Evaluation of U. S. National Toxicology Program (NTP) mouse lymphoma assay data using International Workshop on Genotoxicity Tests (IWGT) and the Organization for Economic Co-Operation and Development (OECD) criteria

The forward gene mutation mouse lymphoma assay (MLA) is widely used, as part of a regulatory test battery, to identify the genotoxic potential of chemicals. It identifies mutagens capable of inducing a variety of genetic events. During the 1980s and early 1990s, the U.S. National Toxicology Program (NTP) developed a publicly available database (https://tools.niehs.nih.gov/cebs3/ui/) of MLA results. This database is used to define the mutagenic potential of chemicals, to develop structure-activity relationships (SAR), and to draw correlations to animal carcinogenicity findings. New criteria for MLA conduct and data interpretation were subsequently developed by the International Workshop for Genotoxicity Testing (IWGT) and the Organization of Economic Cooperation and Development (OECD). These recommendations are included in a new OECD Test Guideline (TG490). It is essential that early experimental data be re-examined and classified according to the current criteria to build a curated database to better inform chemical-specific evaluations and SAR models. We re-evaluated more than 1900 experiments representing 342 chemicals against the newly defined acceptance criteria for background mutant frequency (MF), cloning efficiency (CE), positive control values (modified for this evaluation due to lack of colony sizing), appropriate dose selection, and data consistency. Only 17% of the evaluated experiments met all acceptance criteria used in this re-evaluation. Results from 211 chemicals were determined to be uninterpretable, 92 were positive, and 39 equivocal. The authors could not classify any responses as negative because colony sizing was not performed for any of these experiments and it is clear, based on many experiment with unacceptably low background and positive control MFs, that mutant colony recovery was often suboptimal. This re-evaluation provides a curated database for the MLA. A similar curation should be done for other widely used genetic toxicology assays, but will be more difficult for certain assays (e.g., in vitro chromosomal aberrations) because important parameters such as level of cytotoxicity were often not evaluated/reported. Environ. Mol. Mutagen. 59:829-841, 2018. © 2018 Wiley Periodicals, Inc.