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Abstract: Thioredoxin reductase (TrxR) is critical for cellular redox regulation and is involved in tumor proliferation, apoptosis and metastasis. Its C-terminal redox-active center contains a cysteine (Cys497) and a unique selenocysteine (Sec498), which are exposed to solvent and easily accessible. Thus, it is becoming an important target for anticancer drugs. Selective inhibition of TrxR by 1,2-(bis-1,2-benzisoselenazol-3(2H)-one)ethane (4a) prevents proliferation of several cancer cell lines both in vivo and in vitro. Using the structure of 4a as a starting point, a series of novel bis-1,2-benzisoselenazol-3(2H)-ones was designed, prepared and tested to explore the structure–activity relationships (SARs) for this class of inhibitor and to improve their potency. Notably, 1,2-(5,5′-dimethoxybis(1,2-benzisoselenazol-3(2H)-one))ethane (12) was found to be more potent than 4a in both in vitro and in vivo evaluation. Its binding sites were confirmed by biotin-conjugated iodoacetamide assay and a SAR model was generated to guide further structural modification. [Copyright &y& Elsevier]