## 3 Chemical Approaches to Bioactive Molecule Labelling with Technetium-99m
### The [99 mTc][Tc(CO)\({}_{3}\)]+ Metal Fragment  
The core [99 mTc][Tc(CO)\({}_{3}\)]+ is a chemically robust organometallic moiety formed by technetium in the I oxidation state coordinated to three carbonyl groups. The precursor [99 mTc][Tc(CO)\({}_{3}\)(H\({}_{2}\)O)\({}_{3}\)]+ can be readily prepared from the generator-eluted pertechnate under the reducing conditions developed by [PERSON] et al. [56; 57; 58]. A large number of different Tc(I) complexes can be prepared starting from this stable water-soluble synthon, because the water ligands can be easily replaced by a variety of donor groups [59; 60]. High yields of the [99 mTc][Tc(CO)\({}_{3}\)(H\({}_{2}\)O)\({}_{3}\)]+ metal fragment are obtained in a single-step procedure in the presence of the appropriate buffers, via the reduction of [99 mTc]TcO\({}_{4}\)- by potassium boranocarbonate K\({}_{2}\)[BH\({}_{3}\)CO\({}_{2}\)], which also acts as a source of carbonyl ligands. With the aim of making this labelling chemistry readily accessible, a kit-based formulation for producing [99 mTc][Tc(CO)\({}_{3}\)(H\({}_{2}\)O)\({}_{3}\)]+ synthon is available under the name IsoLink. When the metal has a d\({}^{6}\) low spin configuration, the complexes containing the [99 mTc][Tc(I)(CO)\({}_{3}\)]+ core are typically inert and maintain their integrity under different reaction conditions. While the robustness of complexes is purely kinetic, essentially all types of donor atoms have been used and numerous examples of bidentate and tridentate chelators have been reported, including derivatives that can be linked to targeting molecules [61]. As opposed to the traditional [99 mTc][V)-oxo approach, where the choice of ligand is quite limited, for [99 mTc][Tc(I)(CO)\({}_{3}\)]+ almost any monodentate, bidentate, ot tridentate donor ligands can be used. Among them, anionic bidentate ligands are more efficient than neutral ones, and tridentate ligands react much faster than bidentate ones, providing the additional advantage of shielding the metal centre from cross-reactions, as often occurs for the [99 mTc][V]+ and [99 mTc][CO)\({}_{3}\)Cl(_{B}\)]+ complexes (\(B\), bidentate ligand). The most studied tridentate ligands are aliphatic triamines, trithiacyclonane, bis-benzimidazole amine, pyrazolyl borates, and histidine derivatives, which can be conveniently modified to provide complexes with an anionic, cationic, or neutral overall charge and specific pharmacokinetic profiles to achieve the desired biodistribution when the [99 mTc][Tc(I)(CO)\({}_{3}\)]+ is incorporated into targeting vectors. Of these ligands, the histidine (his) derivative can be easily connected to biomolecules through the introduction of an acetic acid group at N\({}^{\epsilon}\) in histidine to yield a N\({}^{\alpha}\) carboxylate histidine ready to be condensed to a selected biomolecule. The use of histidine has the additional advantage of not being sensitive to reduction and the resulting [99 mTc][Tc(CO)\({}_{3}\)(his)] can be prepared in a single reaction step from [99 mTc]Tc-pertechnente. The Metal Fragments Approach in the Development of New [99 mTc]Tc-Based Radiopharmaceuticals for Prostate Cancer  
Prostate-specific membrane antigen (PSMA), a glutamate carboxopeptidase enzyme overexpressed in 95% of advanced prostate cancer (PCA) cases, is a molecular target for the imaging and radionuclide therapy of PCa using specific radiopharmaceuticals. In particular, the inhibition of its activity has been attributed to the urea-based Lys(b-naphthyl alanine)-NH-CO-NH-Glu(Lys(Nal)-Urea-Glu fragment, which, interacting electrostatically with the active peptide sites of the enzyme, has been shown to image advantageously PSMA-expressing in PCa [62]. Furthermore, when the expression levels in healthy tissue are low, PSMA has the potential for high-dose radiotherapeutic treatment, with minimized radioactivity-related side effects.