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This study confirmed the effect of sodium/iodine symporter (NIS) expression on existing drugs by in vitro and in vivo tests using cultured cell lines. The tumor growth inhibitory effect of sodium astatide ([²¹¹At]NaAt) was evaluated by in vitro and in vivo tests using human thyroid cancer cells (K1, K1/NIS and K1/NIS-DOX). NIS expression in cancer cells was controlled using the Tet-On system. [¹³¹I]NaI was used as control existing drug. From the results of the in vitro studies, the mechanism of [²¹¹At]NaAt uptake into thyroid cancer cells is mediated by NIS, analogous to [¹³¹I]NaI, and the cellular uptake rate correlates with the expression level of NIS. [²¹¹At]NaAt’s ability to inhibit colony formation was more than 10 times that of [¹³¹I]NaI per becquerel (Bq), and [²¹¹At]NaAt’s DNA double-strand breaking (DSB) induction was more than ten times that of [¹³¹I]NaI per Bq, and [²¹¹At]NaAt was more than three times more cytotoxic than [¹³¹I]NaI (at 1000 kBq each). In vivo studies also showed that the tumor growth inhibitory effect of [²¹¹At]NaAt depended on NIS expression and was more than six times that of [¹³¹I]NaI per Bq.