Effect of Misonidazole on Formation of Thymine Damage by Gamma Rays

The effect of the radiosensitizer misonidazole (Ro-07-0582) on the formation of thymine base damage of the 5,6-dihydroxydihydrothymine-type by γ rays was measured under aerobic and hypoxic conditions. HeLa cells, prelabeled with [methyl-3 H]thymidine, were suspended in phosphate-buffered saline in the presence and absence of misonidazole. Concentrations of misonidazole up to 15 mM were used. The cell suspensions were irradiated at ice temperature with 60 Co γ rays. Dose-response curves under aerobic and hypoxic conditions showed a much depressed base damage formation under hypoxia, which was created by blowing a stream of nitrogen across the cell suspensions for 30 min on ice. The presence of misonidazole had little or no detectable effect under hypoxia. It is concluded that an effect on the level of formation of thymine base damage is not primarily responsible for the radiosensitization by misonidazole under hypoxic conditions.