Radiobromine-labelled tracers for positron emission tomography: possibilities and pitfalls

The use of positron emission tomography (PET) for radionuclide imaging provides better sensitivity, better spatial and temporal resolution and better quantification accuracy in comparison with single photon emission computed tomography (SPECT). One limitation of PET is the predominant use of short-lived (with half-life up to 2 h) radionuclides. Extension of PET utility might be achieved by the use of more long-lived, "non-conventional" positron emitters. Two positron-emitting isotopes of bromine, 75Br (T1/2 = 96.7 min) and 76Br (T1/2 = 16.2 h), can be considered as labels for targeting proteins and peptides, and for small molecules, which have an optimal imaging time outside the time frame provided by conventional biogenic positron emitters. Variety of tracers might be labelled by electrophilic bromination of activated phenolic rings, electrophilic bromodestannylation and halogen exchange. A major problem is that in vivo metabolism of tracers might lead to formation of radiobromide as a main radiocatabolite. Radiobromide is very slowly excreted, and is distributed in the extracellular space creating high background. Careful tracer design optimisation is required to avoid this obstacle in the introduction of bromine isotopes into PET practice.