Evaluation of experimental parameters in an S9 /human leukocyte SCE test with cyclophosphamide

The experimental parameters S9 concentration, cofactor concentration and exposure time were investigated in a microsome test with human leukocyte cultures, To evaluate these parameters, their effects on cyclophosphamide-induced SCE frequencies and changes in cell proliferation were analyzed. By varying S9 concentration and exposure time, dose-effect relationships were found with respect to the induction of SCE. A relatively small effect was found when the concentrations of the cofactors NADP and G-6-P were varied. In general, an increase in SCE frequencies was accompanied by delayed cell proliferation. The importance of varying the experimental parameters to avoid false- negative results in microsome tests is discussed.