Evaluation of hypoxia-specific cytotoxic bioreductive agent-sodium borocaptate-10B conjugates as 10B-carriers in boron neutron capture therapy.
OBJECTIVE
To evaluate the usefulness of 5 new 10B-compounds (TX-2091, TX-2095, TX-2097, TX-2100, and TX-2110) as 10B-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 used 10B-carrier, sodium borocaptate-10B (BSH).


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, the 10B concentrations in the tumors and normal tissues were measured by gamma-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 intratumor 10B 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 of 10B in tumors and maintaining a high enough 10B 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 gamma-rays not only on total cells but also on Q and hypoxic tumor cells, which was not achieved by BSH.


CONCLUSIONS
A 10B-carrier that acts as a hypoxic cytotoxin on tumor cells as well as having the potential to keep 10B in tumors and sensitize tumor cells more markedly than conventional 10B-carriers, such as TX-2100, is a promising candidate for use in BNCT.
Masunaga
Shin-Ichiro
S
Radiation Oncology Research Laboratory, Research Reactor Institute, Kyoto University, 2-1010, Asashiro-nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan.
Nagasawa
Hideko
H
Gotoh
Keiko
K
Sakurai
Yoshinori
Y
Uto
Yoshihiro
Y
Hori
Hitoshi
H
Nagata
Kenji
K
Suzuki
Minoru
M
Maruhashi
Akira
A
Kinashi
Yuko
Y
Ono
Koji
K
D000818:Animals; D001894:Borohydrides / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology*; D001895:Boron / Q000737:chemistry; D016754:Boron Neutron Capture Therapy; D002294:Carcinoma, Squamous Cell / Q000532:radiotherapy*; D019084:Fluorescent Antibody Technique, Indirect; D007554:Isotopes; D051379:Mice; D015394:Molecular Structure; D009570:Nitriles / Q000138:chemical synthesis / Q000737:chemistry; D009593:Nitroimidazoles / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology*; D011810:Quinoxalines / Q000138:chemical synthesis / Q000737:chemistry; D013438:Sulfhydryl Compounds / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology*; D014018:Tissue Distribution
D001894:Borohydrides; D007554:Isotopes; D009570:Nitriles; D009593:Nitroimidazoles; D011810:Quinoxalines; D013438:Sulfhydryl Compounds; D001895:Boron