an active vinorelbine metabolite, especially in P-gp/Cyp3a knockout plasma. Using wild-type and Cyp3a knockout liver microsomes, we found that 4'-O-deacetylvinorelbine formation was 4-fold increased in Cyp3a knockout liver and was not mediated by Cyp3a or other cytochrome P450 enzymes. In vitro incubation of vinorelbine with plasma revealed that vinorelbine deacetylation in Cyp3a and especially in P-gp/Cyp3a knockout mice but not in P-gp-deficient mice was strongly up-regulated. Metabolite formation in microsomes and plasma could be completely inhibited with the nonspecific carboxylesterase (CES) inhibitor bis(4-nitrophenyl) phosphate and partly with the CES2-specific inhibitor loperamide, indicating that carboxylesterase Ces2a, which was appropriately up-regulated in Cyp3a and especially in P-gp/Cyp3a knockout liver was responsible for the 4'O-deacetylvinorelbine formation. Such compensatory up-regulation can complicate the interpretation of knockout mouse data. Nonetheless, P-gp, Mrp2, Cyp3a, and Ces2a clearly restricted vinorelbine availability in mice. Variation in activity of their human homologs may also affect vinorelbine pharmacokinetics in patients.

Analysis of the pharmacokinetic boosting effects of ritonavir on oral bioavailability of drugs in mice

  Ritonavir dramatically increases the bioavailability of a variety of concurrently administered drugs by inhibition of metabolic enzymes and drug transporters. The purpose of this study was to investigate the extent to which ritonavir's inhibition of drug transporters and/or CYP3A contributes to the increased oral bioavailability in mice. The area under the plasma concentration-time curves (AUC) for orally administered saquinavir after coadministration with 50 mg/kg ritonavir dramatically increased (325-fold). As a result, the bioavailability, Fa·Fg and Fh increased 75-, 38- and twofold, respectively. In addition, the increase in the AUC predicted from the in vitro Ki value was ninefold, which was derived from the inhibition of metabolic enzymes by ritonavir in the liver. The remaining 36-fold increase in the AUC was considered to be derived from the inhibition in the small intestine. The AUCinf for probe substrate midazolam, fexofenadine, and pravastatin increased after the oral administration of ritonavir by only five-, 13-, and sevenfold, respectively. Moreover, the AUC0-12 for saquinavir was affected negligibly by itraconazole. These results indicate ritonavir mainly affects the first-pass effect of saquinavir in the small intestine, increasing the bioavailability of orally administered saquinavir. Furthermore, cyp isoforms other than CYP3A, which contribute to the metabolism of saquinavir in humans, are involved in the metabolism of saquinavir in mice.

Humanized transgenic mouse models for drug metabolism and pharmacokinetic research

Extrapolation of the metabolic, pharmacokinetic and toxicological data obtained from animals to humans is not always straightforward, given the remarkable species difference in drug metabolism that is due in large part to the differences in drug-metabolizing enzymes between animals and humans. Furthermore, genetic variations in drug-metabolizing enzymes may significantly alter pharmacokinetics, drug efficacy and safety. Thus, humanized transgenic mouse lines, in which the human drug-metabolizing enzymes are expressed in mouse tissues in the presence or absence of mouse orthologues, have been developed to address such challenges. These humanized transgenic mice are valuable animal models in understanding the significance of specific human drug-metabolizing enzymes in drug clearance and pharmacokinetics, as well as in predicting potential drug-drug interactions and chemical toxicity in humans. This review, therefore, aims to summarize the development and application of some humanized transgenic mouse models expressing human drug-metabolizing enzymes. The limitations of these genetically modified mouse models are also discussed.

Complete OATP1B1 and OATP1B3 deficiency causes human Rotor syndrome by interrupting conjugated bilirubin reuptake into the liver

Bilirubin, a breakdown product of heme, is normally glucuronidated and excreted by the liver into bile. Failure of this system can lead to a buildup of conjugated bilirubin in the blood, resulting in jaundice. The mechanistic basis of bilirubin excretion and hyperbilirubinemia syndromes is largely understood, but that of Rotor syndrome, an autosomal recessive disorder characterized by conjugated hyperbilirubinemia, coproporphyrinuria, and near-absent hepatic uptake of anionic diagnostics, has remained enigmatic. Here, we analyzed 8 Rotor-syndrome families and found that Rotor syndrome was linked to mutations predicted to cause complete and simultaneous deficiencies of the organic anion transporting polypeptides OATP1B1 and OATP1B3. These important detoxification-limiting proteins mediate uptake and clearance of countless drugs and drug conjugates across the sinusoidal hepatocyte membrane. OATP1B1 polymorphisms have previously been linked to drug hypersensitivities. Using mice deficient in Oatp1a/1b and in the multispecific sinusoidal export pump Abcc3, we found that Abcc3 secretes bilirubin conjugates into the blood, while Oatp1a/1b transporters mediate their hepatic reuptake. Transgenic expression of human OATP1B1 or OATP1B3 restored the function of this detoxification-enhancing liver-blood shuttle in Oatp1a/1b-deficient mice. Within liver lobules, this shuttle may allow flexible transfer of bilirubin conjugates (and probably also drug conjugates) formed in upstream hepatocytes to downstream hepatocytes, thereby preventing local saturation of further detoxification processes and hepatocyte toxic injury. Thus, disruption of hepatic reuptake of bilirubin glucuronide due to coexisting OATP1B1 and OATP1B3 deficiencies explains Rotor-type hyperbilirubinemia. Moreover, OATP1B1 and OATP1B3 null mutations may confer substantial drug toxicity risks.

A critical analysis of the interplay between cytochrome P450 3A and P-glycoprotein: recent insights from knockout and transgenic mice

CYP3A is one of the most important drug-metabolizing enzymes, determining the first-pass metabolism, oral bioavailability, and elimination of many drugs. It is also an important determinant of variable drug exposure and is involved in many drug-drug interactions. Recent studies with CYP3A knockout and transgenic mice have yielded a number of key insights that are important to consider during drug discovery and development. For instance, studies with tissue-specific CYP3A-transgenic mice have highlighted the importance of intestinal CYP3A-dependent metabolism. They also revealed that intestinal CYP3A plays an important role in the regulation of various drug-handling systems in the liver. Intestinal CYP3A activity can thus have far-reaching pharmacological effects. Besides CYP3A, the active drug efflux transporter P-glycoprotein also has a strong effect on the pharmacokinetics of numerous drugs. CYP3A and P-glycoprotein have an extensive overlap in their substrate spectrum. It has been hypothesized that for many drugs, the combined activity of CYP3A and P-glycoprotein makes for efficient intestinal first-pass metabolism of orally administered drugs as a result of a potentially synergistic collaboration. However, there is only limited in vitro and in vivo evidence for this hypothesis. There has also been some confusion in the field about what synergy actually means in this case. Our recent studies with Cyp3a/P-glycoprotein combination knockout mice have provided further insights into the CYP3A-P-glycoprotein interplay. We here present our view of the status of the synergy hypothesis and an attempt to clarify the existing confusion about synergy. We hope that this will facilitate further critical testing of the hypothesis and improve communication among researchers. Above all, the recent findings and insights into the interplay between CYP3A and P-glycoprotein may have implications for improving oral drug bioavailability and reducing adverse side effects.

P-glycoprotein (ABCB1) transports the primary active tamoxifen metabolites endoxifen and 4-hydroxytamoxifen and restricts their brain penetration

P-glycoprotein (P-gp, ABCB1) is a highly efficient drug efflux pump expressed in brain, liver, and small intestine, but also in tumor cells, that affects pharmacokinetics and confers therapy resistance for many anticancer drugs. The aim of this study was to investigate the impact of P-gp on tamoxifen and its primary active metabolites, 4-hydroxytamoxifen, N-desmethyltamoxifen, and endoxifen. We used in vitro transport assays and Abcb1a/1b(-/-) mice to investigate the impact of P-gp on the oral availability and brain penetration of tamoxifen and its metabolites. Systemic exposure of tamoxifen and its metabolites after oral administration of tamoxifen (50 mg/kg) was not changed in the absence of P-gp. However, brain accumulation of tamoxifen, 4-hydroxytamoxifen, and N-desmethyltamoxifen were modestly, but significantly (1.5- to 2-fold), increased. Endoxifen, however, displayed a 9-fold higher brain penetration at 4 h after administration. Endoxifen was transported by P-gp in vitro. Upon direct oral administration of endoxifen (20 mg/kg), systemic exposure was slightly decreased in Abcb1a/1b(-/-) mice, but brain accumulation of endoxifen was dramatically increased (up to 23-fold at 4 h after administration). Shortly after high-dose intravenous administration (5 or 20 mg/kg), endoxifen brain accumulation was increased only 2-fold in Abcb1a/1b(-/-) mice compared with wild-type mice, suggesting a partial saturation of P-gp at the blood-brain barrier. Endoxifen, the clinically most relevant metabolite of tamoxifen, is a P-gp substrate in vitro and in vivo, where P-gp limits its brain penetration. P-gp might thus be relevant for tamoxifen/endoxifen resistance of P-gp-positive breast cancer and tumors positioned behind a functional blood-brain barrier.

Fatty acid binding proteins: potential chaperones of cytosolic drug transport in the enterocyte?

PURPOSE

Several poorly water-soluble drugs have previously been shown to bind to intestinal (I-FABP) and liver fatty acid binding protein (L-FABP) in vitro. The purpose of this study was to examine the potential role of drug binding to FABPs on intestinal permeability and gut wall metabolism in vivo.

METHODS

The intestinal permeability of ibuprofen, progesterone and midazolam (which bind FABPs) and propranolol (which does not) was examined using an autoperfused recirculating permeability model in control rats and rats where FABP levels were upregulated via pre-feeding a fat-rich diet.

RESULTS

The intestinal permeability of drugs which bind FABPs in vitro was increased in animals where FABP levels were upregulated by prefeeding a high fat diet. The gut wall metabolism of midazolam was also reduced in animals with elevated FABP levels.

CONCLUSIONS

Consistent with their role in the cellular transport of endogenous lipophilic substrates, FABPs appear to facilitate the intracellular disposition of drug molecules that bind FABPs in vitro. Drug binding to FABPs in the enterocyte may also attenuate gut wall metabolism in a manner analogous to the reduction in hepatic extraction mediated by drug binding to plasma proteins in the systemic circulation.

Effect of ABCC2 (MRP2) transport function on erythromycin metabolism

The macrolide antiobiotic erythromycin undergoes extensive hepatic metabolism and is commonly used as a probe for cytochrome P450 (CYP) 3A4 activity. By means of a transporter screen, erythromycin was identified as a substrate for the transporter ABCC2 (MRP2) and its murine ortholog, Abcc2. Because these proteins are highly expressed on the biliary surface of hepatocytes, we hypothesized that impaired Abcc2 function may influence the rate of hepatobiliary excretion and thereby enhance erythromycin metabolism. Using Abcc2 knockout mice, we found that Abcc2 deficiency was associated with a significant increase in erythromycin metabolism, whereas murine Cyp3a protein expression and microsomal Cyp3a activity were not affected. Next, in a cohort of 108 human subjects, we observed that homozygosity for a common reduced-function variant in ABCC2 (rs717620) was also linked to an increase in erythromycin metabolism but was not correlated with the clearance of midazolam. These results suggest that impaired ABCC2 function can alter erythromycin metabolism, independent of changes in CYP3A4 activity.

Drug-metabolizing enzyme, transporter, and nuclear receptor genetically modified mouse models

Determining the in vivo significance of a specific enzyme, transporter, or xenobiotic receptor in drug metabolism and pharmacokinetics may be hampered by gene multiplicity and complexity, levels of expression, and interaction between various components involved. The development of knockout (loss-of-function) and transgenic (gain-of-function) mouse models opens the door to the improved understanding of gene function in a whole-body system. There is also growing interest in the development of humanized mice to overcome species differences in drug metabolism and disposition. This review, therefore, aims to summarize and discuss some successful examples of drug-metabolizing enzyme, transporter, and nuclear-receptor genetically modified mouse models. These genetically modified mouse models have been proven as invaluable models for understanding in vivo function of drug-metabolizing enzymes, transporters, and xenobiotic receptors in drug metabolism and transport, as well as predicting potential drug-drug interaction and toxicity in humans. Nevertheless, concerns remain about interpretation of data obtained from such genetically modified mouse models, in which the expression of related genes is altered significantly.

The nuclear receptors constitutive active/androstane receptor and pregnane x receptor activate the Cyp2c55 gene in mouse liver

Mouse CYP2C55 has been characterized as an enzyme that catalyzes synthesis of 19-hydroxyeicosatetraenoic acid (19-HETE), an arachidonic acid metabolite known to have important physiological functions such as regulation of renal vascular tone and ion transport. We have now found that CYP2C55 is induced by phenobarbital (PB) and pregnenolone 16alpha-carbonitrile (PCN) in both mouse kidney and liver. The nuclear xenobiotic receptors constitutive active/androstane receptor (CAR) and pregnane X receptor (PXR) regulate these drug inductions: CYP2C55 mRNA was increased 25-fold in PB-treated Car(+/+) but not in Car(-/-) mice and was induced in Pxr(+/+) but not Pxr(-/-) mice after PCN treatment. Cell-based promoter analysis and gel shift assays identified the DNA sequence (-1679)TGAACCCAGTTGAACT(-1664) as a DR4 motif that regulates CAR- and PXR-mediated transcription of the Cyp2c55 gene. Chronic PB treatment increased hepatic microsomal CYP2C55 protein and serum 19-HETE levels. These findings indicate that CAR and PXR may play a role in regulation of drug-induced synthesis of 19-HETE in the mouse.

Abcc2 (Mrp2), Abcc3 (Mrp3), and Abcg2 (Bcrp1) are the main determinants for rapid elimination of methotrexate and its toxic metabolite 7-hydroxymethotrexate in vivo

The multidrug transporters ABCC2, ABCC3, and ABCG2 can eliminate potentially toxic compounds from the body and have overlapping substrate specificities. To investigate the overlapping functions of Abcc2, Abcc3, and Abcg2 in vivo, we generated and characterized Abcc3;Abcg2-/- and Abcc2;Abcc3;Abcg2-/- mice. We subsequently analyzed the relative impact of these transport proteins on the pharmacokinetics of the anticancer drug methotrexate (MTX) and its main, toxic, metabolite 7-hydroxymethotrexate (7OH-MTX) after i.v. administration of MTX (50 mg/kg). Whereas in single and double knockout mice, the plasma and liver concentrations of MTX and 7OH-MTX decreased rapidly after MTX administration, in the Abcc2;Abcc3;Abcg2-/- mice, they remained very high. One hour after administration, 67% of the MTX dose was still present in livers of Abcc2;Abcc3;Abcg2-/- mice as MTX or 7OH-MTX versus 7% in wild-type, showing dramatic liver accumulation of these toxic compounds when Abcc2, Abcc3, and Abcg2 were all absent. Furthermore, the urinary and fecal excretion of the nephrotoxic metabolite 7OH-MTX were increased 27- and 7-fold, respectively, in Abcc2;Abcc3;Abcg2-/- mice. Thus, Abcc2, Abcc3, and Abcg2 together mediate the rapid elimination of MTX and 7OH-MTX after i.v. administration and can to a large extent compensate for each other's absence. This may explain why it is still comparatively safe to use a toxic drug such as MTX in the clinic, as the risk of highly increased toxicity due to dysfunctioning of ABCC2, ABCC3, or ABCG2 alone is limited. Nevertheless, cotreatment with possible inhibitors of ABCC2, ABCC3, and ABCG2 should be done with utmost caution when treating patients with methotrexate.

Approaches for minimizing metabolic activation of new drug candidates in drug discovery

A large body of circumstantial evidence suggests that metabolic activation of drug candidates to chemically reactive electrophilic metabolites that are capable of covalently modifying cellular macromolecules may result in acute and/or immune system-mediated idiosyncratic toxicities in humans. Thus, minimizing the potential for metabolic activation of new drug candidates during the drug discovery and lead optimization stage represents a prudent strategy to help discover and develop the next generation of safe and effective therapeutic agents. In the present chapter, we discuss the scientific methodologies that currently are available to industrial pharmaceutical scientists for assessing and minimizing metabolic activation during drug discovery, their attributes and limitations, and future scientific directions that have the potential to help advance progress in this field. We also propose a roadmap that should help utilize the armamentarium of available scientific tools in a logical way and contribute to addressing metabolic activation issues in the drug discovery-setting in a rapid, scientifically appropriate, and resource-conscious manner.

Examination of CYP3A and P-glycoprotein-mediated drug-drug interactions using animal models

With the advent of polytherapy for cancer treatment it has become prudent to minimize, as much as possible, the potential for drug-drug interactions (DDI). Toward this end, the metabolic and transporter pathways involved in the disposition of a drug candidate (phenotyping) and potential for inhibition and induction of drug-metabolizing enzymes and transporters are evaluated in vitro. Such in vitro human data can be made available prior to human dosing and enable in vitro to in vivo-based predictions of clinical outcomes. Despite some success, however, in vitro systems are not dynamic and sometimes fail to predict drug-drug interactions for a variety of reasons. In comparison, relatively less effort has been made to evaluate predictions based on data derived from in vivo animal models. This chapter will attempt to summarize different examples from the literature where animal models have been used to predict cytochrome P450 3A (CYP3A)- and P-glycoprotein-based DDI. When employing data from animal models one needs to be aware of species differences in enzyme- and transporter-activity leading to differences in pharmacokinetics, clearance pathways as well as species differences in selectivity and affinity of probe substrates and inhibitors. Because of these differences, in vivo animal studies alone, cannot be predictive of human DDI. Despite these caveats, the information obtained from validated in vivo animal models may prove useful when used in conjunction with in vitro-in vivo extrapolation methods. Such an integrated data set can be used to select drug candidates with a reduced DDI potential.

Inhibition and stimulation of intestinal and hepatic CYP3A activity: studies in humanized CYP3A4 transgenic mice using triazolam

CYP3A4 is an important determinant of drug-drug interactions. In this study, we evaluated whether cytochrome P450 3A knockout mice [Cyp3a(-/-)] and CYP3A4 transgenic (CYP3A4-Tg) mice can be used to study drug-drug interactions in the liver and intestine. Triazolam was used as a probe drug because it is a highly specific CYP3A substrate and not a P-glycoprotein substrate. Triazolam metabolism was profoundly reduced in Cyp3a(-/-) mice both in vitro and in vivo. In vitro studies revealed clear species differences in humans and mice, but triazolam metabolism in microsomes derived from CYP3A4-Tg "humanized" mice closely resembled that in human microsomes. It is interesting to note that studies with tissue-specific CYP3A4-Tg mice revealed that intestinal CYP3A4 has a major impact on oral triazolam exposure, whereas the effect of hepatic CYP3A4 was limited. To mimic a drug-drug interaction, we coadministered triazolam with the prototypical CYP3A inhibitor ketoconazole, which increased triazolam exposure in all the CYP3A-proficient mouse strains but not in Cyp3a(-/-) mice. We further found that the anticancer drug gefitinib is a potent stimulator of 1'-OH triazolam formation in vitro. It is noteworthy that we could also show in vivo stimulation of triazolam metabolism by gefitinib, resulting in a lower oral triazolam exposure. To our knowledge, this is the first in vivo example of direct stimulation of CYP3A4 activity after oral drug administration. Overall, this study illustrates how Cyp3a(-/-) and CYP3A4-Tg mice can be used to study drug-drug interactions. The data clarify that for drugs that are not P-glycoprotein substrates, intestinal metabolism also can be more important than hepatic metabolism after oral administration.

Safety assessment of drug metabolites: implications of regulatory guidance and potential application of genetically engineered mouse models that express human P450s

Species differences in drug metabolism present two challenges that may confound the nonclinical safety assessment of candidate drugs. The first challenge is encountered when metabolites are formed uniquely or disproportionately in humans. Another challenge is understanding the human relevance of toxicities associated with metabolites formed uniquely or disproportionately in a nonclinical species. One potential approach to minimize the impact of metabolite related challenges is to consider genetically engineered mouse models that express human P450 enzymes. Human P450 expressing mouse models may have the ability to generate major human metabolites and eliminate or reduce the formation of mouse specific metabolites. Prior to determining the utility of any particular model, it is important to qualify by characterizing protein expression, establishing whether the model generates an in vivo metabolite profile more closely related to that of humans than the wild-type mouse, verifying genetic stability, and evaluating animal health. When compared to the current strategy for handling metabolite challenges (i.e., direct administration of metabolite), identifying an appropriate human P450 expressing model could provide a number of benefits. Such benefits include improved scientific relevance of the evaluation, decreased resource needs, and a possible reduction in the number of animals used. These benefits may ultimately improve the quality and speed by which promising new drug candidates are developed and delivered to patients.

Functionally Overlapping Roles of Abcg2 (Bcrp1) and Abcc2 (Mrp2) in the Elimination of Methotrexate and Its Main Toxic Metabolite 7-Hydroxymethotrexate In vivo

Purpose: ABCC2 (MRP2) and ABCG2 (BCRP) transport various endogenous and exogenous compounds, including many anticancer drugs, into bile, feces, and urine. We investigated the possibly overlapping roles of Abcg2 and Abcc2 in the elimination of the anticancer drug methotrexate (MTX) and its toxic metabolite 7-hydroxymethotrexate (7OH-MTX).

Experimental Design: We generated and characterized Abcc2;Abcg2-/- mice, and used these to determine the overlapping roles of Abcc2 and Abcg2 in the elimination of MTX and 7OH-MTX after i.v. administration of 50 mg/kg MTX.

Results: Compared with wild-type, the plasma areas under the curve (AUC) for MTX were 1.6-fold and 2.0-fold higher in Abcg2-/- and Abcc2-/- mice, respectively, and 3.3-fold increased in Abcc2;Abcg2-/- mice. The biliary excretion of MTX was 23-fold reduced in Abcc2;Abcg2-/- mice, and the MTX levels in the small intestine were dramatically decreased. Plasma levels of 7OH-MTX were not significantly altered in Abcg2-/- mice, but the areas under the curve were 6.2-fold and even 12.4-fold increased in Abcc2-/- and Abcc2;Abcg2-/- mice, respectively. This indicates that Abcc2 compensates for Abcg2 deficiency but that Abcg2 can only partly compensate for Abcc2 absence. Furthermore, 21-fold decreased biliary 7OH-MTX excretion in Abcc2;Abcg2-/- mice and substantial 7OH-MTX accumulation in the liver and kidney were seen. We additionally found that in the absence of Abcc2, Abcg2 mediated substantial urinary excretion of MTX and 7OH-MTX.

Conclusions: Abcc2 and Abcg2 together are major determinants of MTX and 7OH-MTX pharmacokinetics. Variations in ABCC2 and/or ABCG2 activity due to polymorphisms or coadministered inhibitors may therefore substantially affect the therapeutic efficacy and toxicity in patients treated with MTX.

An intestinal epithelium-specific cytochrome P450 (P450) reductase-knockout mouse model: direct evidence for a role of intestinal p450s in first-pass clearance of oral nifedipine

To determine the in vivo function of intestinal cytochrome P450 (P450) enzymes, we have generated an intestinal epithelium (IE)-specific P450 reductase gene (Cpr) knockout mouse model (designated IE-Cpr-null). In the IE-Cpr-null mouse, CPR expression was abolished in IE cells; however, CPR expression was not altered in other tissues examined. The loss of CPR expression in the small intestine (SI) led to increased expression of several P450 proteins examined, including CYP1A1, CYP2B, CYP2C, and CYP3A. It is interesting to note that the expression of CYP1A1 was also increased in the liver, kidney, and lung of the IE-Cpr-null mice compared with wild-type (WT) littermates, a result strongly supporting the notion that SI metabolism of putative dietary CYP1A1 inducers can influence the systemic bioavailability of these inducers. The rates of SI microsomal metabolism of nifedipine (NFP) in the IE-Cpr-null mice were approximately 10% of the rates in WT littermates, despite the compensatory expression of multiple P450 enzymes in the SI. Furthermore, the area under the concentration-time curve (AUC) values for blood NFP (dosed at 10 mg/kg) levels were 1.6-fold higher in IE-Cpr-null mice than in WT littermates when NFP was given orally; in contrast, the AUC values were comparable for the two strains when NFP was given intravenously. This result directly showed that P450-catalyzed NFP metabolism in the SI plays an important role in the first-pass clearance of oral NFP. Our findings indicate that the IE-Cpr-null mouse model can be used to study the in vivo function of intestinal P450 enzymes in the clearance of oral drugs and other xenobiotics.

Opportunities and challenges in the discovery of new central nervous system drugs

There have been relatively few new mechanism-based approvals for nervous system relevant drugs over the past 5 years, despite the increasing budgets of pharmaceutical and biotechnology companies. The genomic revolution has provided scientists with many molecular targets for drug discovery and research advances in chemistry, drug metabolism, pharmacology, and toxicology have provided much insight into understanding the pitfalls of the drug discovery and development process. Herein is provided a perspective on both the opportunities and challenges in the discovery and development of novel medicines for the treatment of human CNS disorders.

Methotrexate pharmacokinetics in transgenic mice with liver-specific expression of human organic anion-transporting polypeptide 1B1 (SLCO1B1)

Human organic anion-transporting polypeptide 1B1 (OATP1B1) is an important hepatic uptake transporter that can transport a wide variety of drugs. In the present study, we have generated and characterized a transgenic mouse model with specific and functional expression of human OATP1B1 (SLCO1B1) in the liver. Immunohistochemical staining revealed basolateral localization of transgenic OATP1B1 in the liver, whereas no expression of OATP1B1 was found in the kidney and small intestine. Using this transgenic model, the in vivo role of human OATP1B1 in the disposition of the anticancer drug methotrexate (MTX) was studied. In mice on a semisynthetic diet, the area under the plasma concentration-time curve for intravenous methotrexate in SLCO1B1 transgenic mice was 1.5-fold decreased compared with wild-type mice. Furthermore, the amount of MTX in the liver was markedly higher ( approximately 2-fold) in the SLCO1B1 transgenic mice compared with wild-type mice, resulting in 2- to 4-fold higher liver-plasma ratios of MTX. Some murine liver Slco genes were markedly down-regulated on the semisynthetic diet compared with a standard diet, which probably reduced murine Oatp-mediated MTX uptake in the liver and therefore facilitated detection of the function of the transgenic OATP1B1. Taken together, these data demonstrate a marked and possibly rate-limiting role for human OATP1B1 in MTX elimination in vivo. Variation in OATP1B1 activity due to genetic polymorphisms, drug-drug interactions, and possibly dietary conditions may therefore play a role in the severity of MTX-related toxicity. SLCO1B1 transgenic mice could be a useful tool in studying the in vivo role of human OATP1B1 in drug pharmacokinetics.

Intestinal cytochrome P450 3A plays an important role in the regulation of detoxifying systems in the liver

CYP3A4 is an important xenobiotic metabolizing enzyme. We previously found that CYP2C55 is highly up-regulated in Cyp3a(-/-) mice. Here, we have further investigated the mechanism of regulation of CYP2C55 and other detoxifying systems in Cyp3a(-/-) mice. Induction studies with prototypical inducers demonstrated an important role for the nuclear receptors PXR and CAR in the up-regulation of CYP2C55. Subsequent diet-switch experiments revealed that food-derived xenobiotics are primarily responsible for the increased induction of CYP2C55, as well as of several other primary detoxifying systems in Cyp3a(-/-) mice. Our data suggest that CYP3A normally metabolizes food-derived activators of PXR and/or CAR, explaining the increased levels of such activators in Cyp3a(-/-) mice and subsequent up-regulation of a range of detoxifying systems. Interestingly, our studies with tissue-specific CYP3A4 transgenic Cyp3a(-/-) mice revealed that not only hepatic but also intestinal expression of CYP3A4 could reduce the hepatic expression of detoxifying systems to near wild-type levels. Apparently, intestinal CYP3A4 can limit the hepatic exposure to food-derived activators of nuclear receptors, thereby regulating the expression of a range of detoxifying systems in the liver. This broad biological effect further emphasizes the importance of intestinal CYP3A activity and could have profound implications for the prediction of drug exposure.