Which is most sensitive? Assessing responses of mice and rats in toxicity bioassays

Rodent species are commonly used in traditional toxicology testing guidelines to predict human health toxicity outcomes. The use of a consistent species in test guidelines is important for maintaining consistency and comparability between tests and testing guidelines. This recommendation was operationalized for this study as the implicit assumption of uniform species and species-sex sensitivities. This investigation analyzed the uniformity assumption using data from National Toxicology Program Technical Reports (and where applicable Toxicity Reports), which provide data from both short-term and chronic rodent toxicity tests. These data were extracted and modeled using the Environmental Protection Agency's Benchmark Dose Software. Minimum best-fit benchmark doses (BMD) and benchmark dose lower limits (BMDL) were determined and a minimum best-fit BMD10 and BMDL10 estimated for every chemical and study duration. Endpoints of interest included non-neoplastic lesions, final mean body weights, and mean organ weights. The distribution of findings was then assessed to determine the most sensitive species and species-sex combinations associated with the minimum best-fit BMDL10. Data indicated that species and species-sex sensitivity for this group of chemicals is not uniform and that rats are significantly more sensitive than mice for non-cancerous outcomes observed, depending upon study duration. There are also indications that male rats may be more sensitive than other species-sex groups in certain situations.