Current advances in ligand design for inorganic positron emission tomography tracers 68Ga, 64Cu, 89Zr and 44Sc.

A key part of the development of metal based Positron Emission Tomography probes is the chelation of the radiometal. In this review the recent developments in the chelation of four positron emitting radiometals, 68Ga, 64Cu, 89Zr and 44Sc, are explored. The factors that effect the chelation of each radio metal and the ideal ligand system will be discussed with regards to high in vivo stability, complexation conditions, conjugation to targeting motifs and complexation kinetics. A series of cyclic, cross-bridged and acyclic ligands will be discussed, such as CP256 which forms stable complexes with 68Ga under mild conditions and PCB-TE2A which has been shown to form a highly stable complex with 64Cu. 89Zr and 44Sc have seen significant development in recent years with a number of chelates being applied to each metal - eight coordinate di-macrocyclic terephthalamide ligands were found to rapidly produce more stable complexes with 89Zr than the widely used DFO.