Inhibition of base excision repair potentiates iododeoxyuridine-induced cytotoxicity and radiosensitization

5-Iodo-2'-deoxyuridine (IdUrd) is a halogenated thymidine analogue recognized as an effective in vitro and in vivo radiosensitizer in human cancers. IdUrd-related cytotoxicity and/or radiosensitization are correlated with the extent of IdUrd-DNA incorporation replacing thymidine. IdUrd cytotoxicity and radiosensitization result, in part, from induction of DNA single-strand breaks (SSB) with subsequent enhanced DNA double-strand breaks leading to cell death. Because base excision repair (BER) is a major DNA repair pathway for SSB induced by chemical agents and ionizing radiation, we initially assessed the role of BER in modulating IdUrd cytotoxicity and radiosensitization using genetically matched Chinese hamster ovary cells, with (AA8 cells) and without (EM9 cells) XRCC1 expression. XRCC1 plays a central role in processing and repairing SSBs and double-strand breaks. We found that EM9 cells were significantly more sensitive than parental AA8 cells to IdUrd alone and to IdUrd + ionizing radiation. The EM9 cells also demonstrate increased DNA damage after IdUrd treatment as evaluated by pulse field gel electrophoresis and single cell gel electrophoresis (Comet Assay). BER-competent EM9 cells, which were stably transfected with a cosmid vector carrying the human XRCC1 gene, showed responses to IdUrd similar to AA8 cells. We also assessed the role of methoxyamine, a small molecule inhibitor of BER, in the response of human colon cancer cells (HCT116) to IdUrd cytotoxicity and radiosensitization. Methoxyamine not only was able to increase IdUrd cytotoxicity but also increased the incorporation of IdUrd into DNA of HCT116 human colon cancer cells leading to greater radiosensitization. Thus, a genetic or biochemical impairment of BER results in increased IdUrd-induced cytotoxicity and radiosensitization in mammalian cells.