## 3 Chemical Approaches to Bioactive Molecule Labelling with Technetium-99m
### The \({}^{99m}\)Tc(V)-Oxo Core  
The \({}^{99\text{m}}\)Tc(V)-oxo core represents the most extensively studied metal fragment. Complexes based on this core are generally pentacoordinated, adopting square pyramidal geometry with the \(\pi\)-bonding oxo-group in the apical position. Due to the strong _trans_ influence of the oxo-group, six coordinated technetium-oxo compounds are relatively uncommon. The oxo-core is stabilized by \(\sigma\)- and \(\pi\)-donating atoms coming from amino, amido, and thiolate ligands, as well as the tetradentate ligands of the N4-xSx class [29; 30; 31; 32]. These ligands are extremely efficient at forming more stable \({}^{99\text{m}}\)Tc-oxo complexes compared with those derived from bidentate ligands. These compounds can be prepared through the direct reduction of the \({}^{99\text{m}}\)Tc-pertechnate anion in the presence of tin chloride, which is the ubiquitous preliminary step in the preparation of conventional \({}^{99\text{m}}\)Tc-radiopharmaceuticals, or via a ligand exchange reaction with [\({}^{99\text{m}}\)Tc]Tc-glucoheptonate (Figure 5). Figure 4: Schematic representation of the metal fragment strategy for bioactive molecule labelling. Figure 3: Inorganic technetium functional groups useful for labelling bioactive molecules. HYNIC=6-hydrazinonicotinamide. A representative example is provided by the complex technetium-99m N,N-1,2-ethylene diylbis-L-cysteine diethyl ester dihydrochloride (_l_-_[\({}^{99\text{m}}\)Tc]Tc-ECD), a brain perfusion imaging agent on the market with the name Neurolite\({}^{\text{\textregistered}}\). The complex is neutral and formed by multiple Tc=O bonds coordinated by a tetradentate ligand with two nitrogen and two sulphur donor atoms in a square/pyramidal arrangement. The high symmetry makes the complex hydrophobic. Because of its lipophilicity, it can cross the blood-brain barrier (BBB). The mechanism of brain localization is based on the transformation by esterase enzymes of one of the lateral ester groups (-COOEt) in a carboxylic group (-COOH), with the subsequent formation of a negatively charged more hydrophilic compound that is unable to cross the BBB and thus is trapped. It is important to note that only \({}^{99\text{m}}\)Tc-ECD having a \(l\),\(l\) configuration undergoes ester hydrolysis, making this compound a true metabolic marker of the brain's esterase enzymes. Mixed aminothi-based chelators, most notably N\({}_{2}\)S\({}_{2}\) [e.g., bis(aminoethanediol) (BAT)] and N\({}_{3}\)S [e.g., mercaptoacetylglyglyglyglyglycine (MAG\({}_{3}\))], have been used to label a selected bioactive molecule with technetium-99m [33; 34; 35; 36; 37]. These tetradentate chelating systems allow us to produce more stable Tc(V)-oxo complexes in comparison with those produced with similar bidentate chelators. Representative examples from this class of compounds are \({}^{99\text{m}}\)Tc-TRODAT-1, a probe for the diagnosis of [PERSON]'s disease, and \({}^{99\text{m}}\)Tc-Deproeotide (P829), a Tc(V)-oxo somatostatin analogue complex [38; 39; 40; 41; 42]. TRODAT-1 (Figure 6) is currently considered a useful imaging agent for the early detection of [PERSON]'s disease, where a phenyltropanepharmacophore is used as a targeting vector for specific monoamine transporter proteins, and the N\({}_{2}\)S\({}_{2}\) chelate system, connected to the tropane molecule, has been introduced to coordinate the technetium-oxo core without affecting the affinity for the biological target. Likewise, a tripeptide chain provides a N\({}_{3}\)S binding group for (Tc=O)\({}^{3+}\) core in the \({}^{99\text{m}}\)Tc-Deproetide radiopharmaceutical (Figure 6) and the bioactive part of the original octreeotide structure has been modified to eliminate the disulphide bridge, which would be reduced during the formation of the (Tc=O)\({}^{3+}\) core from the generator-eluted \({}^{99\text{m}}\)Tc-perpetchnetate. Figure 5: Reaction scheme of the preparation of the technetium-99m N,N-1,2-ethylene diylbis-L-cysteine diethyl ester dihydrochloride from \({}^{99\text{m}}\)Tc-perpetechnate.