Effect of Tirapazamine and Mild Temperature Hyperthermia on the Recovery from Radiation-Induced Damage in Pimonidazole-Unlabeled Quiescent Tumor Cell Population

Shin-Ichiro Masunaga, Yoshinori Sakurai, Hiroki Tanaka, Minoru Suzuki, Natsuko Kondo, Masaru Narabayashi, Keizo Tano, Akira Maruhashi, Koji Ono

Radiation Life and Medical Science Research Division, Research Reactor Institute, Kyoto University, Kyoto, Japan.
Radiation Life and Medical Science Research Division, Research Reactor Institute, Kyoto University, Kyoto, Japan..

DOI: 10.4236/jct.2013.42065                

Abstract

The aim in this study is to examine the effect of tirapazamine (TPZ) and mild temperature hyperthermia (MTH) on the repair of radiation-induced damage in pimonidazole-unlabeled quiescent (Q) tumor cells. Labeling of proliferating (P) cells in C57BL/6J mice bearing EL4 tumors was achieved by continuous administration of 5-bromo-2-deoxyuridine (BrdU). Tumors were irradiated with γ-rays at 1 h after the administration of pimonidazole followed by TPZ treatment or MTH. Twenty-four hours later, assessment of the responses of Q and total (= P + Q) cells were based on the frequencies of micronucleation and apoptosis using immunofluorescence staining for BrdU. The response of the pimonidazole-unlabeled tumor cell fractions was assessed by means of apoptosis frequency using immunofluorescence staining for pimonidazole. With γ-rays only, the pimonidazole-unlabeled cell fraction showed significantly enhanced radio-sensitivity compared with the whole cell fraction more remarkably in Q cells than total cells. However, a significantly greater decrease in radio-sensitivity in the pimonidazole-unlabeled than the whole cell fraction, evaluated using a delayed assay, was more clearly observed in Q cells than total cells. Post-irradiation MTH more remarkably repressed the decrease in radio-sensitivity in the Q cell than the total cells. Post-irradiation TPZ administration produced a large radio-sensitizing effect on both total and Q cells, especially on Q cells. On the other hand, in pimonidazole-unlabeled cell fractions in both total and Q cells, TPZ suppressed the reduction in sensitivity due to delayed assay much more efficiently than MTH, whereas no radio-sensitizing effect was produced. Not only through suppressing the recovery from radiation-induced damage but also through radio-sensitizing effect, post-irradiation TPZ administration is very useful for repressing the increase in the difference in radio-sensitivity due to the delayed assay not only between total and Q tumor cells but also between the pimonidazole-unlabeled and the whole cell fractions within the total and Q tumor cells.

Keywords

Quiescent Cell; Recovery from Radiation-Induced Damage; Tirapazamine; Pimonidazole; Mild Temperature Hyperthermia

Conflicts of Interest

The authors declare no conflicts of interest.

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