Radiosensitization by cisplatin of RIF1 tumour cells in vitro.

The ability of cis-diamminedichloroplatinum (II) (c-DDP) to enhance radiation-induced cell killing was tested on oxic RIF1 tumour cells in monolayer culture. Marked radiosensitization of the survivors of a 1 h drug treatment was found with all c-DDP doses tested, with enhancement ratios increasing from 1.2 to 2.2 with increasing drug dose. Isobologram analyses showed that the interactions of c-DDP with X-rays were supra-additive. To test whether part of the enhancement was due to a selection of subpopulations, the diploid and tetraploid RIF1 cells, which normally coexist in culture, were separated by unit gravity velocity sedimentation, and by developing diploid and tetraploid clones. Both methods showed that there was little difference in either drug sensitivity or radiation sensitivity between diploid and tetraploid cells. DNA histograms obtained by flow cytometry showed little or no cycle progression during the 1 h drug treatment. These data indicate that the radiosensitization was not the result of the drug exposure leaving cells in a radiosensitive phase. The observed radiosensitization, therefore, appears to have resulted from a true drug/X-ray interaction.