The usefulness of mild temperature hyperthermia combined with continuous tirapazamine administration under reduced dose–rate irradiation with γ-rays

Purpose: We clarified the usefulness of mild temperature hyperthermia (MTH) in combination with the continuous administration of tirapazamine (TPZ) under reduced dose-rate irradiation (RDRI) using γ-rays. Materials and Methods: SCC VII tumour-bearing mice received a continuous administration of 5-bromo-2′-deoxyuridine (BrdU) to label all proliferating (P) cells. Then, they received a 24 h continuous subcutaneous infusion of TPZ either with or without MTH under high dose-rate irradiation (HDRI) or RDRI using γ-rays. After the irradiation, the tumour cells were isolated and incubated with a cytokinesis blocker, and the micronucleus (MN) frequency in non-proliferating tumour cells without BrdU labeling (= quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. The MN frequency in the total tumour cell populations was determined using tumours that were not pretreated with BrdU. Results: The sensitivity of both the total and Q cell populations, especially the latter, was significantly reduced with RDRI compared with HDRI. TPZ increased the sensitivity of both populations, with a slightly more remarkable increase in Q cells. Further, MTH combined with TPZ raised the sensitivity of both the total and Q cell populations, especially the latter, under RDRI more markedly than under HDRI. Conclusion: From the viewpoint of solid tumour control as a whole, including intratumour Q-cell control, the use of TPZ, especially in combination with MTH, is useful for suppressing the reduction in the sensitivity of tumour cells caused by the decrease in irradiation dose rate in vivo.