In Vitro Mouse Lymphoma Cell (L5178Y Tk+/- -3.7.2.C) Forward Mutation Assay

Key Challenges for In Vitro Testing of Tobacco Products for Regulatory Applications: Recommendations for the In Vitro Mouse Lymphoma Assay

The Institute for In Vitro Sciences (IIVS) is sponsoring a series of workshops to develop recommendations for optimal scientific and technical approaches for conducting in vitro assays to assess potential toxicity within and across traditional tobacco and various tobacco and nicotine next-generation products (NGPs), including Heated Tobacco Products (HTPs) and Electronic Nicotine Delivery Systems (ENDS). This report was developed by a working group composed of attendees of the seventh IIVS workshop, 'Approaches and recommendations for conducting the mouse lymphoma gene mutation assay (MLA) and introduction to in vitro disease models', which was held virtually on 21-23 June 2022. This publication provides a background overview of the MLA, and includes the description of assay conduct and data interpretation, key challenges and recommended best practices for evaluating tobacco and nicotine products, with a focus on the evaluation of NGPs, and a summary of how the assay has been used to evaluate and compare tobacco and nicotine products.

Whole-genome high-fidelity sequencing: A novel approach to detecting and characterization of mutagenicity in vivo

Direct DNA sequencing can be used for characterizing mutagenicity in simple and complex biological models. Recently we described a method of whole-genome sequencing for detecting mutations in simple models of cultured bacteria, mammalian cells, and nematode. In the current proof-of-concept study, we expand and improve our method for evaluating a more complex mammalian biological model in outbred mice. We detail the method by applying it to a small set of animals treated with a mutagen with known mutagenicity profiles, N-ethyl-N-nitrosourea (ENU), for consistency with the known data. Whole-genome high-fidelity sequencing (HiFi Sequencing) showed frequencies and spectra of background mutations in tissues of untreated mice that were consistent with normal ageing and characterized by spontaneous or enzymatic deamination of 5-methylcytosine. In mice treated with a single 40 mg/kg dose of ENU, the frequency of mutations in the genomic DNA of solid tissues increased up to 7-fold, with the greatest increase observed in the spleen and the smallest increase in the liver. The most common mutations detected in ENU-treated mice were T > A transitions and T > C transversions, consistent with the types of mutations caused by alkylating agents. The data suggest that HiFi Sequencing may be useful for characterizing mutagenicity of novel compounds in various biological models.

In vitro and in vivo assessments of the genotoxic potential of 3-chloroallyl alcohol

3-Chloroallyl alcohol (3-CAA) can be found in the environment following the application of plant protection products. 3-CAA is formed in groundwater following the injection of 1,3-dichloropropene, a fumigant used to control nematodes. 3-CAA is also formed, in leafy crops, as a glycoside conjugate following application of the herbicide, clethodim. Human exposure may occur from groundwater used as drinking water or through dietary consumption. To characterize 3-CAA's potential to cause genotoxicity in mammals, in vitro and in vivo studies were conducted. 3-CAA was negative in an Ames test and positive in a mouse lymphoma forward mutation assay. 3-CAA was negative in an acute in vivo CD-1 mouse bone marrow micronucleus assay when administered up to a dose level of 125 mg/kg/day for two consecutive days. In a combined gene mutation assay and erythrocyte micronucleus assay, using transgenic Big Blue® Fischer 344 rats, 3-CAA was administered via drinking water at targeted dose levels of 0, 10, 30, and 100 mg/kg/day for 29 days. Peripheral blood samples, collected at the end of treatment, were analyzed for micronucleus induction in reticulocytes using flow cytometry. Liver and bone marrow samples, collected 2 days after the termination of the treatment, were analyzed for the induction of mutations at the cII locus. 3-CAA did not induce an increase in mutant frequency or micronuclei under the experimental conditions. In conclusion, the mutagenic response observed in the in vitro mouse lymphoma assay is not confirmed in the whole animal. 3-CAA is not considered to pose a mutagenic risk.