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Accumulation of ¹⁰B in the Central Degenerative Areas of Human Glioma and Colon Carcinoma Spheroids after Sulfhydryl Boron Hydride Administration¹

Division of Physical Biology, Department of Radiation Sciences, Uppsala University, Box 535, S-751 21 Uppsala, Sweden

Sulfhydryl boron hydride (BSH) (¹⁰B enriched) is presently used for boron neutron capture therapy of malignant gliomas. BSH must be close to the target cells to be effective in the inactivation of cell proliferation because of the short range of the reaction products (5–9 µm). Clinical experience indicates that BSH is taken up in gliomas but it is not known to which structures it binds at the cellular level. In vitro tests on monolayer cultured cells have indicated that BSH does not bind, or only shows very weak binding, to single isolated cells. It is possible that BSH accumulates in tumor regions due to the special conditions in poorly vascularized tumor tissue, such as low pO₂, low extracellular pH, metabolic gradients, and degenerative changes. To test this we incubated three types of multicellular tumor spheroids with BSH for different times and analyzed both penetration and binding. The spatial distribution of ¹⁰B in sections of the spheroids was analyzed by neutron capture autoradiography. We found extensive accumulation of ¹⁰B in the central regions of both glioma and colon carcinoma spheroids. The accumulation closely followed the pattern of the degenerative changes which were characterized by massive necrosis in the central regions of the colon carcinoma spheroids and by a continuously increasing frequency of pyknotic nuclei as a function of depth in the glioma spheroids. The accumulation of ¹⁰B in the prostatic carcinoma spheroids was much lower. The penetration assay, based on freeze-drying and vapor fixation, showed that BSH penetrated easily since ¹⁰B equilibrated within 5–15 min in the studied spheroids. Thus, the low accumulation in the prostatic carcinoma spheroids was not due to penetration difficulties. The results of the present study on cellular spheroids and the results from previous studies on transplanted tumors support the observation that BSH penetrates easily into the degenerative tumor areas and that ¹⁰B, for some tumor types, might accumulate in these regions as a result of the BSH administration.

¹ The work was financially supported by three grants from the Swedish Cancer Society: 980-B90-02 XB; 3009-B91-01XAB; and 1176-B91-05XAB.

² To whom requests for reprints should be addressed.

Received 9/ 9/91. Accepted 1/ 7/92.