Exclusion of an interactive effect of combined x-irradiation and activated cyclophosphamide in tissue culture.

The effect of Cyclophosphamide (CY) on the X ray survival of clonogenic tumor cells has been studied in vitro. Two activated derivatives of the drug, Peroxycyclophosphamide and Hydroperoxycyclophosphamide, were employed. Two cell lines, repair-competent human HeLa cells and the repair-deficient rat REQ line, were investigated. Neither form of CY had any effect on the X ray survival curve of either cell line, indicating that any interaction anticipated in vivo could be expected to be additive.