Species differences in the genotoxicity of cyclophosphamide and styrene in three in vivo assays.

Species differences in dispositional factors such as distribution, metabolism and excretion may often account for species differences in the toxic responses to foreign chemicals. In this study we compared the genotoxic responses of cyclophosphamide (CP) and styrene (ST) between Porton rats and LACA Swiss mice in three in vivo assays (bone marrow micronucleus (MN), sperm morphology (SM) and sister-chromatid exchange (SCE) assays). The sensitivities of the three assays were compared by the doses of the compounds required to elicit a significant genotoxic response. The baseline levels for the MN, SCE and SM assays were 1.1-1.4 and 1.2-1.3 MNPCEs/1000 PCEs, 0.23-0.24 and 0.20-0.21 SCEs/chromosome, 3.5-5.7% and 1.6-1.9% abnormal sperm in mice and rats, respectively. CP was a potent genotoxin in the MN and SCE assays but weakly genotoxic in the SM assay. At comparable doses, the rat was approximately 3-, 2.5- and 1.8-fold more sensitive to CP than mice in the MN, SM and SCE assays, respectively. ST produced weak genotoxic responses in all assays in mice and only in the SM and SCE assays in rats. The mice were more sensitive to ST in the MN and SM assays, while it was difficult to compare the species in the SCE assay. For both compounds the sensitivity of the three assays, in decreasing order, were SCE greater than MN much greater than SM. For CP the relative responses in the Porton rats and LACA Swiss mice were qualitatively similar to previous reports. Although the use of different strains may explain differences between the studies in the magnitude of the responses observed. The results for ST in the rat shows that the choice of genotoxic endpoint can determine whether a response is detectable. Moreover, the discrepancies between the results for ST in this study and others, suggest that as well as using a battery of in vivo tests, it may be prudent to select more that one strain or species to fully assess a compound's ability to produce DNA damage.