Technetium-99m labeling of a molecule bearing easily reducible groups

Chemical stability of the naphthol-azo dye Evans Blue (EB) was examined in the presence of acidic stannous chloride (SnCl₂), with a view to preparing an instant cold kit. EB was found to be reactive toward this reducing agent, yielding the metal-chelating molecule 1,7-diamino-8-naphthol-2,4-disulfonic acid at high acidity and high stannous concentrations. This reduction reaction was undesirable in the cold kit preparation. The conditions were determined where reduction was inhibited, at pH = 5.0 and with a mole ratio of EB to SnCl₂ = [10:1], effecting the facile preparation of stable cold kits. Successful ^99mTc-labeling of an EB cold kit using these conditions resulted in the desired product with 98% radiochemical purity. Based on the radiolabeling efficiencies of chosen model compounds, it was rationalized that ^99mTc metal predominantly coordinated with the 1-amino-8-hydroxy groups in the EB molecule to form ^99mTc-EB.