It depends largely on what you are trying to model. Mice are a useful model because of the genetic variants available and the large number of cancer models, both genetic and xenograft. Rats were classically used because their larger size made sampling different tissues easier. You will need both ethics approval to use a cytotoxic in either animal model and appropriate protocols to minimise the exposure of anyone in your lab.

Thank you,

Cytotoxicity study is best in Rats model