The usefulness of mild temperature hyperthermia combined with a newly developed hypoxia-oriented ¹⁰B conjugate compound, TX-2100, for boron neutron capture therapy

Abstract

Purpose: To evaluate the usefulness of a new ¹⁰B-compound (TX-2100) as a ¹⁰B-carrier in boron neutron capture therapy (BNCT), compared with the simultaneous use of its component drugs, sodium borocapate-¹⁰B (BSH) and 3-amino-2-quinoxalinecarbonitrile 1,4-dioxide (TX-402). Further, the usefulness of mild temperature hyperthermia (MTH, 40°C, 30 min) combined with TX-2100 was also examined compared with MTH combined with the concurrent administration with its component drugs.

Materials and methods: TX-2100 is a hybrid compound that has both a hypoxic cytotoxin unit (TX-402) and a thermal neutron-sensitizing unit (BSH). TX-2100 or both TX-402 plus BSH in combination with MTH or not was administered to SCC VII tumour-bearing mice intra-peritoneally. Then, the ¹⁰B concentrations in the tumours and normal tissues were measured by γ-ray spectrometry. Meanwhile, SCC VII tumour-bearing mice were continuously given 5-bromo-2′-deoxyuridine (BrdU) to label all proliferating (P) cells in the tumours, then treated with TX-2100, TX-402 plus BSH or BSH only, in the same manner as in the biodistribution experiments, either with or without MTH. Right after thermal neutron irradiation during which intra-tumour ¹⁰B concentrations remained at similar levels, the tumours were excised, minced and trypsinized. The tumour 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 the total (P + Q) tumour cell population was determined from the tumours that were not pre-treated with BrdU. The clonogenic cell survival was also determined in mice given no BrdU.

Results: ¹⁰B biodistribution analyses in tumours, brain, skin, muscles, blood and liver indicated that the administration of TX-2100 plus MTH is most favourable for concentrating a sufficient amount of ¹⁰B in tumours and maintaining a high enough ¹⁰B concentration during irradiation. In addition, MTH had a stronger sensitizing effect when combined with TX-2100 than with the concurrent administration of its components TX-402 and BSH on both the total and Q cell populations in solid tumours.

Conclusion: MTH was very effective in combination with the newly-developed TX-2100. The sensitizing effect in combination with MTH should be examined when new ¹⁰B-carriers are designed.

• Boron neutron capture therapy
• sodium borocaptate-¹⁰B
• hypoxic cytotoxin
• quiescent cell
• thermal neutrons