Technetium and Rhenium Tricarbonyl Core

As mentioned above, [M(OH2)3(CO)3] can be considered as a normal aqua ion with only three available coordination sites. Substitution of the water molecules with almost any type of chelator (classic/nonclassic) forms kinetically stable coordination compounds. This holds true for mono-, bi-, and tridentate ligands, regardless of their hardness or softness. This behavior also represents the distinct feature of [M(OH2)3(CO)3] as compared with other technetium and rhenium metal centers, and is one of its major advantages for the labeling of molecules for imaging and therapeutic purposes. 

Valliant and Zubieta developed BFCAs for the Tc(CO)3 core, based on a lysine backbone comprising pyridyl, imidazole, thiolate, carboxylate groups, etc., for the specific purpose of conjugation to small peptides by solid-phase synthetic methods (Banerjee et 

The tricarbonyl technology not only provides new opportunities with respect to the use of exotic ligand systems, hut it also opens perspectives for novel labeling strategies. 

The group at the Paul Scherrer Institute and Pliickthun et al. have successfully developed a direct labeling protocol employing [ Tc(OH2)3(CO)3] for scFvs and mini-an-tihodies (hi- and trivalent constructs of scFv) carrying an N- or a C-terminal His-tag (Deyev et al. 2003 Waihel et al. 2000 Willuda et al. 1999, 2001). The method is particu- 

Zubieta and coworkers have recently taken advantage of the fluorescent and luminescent properties of organometallic complexes of rhenium (and technetium) comprising certain aromatic ligand systems (Fig. 2.2.6). The nonradioactive Re(CO)3 bioconjugates with a formyl peptide receptor-targeting peptide (flVlLF), enabeled visualization 

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The number of technetium and rhenium tricarbonyl compounds in preclinical evaluation is remarkable. These efforts comprise small molecules as well as macromolecules useful in diagnostic and/or therapeutic nuclear medicine. There are also clinical data available with tumor affine peptides such as neurotensin receptor- and somatostatin receptor-targeting peptides radiolabeled with the Tc(CO)3 core. 

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