Establishment of radioactive astatine and iodine uptake in cancer cell lines expressing the human sodium/iodide symporter

The sodium/iodide symporter (NIS) has been recognized as an attractive target for radioiodine-mediated cancer gene therapy. In this study we investigated the role of human NIS for cellular uptake of the high LET α-emitter astatine-211 (211At) in comparison with radioiodine as a potential radionuclide for future applications. A mammalian NIS expression vector was constructed and used to generate six stable NIS-expressing cancer cell lines (three derived from thyroid carcinoma, two from colon carcinoma, one from glioblastoma). Compared with the respective control cell lines, steady state radionuclide uptake of NIS-expressing cell lines increased up to 350-fold for iodine-123 (123I), 340-fold for technetium-99m pertechnetate (99mTcO4-) and 60-fold for 211At. Cellular 211At accumulation was found to be dependent on extracellular Na+ ions and displayed a similar sensitivity towards sodium perchlorate inhibition as radioiodide and 99mTcO4- uptake. Heterologous competition with unlabelled NaI decreased NIS-mediated 211At uptake to levels of NIS-negative control cells. Following uptake both radioiodide and 211At were rapidly (apparent t1/2 3-15 min) released by the cells as determined by wash-out experiments. Data of scintigraphic tumour imaging in a xenograft nude mice model of transplanted NIS-modified thyroid cells indicated that radionuclide uptake in NIS-expressing tumours was up to 70 times (123I), 25 times (99mTcO4-) and 10 times (211At) higher than in control tumours or normal tissues except stomach (3-5 times) and thyroid gland (5-10 times). Thirty-four percent and 14% of the administered activity of 123I and 211At, respectively, was found in NIS tumours by region of interest analysis (n=2). Compared with cell culture experiments, the effective half-life in vivo was greatly prolonged (6.5 h for 123I, 5.2 h for 211At) and preliminary dosimetric calculations indicate high tumour absorbed doses (3.5 Gy/MBqtumour for 131I and 50.3 Gy/MBqtumour for 211At). In conclusion, NIS-expressing tumour cell lines of different origin displayed specific radionuclide uptake in vitro and in vivo. We provide first direct evidence that the high-energy α-emitter 211At is efficiently transported by NIS. Application of 211At may direct higher radiation doses to experimental tumours than those calculated for 131I. Thus, 211At may represent a promising alternative radionuclide for future NIS-based tumour therapy. (orig.)