## 3 Chemical Approaches to Bioactive Molecule Labelling with Technetium-99m
### The \({}^{99m}\)Tc(V)- Hydrazido Metal Fragment  
Metalorganohydrazine chemistry is an alternative approach to the design of radiopharmaceuticalesticals based on a stable and substitution inert \({}^{99m}\)Tc-metal fragment [43]. \({}^{99m}\)Tc- hydrazido metal fragment is formed by a technetium-HYNIC (HYNIC = 6-hydrazinonicotinamide) core composed of the metal in the V oxidation state and occupying the ligand one or two coordination sites. HYNIC ligands are particularly convenient for radiolabelling biomolecules, with \({}^{99m}\)Tc being the active ester most readily combined with small molecules, proteins, or different targeting vectors. However, even if the metal-organohydrazino unit can be easily prepared from the metal-oxo core by a simple condensation reaction, the intimate details of the chemistry are more complex and dependent upon reaction conditions and the presence of co-ligands, which can influence the stability, hydrophilicity, and pharmacokinetics of the resulting \({}^{99m}\)Tc-compounds [44].  
A large number of co-ligands have been tested, including ethylenediaminediacetic acid (EDDA), tricine, glucoheptonate, mannitol, thiolates, glucamine, and phosphines forming different binary and ternary systems [45]. The EDDA co-ligand is in general preferred to tricine when preparing \({}^{99m}\)Tc-HYNIC complexes with higher stability in vivo and fewer isomeric forms. Unfortunately, while many efforts have been made to completely establish the identities of these complexes, the chemistry remains unclear. The dependence of the coordination chemistry on the reaction condition of this technology has led to a certain difficulty in the development of \({}^{99m}\)Tc-HYNIC radiopharmaceuticals; moreover, the increasing regulators' requirements of providing fully characterized products does not seem to have promoted HYNIC inclusion in clinical practice. Nevertheless, a wide range of bioactive molecules have been labelled with the HYNIC strategy, among which the most investigated are undoubtedly the somatostatin derivatives for neuroendocrine tumour (NETs) imaging. Most published data concern the commercially available \({}^{99m}\)Tc-[HYNIC, Tyr(3)]octreotide (\({}^{99m}\)Tc-HYNIC-TOC, Figure 7) prepared through a two-vial kit formulation containing EDDA as a co-ligand [46; 47; 48].  
The complex is formed by a technetium atom bound to a ligand composed of two functional parts, and a HYNIC group linked to an octapeptide. The EDDA ancillary ligand is required to stabilize the coordination geometry. \({}^{99m}\)Tc-HYNIC-TOC represents a selective receptor imaging agent for neuroendocrine tumours and, even if the true molecular structure of \({}^{99m}\)Tc-HYNIC-TOC is not exactly determined, its biological properties can be understood by considering the bioactive group  
Figure 7: The figure illustrates the chemical structure of the complex \({}^{99m}\)Tc-HYNIC-TOC, an imaging agent for neuroendocrine tumours.  
ocreotide and selectively targeting the somatostatin receptors (SSR) overexpressed on the membranes of neuroendocrine tumours.  
This tracer showed greater sensitivity compared to \({}^{111}\)In-ocreotide, such as specific and high receptor affinity, good biodistribution, faster renal excretion, lower radiation exposure, and high imaging quality as well as the superior capability of \({}^{99\text{m}}\)Tc-TOC to visualize extrahepatic lesions [49].