Co, Fe and Ga chelates for cell labelling: a potential use in PET imaging?

The radiolabelling of blood cellular elements with PET radionuclides offers a higher sensitivity and resolution than conventional imaging, but short-lived PET radionuclides have limited clinical use in cell labelling. Medium half-life PET radionuclides, such as 55Co (t1/2 = 18.2 h), 52Fe (t1/2 = 8.3 h) and 66Ga (t1/2 = 9.4 h), enable quantitative uptake and cell kinetic studies with radiolabelled blood cellular elements. Co(II) oxine, Co(III) tropolonate, Fe(III) oxine, Ga(III) oxine and Ga(III) MPO were prepared using gamma-emitting radionuclides and a number of cell labelling parameters were investigated. The uptake of 57Co(II) oxine into erythrocytes was only 37%, and 65% of the activity eluted from the cells in cell-free plasma within 30 min. In contrast, high leukocyte and erythrocyte labelling efficiencies (> 90%) were obtained with 57Co(III) tropolonate containing cobalt carrier and the elution in cell-free plasma over 4 h was < 8%. High labelling efficiencies were also observed with 59Fe(III) oxine and 67Ga-MPO and the elution from leukocytes over 4 h was < 25%. We conclude that Co(III) tropolonate, Fe(III) oxine and Ga-MPO may be useful for radiolabelling leukocytes for PET investigations.