Oxidation states of technetium in diphosphonate complexes

Valence states of technetium in two most prominent bone imaging agents. i.e., their model complexes with 99Tc (99Tc-MDP and 99Tc-DPD) were determined by redox potentiometric titration. Titrations of 99Tc-pertechnetate by Sn(II) solutions were performed in the presence of a ligand (MDP or DPD), as well as without them. Tc(III) was obtained in complexes with MDP and DPD in strongly alkaline medium (pH 12-13) while in the absence of the ligands Tc(IV) was formed. In strongly acidic medium (pH 2-3) the redox process resulted Tc(IV) in both complexes, while Tc(III) was formed in the absence of the ligands. At pH values near neutral (important for radiopharmaceutical complexes 99mTc-MDP and 99mTc-DPD), it was possible to perform titrations in the ligand absence only at pH values from 7 to 11.8 (due to in hydrolysis) while the resulting oxidation state of Tc in the presence of ligand depended on the used ligand: Tc(IV)-MDP (pH 5.4-5.9 and pH 7.0-7.4), and Tc(III)-DPD (pH 8.0). (author) 12 refs.; 3 figs.; 1 tab