Is Tumour Radiosensitization by Misonidazole a General Phenomenon?

The response of 14 mouse tumour sub-lines to the radiosensitizing action of a large single dose of misonidazole (MISO) has been assessed by regrowth delay. In 13 of these, significant enhancement of radiation effect occurred under ambient conditions, indicating sensitization of naturally hypoxic cells. The enhancement observed (SER') varied with the radiation dose, as would be predicted for a mixed oxic/hypoxic cell population. The maximum SER' in these 13 tumours did not depend on histology or regrowth rate. The 14th tumour, a slow-growing sarcoma, was not sensitized under ambient conditions, but showed marked sensitization when clamped to produce acutely hypoxic cells. This is consistent with no hypoxic cells occurring naturally in a sarcoma with a slow rate of growth. Faster-growing variants of this tumour showed radiosensitization under ambient conditions. The slow-growing carcinoma, RH, however, appears to contain hypoxic cells and did show sensitization. The cytotoxic action of MISO was compared with the radiosensitization by administering it after irradiation in 8 of the tumour lines. In 2 tumours no cytotoxicity was observed. In the rest cytotoxicity was significant, but much smaller than the sensitization observed when MISO was administered before irradiation. These regrowth-delay data have been used to calculate hypoxic fractions in 3 ways. Estimates of hypoxic fraction ranged from less than 0.1% in the slow sarcoma to greater than or equal to 30% in several tumours. There is considerable variation in the estimate, according to the technique used.