A new bifunctional tridentate NSN ligand leading to cationic tricarbonyl fac-[M(NSN)(CO)3]+ (M = Re, 99mTc) complexes

The synthesis and characterization of the new NSN tridentate bifunctional chelating agent 3-[2-(2′-pyridin-2-yl-ethylsulfanyl)ethylamino] propionic acid (as its hydrochloric salt, 1) and of its corresponding rhenium complex fac-[Re(NSN)(CO)3]+, 2, is described. Both compounds were characterized by elemental analysis, IR and NMR spectroscopies and X-ray crystallography. In complex 2 the coordination geometry around rhenium is distorted octahedral with the NSN atoms participating in the coordination sphere while the carboxylate group remains free. At tracer level, the analogous complex fac-[99mTc(NSN)(CO)3]+, 3, was obtained in high yield by reacting ligand 1 with the fac-[99mTc(H2O)3(CO)3]+ precursor at pH 6.5. The structure of 3 was established by HPLC comparison to the prototype rhenium complex 2. Complex 3 is stable in solution as well as in the presence of strongly coordinating agents like histidine or cysteine. Our data indicate that ligand 1 can be used as a bifunctional NSN chelating agent in the design of potential 99mTc-radiopharmaceuticals.