Enhancement of X-ray induced cytotoxicity by neocarzinostatin in asynchronous and synchronous rat 3Y1 fibroblasts.

A polypeptide antibiotic, neocarzinostatin (NCS), has recently been shown to fix potentially lethal damage and inhibit repair of sublethal damage. To extend these observations, the effects of NCS on X-ray induced cytotoxicity at different phases of the cell cycle and the induction of DNA double strand breaks (dsb) were examined using rat 3Y1 fibroblasts. Colony survival assays showed that postirradiation treatment with 0.019 micrograms/ml NCS for 20 min enhanced X-ray cell killing at all phases of the cell cycle. Neutral filter elution analyses showed that NCS increased DNA dsb induction after X-irradiation. Thus, NCS appears to enhance the radiosensitivity by increasing the initial level of radiation-induced DNA dsb.