Mechanism of x-ray sensitization of bacteriophage T7 by 5-bromouracil

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 radiosensitization in exponentially growing (DNA dsb measurements only) and plateau-phase Chinese hamster ovary cells using an approach developed by Webb et al. (1993). The approach is based on the scavenging capacity of acetone for hydrated electrons, which are thought to react with bromine 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 microM BrdU (approximately 40% replacement of thymidine by BrdU), but does not restore cell radiosensitivity to the levels observed in BrdU-free cells. Although BrdU radiosensitizes cells by decreasing both D0 and Dq of the survival curve, acetone only restores D0 to levels measured in BrdU-free cells, but leaves Dq at levels measured in BrdU-containing cells. In plateau-phase cells, acetone removes the majority of excess DNA dsb and interphase chromosome breaks induced in the presence of 4 microM BrdU (approximately 50% replacement of thymidine by BrdU) but has only a small effect on BrdU-mediated radiosensitization to killing. These observations suggest that increased DNA damage production has a variable contribution in BrdU radiosensitization: it constitutes a major, albeit not the sole, component in the radiosensitization of exponentially growing cells, but only a minor component in the radiosensitization of plateau-phase cells. The results suggest that BrdU radiosensitization does not derive exclusively from increased DNA damage induction and support our previous hypothesis invoking repair inhibition/damage fixation as a component in the mechanism of radiosensitization. The results further suggest that repair inhibition is a major component in BrdU radiosensitization in exponentially growing cells, but the main cause of radiosensitization in plateau-phase cells.

Enhancement of X ray induced DNA damage by pre-treatment with halogenated pyrimidine analogs

The use of the halogenated pyrimidine analogs, including bromodeoxyuridine (BrdUrd) and iododeoxyuridine (IdUrd), as potential clinical radiosensitizers is an area of renewed clinical research. Cellular radiation effects of these analogs using clinically achievable steady state plasma levels (10(-7)-10(-5) M) were measured in vitro using exponentially growing Chinese hamster V79 cells. Radiation response was determined by clonogenic survival and by the production of DNA single strand (SSB) and double strand breaks (DSB) using filter elution techniques. Replacement of thymidine in DNA by BrdUrd or IdUrd was also determined. Drug exposure of BrdUrd or IdUrd for two population doublings (17 hr) prior to irradiation resulted in a progressive reduction of n and D0 compared to untreated controls over the drug dose range of 10(-7)-10(-5) M. The percent thymidine replacement increased from 1% at 10(-7) M to 5% at 10(-6) M to 23% at 10(-5) M. Using a 17 hr exposure at 10(-5) of BrdUrd or IdUrd, the production of SSB and DSB was increased by greater than or equal to 2X and greater than or equal to 1.5X respectively. No differences in the kinetics of repair of SSB and DSB were found following 3 hr of post-irradiation repair. We conclude that in this in vitro model, there appears to be a direct relationship between percent thymidine replacement, reduction of radiation survival parameters (n, D0), and the production of DNA strand breaks in the clinically achievable dose range of these halogenated pyrimidine analogs. These filter elution assays may be adapted to in vivo studies and possibly may allow for monitoring of radiation-induced DNA damage and repair in clinical tumor specimens treated with these radiosensitizers.

Potentiation of chemotherapy cytotoxicity following iododeoxyuridine incorporation in Chinese hamster cells

This study reports on the potentiation of selected chemotherapy drugs following halogenated pyrimidine incorporation into cellular DNA. Exponentially growing cultures of Chinese hamster V79 cells were exposed to 10(-5) M IdUrd or BrdUrd for 17 hr (approximately 2 cell doublings). This exposure resulted in approximately 16% replacement of thymidine by IdUrd or BrdUrd in the cellular DNA. Following the IdUrd exposure, dose response curves were determined for a 1 hr exposure to the various drugs. IdUrd pre-treatment was shown to enhance the cytotoxicity of melphalan, adriamycin, cisplatin, and neocarzinostatin with enhancement ratios at 1% survival of 1.5, 1.8, 1.5, and 1.4 respectively. BrdUrd pre-treatment also enhanced cisplatin cytotoxicity. A combination of BrdUrd pretreatment, cisplatin, and X rays was shown to yield additive survival effects. These findings are discussed in the context of possible clinical application of local drug perfusion of tumor-containing organs.

Incorporation of 5-substituted uracil derivatives into nucleic acids. The isolation and gamma-radiation-sensitivity of bacteriophage T3 containing the thymine analogue 5-vinyluracil

Bacteriophage T3 was produced in a form that contained 32% of its normal DNA thymine residues replaced with 5-vinyluracil residues by infecting a thymine-requiring strain of Escherichia coli with phage T3 in a medium containing 5-vinyluracil. When 2'-deoxy-5-vinyluridine was added to the medium instead, no incorporation was observed into the phage DNA, and the presence of the deoxyribonucleoside severely decreased the number of viable phage particles produced. The analogue-containing phage, although initially viable, rapidly lost viability when stored, but it was no more sensitive than was normal phage T3 to the effect of gamma-radiation.

Sensitization of Escherichia coli C to gamma-radiation by 5-bromouracil incorporation

Escherichia coli C cells, unifilarly substituted with 5-bromouracil (BrUra) were 2-25 times as sensitive as unsubstituted cells to killing by gamma-irradiation under aerobic conditions. The yield of DNA double-strand breaks in BrUra-substituted cells was increased by a factor only 1-55, suggesting that other lesions also contribute to cell-killing. Alkaline sucrose density gradient analysis of the 3H-thymine labelled DNA strand showed there was less repair of gamma-ray-induced single-strand breaks when BrUra was in the complementary strand. Since there are more of these unrepaired breaks than can be accounted for by BrUra-induced DNA double-strand breakage, some fraction of the lethal events in BrUra-substituted E. coli cells may be unrepaired DNA single-strand breaks.