Effect of Oxygen-Radiosensitizer Mixtures on the Radiation Response of Chinese Hamster Cells, Line V-79-753B, in Vitro: II. Determination of the Initial Yield of Single-Strand Breaks in the Cellular DNA Using a Rapid Lysis Technique

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Abstract
The maximum yield of single-strand breaks (ssb) in the cellular DNA of Chinese hamster cells V-79-753B is produced at a concentration of O2 that produces an enhancement ratio for cell survival of 1.9. The relationship between the O2 concentration and enhancement ratio for survival in this cell line is biphasic with a plateau at ER [enhancement ratio] = 1.9 over the range of 1.5-7 .mu.M O2. For concentrations of O2 below 1.5 .mu.M, a linear relationship between 1/D0 [median lethal dose] and the initial yield of ssb is found. Electron affinic and free radical radiosensitizers operate by different mechanisms which are reflected at the level of ssb production; electron affinic compounds increase the yield of ssb in anoxia and in the presence of low concentrations of O2; free radical radiosensitizers do not. Since TMPN [2,2,6,6-tetramethylpiperidin-4-ol-N-oxy] can compete with O2 or misonidazole in reactions that lead to changes in radiosensitivity but not ssb production, the relationship between the 2 parameters must be casual and not causal.