Current use and future potential of organometallic radiopharmaceuticals

Contrary to common belief, organometallic compounds exhibit remarkable stability in aerobic and even diluted aqueous solutions. Technetium-sestamibi (Cardiolite) is one of the most prominent examples of this class of compounds routinely used in nuclear medicine. This review summarises the recent progress in labelling of biomolecules with organometallic complexes for diagnostic and therapeutic application in radiopharmacy and exemplifies in detail developments focussing on organometallic technetium- and rhenium-tricarbonyl technologies. The value of such technologies has been recognised and they have become a valuable alternative to common labelling methodologies. An increasing number of groups have started to employ an organometallic precursor for the purpose of radioactive labelling of various classes of biomolecules, and the advantages and limitations of this new technique are compared with those of other labelling methods. The synthetic access to appropriate precursors via double-ligand exchange or aqueous carbonyl kit preparation for routine application is described. Strategies and examples for the design of appropriate bifunctional chelating agents for the Tc/Re-tricarbonyl core are given. The functionalization of biomolecules such as tracers for the central nervous system (dopaminergic and serotonergic), tumour affine peptides (somatostatin receptors, neuroreceptors) and tumour binding single-chain antibody fragments is summarised. Where possible and appropriate, the in vitro and in vivo results in respect of these examples are compared with those obtained with classical (99m)Tc/(188)Re(V)- and (111)In-labelled analogues. The preclinical results show the in many ways superior characteristics of organometallic labelling techniques.