Combination of cisplatin and radiation in cell culture: Effect of duration of exposure to drug and timing of irradiation

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Abstract
Responses to the combination of cisplatin (CDDP) and radiation in experimental and clinical studies have been reported to vary from high radiosensitization to clear sub‐additivity. We examined the combined effect of CDDP with ionizing radiation in both murine mammary adenocarcinoma (EMT‐6) and human ovarian carcinoma (OV‐1063) cells with special reference to the duration of CDDP exposure and timing of irradiation. Cell survival was measured with a colorimetric assay of cell density. The nature of interaction of cisplatin and radiation was evaluated using isobolograms and a combination index (CI). Exposure of both cell lines to CDDP for 24 hr before irradiation yielded an additive or slightly sub‐additive response only if the exposure was extended for a few more hours after irradiation. In EMT‐6 cells, the combination of radiation with subsequent continuous as well as short‐term (4 to 6 hr) CDDP treatment was found to have a clear sub‐additive effect; dose escalation of each modality reduced the additional effect of the other. The sub‐additive effect may be explained by a radiation‐induced arrest of cells in late S phase, which was dose‐ and time‐dependent. Post‐radiation exposure to CDDP further increased the S‐phase arrest. In contrast, a 2 hr post‐radiation drug exposure resulted in a supra‐additive combined effect. Our results stress the crucial role of the timing and the doses of both modalities as well as the duration of post‐radiation drug exposure on their combined effect. Int. J. Cancer 75:635–642, 1998.

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