Evaluation of the genotoxicity of tamoxifen in the liver and kidney of F344 gpt delta transgenic rat in 3-week and 13-week repeated dose studies

Transgenic rat gene mutation assays can be used to assess genotoxicity of chemicals in target organs for carcinogenicity. Mutations in transgenes are genetically neutral and accumulate during a treatment period; thus, the assays are suitable for assessment of the genotoxicity risk of chemicals using a repeated-dose treatment paradigm. However, few such studies have been conducted. To examine the utility of the transgenic rat assays in repeated-dose studies, we treated female F344 gpt delta rats with tamoxifen (TAM) at 20 and 40 mg/kg, or toremifene (TOR) at 40 mg/kg by gavage daily for 3 weeks. We also fed gpt delta rats with TAM at either 250 ppm (15.4-17.6 mg/kg) or 500 ppm (30.0-32.9 mg/kg) for 13 weeks. TAM is carcinogenic in the rat liver and TOR is not carcinogenic. TAM administration significantly increased gpt (point mutations) and Spi− (deletions) mutant frequencies (MFs) in the liver, the target organ of carcinogenesis; MFs were higher after treatment for 13 weeks than after treatment for 3 weeks. The MFs in the kidney did not increase in any of the TAM treatment groups. TOR had no effect on MFs (gpt and Spi−) in either the liver or the kidney. We conclude that the gpt delta rat assay in the repeated-dose treatment paradigm is sensitive enough to detect gene mutations induced by TAM in the target organ for carcinogenesis. Furthermore, the assay can be integrated into a 13-week dose-finding study for a 2-year cancer bioassay.