Radiolabeling Human Peripheral Blood Stem Cells for Positron Emission Tomography (PET) Imaging in Young Rhesus Monkeys

These studies focused on a new radiolabeling technique with copper ( 64 Cu) and zirconium ( 89 Zr) for positron emission tomography (PET) imaging using a CD45 antibody. Synthesis of 64 Cu-CD45 and 89 Zr-CD45 immunoconjugates was performed and the evaluation of the potential toxicity of radiolabeling human peripheral blood stem cells (hPBSC) was assessed in vitro (viability, population doubling times, colony forming units). hPBSC viability was maintained as the dose of 64 Cu-TETA-CD45 increased from 0 (92%) to 160 µCi/mL (76%, p >0.05). Radiolabeling efficiency was not significantly increased with concentrations of 64 Cu-TETA-CD45 >20 µCi/mL ( p >0.50). Toxicity affecting both growth and colony formation was observed with hPBSC radiolabeled with ≥40 µCi/mL ( p <0.05). For 89 Zr, there were no significant differences in viability ( p >0.05), and a trend towards increased radiolabeling efficiency was noted as the dose of 89 Zr-Df-CD45 increased, with a greater level of radiolabeling with 160 µCi/mL compared to 0–40 µCi/mL ( p <0.05). A greater than 2,000 fold-increase in the level of 89 Zr-Df-CD45 labeling efficiency was observed when compared to 64 Cu-TETA-CD45. Similar to 64 Cu-TETA-CD45, toxicity was noted when hPBSC were radiolabeled with ≥40 µCi/mL ( p <0.05) (growth, colony formation). Taken together, 20 µCi/mL resulted in the highest level of radiolabeling efficiency without altering cell function. Young rhesus monkeys that had been transplanted prenatally with 25×10 6 hPBSC expressing firefly luciferase were assessed with bioluminescence imaging (BLI), then 0.3 mCi of 89 Zr-Df-CD45, which showed the best radiolabeling efficiency, was injected intravenously for PET imaging. Results suggest that 89 Zr-Df-CD45 was able to identify engrafted hPBSC in the same locations identified by BLI, although the background was high.