A comparison of two cytotoxicity assays for the detection of metabolism-mediated toxicity in vitro: a study with cyclophosphamide.

The cytotoxicity to V79 Chinese hamster fibroblasts of cyclophosphamide (CPA) metabolites, generated by rat-liver S9 fractions, has been compared in two assay systems with different endpoints of toxicity, namely, reduction of cloning efficiency and inhibition of cell growth. The two assay systems were found to be equally sensitive in detecting the metabolism-mediated cytotoxicity of CPA and gave similar ID50 values. Further studies confirmed the cytochrome P-450 enzyme requirement for the bioactivation of CPA to cytotoxic metabolites. CPA activation was mediated by phenobarbitone-inducible forms of cytochrome P-450, but not by beta-naphthoflavone-inducible forms.