99mTc(I) tricarbonyl complexes with N-methylamides of picolinic and thiopicolinic acids

Tricarbonyl complexes of technetium(I) and rhenium(I) were introduced to radiopharmaceutical chemistry by Alberto et al. who elaborated an easy method for no-carrier-added level (n.c.a.) synthesis of their precursors, fac-[99mTc(CO)3(H2O)3]+ (1) and its rhenium analog. Numerous advantages of tricarbonyltechnetium(I) and analogous tricarbonylrhenium(I) cores in radiopharmaceuticals, in particular kinetic inertness of the central metal ions with d6 electronic configuration, well justify a great research interest in these complexes, especially in those obtained by substitution of a bi- or tridentate chelating ligand for two or three labile water molecules in 1. This interest was manifested e.g. by a significant number of 27 relevant presentations at the recent conference on metal complexes in nuclear medicine. Due to the softness (HSAB concept) of Tc(I) and Re(I) centres, chelators with soft donor atoms are preferred as their ligands. Widely studied in this respect were bi- and tridentate derivatives of pyridine and/or imidazole (aromatic N-donors) with other donor atoms, in particular nitrogen and/or sulphur. The aim of our studies is to find novel ligands that form very stable tricarbonyl complexes of Tc(I) and Re(I), which after further functionalization of the ligand can work as precursors of novel radiopharmaceuticals of the 2nd generation. (orig.)