Comparison of in vivo Efficacy of Hypoxic Cytotoxin Tirapazamine and Hypoxic Cell Radiosensitizer KU‐2285 in Combination with Single and Fractionated Irradiation

Development of strategies to eradicate radioresistant hypoxic cells would be of great benefit for clinical radiotherapy. In the present study, the in vivo effects of a promising hypoxic cytotoxin, tirapazamine (3‐amino‐1,2,4‐benzotriazine 1,4‐di‐N‐oxide), were examined in comparison with those of KU‐2285, one of the best hypoxic cell radiosensitizers, in combination with both single and fractionated irradiation. The tumor response was assessed by the standard in vivo‐in vitro clonogenic assay using SCCVII tumors in C3H mice and EMT‐6/KU tumors in Balb/c mice with different characteristics of tumor hypoxia. With single‐dose irradiation (18 Gy), both tirapazamine and KU‐2285 showed significant enhancement of cell killing in a dose‐dependent manner, but tirapazamine was more effective for SCCVII tumors with acutely hypoxic cells, while KU‐2285 was more effective for EMT‐6/KU tumors predominantly with chronically hypoxic cells. In fractionated irradiation regimens (4 fractions of 5 Gy at 12 h intervals), tirapazamine showed more marked combined effects at 10 and 20 mg/kg than KU2285 at 100–200 mg/kg in both SCCVII and EMT‐6/KU tumors. We concluded that the effectiveness of KU‐2285 and tirapazamine was correlated with the nature of tumor hypoxia with single‐dose irradiation, whereas tirapazamine appeared more potent than KU‐2285 with fractionated irradiation. These findings suggest the potential usefulness of tirapazamine in clinical fractionated radiotherapy.