The biradical Ro-03-6061 removes the shoulder from the survival curve of hypoxic V-79 whereas both NPPN and PNAP are dose-modifying sensitizers like oxygen. When hypoxic cells were irradiated in the presence of a combination of NPPN and the biradical at equivalent radical concentrations, the survival curve was similar to that for cells irradiated in the presence of NPPN alone. When a combination of PNAP and the biradical were used at a concentration where the number of electron affinic (PNAP) molecules was equal to the number of free radical moieties, the resultant D0 value was similar to that for PNAP alone but the shoulder of the curve was not completely restored. In experiments where the biradical was used in combination with diamide the effect was additive. Cell kill was exponential even at low doses when 0.05 mM diamide was used in combination with 0.1 mM biradical; the extrapolation number was 1.0 and the D0 was 240 rad. The results suggest mechanistic differences between electron affinic, free radical, and sulfhydryl-binding radiosensitizers, and also differences between monofunctional and polyfunctional sensitizers.

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