Applicability of the 2-nitroimidazole-sodium borocaptate-B-10 conjugate, TX-2060, as a B-10-carrier in boron neutron capture therapy

Background: It is difficult to deliver a therapeutic amount of B-10 from conventional B-10-carriers for boron neutron capture therapy (BNCT) throughout the target tumors, especially into the intratumor hypoxic cells which have low uptake capacities. We evaluated the usefulness of 5 new B-10-compounds (TX-2041, TX-2042, TX-2058, TX-2059 and TX-2060) as B-10-carriers in BNCT They are 2-nitroimidazole-sodium borocaptate-B-10 (BSH) conjugates, that is, hybrid compounds that have both a hypoxic tumor cell sensitizing unit under gamma-ray irradiation, 2-nitroimidazoles and a thermal neutron-sensitizing unit, BSH. Materials and Methods: The 5 new compounds were administered to SCC VII tumor-beating mice intrapetitoneal. As a control, BSH was also administered in the same manner. Then, the B-10 concentrations in the tumors and normal tissues were measured by gamma-ray spectrometry. Based on the data of the pharmacokinetics analyses, TX-2060 was chosen for a subsequent tumor-irradiation study. SCC VII tumor-beating mice were continuously given 5-bromo-2-deoxyuridine (BrdU) to label all proliferating (P) cells in the tumors, then treated with TX-2060 or BSH in the same manner as in the pharmacokinetics analyses. To obtain similar intratumor B-10 concentrations during radiation exposure, irradiation with thermal neutrons or gamma-rays was started from 60 min after administration of the B-10-carrier. Right 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-labelling (= quiescent (Q) cells) was determined using immunofluorescence staining for BrdU Meanwhile, the MN frequency in total (P + Q) tumor cells was determined from the tumors that were not pretreated with BrdU The clonogenic cell survival was also determined in mice given no BrdU Results: B-10 distribution analyses in tumors, muscles, blood and liver indicated that TX-2060 has the most favorable characteristics for concentrating a sufficient amount of B-10 in tumors and maintaining a high enough B-10 concentration during irradiation. In addition, TX-2060 had a significantly stronger radio-sensitization effect with reactor thermal neutron beams than BSH on both total and Q cells in solid tumors. Further, TX-2060 clearly exhibited a radio-sensitization effect with gamma-rays, not only on total cells but also on Q and hypoxic tumor cells, which was not achieved by BSH. Conclusion: B-10-carrier, with a gamma-ray-sensitizing effect on tumor cells as well as the potential to keep B-10 in tumors and sensitize tumor cells more markedly than conventional B-10-carriers, such as TX-2060, is a promising candidate for use in BNCT.