Inter-laboratory evaluation of the response of primary human hepatocyte cultures to model CYP inducers - a European Centre for Validation of Alternative Methods (ECVAM) - funded pre-validation study

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.

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.

Drug biokinetic and toxicity assessments in rat and human primary hepatocytes and HepaRG cells within the EU-funded Predict-IV project

The overall aim of Predict-IV (EU-funded collaborative project #202222) was to develop improved testing strategies for drug safety in the late discovery phase. One major focus was the prediction of hepatotoxicity as liver remains one of the major organ leading to failure in drug development, drug withdrawal and has a poor predictivity from animal experiments. In this overview we describe the use and applicability of the three cell models employed, i.e., primary rat hepatocytes, primary human hepatocytes and the human HepaRG cell line, using four model compounds, chlorpromazine, ibuprofen, cyclosporine A and amiodarone. This overview described the data generated on mode of action of liver toxicity after long-term repeat-dosing. Moreover we have quantified parent compound and its distribution in various in vitro compartments, which allowed us to develop biokinetic models where we could derive real exposure concentrations in vitro. In conclusion, the complex data set enables quantitative measurements that proved the concept that we can define human relevant free and toxic exposure levels in vitro. Further compounds have to be analyzed in a broader concentration range to fully exploit these promising results for improved prediction of hepatotoxicity and hazard assessment for humans.

Biotransformation in vitro: An essential consideration in the quantitative in vitro-to-in vivo extrapolation (QIVIVE) of toxicity data

Early consideration of the multiplicity of factors that govern the biological fate of foreign compounds in living systems is a necessary prerequisite for the quantitative in vitro-in vivo extrapolation (QIVIVE) of toxicity data. Substantial technological advances in in vitro methodologies have facilitated the study of in vitro metabolism and the further use of such data for in vivo prediction. However, extrapolation to in vivo with a comfortable degree of confidence, requires continuous progress in the field to address challenges such as e.g., in vitro evaluation of chemical-chemical interactions, accounting for individual variability but also analytical challenges for ensuring sensitive measurement technologies. This paper discusses the current status of in vitro metabolism studies for QIVIVE extrapolation, serving today's hazard and risk assessment needs. A short overview of the methodologies for in vitro metabolism studies is given. Furthermore, recommendations for priority research and other activities are provided to ensure further widespread uptake of in vitro metabolism methods in 21st century toxicology. The need for more streamlined and explicitly described integrated approaches to reflect the physiology and the related dynamic and kinetic processes of the human body is highlighted i.e., using in vitro data in combination with in silico approaches.

The utility of cold-preserved human hepatocytes in studies on cytochrome P450 induction and hepatic drug transport

Human hepatocytes that had been cold-preserved in SureTran(TM) matrix (Abcellute Ltd, Cardiff, UK) were used for studies on cell viability, cytochrome P450 (CYP) 3A4, 2B6 and 1A2 induction and hepatic drug transporters. It has recently been shown that basal CYP activities are maintained in cold-preserved hepatocytes (Palmgren et al., 2012). After 5 d of cold preservation, the viability was still more than 70%, and after 8 d it was around 60%. In hepatocytes that had been cold-preserved for 3 d, the activity of CYP3A4 was induced around 15-fold upon treatment with 8 µM rifampicin for 72 h. For CYP2B6, the activity was induced 4- to 16-fold in hepatocytes that had been cold-preserved for 3 d and thereafter treated with 1 mM phenobarbital for 72 h. The activity of CYP1A2 was low and close to the limit of detection in non-treated cells that had been cold-preserved for up to 3 d, while the activity increased in cells treated with 0.3-25 µM β-naphthoflavone for 72 h. CYP3A4, 2B6 and 1A2 mRNA levels were only determined with hepatocytes from one donor and increased upon treatment with the inducers. Hepatic uptakes of estrone-3-sulfate, taurocholate, ipratropium and rosuvastatin were stable in human hepatocytes that had been cold-preserved for up to 2 d. In summary, cold-preserved human hepatocytes demonstrate retained viability and can advantageously be used for in vitro induction studies and for studies of hepatic uptake transporters.

Assessment of cytochrome P450 (1A2, 2B6, 2C9 and 3A4) induction in cryopreserved human hepatocytes cultured in 48-well plates using the cocktail strategy

1. A fast, straightforward and cost-effective assay was validated for the assessment of CYP induction in cryopreserved human hepatocytes cultured in 48-well plates. The cocktail strategy (in situ incubation) was used to assess the induction of CYP1A2, CYP2B6, CYP2C9 and CYP3A4 by using the recommended probe substrate, i.e. phenacetin, bupropion, diclofenac and midazolam, respectively. 2. Cryopreserved human hepatocytes were treated for 72 h with prototypical reference inducers, β-naphthoflavone (25 µM), phenobarbital (500 µM) and rifampicin (10 µM) as positive controls for CYP induction. The use of a cocktail strategy has been validated and compared to the classical approach (single incubation). The need of using phase II inhibitor (salicylamide) in CYP induction assay was also investigated. 3. By using three different batches of cryopreserved human hepatocytes and our conditions of incubations, we showed that there was no relevant drug-drug interaction using the cocktail strategy. The same conclusions were observed when a broad range of enzyme activity has to be assessed (wide range of reference inducers, i.e. EC50-Emax experiment). In addition, the interassay reproducibility assessment showed that the day-to-day variability was minimal. 4. In summary, the study showed that the conditions used (probe substrates, concentration of probe substrate and time of incubation) for the cocktail approach were appropriate for investigations of CYP induction potential of new chemical entities. In addition, it was also clear that the use of salicylamide in the incubation media was not mandatory and could generate drug-drug interactions. For this reason, we recommend to not use salicylamide in CYP induction assay.

The nude mouse as model for liver deficiency study and treatment and xenotransplantation

We aimed at reviewing the various uses of Nude mouse for the development of liver deficiency models and evaluation of efficacy of hepatic cell xenotransplantation. The first part records the large range of liver deficiency models that can be developed in Nude mice: surgical partial hepatectomy, acute toxic liver deficiency, chronic cirrhosis, and transgenic liver injury. The second part tackles the outcome of rat hepatocyte as well as human cell transplantation, both mature hepatocyte and hepatic progenitor, into Nude mouse submitted to liver injury. Results are discussed and compared to other available immunodeficient mouse models. The issue of humanized liver creation is also addressed. Altogether, these results show that Nude mouse appears to be a suitable small animal model to expand our insight into liver cell engraftment and regeneration.

Toward Preclinical Predictive Drug Testing for Metabolism and Hepatotoxicity by Using In Vitro Models Derived from Human Embryonic Stem Cells and Human Cell Lines — A Report on the Vitrocellomics EU-project

Drug-induced liver injury is a common reason for drug attrition in late clinical phases, and even for post-launch withdrawals. As a consequence, there is a broad consensus in the pharmaceutical industry, and within regulatory authorities, that a significant improvement of the current in vitro test methodologies for accurate assessment and prediction of such adverse effects is needed. For this purpose, appropriate in vivo-like hepatic in vitro models are necessary, in addition to novel sources of human hepatocytes. In this report, we describe recent and ongoing research toward the use of human embryonic stem cell (hESC)-derived hepatic cells, in conjunction with new and improved test methods, for evaluating drug metabolism and hepatotoxicity. Recent progress on the directed differentiation of human embryonic stem cells to the functional hepatic phenotype is reported, as well as the development and adaptation of bioreactors and toxicity assay technologies for the testing of hepatic cells. The aim of achieving a testing platform for metabolism and hepatotoxicity assessment, based on hESC-derived hepatic cells, has advanced markedly in the last 2–3 years. However, great challenges still remain, before such new test systems could be routinely used by the industry. In particular, we give an overview of results from the Vitrocellomics project (EU Framework 6) and discuss these in relation to the current state-of-the-art and the remaining difficulties, with suggestions on how to proceed before such in vitro systems can be implemented in industrial discovery and development settings and in regulatory acceptance.

HNF-4α determines hepatic differentiation of human mesenchymal stem cells from bone marrow

AIM: To investigate the differentiation status and key factors to facilitate hepatic differentiation of human bone-marrow-derived mesenchymal stem cells (MSCs).

METHODS: Human MSCs derived from bone marrow were induced into hepatocyte-like cells following a previously published protocol. The differentiation status of the hepatocyte-like cells was compared with various human hepatoma cell lines. Overexpression of hepatocyte nuclear factor (HNF)-4α was mediated by adenovirus infection of these hepatocyte-like cells. The expression of interesting genes was then examined by either reverse transcription-polymerase chain reaction (RT-PCR) or real-time RT-PCR methods.

RESULTS: Our results demonstrated that the differentiation status of hepatocyte-like cells induced from human MSCs was relatively similar to poorly differentiated human hepatoma cell lines. Interestingly, the HNF-4 isoform in induced MSCs and poorly differentiated human hepatoma cell lines was identified as HNF-4γ instead of HNF-4α. Overexpression of HNF-4α in induced MSCs significantly enhanced the expression level of hepatic-specific genes, liver-enriched transcription factors, and cytochrome P450 (P450) genes.

CONCLUSION: Overexpression of HNF-4α improves the hepatic differentiation of human MSCs from bone marrow and is a simple way of providing better cell sources for clinical applications.

Follow-up to the pre-validation of a harmonised protocol for assessment of CYP induction responses in freshly isolated and cryopreserved human hepatocytes with respect to culture format, treatment, positive reference inducers and incubation conditions

We have compared induction responses of human hepatocytes to known inducers of CYP1A2, CYP2B6, CYP2C and CYP3A4/5 to determine whether the culture format, treatment regimen and/or substrate incubation conditions affected the outcome. CYP induction responses to prototypical inducers were equivalent regardless of pre-culture time (24h or 48h), plate format (60mm or 24-well plates) used or whether CYP activities were measured in microsomes or whole cell monolayers. Fold-induction of CYP3A4/5 by 1000muM PB and 10microM RIF were equivalent. In contrast, the fold-induction of CYP2B6 by PB was 3-fold higher that by 10microM RIF. In addition to inducing CYP1A2, 50microM OME also induced CYP3A4/5 in 50% of the donors tested. CYP2B6 was induced in 14 out of 21 donors by BNF; however CYP3A4/5 was unaffected by BNF in these donors. In order to confirm that donor-to-donor variation was not due to inter-laboratory differences, the induction responses of 5 different batches of cryopreserved human hepatocytes were compared in two different laboratories. The induction of CYP1A2, CYP2B6 and CYP3A4 measured in our laboratory were equivalent to those obtained by the commercial companies, proving good between-laboratory reproducibility. In conclusion, there is some flexibility in the treatment and incubation protocols for classical CYP induction assays on human hepatocytes. Both RIF and PB are suitable positive control inducers of CYP3A4/5 but PB may be more appropriate for CYP2B6 induction. BNF may be more appropriate for CYP1A2 induction than OME since, in contrast to the latter, it does not induce CYP3A4. Induction responses using hepatocytes from the same donor but in different labs can be expected to be similar. The good reproducibility of induction responses between laboratories using cryopreserved hepatocytes underlines the usefulness of these cells for these types of studies.