Radiolabelled leukocytes for imaging inflammation: how radiochemistry affects clinical use

Indium-111((111)In)-labelled leukocytes were introduced for imaging inflammation about 25 years ago. A few years later methods to label leukocytes with Technetium-99m ((99m)Tc) were developed, but the two radiolabels cannot be used interchangeably. The amount of radioactivity which can be administered with (111)In is low, because of its 67-h half-life and associated radiation dose. This results in low count density in images. However, (111)In labelling is very stable, with binding to intracellular macromolecules and particulates, and there is minimal urinary or faecal excretion. In contrast, (99m)Tc has a half-life of 6 h and can be administered in higher doses, resulting in improved image quality. However, (99m)Tc labelling is less stable because the trapped form is soluble and there is excretion of (99m)Tc through both the kidneys and intestine, which limits imaging of disease in the abdomen except at early times. There is interest in extending inflammation imaging to PET. Although leukocytes can be labelled with (18)F-FDG, its half-life and stability are not optimal and radiometals such as Copper-64 are being evaluated. Despite the laborious nature of leukocyte labelling, it has yet to be replaced by direct injection agents.