Chemical and biological characterization of different Tc complexes of cysteine and cysteine derivatives.

The labeling of cysteine and its derivatives (penicillamine, N-acetylcysteine, cysteine ethyl ester) with 99Tc (99mTc) was studied as a model for the radiopharmaceutical preparation of Tc-99m mercaptide complexes. After the use of TcO4-, TcOCl52-, and TcBr62- as labeling agents for the Tc oxidation states VII, V, and IV, respectively, complexes of Tc(V) and Tc(IV) were prepared and characterized. The biologic behavior of these complexes was studied in rats. The substitutions in cysteine are responsible for substantial changes of net charge and lipophilicity in the Tc complexes, and the consequences on the excretion patterns in Wistar rats are discussed.