Repair of DNA and Chromosome Breaks in Cells Exposed to SR 4233 under Hypoxia or to Ionizing Radiation¹

3-Amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233) is a bioreductive anticancer drug which has a high selective toxicity to hypoxic cells. We have characterized the DNA and chromosome damage in wild-type Chinese hamster ovary (CHO) cells and mutant XR-1 cells after exposure to SR 4233 under hypoxia and compared it to the damage produced by ionizing radiation (γ-rays). Using the technique of pulsed field gel electrophoresis, we found that the kinetics of rejoining of DNA double-strand breaks in CHO cells after treatment with SR 4233 was concentration dependent, varying from 95% (<50 µm) to 10% (200 µm) by 24 h. This contrasted with the dose-independent kinetics exhibited in cells after γ-ray exposure. XR-1 cells were deficient in rejoining double-strand breaks produced by either SR 4233 or γ-rays. XR-1 cells were 2-fold more sensitive than wild-type CHO cells to SR 4233 but were 10-fold more sensitive than CHO to γ-rays. These results suggested that DNA double-strand breaks are involved in hypoxic cell killing by SR 4233, but the specific type of lesion produced is not identical with that causing cell killing by γ-rays. To further investigate this, we measured chromosome breaks in CHO cells by premature chromosome condensation after equitoxic doses of SR 4233 under hypoxia and γ-rays. SR 4233 produced lower initial but similar final (after 6 h of repair) numbers of chromosome breaks compared to γ-rays at equitoxic doses. These results suggest that, at low doses, chromosome breaks can entirely account for hypoxic cell killing by SR 4233 and that chromosome breaks produced by SR 4233 are more damaging and/or more difficult to repair than those produced by γ-rays.