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A New Electrophoretic Method for Determining Ligand: Technetium Stoichiometry in Carrier Free ^99mTc-Radiopharmaceuticals

Johns Hopkins University, Baltimore, Maryland

Correspondence: For reprints contact: H. Donald Burns, Division of Nuclear Medicine, The Johns Hopkins Medical Institutions, 615 N. Wolfe St., Baltimore, MD 21205.

ABSTRACT

An electrophoretic procedure is outlined for the determination of the number of ligands bound to technetium-99m radiopharmaceuticals. The approach involves use of ligands that will complex technetium in a similar fashion but that differ in charge. This approach was applied experimentally to dimercapto ligands in which the ligating sulfur atoms are separated by a flexible three-carbon chain (1,3-dimercapto compounds). Two such ligands studied are 1,3-dimercaptopropane (DMP) and dihydrothioctic acid (DHTA). The Tc compound of DHTA migrates much farther on electrophoresis than the Tc complex of DMP. However, when TcO₄^– is reduced by SnCl₂ or NaBH₄ in the presence of equimolar quantities of DHTA and DMP, a new compound is formed being twice as abundant as either the TcDMP or the TcDHTA compound and migrating an intermediate distance. The formation of this new complex and the 1:2:1 distribution indicates that two 1,3-dimercapto compounds are attached to the Tc-center in all three compounds.