Evaluation of Strategies to Improve In Vitro Mutagenicity Assessment: Alternative Sources of S9 Exogenous Metabolic Activation and the Development of an In Vitro Assay Based on MutaMouse Primary Hepatocytes

Abstract

In vitro genetic toxicity tests using cultured bacterial or mammalian cells provide a cost- and time-effective alternative to animal tests. Unfortunately, existing in vitro assays are not always reliable. This is in part due to the limited metabolic capacity of the cells used, which is often critical to accurately assess chemical genotoxicity. This limited metabolic capacity necessitates the use of exogenous sources of mammalian metabolic enzymes that can simulate in vivo mammalian metabolic activation reactions. In response to this, and other limitations, alongside the worldwide trend to reduce animal testing, there is an acute need to consider various strategies to improve in vitro mutagenicity assessment. This thesis first examined the utility of exogenous metabolic activation systems based on human hepatic S9, relative to conventional induced rat liver S9, for routine genetic toxicity assessment. This was accomplished by critically evaluating existing literature, as well as new experimental data. The results revealed the limitations of human liver S9 for assessment of chemical mutagenicity. More specifically, the analyses concluded that, due to the increased risk of false negative results, human liver S9 should not be used as a replacement for induced rat liver S9. To address the limitations of conventional mammalian cell genetic toxicity assays that require exogenous hepatic S9, the thesis next evaluated the utility of an in vitro mutagenicity assay based on metabolically-competent primary hepatocytes (PHs) derived from the transgenic MutaMouse. Cultured MutaMouse PHs were thoroughly characterized, and found to temporarily retain the phenotypic attributes of hepatocytes in vivo; they express hepatocyte-specific proteins, exhibit the karyotype of typical hepatocytes, and maintain metabolic activity for at least the first 24 hours after isolation. Preliminary validation of the in vitro MutaMouse PH gene mutation assay, using a panel of thirteen mutagenic and non-mutagenic chemicals, demonstrated excellent sensitivity and specificity. Moreover, inclusion of substances requiring a diverse array of metabolic activation pathways revealed comprehensive metabolic competence. Finally, the thesis further investigated the applicability domain of the in vitro MutaMouse PH assay by challenging the assay with selected azo compounds. Comparison of these results with those obtained using the in vivo MutaMouse TGR (transgenic rodent) assay revealed that MutaMouse PHs can carry out some forms of reductive metabolism. Overall, this thesis demonstrated that a gene mutation assay based on MutaMouse PHs holds great promise for routine assessments of chemical mutagenicity.