Usefulness of Tirapazamine as a Combined Agent in Chemoradiation and Thermo‐chemoradiation Therapy at Mild Temperatures: Reference to the Effect on Intratumor Quiescent Cells

C3H/He mice bearing SCC VII tumors received 5‐bromo‐2′‐deoxyuridine (BrdU) continuously for 5 days via implanted mini‐osmotic pumps to label all proliferating (P) cells. The mice then received one of six different DNA‐damaging agents with or without mild temperature hyperthermia (40°C, 30 min, MTH). These agents were adriamycin (ADM), mitomycin C (MMC), cyclophosphamide (CPA), bleomycin (BLM), cisplatin (CDDP), and tirapazamine (TPZ). After the drug treatment, the tumor‐bearing mice were irradiated with a series of doses of γ‐rays. Immediately after irradiation, the tumors were excised, minced and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin‐B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (=quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P+Q) tumor cells was determined from the tumors that had not been pretreated with BrdU. MTH significantly increased the MN frequency of total cells in tumors irradiated with γ‐rays combined with CPA, BLM, CDDP or TPZ, and that of Q cells in tumors irradiated with γ‐rays combined with BLM or TPZ. The sensitivity difference in the MN frequency between total and Q tumor cells was significantly decreased by the combination with TPZ. TPZ combined with radiotherapy and TPZ combined with thermo‐radiotherapy at mild temperatures appear to be promising modalities for sensitizing tumor cells in vivo, including Q tumor cells.