MECHANISM OF RADIOSENSITIZATION BY HALOGENATED PYRIMIDINES - THE CONTRIBUTION OF EXCESS DNA AND CHROMOSOME-DAMAGE IN BRDU RADIOSENSITIZATION MAY BE MINIMAL IN PLATEAU-PHASE CELLS

We measured the contribution of increased DNA double-strand break (dsb ) and interphase chromosome break induction in BrdU-mediated radiosens itization in exponentially growing (DNA dsb measurements only) and pla teau-phase Chinese hamster ovary cells using an approach developed by Webb et al. (1993). The approach is based on the scavenging capacity o f acetone for hydrated electrons, which are thought to react with brom ine and form excess DNA and chromosome damage in BrdU-containing cells . In irradiated exponentially growing cells, acetone (1 M) removes the majority of excess DNA dsb induced in the presence of 4 mu M BrdU (si milar to 40% replacement of thymidine by BrdU), but does not restore c ell radiosensitivity to the levels observed in BrdU-free cells. Althou gh BrdU radiosensitizes cells by decreasing both D-0 and D-q of the su rvival curve, acetone only restores D-0 to levels measured in BrdU-fre e cells, but leaves D-q at levels measured in BrdU-containing cells. I n plateau-phase cells, acetone removes the majority of excess DNA dsb and interphase chromosome breaks induced in the presence of 4 mu M Brd U (similar to 50% replacement of thymidine by BrdU) but has only a sma ll effect on BrdU-mediated radiosensitization to killing. These observ ations suggest that increased DNA damage production has a variable con tribution in BrdU radiosensitization: it constitutes a major, albeit n ot the sole, component in the radiosensitization of exponentially grow ing cells, but only a minor component in the radiosensitization of pla teau-phase cells. The results suggest that BrdU radiosensitization doe s not derive exclusively from increased DNA damage induction and suppo rt our previous hypothesis invoking repair inhibition/damage fixation as a component in the mechanism of radiosensitization. The results fur ther suggest that repair inhibition is a major component in BrdU radio sensitization in exponentially growing cells, but the main cause of ra diosensitization in plateau-phase cells.