## 3 Chemical Approaches to Bioactive Molecule Labelling with Technetium-99m  
In the last decay, the labelling of targeting vectors has become the main object of modern radiopharmaceutical research. Technetium is a transition metal and, as such, presents a major disadvantage with respect to other radionuclides in combining with biologically active molecules. For example, \({}^{99m}\)Tc cannot be a substitute for a carbon or a hydrogen atom in a targeting molecule, as happens for the labelling with carbon-11, or fluorine-18 and iodine-123 respectively. The development of technetium imaging agents requires both a familiarity with the coordination chemistry of the group 7 metals and an understanding of the design of suitable ligands that provide robust molecular imaging probes. Deep knowledge of inorganic chemistry allows for developing convenient approaches to introduce stable technetium-99m into a bioactive molecule with the aim of not affecting its bioactivity. A number of inorganic technetium functional groups, also called \"cores\" or \"metal fragments,\" have been identified so far (Figure 3). These groups are chemical motifs comprising a characteristic arrangement of atoms bonded to the metallic centre and determining the formation of a variety of coordination complexes and molecular geometries. They can be prepared in physiological solution, have technetium  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reactors** & **Countries** & **Six-Day GBq/Mo-99 GBq/year** & **Start of Service** & **End of Service** \\ \hline
**LVR-15** & Czech Republic & 3330000 & 1957 & 2028 \\
**MARIA** & Poland & 3515000 & 1974 & 2030 \\
**OPAL** & Australia & 3420650 & 2007 & 2055 \\
**HRR** & The Netherlands & 8946600 & 1961 & 2024 \\
**BR2** & Belgium & 6060600 & 1961 & \textgreater{}2026 \\
**RA-3** & Argentina & 680800 & 1967 & 2027 \\
**SAFARI-1** & South Africa & 4835900 & 1965 & 2030 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Nuclear reactors involved in \({}^{99}\)Mo supply for generator medical devices.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Reactor Based** & **Accelerator Based** \\ \hline LEU-235U(n,f)\({}^{99}\)Mo & \({}^{238}\)U(\(\gamma\),f)\({}^{99}\)Mo \\ nat\({}^{98}\)Mo(n,\(\gamma\))\({}^{99}\)Mo & \({}^{96}\)Zr(\(\alpha\),n)\({}^{99}\)Mo \\  & \({}^{100}\)Mo(\(\gamma\),\(n\))\({}^{99}\)Mo \\  & \({}^{100}\)Mo(p,pn)\({}^{99}\)Mo \\  & \({}^{100}\)Mo(p,2n)\({}^{99m}\)Tc \\ \hline \hline \end{tabular}
\end{table}
Table 2: Tc-99m alternative production routes.  
in a reduced oxidation state, and have labile coordination positions, which can be conveniently exploited to introduce the selected bioactive molecule. As previously described, technetium-99m, whether produced by a \({}^{99}\)Mo/\({}^{99\text{m}}\)Tc generator or by a cyclotron, is obtained as [\({}^{99\text{m}}\)Tc]NaTo\({}_{4}\) in a physiological solution. Consequently, in order to link the radionuclide to a bioactive molecule, the Tc(VII) metal must be reduced to a suitable oxidation state in the presence of appropriate ligands.  
The metal fragments reported in Figure 3, in combination with an appropriate set of coordinating atoms, provide an effective strategy for tethering a biologically active moiety with a technetium-99m complex. The metal fragment strategy mainly involves a two-component system composed of (1) the radioactive metal fragment and (2) an appropriate chelating group eventually bound to the selected bioactive molecule through a spacer group. The high affinity of the precursor metal fragment for the specific binding sites on the bioactive ligand allows for obtaining a conjugate complex resulting from the fitting of the two carefully selected molecular building blocks (Figure 4).  
In the following section, descriptions of complexes containing the most important technetium functional groups are given. Some of these complexes are also examples of \({}^{99\text{m}}\)Tc-radiopharmaceuticals that have been employed in clinical studies or are currently used as diagnostic agents in clinical practice.