Applicability of Combination with Tirapazamine in Boron Neutron Capture Therapy

SCC VII tumor‐bearing mice were continuously given 5‐bromo‐2′‐deoxyuridine (BrdU) to label all proliferating cells. After injection of tirapazamine (TPZ), a bioreductive agent, combined with sodium borocaptate‐(10)B (BSH) or dl‐p‐boronophenylalanine‐(10)B (BPA) administration, the tumors were irradiated with thermal neutrons, and then isolated and incubated with cytochalasin‐B (a cytokinesis blocker). The micronucleus (MN) frequency in cells without BrdU labeling (quiescent (Q) cells) was determined by means of immunofluorescence staining for BrdU, and that for total cells was obtained from tumors not pretreated with BrdU. Even when no (10)B‐compound was administered, TPZ increased the MN frequency of tumor cells including Q cells, resulting in reduction of the difference in MN frequency between total and Q cells, mainly by increasing the MN frequency of Q cells. TPZ increased the MN frequency of Q cells when combined with BPA administration, but TPZ showed no apparent effect on each cell population when combined with BSH. Namely, TPZ reduced the difference in MN frequency between total and Q cells caused by (10)B‐compound administration, especially when BPA was administered. From the viewpoint of the overall cell killing effect in boron neutron capture therapy (BNCT), combination with TPZ appeared to be useful in BPA‐BNCT, but not in BSH‐BNCT.