The potentiation of radiation damage by nicotinamide in the SCCVII tumour in vivo.

We have continued our assessment of the ability of nicotinamide to sensitize tumours to radiation. Using the SCCVII carcinoma and estimating tumour response by either a regrowth delay or an in vivo/in vitro survival assay, it was found that a large single dose of nicotinamide (1000 mg/kg) increased radiation-induced tumour damage. This effect was observed regardless of whether the tumour was grown intramuscularly, subcutaneously or intradermally, or whether the nicotinamide was administered intraperitoneally, intravenously or orally. The enhancement was maximal when the drug was given between 30 min and 2 h prior to irradiation and resulted in enhancement ratios ranging from 1.1 to 1.7. Although the radiation response of tumours was dependent on tumour size, the radiation enhancement produced by nicotinamide was not. Utilizing the technique of labelling tumour cells with the fluorescent stain Hoechst 33342, we were able to identify the presence of both chronic and acutely hypoxic cells in this tumour model and obtained results suggesting that apart from reducing chronic hypoxia, nicotinamide may also have the ability to decrease the level of radioresistant acute hypoxia.