Technetium essential radiopharmaceuticals

The Class II formulations are the so called technetium essential radiopharmaceuticals whose biodistribution depends not only on the nature of the substrate to be labelled, but also on the presence of "mTc itself. Some examples of the major types of "mTc radiopharmaceutical agents in use, or under investigation, are summarized in Table 22. The nature of the "mTc binding site... 

Fig. 2 The three fundamental building blocks and precursor complexes in organometallic radiopharmaceuticals in red the sites which can be substituted by targeting molecules (second generation) or ligands for technetium essential radiopharmaceuticals (first generation)...

Perfusion agents are the most commonly used radiopharmaceuticals containing Tc. They are also referred to as first-generation radiopharmaceuticals or technetium-essential radiopharmaceuticals. As described in the preceding chapters, they have been developed over the past decades with great success and are in manifold clinical use for the diagnosis of, e.g., renal and heart function, cerebral blood flow, and many others. The radiopharmaceutical is "Tc essential since it would obviously not work without technetium The pharmaceutical has to be considered as an entity in which the Tc-center plays the role of an important structure and... 

Metal-essential radiopharmaceuticals are those whose targeting properties are inherent in the structure and chemistry of the complex itself rather than a biomolecule whose targeting ability is independent of the attachment of the metal. While this class of tracers includes many well-developed technetium complexes, it includes relatively few rhenium complexes. 

Radiopharmaceutical formulations based on "mTc may be divided into two major classes.635 The Class I formulations include what may be described as technetium tagged materials. These may be particles such as cells or colloidal substances, or they may be molecular species such as proteins or other large biological molecules. The essential feature of these agents is that their biodistribution behaviour is determined by the nature of the substrate material. Thus the attachment of a "mTc label should have little or no effect on their biodistribution. However, it cannot necessarily be assumed that this will be the case, especially when relatively low molecular weight substrates are involved. 

Finally, while the new synthetic methods for technetium compounds in the oxidation states I, II and V will undoubtedly play an important role in the development of improved radioscintigraphic agents, there remains a need to understand the chemical mechanisms involved in these syntheses, particularly with regard to ligand substitution reactions and atom transfer redox processes. Mechanistic studies to complement the increasing body of structural knowledge is essential to the further development of technetium radiopharmaceuticals. 

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