Oncogenic transformation systems involving mammalian cells in vitro to determine the relative risks of different treatment modalities.

Systems developed to assay oncogenic transformation in vitro represent a rapid and powerful tool to screen and compare carcinogens for their relative oncogenic potential as well as to search for compounds that reduce or inhibit carcinogenesis produced by both physical and chemical agents. An established line of mouse fibroblasts (10T1/2 cells) cultured in vitro were used to compare the incidence of oncogenic transformation produced by X-rays, heat, various hypoxic cell radiosensitizers and a range of commonly used chemotherapy agents. A variety of 2 and 5 substituted nitroimidazoles were tested; these included metronidazole, desmethylmisonidazole, misonidazole, SR-2508, SR-2555, Ro-0741, RSU-1047 and RSU-1021. Most of these sensitizers produced a similar level of transformation; for example a three day exposure of aerated cells to a concentration of 1 mM of the drug resulted in a transformation incidence comparable to 1 Gy of X-rays. The notable exception was SR-2508 which produced a five-fold higher incidence of transformation. The potential carcinogenicity of sensitizers must be considered in choosing which of the currently available new drugs is to be used in clinical trials as an alternative to misonidazole. A variety of chemotherapy agents were tested at a concentration comparable to those used clinically and matched to produce about the same amount of cell killing. It was found that the incidence of oncogenic transformation varied widely. Some agents, such as Vincristine did not produce transformation at a level that could be detected above background, while others such as cis-Platinum appear to be potent carcinogens and produced transformation at a rate an order of magnitude higher than can be achieved by any dose of X-rays. Hyperthermia in the range of 40 degrees C caused no transformation, whether or not the heat treatment was associated with significant cell killing. In addition modest hyperthermia appreciably reduced the transformation frequencies associated with X-ray doses. Superoxide dismutase (SOD), a known space free radical scavenger, has been shown to reduce the level of transformation produced by radiation and sensitizers. To be effective, SOD must be present for prolonged periods during the fixation and expression period of the transformation process.