Development of Rhenium Radiopharmaceuticals from Coordination Chemistry View Point

Favourable nuclear properties of radiorhenium (¹⁸⁸Re and ¹⁸⁶Re) are of interest in the development of new radiotherapeutic and molecular imaging agents. A fundamental critical component of a radio-metal based radiopharmaceutical is the chelator, the ligand system that binds the radio metal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. The design of a robust Re conjugate can be achieved through a careful selection of the most stable rhenium cores like [Re≡N]²⁺ and [Re(CO)₃]⁺ that usually are strongly resistant to hydrolysis by water molecules. The chemistry of rhenium has distinct parallels with its group 7 congener technetium in respect of radiopharmaceutical designing and the resulting species are expected to exhibit similar "in vivo" biological behaviour. The labelling of biologically active molecules to generate affinity of radiorhenium complexes for a specific biological target can be achieved by 'bifunctional approach' or by 'metal fragment' strategies. This review highlights the role of coordination chemistry in the design of rhenium radiopharmaceuticals engineered to selectively target tissue of interest such as cancer cells. A range of rhenium complexes with different ligands with rhenium in different oxidation states are introduced and their potential to be translated to new radiotherapeutic agents discussed.