| Summary: | The diaminedithiol (N<sub>2</sub>S<sub>2</sub>) tetradentate ligand constitutes a useful chelating molecule for preparing <sup>99m</sup>Tc-labeled compounds of high in vivo stability in high radiochemical yields. However, since the thiol groups in the N<sub>2</sub>S<sub>2</sub> ligand are easy to be oxidized to disulfide bonds, they need to be protected with an appropriate protecting group, which hinders the broad applications of the N<sub>2</sub>S<sub>2</sub> ligand for radiopharmaceuticals. In this study, a Zn chelate of N<sub>2</sub>S<sub>2</sub> was evaluated as a precursor for purification-free <sup>99m</sup>Tc-labeled N<sub>2</sub>S<sub>2</sub> under the mild and simple procedure. Zn-N<sub>2</sub>S<sub>2</sub> was prepared by reacting Zn acetate with N<sub>2</sub>S<sub>2</sub>, and the Zn-N<sub>2</sub>S<sub>2</sub> remained stable under aerobic conditions at room temperature. <sup>99m</sup>Tc-N<sub>2</sub>S<sub>2</sub> was obtained over 90% radiochemical yields at room temperature by a one-pot reaction, consisting of Zn-N<sub>2</sub>S<sub>2</sub> (10<sup>−5</sup> M), <sup>99m</sup>TcO<sub>4</sub><sup>−</sup>, ethylenediaminetetraacetic acid (EDTA), and a reducing agent (Sn<sup>2+</sup>) at pH = 5.5 to 7.5. <sup>99m</sup>Tc-N<sub>2</sub>S<sub>2</sub> was also obtained over 90% radiochemical yields when the reaction was conducted in the presence of an equimolar amount of IgG antibody. These findings indicate the Zn complex of N<sub>2</sub>S<sub>2</sub> ligand constitutes a stable and useful precursor to prepare <sup>99m</sup>Tc-labeled N<sub>2</sub>S<sub>2</sub> compounds in high yields under the mild and simple procedure.
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