Comparative studies of total cross-linking, cell survival and cell cycle perturbations in Chinese hamster cells treated with alkylating agents in vitro.

The toxicities of 4-hydroperoxycyclophosphamide (4-OOH CY), phosphoramide mustard (PM), melphalan (MEL) and busulphan (BU) have been compared in Chinese hamster cells, V-79-753B. The initial total amount of cross-linking was a determining factor for the clonogenic survival of cells treated with MEL or PM. Although 4-OOH CY generated cross-links in this cell line, this damage did not account for the toxicity of the compound. There was no evidence for cross-link formation in cells treated with BU, even at a dose of the drug (1000 micrograms/ml) that was too toxic to measure clonogenic survival. Comparison for the four compounds at equitoxic doses showed that both PM and MEL caused the arrest of the cell cycle at G2 which persisted after drug removal. This was accompanied by a decline in the population growth rate and a decrease in total cell count. In contrast, both BU and 4-OOH CY caused a temporary arrest of the cell cycle G2, 24 hr after drug removal. However, the cell cycle distribution returned the control values within 3-4 days after treatment. Both BU and 4-OOH CY showed little effect on the initial growth rate of the cells. It is concluded that the initial amount of cross-links contributes to the toxicity of PM and MEL. However, it is unlikely that the generation of cross-links is of major importance for the toxicity of either 4-OOH CY or BU.