Modulation of radiation response by inhibiting topoisomerase II catalytic activity

Due to the essential role played by DNA topoisomerases (topos) in cell survival, the use of topoisomerase inhibitors as chemotherapeutic drugs in combination with radiation has become a common strategy for the treatment of cancer. Catalytic inhibitors of these enzymes would be promising to improve the effectiveness of radiation and therefore, it appears reasonable to incorporate them in combined modality trials. In this work, we have investigated the capacity of both ICRF-193 and Aclarubicin (ACLA), two catalytic inhibitors of topoisomerase II (Topo II), to modulate radiation response in Chinese hamster V79 cell line and its radiosensitive mutant irs2. We also have explored potential mechanisms underlying these interactions. Experiments were performed in the presence and absence of either ICRF-193 or ACLA, and topo II activity was measured using an assay based upon decatenation of kinetoplast DNA (kDNA). For the combined experiments cells were incubated for 3 h in the presence of various inhibitor concentrations and irradiated 30 min prior to the end of treatments and cell survival was determined by clonogenic assay. DNA-damaging activity was measured by single-cell gel electrophoresis. Our results demonstrate that combinations of catalytic inhibitors of topo II and radiation produce an increase in cell killing induced by ionising radiation. The mechanism of radiation enhancement may involve a direct or indirect participation of topo II in the repair of radiation-induced DNA damage.