Modeled Hepatic/Plasma Exposures of Omeprazole Prescribed Alone in Cytochrome P450 2C19 Poor Metabolizers Are Likely Associated with Hepatic Toxicity Reported in a Japanese Adverse Event Database

Omeprazole, a gastric acid pump inhibitor, is repeatedly administered and is oxidatively metabolized mainly by polymorphic cytochrome P450 2C19. The prescribed dosage of omeprazole was discontinued or reduced in 47 of the 135 patients who received omeprazole alone in this survey, as recorded in the Japanese Adverse Drug Event Report database. The days to onset of omeprazole-related disorders were 3-4 d (median) and 16 d for intravenous 20-40 mg and oral 20 mg daily doses, respectively, in 34 patients for whom relevant data were available. The maximum plasma concentration of omeprazole was pharmacokinetically modeled after a single oral 40-mg dose in P450 2C19-defective poor metabolizers and was 2.4-fold higher than that in extensive metabolizers. The modeled area under the hepatic concentration curves of omeprazole in P450 2C19 poor metabolizers after virtual daily 40-mg doses for 7 d was 5.2-fold higher than that in the extensive metabolizers. Omeprazole-induced P450 2C19 (approx. 2-fold), resulting in increased hepatic intrinsic clearance in repeated doses, was considered after the second day. Virtual plasma/hepatic exposure estimated using pharmacokinetic modeling in subjects with P450 2C19 poor metabolizers indicated that these exposure levels virtually estimated could be one of causal factors for unexpected hepatic disorders induced by prescribed omeprazole, such as those resulting from drug interactions with repeatedly co-administered medicines.

Cytochrome P450 1A2 and 2C enzymes autoinduced by omeprazole in dog hepatocytes and human HepaRG and HepaSH cells are involved in omeprazole 5-hydroxylation and sulfoxidation

The induction assay for the cytochromes P450 (P450s) is an important tool in drug discovery and development. The inductions of dog P450 1A2 and 3A12 by omeprazole and rifampicin were functionally characterised in dog hepatocytes and were compared with induction in human HepaRG and HepaSH cells.P450 1A2-dependent ethoxyresorufin O-deethylation was induced by R,S-omeprazole and P450 3 A-dependent midazolam 1'-hydroxylation was induced by rifampicin, and both reactions were significantly enhanced in cultured dog hepatocytes and human HepaRG and HepaSH cells.Recombinant dog P450 1A2 exhibited activities towards R- and S-omeprazole 5-hydroxylation with low Km values of 23-28 µM, whereas dog P450 2C21 and 3A12 efficiently mediated S-omeprazole 5-hydroxylation and sulfoxidation, respectively, with high Vmax values of 12-17 min-1.Although omeprazole 5-hydroxylation by human P450 2C19 (and sulfoxidation by P450 3A4) in human HepaSH cells were slightly (∼2-fold) induced by R,S-omeprazole, dog P450 1A2 was autoinduced by omeprazole in dog hepatocytes and showed enhanced R-omeprazole 5-hydroxylation activity (∼5-fold).These results indicate that omeprazole can be an autoinducer of P450 1A2 in hepatocytes, and this enzyme was found to be involved in omeprazole 5-hydroxylation and sulfoxidation in dog hepatocytes and human HepaRG and HepaSH cells.

Candidate Proficiency Test Chemicals to Address Industrial Chemical Applicability Domains for in vitro Human Cytochrome P450 Enzyme Induction

Cytochrome P450 (CYP) enzymes play a key role in the metabolism of both xenobiotics and endogenous chemicals, and the activity of some CYP isoforms are susceptible to induction and/or inhibition by certain chemicals. As CYP induction/inhibition can bring about significant alterations in the level of in vivo exposure to CYP substrates and metabolites, CYP induction/inhibition data is needed for regulatory chemical toxicity hazard assessment. On the basis of available human in vivo pharmaceutical data, a draft Organisation for Economic Co-operation and Development Test Guideline (TG) for an in vitro CYP HepaRG test method that is capable of detecting the induction of four human CYPs (CYP1A1/1A2, 2B6, and 3A4), has been developed and validated for a set of pharmaceutical proficiency chemicals. However to support TG adoption, further validation data was requested to demonstrate the ability of the test method to also accurately detect CYP induction mediated by industrial and pesticidal chemicals, together with an indication on regulatory uses of the test method. As part of "GOLIATH", a European Union Horizon-2020 funded research project on metabolic disrupting chemical testing approaches, work is underway to generate supplemental validated data for an additional set of chemicals with sufficient diversity to allow for the approval of the guideline. Here we report on the process of proficiency chemical selection based on a targeted literature review, the selection criteria and considerations required for acceptance of proficiency chemical selection for OECD TG development (i.e. structural diversity, range of activity, relevant chemical sectors, global restrictions etc). The following 13 proposed proficiency chemicals were reviewed and selected as a suitable set for use in the additional validation experiments: tebuconazole, benfuracarb, atrazine, cypermethrin, chlorpyrifos, perfluorooctanoic acid, bisphenol A, N,N-diethyl-m-toluamide, benzo-[a]-pyrene, fludioxonil, malathion, triclosan, and caffeine. Illustrations of applications of the test method in relation to endocrine disruption and non-genotoxic carcinogenicity are provided.

Pharmacokinetic Estimation Models-based Approach to Predict Clinical Implications for CYP Induction by Calcitriol in Human Cryopreserved Hepatocytes and HepaRG Cells

Calcitriol, a vitamin D₃ metabolite, is approved for various indications because it is the bioactive form of vitamin D in the body. The purpose of this study was to predict the clinical significance of cytochrome P450 (CYP) induction by calcitriol using in vitro human cryopreserved hepatocytes, HepaRG experimental systems, and various pharmacokinetic estimation models. CYP2B6, 3A4, 2C8, and 2C9 mRNA levels increased in a concentration-dependent manner in the presence of calcitriol in human cryopreserved hepatocytes and HepaRG cells. Using the half maximal effective concentration (EC₅₀) and maximum induction effect (E_max) obtained from the in vitro study, a basic kinetic model was applied, suggesting clinical relevance. In addition, a static mechanistic model showed the improbability of a clinically significant effect; however, the calculated area under the plasma concentration-time curve ratio (AUCR) was marginal for CYP3A4 in HepaRG cells. To clarify the effect of CYP3A4 in vivo, physiologically based pharmacokinetic (PBPK) modeling was applied as a dynamic mechanistic model, revealing a low clinically significant effect of CYP3A4 induction by calcitriol. Therefore, we conclude that CYP induction by calcitriol treatment would not be clinically significant under typical clinical conditions.

Inhibitory and inductive effects of 4- or 5-methyl-2-mercaptobenzimidazole, thyrotoxic and hepatotoxic rubber antioxidants, on several forms of cytochrome P450 in primary cultured rat and human hepatocytes

Effects of 4-methyl-2-mercaptobenzimidazole (4-MeMBI) and 5-methyl-2- mercaptobenzimidazole (5-MeMBI) on cytochrome P450 (CYP) activity were examined in primary cultured rat hepatocytes. Hepatocytes from male Wistar rats were cultured in the presence of 4-MeMBI or 5-MeMBI (0-400 μM), and the activity of CYPs 3A2/4 (48 and 96 h) and 1A1/2 (48 h) was determined by measuring the activity of testosterone 6β-hydroxylation and 7-ethoxyresorufin O-deethylation, respectively. As a result, 4-MeMBI and 5-MeMBI (≥12.5 μM) inhibited CYP3A2 activity. On the other hand, 4-MeMBI (≥25 μM) and 5-MeMBI (≥100 μM) induced CYP1A1/2 activity, being consistent with the previous in vivo results. In a comparative metabolism study using primary cultured human hepatocytes from two Caucasian donors, 4-MeMBI and 5-MeMBI induced the activity of CYPs 3A4 and 1A1/2 with individual variability. It was concluded from these results that 4-MeMBI, 5-MeMBI and MBI caused inhibition of CYP3A2 activity in primary cultured rat hepatocytes, suggesting their potential for metabolic drug-drug interactions. Primary cultured rat and human hepatocytes were considered to be useful for the evaluation of effects of the benzimidazole compounds on their inducibility and inhibitory activities of cytochrome P450 forms.

Comparison of the inducibility of CYP mRNA exposed to typical inducers in fresh and cryopreserved cynomolgus monkey hepatocytes

Herein, we evaluated CYPs and their nuclear receptor mRNA induction by exposure to typical inducers, omeprazole, rifampicin, and phenobarbital in cynomolgus monkey hepatocytes. Six freshly-isolated hepatocytes and 6 cryopreserved hepatocytes from cynomolgus monkey liver were prepared for a 14-day monolayer culture, 28-day co-culture with feeder cells, and 28-day 3D spheroid culture with feeder cells. Omeprazole and rifampicin respectively induced CYP1A1 and CYP3A8 mRNAs, while phenobarbital induced CYP2C43, CYP2C75, and CYP3A8, and slightly induced CYP2B6. The nuclear receptors AHR, PXR, and CAR mRNA levels, which were activated by omeprazole, rifampicin, and phenobarbital, respectively, tended to decrease via exposure to inducers despite the increase in CYP mRNA levels. These trends were similar for all three culture methods. No evident difference was observed in CYP mRNA induction between fresh and cryopreserved hepatocytes. Based on mRNA levels, the co-culture and 3D spheroid culture methods are more reasonable than monolayer culture for CYP evaluation, because the use of feeder cells can reduce the number of hepatocytes, improve the cell adhesion, and maintain the mRNA expression levels. In addition, co-culture method is more cost-effective, as common culture plates can be used.

Interleukin-1β and tumor necrosis factor-α affect cytochrome P450 expression in cynomolgus macaque hepatocytes

The cynomolgus macaque, partly due to its evolutionary closeness to humans, is an important nonhuman primate species used in drug metabolism studies. In humans, expressions of cytochromes P450 (P450s), including the important drug-metabolizing enzyme P450 3A4, are affected by various cytokines. However, this phenomenon has not been fully investigated in cynomolgus macaques. In this study, the effects of cytokines on P450 expression were investigated using the quantitative polymerase chain reaction to evaluate mRNA expression. Hepatocytes from cynomolgus macaques were treated with lipopolysaccharide and various cytokines, including interleukin (IL)-1β, IL-2, IL-6, interferon-γ, and tumor necrosis factor-α, and the expression levels of 11 P450s were compared with those of solvent-treated controls. Tumor necrosis factor-α significantly decreased cynomolgus P450 2C8 and 2C76 mRNA expression in multiple lots of cynomolgus hepatocytes investigated. IL-1β significantly decreased cynomolgus P450 1A1, 2C8, 2C19, and 2C76 mRNA expression, but increased P450 3A5 mRNA expression in multiple lots of hepatocytes. Moreover, P450 1A1-and 2C19-mediated drug oxidations were significantly and dose-dependently suppressed by IL-1β, under the present limited conditions. These results suggest that cytokines can influence hepatic P450 mRNA expression levels in cynomolgus macaques, just as cytokines are reported to affect P450 expression in humans.

Validation of in vitro methods for human cytochrome P450 enzyme induction: Outcome of a multi-laboratory study

CYP enzyme induction is a sensitive biomarker for phenotypic metabolic competence of in vitro test systems; it is a key event associated with thyroid disruption, and a biomarker for toxicologically relevant nuclear receptor-mediated pathways. This paper summarises the results of a multi-laboratory validation study of two in vitro methods that assess the potential of chemicals to induce cytochrome P450 (CYP) enzyme activity, in particular CYP1A2, CYP2B6, and CYP3A4. The methods are based on the use of cryopreserved primary human hepatocytes (PHH) and human HepaRG cells. The validation study was coordinated by the European Union Reference Laboratory for Alternatives to Animal Testing of the European Commission's Joint Research Centre and involved a ring trial among six laboratories. The reproducibility was assessed within and between laboratories using a validation set of 13 selected chemicals (known human inducers and non-inducers) tested under blind conditions. The ability of the two methods to predict human CYP induction potential was assessed. Chemical space analysis confirmed that the selected chemicals are broadly representative of a diverse range of chemicals. The two methods were found to be reliable and relevant in vitro tools for the assessment of human CYP induction, with the HepaRG method being better suited for routine testing. Recommendations for the practical application of the two methods are proposed.

[In silico prediction models of the induction of drug-metabolizing enzymes for drug discovery]

Drug metabolism in the liver is a major factor affecting pharmacokinetics of drugs, and cytochrome P450s (P450s) are major enzymes responsible for it. Since drug-drug interactions (DDIs) can affect the pharmacokinetics of concomitantly administrated drugs, it may limit the drug therapy such as dose adjustment and contraindications for co-administration and lead to dose adjustment and contraindications for co-administration. DDI is thus one of the risk factors to be reduced in the lead-optimization stage. Therefore, it is important to estimate DDI risk in the early drug discovery stage and develop candidates with low DDI risk. P450 induction is one of the important mechanisms causing DDIs and the activation of nuclear receptors is involved in this phenomenon. In this manuscript, the mechanism and evaluation methods of P450 induction are briefly reviewed, and then the new in silico methods for the prediction of P450 induction, which have been recently established by us, and its application to drug development are introduced.

Expression and inducibility of cytochrome P450s in human hepatocytes isolated from chimeric mice with humanised livers

The evaluation of drug-mediated cytochrome P450 (P450) induction using human hepatocytes is important for predicting drug interactions. In this study, we prepared hepatocytes from chimeric mice with humanised livers (Hu-Liver mice) and evaluated the expression and inducibility of P450s in these hepatocytes. Up to 95% of the Hu-Liver cells stained positive for human leukocyte antigen and the mean viability exceeded 85% (n = 10). Monolayer-cultured Hu-Liver cells displayed a similar morphology to cultures of the corresponding human hepatocytes used as transplantation donors. The mRNA expression levels in Hu-Liver cells of 16 P450 forms belonging to P450 subfamilies 1-4 correlated well with the expression levels of the same enzymes in human hepatocytes. The variations in individual P450 mRNA levels between Hu-Liver cells and the corresponding human hepatocytes were within five-fold for 13 P450 forms. The production of 6β-hydroxytestosterone in Hu-Liver cells was significantly increased (p < .05) following treatment with the CYP3A inducer, rifampicin. Hu-Liver cells have characteristics similar to those of human hepatocytes in terms of mRNA expression levels and the inducibility of the various P450 forms. Thus, Hu-Liver cells can potentially be used for in vitro drug-mediated induction assays of human hepatic P450s.

Critical differences in drug metabolic properties of human hepatic cellular models, including primary human hepatocytes, stem cell derived hepatocytes, and hepatoma cell lines

Primary human hepatocytes (PHH), HepaRG™, HepG2, and two sources of induced pluripotent stem cell (iPSC) derived hepatocytes were characterized regarding gene expression and function of key hepatic proteins, important for the metabolic fate of drugs. The gene expression PCA analysis showed a distance between the two iPSC derived hepatocytes as well as the HepG2 and HepaRG™ cells to the three PHH donors and PHH pool, which were clustered more closely together. Correlation-based hierarchical analysis clustered HepG2 close to the stem cell derived hepatocytes both when the expression of 91 genes related to liver function or only cytochrome P450 (P450) genes were analyzed indicating the non-liver feature and a similar low P450 profile in these cell models. The specific P450 activities and the metabolic pattern of well-characterized drug substances in the cell models demonstrated that iPSC derived hepatocytes had modest levels of CYP3A and CYP2C9, while CYP1A2, 2B6, 2C8, 2C9, 2C19, and 2D6 were barely detectable. High expression of several extrahepatic P450s such as CYP1A1 and 1B1 detected in the stem cell derived hepatocytes may have significant effects on metabolite profiles. However, one of the iPSC derived hepatocytes demonstrated significant combined P450 and conjugating enzyme activity of certain drugs. HepaRG™ cells showed many metabolic properties similar to PHHs and will in many respects be a good model in studies of metabolic pathways and induction of drug metabolism whereas there is still ground to cover before iPSC derived hepatocytes will be seen as a substitute to PHH in drug metabolism studies.

Establishing Transcriptional Signatures to Differentiate PXR-, CAR-, and AhR-Mediated Regulation of Drug Metabolism and Transport Genes in Cryopreserved Human Hepatocytes

The potential for drug-drug interactions (DDIs) arising from transcriptional regulation of drug-disposition genes via activation of nuclear receptors (NRs), such as pregnane X receptor (PXR), constitutive androstane receptor (CAR), and aryl hydrocarbon receptor (AhR), remains largely unexplored, as highlighted in a recent guidance document from the European Medicines Agency. The goal of this research was to establish PXR-/CAR-/AhR-specific drug-metabolizing enzyme (DME) and transporter gene expression signatures in sandwich-cultured cryopreserved human hepatocytes using selective activators of PXR (rifampin), CAR (CITCO), and AhR (omeprazole). Dose response for ligand-induced changes to 38 major human DMEs and critical hepatobiliary transporters were assessed using a custom gene expression array card. We identified novel differentially expressed drug-disposition genes for PXR (↑ABCB1/MDR1, CYP2C9, CYP2C19, and EPHX1, ↓ABCB11), CAR [↑sulfotransferase (SULT) 1E1, uridine glucuronosyl transferase (UGT) 2B4], and AhR (↑SLC10A1/NTCP, SLCO1B1/OATP1B1], and coregulated genes (CYP1A1, CYP2B6, CYP2C8, CYP3A4, UGT1A1, UGT1A4). Subsequently, DME gene expression signatures were generated for known CYP3A4 inducers PF-06282999 and pazopanib. The former produced an induction signature almost identical to that of rifampin, suggesting activation of the PXR pathway, whereas the latter produced an expression signature distinct from those of PXR, CAR, or AhR, suggesting involvement of an alternate pathway(s). These results demonstrate that involvement of PXR/CAR/AhR can be identified via expression changes of signature DME/transporter genes. Inclusion of such signature genes could serve to simultaneously identify potential inducers and inhibitors, and the NRs involved in the transcriptional regulation, thus providing a more holistic and mechanism-based assessment of DDI risk for DMEs and transporters beyond conventional cytochrome P450 isoforms.

Induction of human cytochrome P450 3A enzymes in cultured placental cells by thalidomide and relevance to bioactivation and toxicity

Evidence has been presented for auto-induced human cytochrome P450 3A enzyme involvement in the teratogenicity and clinical outcome of thalidomide due to oxidation to 5-hydroxythalidomide and subsequent metabolic activation in livers. In this study, more relevant human placenta preparations and placental BeWo cells showed low but detectable P450 3A4/5 mRNA expression and drug oxidation activities. Human placental microsomal fractions from three subjects showed detectable midazolam 1´- and 4-hydroxylation and thalidomide 5-hydroxylation activities. Human placental BeWo cells, cultured in the recommended media, also indicated detectable midazolam 1´- and 4-hydroxylation and thalidomide 5-hydroxylation activities. To reduce any masking effects by endogenous hormones used in the recommended media, induction of P450 3A4/5 mRNA and oxidation activities were measured in placental BeWo cells cultured with a modified medium containing 5% charcoal-stripped fetal bovine serum. Thalidomide significantly induced P450 3A4/5, 2B6, and pregnane X receptor (PXR) mRNA levels 2 to 3-fold, but rifampicin only enhanced P450 3A5 and PXR mRNA under the modified media conditions. Under these modified conditions, thalidomide also significantly induced midazolam 1´-hydroxylation and thalidomide 5-hydroxylaion activities 3-fold but not bupropion hydroxylation activity. Taken together, activation of thalidomide to 5-hydroxythalidomide with autoinduction of P450 3A enzymes in human placentas, as well as livers, is suggested in vivo.

Considerations from the IQ Induction Working Group in Response to Drug-Drug Interaction Guidance from Regulatory Agencies: Focus on Downregulation, CYP2C Induction, and CYP2B6 Positive Control

The European Medicines Agency (EMA), the Pharmaceutical and Medical Devices Agency (PMDA), and the Food and Drug Administration (FDA) have issued guidelines for the conduct of drug-drug interaction studies. To examine the applicability of these regulatory recommendations specifically for induction, a group of scientists, under the auspices of the Drug Metabolism Leadership Group of the Innovation and Quality (IQ) Consortium, formed the Induction Working Group (IWG). A team of 19 scientists, from 16 of the 39 pharmaceutical companies that are members of the IQ Consortium and two Contract Research Organizations reviewed the recommendations, focusing initially on the current EMA guidelines. Questions were collated from IQ member companies as to which aspects of the guidelines require further evaluation. The EMA was then approached to provide insights into their recommendations on the following: 1) evaluation of downregulation, 2) in vitro assessment of CYP2C induction, 3) the use of CITCO as the positive control for CYP2B6 induction by CAR, 4) data interpretation (a 2-fold increase in mRNA as evidence of induction), and 5) the duration of incubation of hepatocytes with test article. The IWG conducted an anonymous survey among IQ member companies to query current practices, focusing specifically on the aforementioned key points. Responses were received from 19 companies. All data and information were blinded before being shared with the IWG. The results of the survey are presented, together with consensus recommendations on downregulation, CYP2C induction, and CYP2B6 positive control. Results and recommendations related to data interpretation and induction time course will be reported in subsequent articles.

Role of Cytochrome P450 2C8 in Drug Metabolism and Interactions

During the last 10-15 years, cytochrome P450 (CYP) 2C8 has emerged as an important drug-metabolizing enzyme. CYP2C8 is highly expressed in human liver and is known to metabolize more than 100 drugs. CYP2C8 substrate drugs include amodiaquine, cerivastatin, dasabuvir, enzalutamide, imatinib, loperamide, montelukast, paclitaxel, pioglitazone, repaglinide, and rosiglitazone, and the number is increasing. Similarly, many drugs have been identified as CYP2C8 inhibitors or inducers. In vivo, already a small dose of gemfibrozil, i.e., 10% of its therapeutic dose, is a strong, irreversible inhibitor of CYP2C8. Interestingly, recent findings indicate that the acyl-β-glucuronides of gemfibrozil and clopidogrel cause metabolism-dependent inactivation of CYP2C8, leading to a strong potential for drug interactions. Also several other glucuronide metabolites interact with CYP2C8 as substrates or inhibitors, suggesting that an interplay between CYP2C8 and glucuronides is common. Lack of fully selective and safe probe substrates, inhibitors, and inducers challenges execution and interpretation of drug-drug interaction studies in humans. Apart from drug-drug interactions, some CYP2C8 genetic variants are associated with altered CYP2C8 activity and exhibit significant interethnic frequency differences. Herein, we review the current knowledge on substrates, inhibitors, inducers, and pharmacogenetics of CYP2C8, as well as its role in clinically relevant drug interactions. In addition, implications for selection of CYP2C8 marker and perpetrator drugs to investigate CYP2C8-mediated drug metabolism and interactions in preclinical and clinical studies are discussed.

Developmental Expression of CYP2B6: A Comprehensive Analysis of mRNA Expression, Protein Content and Bupropion Hydroxylase Activity and the Impact of Genetic Variation

Although CYP2B6 catalyzes the biotransformation of many drugs used clinically for children and adults, information regarding the effects of development on CYP2B6 expression and activity are scarce. Utilizing a large panel of human liver samples (201 donors: 24 fetal, 141 pediatric, and 36 adult), we quantified CYP2B6 mRNA and protein expression levels, characterized CYP2B6 (bupropion hydroxylase) activity in human liver microsomes (HLMs), and performed an extensive genotype analysis to differentiate CYP2B6 haplotypes such that the impact of genetic variation on these parameters could be assessed. Fetal livers contained extremely low levels of CYP2B6 mRNA relative to postnatal samples and fetal HLMs did not appear to catalyze bupropion hydroxylation; however, fetal CYP2B6 protein levels were not significantly different from postnatal levels. Considerable interindividual variation in CYP2B6 mRNA expression, protein levels, and activity was observed in postnatal HLMs (mRNA, ∼40,000-fold; protein, ∼300-fold; activity, ∼600-fold). The extremely wide range of interindividual variability in CYP2B6 expression and activity was significantly associated with age (P < 0.01) following log transformation of the data. Our data suggest that CYP2B6 activity appears as early as the first day of life, increases through infancy, and by 1 year of age, CYP2B6 levels and activity may approach those of adults. Surprisingly, CYP2B6 interindividual variability was not significantly associated with genetic variation in CYP2B6, nor was it associated with differences in gender or ethnicity, suggesting that factors other than these are largely responsible for the wide range of variability in CYP2B6 expression and activity observed among a large group of individuals/samples.

Opportunities and challenges in using human hepatocytes in cytochromes P450 induction assays

INTRODUCTION

Identification of inducers of xenobiotic-metabolizing cytochromes P450 (CYP) is of topical interest. The issue mainly concerns three sectors: (i) preclinical testing of drug candidates and testing existing drugs and their combinations; (ii) food safety applications with regard to additives, contaminants, and adulterants; (iii) environmental applications, comprising detection and identification of endocrine disruptors.

AREAS COVERED

A literature search was performed using the PubMed database, covering state-of-the-art of human hepatocyte (HH) culture use, and their exploitation for the identification of P450 inducers. A list of CYP inducers identified by HHs is provided.

EXPERT OPINION

Primary cultures of HHs had long been considered as a gold standard for induction assays of xenobiotic-metabolizing enzymes. Owing to several shortcomings of HHs, alternative approaches such as immortalization of HHs, use of cell lines, generation of clonal cell lines from HHs, use of induced pluripotent stem (iPS) cells, cells from humanized animals, etc., were employed. While yielding particular advantage, overall, alternatives to HHs still remain an avenue for discrete applications or technical situations. Thus, HHs remain the most suitable model for complex CYP induction studies. The summary may be effectively expressed by strength/weakness/opportunity/threats analysis.

Evaluation of cytochrome P450 inductions by anti-epileptic drug oxcarbazepine, 10-hydroxyoxcarbazepine, and carbamazepine using human hepatocytes and HepaRG cells

Anti-epileptic drug oxcarbazepine is structurally related to carbamazepine, but has reportedly different metabolic pathway. Auto-induction potentials of oxcarbazepine, its pharmacologically active metabolite 10-hydroxyoxcarbazepine and carbamazepine were evaluated by cytochrome P450 (CYP) 1A2, CYP2B6 and CYP3A4 mRNA levels and primary metabolic rates using human hepatocytes and HepaRG cells. For the CYP1A2 the induction potential determined as the fold change in mRNA levels was 7.2 (range: 2.3-11.5) and 10.0 (6.2-13.7) for oxcarbazepine and carbamazepine, respectively, while 10-hydroxyoxcarbazepine did not induce. The fold change in mRNA levels for CYP2B6 was 11.5 (3.2-19.3), 7.0 (2.5-10.8) and 14.8 (3.1-29.1) for oxcarbazepine, 10-hydroxyoxcarbazepine and carbamazepine, respectively. The fold change for CYP3A4 induction level by oxcarbazepine, 10-hydroxyoxcarbazepine and carbamazepine was 3.5 (1.2-7.4), 2.7 (0.8-5.7) and 8.3 (3.5-14.5), respectively. The data suggest lower induction potential of oxcarbazepine and 10-hydroxyoxcarbazepine relative to carbamazepine. The results in HepaRG cells showed similar trend as the human hepatocytes. After incubation for 72 h in hepatocytes and HepaRG cells, auto-induction was evident for only carbamazepine metabolism. The 10-keto group instead of double bond at C10 position is evidently a determinant factor for limited auto-induction of P450 enzymes by oxcarbazepine.

Application of Micropatterned Cocultured Hepatocytes to Evaluate the Inductive Potential and Degradation Rate of Major Xenobiotic Metabolizing Enzymes

Long-term coculture models of hepatocytes are promising tools to study drug transport, clearance, and hepatoxicity. In this report we compare the basal expression of drug disposition genes and the inductive response of prototypical inducers (rifampin, phenobarbital, phenytoin) in hepatocyte two-dimensional monocultures and the long-term coculture model (HepatoPac). All the inducers used in the study increased the expression and activity of CYP3A4, CYP2B6 and CYP2C enzymes in the HepatoPac cultures. The coculture model showed a consistent and higher induction of CYP2C enzymes compared with the monocultures. The EC50 of rifampin for CYP3A4 and CYP2C9 was up to 10-fold lower in HepatoPac than the monocultures. The EC50 of rifampin calculated from the clinical drug interaction studies correlated well with the EC50 observed in the HepatoPac cultures. Owing to the long-term stability of the HepatoPac cultures, we were able to directly measure a half-life (t1/2) for both CYP3A4 and CYP2B6 using the depletion kinetics of mRNA and functional activity. The t1/2 for CYP3A4 mRNA was 26 hours and that for the functional protein was 49 hours. The t1/2 of CYP2B6 was 38 hours (mRNA) and 68 hours (activity), which is longer than CYP3A4 and shows the differential turnover of these two proteins. This is the first study to our knowledge to report the turnover rate of CYP2B6 in human hepatocytes. The data presented here demonstrate that the HepatoPac cultures have the potential to be used in long-term culture to mimic complex clinical scenarios.

Differentiation-Promoting Medium Additives for Hepatocyte Cultivation and Cryopreservation

Isolated primary hepatocytes are considered as the reference system for in vitro hepatic methods. Following the isolation of primary hepatocytes from liver tissue, an unfavorable process named dedifferentiation is initiated leading to the attenuation of the hepatocellular phenotype both at the morphological and functional level. Freshly isolated hepatocytes can be used immediately or can be cryopreserved for future purposes. Currently, a number of antidedifferentiation strategies exist to extend the life span of isolated hepatocytes. The addition of differentiation-promoting compounds to the hepatocyte culture medium is the oldest and simplest antidedifferentiation approach applied. In the present chapter, the most commonly used medium additives for cultivation and cryopreservation of primary hepatocytes are reviewed.