Zn Complex of Diaminedithiol Tetradentate Ligand as a Stable Precursor for 99mTc-Labeled Compounds

The diaminedithiol (N2S2) tetradentate ligand constitutes a useful chelating molecule for preparing 99mTc-labeled compounds of high in vivo stability in high radiochemical yields. However, since the thiol groups in the N2S2 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 N2S2 ligand for radiopharmaceuticals. In this study, a Zn chelate of N2S2 was evaluated as a precursor for purification-free 99mTc-labeled N2S2 under the mild and simple procedure. Zn-N2S2 was prepared by reacting Zn acetate with N2S2, and the Zn-N2S2 remained stable under aerobic conditions at room temperature. 99mTc-N2S2 was obtained over 90% radiochemical yields at room temperature by a one-pot reaction, consisting of Zn-N2S2 (10−5 M), 99mTcO4−, ethylenediaminetetraacetic acid (EDTA), and a reducing agent (Sn2+) at pH = 5.5 to 7.5. 99mTc-N2S2 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 N2S2 ligand constitutes a stable and useful precursor to prepare 99mTc-labeled N2S2 compounds in high yields under the mild and simple procedure