Effects of Bioreductive Agents, Tirapazamine and Mitomycin C, on Quiescent Cell Populations in Solid Tumors, Evaluated by Micronucleus Assay

Mice bearing transplantable solid tumors received 10 intraperitoneal administrations of 5‐bromo‐2′‐deoxyuridine (BrdU) to label the proliferating (P) tumor cells, and were then irradiated with (60)Cogamma‐rays or injected with cis‐diamminedichloroplatinum (II) (cisplatin). The tumor cells were isolated and incubated with cytochalasin‐B (a cytokinesis blocker). The micronucleus (MN) frequency in the cells without BrdU labeling, which were regarded as quiescent (Q) cells in the tumor, was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P+Q) tumor cell population was determined from tumors that were not pretreated with BrdU. Pretreatment with tirapazamine, a bioreductive agent, could enhance the sensitivity of tumor cells, including Q cells, to radiation more markedly than mitomycin C pretreatment as judged from an in vivo assay immediately after irradiation. Post‐irradiation administration of tirapazamine produced a large post‐irradiation radiosensitizing effect on both the total and Q tumor cell populations in vivo. Cisplatin treatment combined with tirapazamine demonstrated that tirapazamine also has a chemosensitizing potential for both the total and Q tumor cell populations. We confirmed that the sensitivity of Q cell populations to radiation and chemotherapy using cisplatin can be enhanced by combined treatment with tirapazamine.