The in vitro sensitivity of chronically hypoxic EMT6/SF cells to X-radiation and hypoxic cell radiosensitizers.

We have investigated the effect of extreme, prolonged hypoxia on the radiosensitivity of EMT6/SF cells in vitro. As cells were kept hypoxic for 1-24 h, their radiosensitivity increased, but no further change was noted for hypoxic incubation beyond 24 h. Chronically hypoxic (45 h) cells were more radiosensitive than acutely hypoxic (1 h) cells by a factor of 1.43. When chronically hypoxic cells were re-aerated, the increased radiosensitivity persisted, although it was reduced. Misonidazole (MISO) radiosensitization was equally effective under conditions of acute and chronic hypoxia. In contrast, MISO, SR2555 and SR2508 were more cytotoxic in chronically hypoxic cultures than in acutely hypoxic cells. Measurements suggested that intracellular thiols may play an important role in the effects observed.