Evaluation of hypoxia-specific cytotoxic bioreductive agent-sodium borocaptate-10B conjugates as 10B-carriers in boron neutron capture therapy

Purpose To evaluate the usefulness of 5 new¹⁰B-compounds (TX-2091, TX-2095, TX-2097, TX-2100, and TX-2110) as¹⁰B-carriers in boron neutron capture therpy (BNCT). They were conjugates that had been synthesized from a hypoxia-specific cytotoxic bioreductive agent, quinoxaline oxide TX-402 and a clinically used¹⁰B-carrier, sodium borocaptate-¹⁰B (BSH). Materials and Methods The 5 new compounds were hybrid compounds that have both a hypoxic cytotoxin unit and a thermal neutron-sensitizing unit, BSH. These new compounds and BSH were administered intraperitoneally to SCC VII tumor-bearing mice. Then, the¹⁰B concentrations in the tumors and normal tissues were measured by γ-ray spectrometry. Subsequently, SCC VII tumor-bearing mice were continuously given 5-bromo-2′-deoxyuridine (BrdU) to label all proliferating (P) cells in the tumors, then treated with TX-2100, which was chosen based on the results of the above-mentioned biodistribution analyses, or BSH in the same manner as in the biodistribution studies. Right after irradiation, during which intratumor¹⁰B concentrations were kept at levels similar to each other, 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. Meanwhile, the MN frequency in the total (P+Q) tumor cell population was determined from the tumors that were not pretreated with BrdU. Clonogenic cell survival was also determined in mice given no BrdU. Results ¹⁰B biodistribution analyses in tumors, brain, skin, muscles, blood, and liver indicated that TX-2100 has the most favorable characteristics for concentrating a sufficient amount of¹⁰B in tumors and maintaining a high enough¹⁰B concentration during irradiation. In addition, TX-2100 had a significantly stronger radio-sensitizing effect with reactor thermal neutron beams than BSH on both total and Q cells in solid tumors. Further, TX-2100 clearly exhibited a radio-sensitizing effect with γ-rays not only on total cells but also on Q and hypoxic tumor cells, which was not achieved by BSH. Conclusion A¹⁰B-carrier that acts as a hypoxic cytotoxin on tumor cells as well as having the potential to keep¹⁰B in tumors and sensitize tumor cells more markedly than conventional¹⁰B-carriers, such as TX-2100, is a promising candidate for use in BNCT. A part of this paper was presented at the 64th JRS annual meeting in Yokohama on April 8, 2005.