Metabolism Comparative Cytotoxicity Assay (MCCA) and Cytotoxic Metabolic Pathway Identification Assay (CMPIA) with cryopreserved human hepatocytes for the evaluation of metabolism-based cytotoxicity in vitro: proof-of-concept study with aflatoxin B1.

Recently, we have improved the cryopreservation procedures for human hepatocytes, leading to cells that can be cultured after thawing ("plateable" cryopreserved human hepatocytes). The ability to culture cryopreserved human hepatocytes allows application of the cells for prolonged incubations such as long-term (days) metabolism studies, enzyme induction studies, and cytotoxicity studies. We report here the application of the plateable cryopreserved human hepatocytes to evaluate the relationship between xenobiotic metabolism and toxicity. Two assays were developed: The Metabolism Comparative Cytotoxicity Assay (MCCA) and the Cytotoxic Metabolic Pathway Identification Assay (CMPIA). The MCCA was designed for the initial identification of the role of metabolism in cytotoxicity by comparing the cytotoxic potential of a toxicant in a metabolically competent (primary human hepatocytes) and a metabolically incompetent (Chinese hamster ovary (CHO)) cell type, as well as the evaluation of the role of P450 metabolism by comparing the cytotoxicity of the toxicant in question in human hepatocytes in the presence and absence of a nonspecific, irreversible P450 inhibitor, 1-aminobenzotriazole (ABT). The CMPIA was designed for the identification of the P450 isoforms involved in metabolic activation via the evaluation of the cytotoxicity of the toxicant in the presence and absence of isoform-selective P450 inhibitors. Results of a proof-of-concept study with the MCCA and CMPIA with a known hepatotoxicant, aflatoxin B1 (AFB1), are reported. AFB1 is known to require P450 metabolism for its toxicity. In the MCCA, AFB1 was found to have significantly higher cytotoxicity in human hepatocytes than CHO cells, therefore confirming its requirement for biotransformation to be toxic. ABT was found to effectively attenuate AFB1 cytotoxicity, confirming that P450 metabolism was involved in its metabolic activation. In the CMPIA, AFB1 cytotoxicity was found to be attenuated by ketoconazole and diethyldithiocarbamate, but not by furafylline, quinidine, and sulfaphenazole. Results with the isoform-selective inhibitors suggest that the isoforms inhibited by ketoconazole (mainly CYP3A4) and diethyldithiocarbamate (mainly CYP2A6, and CYP2E1), but not the isoforms inhibited by furafylline (mainly CYP1A2), sulfaphenazole (mainly CYP2C9) and quinidine (mainly CYP2D6) are involved in the metabolic activation of AFB1. This proof-of-concept study suggests that MCCA and CMPIA with cryopreserved human hepatocytes are potentially useful for the evaluation of the relationship between human xenobiotic metabolism and toxicity.