## 3 Chemical Approaches to Bioactive Molecule Labelling with Technetium-99m
### The [99 mTc][Tc(CO)\({}_{3}\)]+ Metal Fragment
Recent studies on [177]Lu-PSMA-617 have demonstrated a high tumour to normal tissue uptake, with prolonged retention of radionuclides in tumour-bearing areas in men with metastatic prostate cancer [63; 64; 65; 66]. Before therapeutic treatment, the imaging of radiopharmaceutical uptake in tumours or metastases is essential and can be performed by positron emitter or single-photon emitter compounds. For these two categories of radiopharmaceuticals, \(\beta^{+}\) particle emitter [68]Ga]Ga-DOTA derivatives for PET imaging have been developed [67; 68], while in the case of \(\gamma\)-emitter compounds, different \({}^{99\mathrm{m}}\)Tc-radiopharmaceuticals have been reported for SPECT PCa imaging. In this last case, the possibility of choosing from different synthesis strategies to ensure robust radiolabelling has been the driving force in the development of compounds with improved biodistribution properties. Based on the chemistry of the organometallic fragment [\({}^{99\mathrm{m}}\)Tc][Tc(CO)\({}_{3}\)(H\({}_{2}\)O)\({}_{3}\)]\({}^{+}\), two promising compounds, MIP-1404 and MIP-1405, both based on an imidazole modification of the PSMA inhibitor, were developed (Figure 9). The preparation of the metal complexes [\({}^{99\mathrm{m}}\)Tc][Tc(CO)\({}_{3}\)-MIP-1404, MIP-1405] was accomplished using a standard methodology and commercially available IsoLink kits (Covidien, Dublin, Ireland) and the imidazole chelator, which contains three nitrogen atom suitable for binding to the \({}^{99\mathrm{m}}\)Tc(I)-tricarbonyl-core. The radiochemical yield of the final radiopharmaceuticals is only 70% and post-labelling purification is needed [69]. Both compounds were used to visualize tumours in PCa and metastases in lymph nodes and bones. In particular, the MIP-1404 compound, also known as [\({}^{99\mathrm{m}}\)Tc]Tc-trofolateat, has demonstrated superior biodistribution properties with low uptake in the kidney and more lesions than MIP-1405 in a phase II clinical trial [58]. The different pharmacokinetics has been attributed to the carboxymethyl group attached to the imidazole. In the MIP-1405 compound there is only a terminal carboxymethyl group on the imidazole moiety, while MIP-1404 has a biscarboxymethyl amino-2-oxoethyl group linked to each imidazole (see Figure 9). The technology of \({}^{99\mathrm{m}}\)Tc-hydrazinonicotinyl was also applied in the development of a PSMA inhibitor for \({}^{99\mathrm{m}}\)Tc-based SPECT. [PERSON] et al. [70] reported in vitro and in vivo studies of \({}^{99\mathrm{m}}\)Tc-EDDA-HYNIC-iPSMA demonstrating that this radiopharmaceutical could detect tumour and metastases of PCa as well as \({}^{68}\)Ga-PSMA-617. \({}^{99\mathrm{m}}\)Tc-EDDA-HYNIC-iPSMA was quickly prepared from a kit with radiochemical yield \(>\)95%. Recently, a new \({}^{99\mathrm{m}}\)Tc radiopharmaceutical, known as \({}^{99\mathrm{m}}\)Tc-PSMA-I&S (I&S = investigation and surgery), is available for SPECT imaging and radio-guided surgery (RGS) for clinical application in urology [71]. This compound can be produced through a robust and reliable freeze-dried kit, facilitating the on-site production. The radiopharmaceutical is based on the molecule DOTAGA-(3-iodo-y)-f-k-Sub(KuE), in which the DOTA chelator is present to coordinate indium-111. Due to the limitations associated with the high cost, poor availability, and non-ideal nuclear properties of this radionuclide, a technetium-99m analogue has been proposed. The DOTA chelator, specifically used for indium-111 labelling, has been replaced with the hydrophilic MAG\({}_{3}\)- analogue MAS\({}_{3}\) (mercaptoethanol triserina), with the aim of stably binding the technetium-99m in the V oxidation state (Figure 10).