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In vitro genotoxicity assessment routinely employs an exogenous metabolic activation mixture to simulate mammalian metabolism. Activation mixtures commonly contain post-mitochondrial liver supernatant (i.e. S9) from chemically induced Sprague Dawley rats. Although Organization for Economic Cooperation and Development (OECD) test guidelines permit the use of other S9 preparations, assessments rarely employ human-derived S9. The objective of this study is to review and evaluate the use of human-derived S9 for in vitro genetic toxicity assessment. All available published genotoxicity assessments employing human S9 were compiled for analysis. To facilitate comparative analyses, additional matched Ames data using induced rat liver S9 were obtained for certain highly cited chemicals. Historical human and induced rat S9 quality control reports from Moltox were obtained and mined for enzyme activity and mutagenic potency data. Additional in vitro micronucleus data were experimentally generated using human and induced rat S9. The metabolic activity of induced rat S9 was found to be higher than human S9, and linked to high mutagenic potency results. This study revealed that human S9 often yields significantly lower Salmonella mutagenic potency values, especially for polycyclic aromatic hydrocarbons, alatoxin B1 and heterocyclic amines (~3- to 350-fold). Conversely, assessment with human S9 activation yields higher potency for aromatic amines (~2- to 50-fold). Outliers with extremely high mutagenic potency results were observed in the human S9 data. Similar trends were observed in experimentally generated mammalian micronucleus cell assays, however human S9 elicited potent cytotoxicity L5178Y, CHO and TK6 cell lines. Due to the potential for reduced sensitivity and the absence of a link between enzyme activity levels and mutagenic potency, human liver S9 is not recommended for use alone in in vitro genotoxicity screening assays; however, human S9 may be extremely useful in follow-up tests, especially in the case of chemicals with species-specific metabolic differences, such as aromatic amines. [ABSTRACT FROM AUTHOR]

In this study, we explored the applicability of using in vitro micronucleus (MN) data from human lymphoblastoid TK6 cells to derive in vivo genotoxicity potency information. Nineteen chemicals covering a broad spectrum of genotoxic modes of action were tested in an in vitro MN test using TK6 cells using the same study protocol. Several of these chemicals were considered to need metabolic activation, and these were administered in the presence of S9. The Benchmark dose (BMD) approach was applied using the dose-response modeling programPROAST to estimate the genotoxic potency from the in vitro data. The resulting in vitro BMDs were compared with previously derived BMDs from in vivo MN and carcinogenicity studies. A proportional correlation was observed between the BMDs from the in vitro MN and the BMDs from the in vivo MN assays. Further, a clear correlation was found between the BMDs from in vitro MN and the associated BMDs for malignant tumors. Although these results are based on only 19 compounds, they show that genotoxicity potencies estimated from in vitro tests may result in useful information regarding in vivo genotoxic potency, as well as expected cancer potency. Extension of the number of compounds and further investigation of metabolic activation (S9) and of other toxicokinetic factors would be needed to validate our initial conclusions. However, this initial work suggests that this approach could be used for in vitro to in vivo extrapolations which would support the reduction of animals used in research (3Rs: replacement, reduction, and refinement). [ABSTRACT FROM AUTHOR]