Complexation of 99, 99m Tc(CO) 3 + organometallic fragment with 2 + 1 chelating systems was studied. HPLC analysis showed that the dithiocarbamate complexes of 99m Tc are stronger than their xanthate, acetylacetonate, and C-methoxycarbonylmethylacetylacetonate analogs. To block the third vacancy in the technetium coordination sphere, we studied thiols, thioethers, primary and secondary amines, hydroxyl ion, imidazole, phosphines, and isocyanides. As determined by 99m Tc NMR spectroscopy, the mixed-ligand complexes Tc(CO) 3 (DTC)L (DTC is dithiocarbamate and L is isocyanide, imidazole, and phosphine) are the most stable to the histidine challenge reaction. HPLC analysis showed that, among these 2 + 1 systems, only the DTC!isonitrile system forms a single complex with 99m Tc(CO) 3 + . The minimal dithiocarbamate!isocyanide concentration providing complete binding of 99m Tc(CO) 3 + is 10 !4 M.The organometallic fragment M(CO) 3 + (M = Tc, Re) attracts growing interest of nuclear medical scientists. This single-charged small fragment has three coordination vacancies (unlike the conventional highvalence technetium and rhenium compounds currently used in the clinical practice) occupied by weak electron donors [e.g., water molecules: M(CO) 3 (H 2 O) 3 + ]. The remaining three coordination sites are occupied by CO groups in the stable fac-configuration. These properties make the tricarbonyl complex promising for preparing so-called third generation of radiopharmaceuticals labeled with 188 Re (therapy) and 99m Tc (diagnostics). In this connection, one of the urgent problems is a search for appropriate chelating systems for the M(CO) 3 + fragment. The chelating core should exhibit the following properties: (1) it should show high affinity for the organometallic fragment, (2) its complex with M(CO) 3 + fragment should be thermodynamically stable and kinetically resistant to ligand exchange in biological media, (3) the complex should not contain labile ligands displaceable by coordinating fragments of protein molecules, and (4) the ligand concentration providing complete binding of [hot] technetium and rhenium should not exceed 10 !5 3 10 !4 M. The latter requirement is the most critical for labeling receptor ligands (steroids, small peptides, etc.). At high ligand concentration, the receptor vacancies can be occupied by free ligands, thus decreasing selective accumulation of the isotope in the target organ.At the moment, tridentate chelators are most frequently used for tethering the M(CO) 3 + fragment to biomolecules [1,2]. Although these ligands meet all the above requirements, the procedure of their conjugation with biomolecules requires multistep organic synthesis. In this connection, bidentate chelators are more convenient. However, their complexes with M(CO) 3 + are unstable in vitro and in vivo, except for complexes with diphosphine ligands [4].So-called [n + 1] approach is extensively used for labeling biomolecules with high-valence technetium and rhenium. In this case, a combination of polydentate and monodentat...
