Dicarboxylate diamide dimercaptide (N2S2) technetium-99m complexes: synthesis and biological evaluation as potential...

Novel diamide dimercaptide (N2S2) ligands 4, 5, and 8 have been synthesized and evaluated as potential renal radiopharmaceuticals. The target compounds were prepared in modest overall yields of 22%, 19%, and 20%, respectively, using readily available starting materials. Following in situ deprotection, 99mTc complexes of high radiochemical purity were obtained in excellent yield and were found to be stable for up to 6 h. The 99Tc complex of ligand 8 was isolated as the AsPh4 salt. The X-ray crystallographic data for [99TcO(8)]AsPh4 (space group P2(1)/n: Z = 4, a = 9.342(3) A; b = 18.594(5) A; c = 18.417(7) A; beta, deg = 90.61(3); V, A3 = 3199.1(20)) show that the Tc is bound to both thiolate sulfur atoms and to two deprotonated amide nitrogen atoms. The coordination geometry about the Tc is square-pyramidal with an -yl oxygen atom in the apical position. The Tc-N bond distances (2.002(12) and 1.984(12) A), the Tc-S bond distances (2.300(5) and 2.286(5) A), and the Tc-O bond distance (1.667(11) A) are in good agreement with bond lengths reported for similar complexes. The carboxylate groups are not bonded to the Tc atom in the solid state, nor in CDCl3 solution, as evidenced by X-ray crystal data and solution NMR data, respectively. In the solid state, [99TcO(8)]AsPh4 is monoanionic, therefore, at physiological pH, [99mTcO(8)] is presumably trianionic. Biodistribution studies performed in rats with the 99mTc complexes revealed slow blood clearance and high muscle uptake for these agents. Modest hepatobiliary excretion was observed, and low quantities of the complexes were found in the heart, lungs, and spleen after 1 h. The urinary excretion of the 99mTc complexes of ligands 4, 5, and 8 was found to be slow when compared to the excretion of [131I]OIH in rats (22%, 22%, and 32% vs 85-86%, respectively). Protein binding of 99mTc complexes of ligands 4, 5, and 8 in both rat and monkey plasma was found to be similar to MAG3. While the synthetic schemes reported here supply facile routes to novel N2S2 ligands, biodistribution studies of the 99mTc complexes performed on rats revealed slow renal excretion rates, accompanied by slow blood clearance and high uptake in muscle tissue. Preliminary planar imaging studies in monkeys also revealed slow renal excretion for these agents. The 99mTc complexes evaluated here are poor candidates as renal radiopharmaceuticals.