Predicting in vitro genotoxicity by mouse lymphoma L5178Y thymidine kinase mutation assay (MLA): Accounting for simulated metabolic activation of chemicals

Abstract The mouse lymphoma L5178Y thymidine kinaselocus gene mutation assay (MLA) is typically part of regulatory batteries of methods used for in vitro evaluation of substances eliciting small gene mutations, and gross structural alterations at the chromosomal level. In an effort to make the MLA endpoint amenable to category formation to support assessments of mutagenesis, a model with 52 DNA and 32 protein binding alertsparticular to the MLA endpoint has been developed. Each alertis supported by a mechanistic justification, an alert-specific training setand an alertperformance evaluation. Subsequently, these alertshave been used in combination with an in vitro rat liver S9 fractionmetabolic simulator (the TIssue MEtabolism Simulator (TIMES) platform) to evaluate MLA mutagenicity of likely metabolites of MLA negative parent compounds. The resulting system provides for transparent in silico identification of structural and general parametric requirements, as well as, binding mechanisms in parent chemicals and their simulated metabolites. When applied to the training sets, performance of the TIMES_MLA models, both with and without rat liver S9 activation show high (≥80%) sensitivity, specificity and concordance, which is consistent with the repeatability and reproducibility of experimental MLA results.