The Combined Effect of Electroporation and Borocaptate in Boron Neutron Capture Therapy for Murine Solid Tumors

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Abstract
10B‐Enriched borocaptate (BSH) was administered intraperitoneally to SCCVII tumor‐bearing C3H/He mice. Electroporation (EP) was conducted by using a tweezers‐type electrode. The 10B contents in tumors were measured by prompt γ‐ray spectrometry. The colony formation assay was applied to investigate the antitumor effects of boron neutron capture therapy (BNCT) and thereby to estimate the intratumor localization of BSH. The 10B concentrations in tumors decreased with time following BSH administration, falling to 5.4(±0.1) ppm at 3 h, whereas EP treatment (3 repetitions) 15 min after BSH injection delayed the clearance of BSH from tumors, and the 10B level remained at 19.4(±0.9) ppm at 3 h. The effect of BNCT increased with the 10B concentration in tumors, and the combination with EP showed a remarkably large cell killing effect even at 3 h after BSH injection. The effect of BNCT, i.e., slope coefficient of the cell survival curve of tumors, without EP was proportional to tumor 10B level (r=0.982), and that of BSH‐BNCT combined with EP lay close to the same correlation line. However, tumors subjected to EP after BSH injection did not show high radiosensitivity when irradiated after conversion to a single cell suspension by enzymatic digestion. This indicates that the increase of the BNCT effect by EP was a consequence of enclosure of BSH in the interstitial space of tumor tissue and not within tumor cells. This is different from a previous in vitro study. The combination of EP and BNCT may be clinically useful, if a procedure to limit EP to the tumor region becomes available or if an alternative similar method is employed.

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