Enhancement of radiosensitivity of cultured mammalian cells by neocarzinostatin. II. Fixation of potentially lethal damage.

The effects of neocarzinostatin (NCS), an anti-tumour drug, on the repair of potentially lethal damage (PLD) were studied using cultured Chinese hamster V79, malignant human melanoma and mouse lymphoma L5178Y cells in the stationary phase. The repair of PLD was observed in the melanoma and L5178Y cells but no such repair was observed in the V79 cells, when studied by delayed plating. NCS added to the culture medium immediately after X-irradiation evoked fixation of PLD within 10 min of the addition of NCS. The ratios of D0 values of the survival curves of the cells treated with NCS to those plated immediately after X-irradiation were 0.78, 0.88 and 0.85 for V79, melanoma and L5178Y cells, respectively. The extent of the fixation by NCS was similar to that caused by 0.5 M NaCl solution. The results in the present study and the inhibition of sublethal damage (SLD) by NCS reported previously, suggest that NCS might react with the DNA damage induced by radiation and modify it to lethal damage. The study indicates that SLD and PLD appear to be closely related to one another.