Gallium 67 in the human lymphoid cell line U-715: uptake, cytotoxicity and intracellular localization

The selective uptake of the Auger- and internal conversion electrons emitting radionuclide 67Ga in malignant tumours may have therapeutic potential. We studied several factors which might affect the uptake and radiotoxicity of 67Ga in the human lymphoma cell line U-715. The 67Ga uptake was dependent on transferrin in a dose-dependent manner. The highest 67Ga uptake was found in the presence of 50 micrograms/ml purified human transferrin. Serum components other than transferrin negatively influenced the 67Ga uptake. Cells were adapted to a serum-free medium in which cells could be maintained for months and without factors disturbing 67Ga uptake. We demonstrated that there was a positive correlation between cell viability and 67Ga uptake (r = 0.97). Preculturing of cells in iron- and transferrin-deficient medium prior to 67Ga uptake led to upregulation of the transferrin receptor and a three-fold increase of 67Ga uptake. Uptake in these cells could be blocked by 72% by anti-transferrin-receptor monoclonal antibodies. Autoradiography of U-715 cells after 67Ga incubation showed intracellular 67Ga both in the cytoplasm and nucleus. Cell fractionation of 67Ga-loaded cells showed 27% of 67Ga present in the nuclei. Culturing of cells for 4 days in the presence of 3 MBq/ml 67Ga resulted in a 45% decrease of cell proliferation. The clonogenic capacity was diminished by 91%. In conclusion, we have demonstrated that 67Ga uptake is a transferrin-receptor-dependent mechanism of vital cells, and that after uptake 67Ga enters the cytosol and nucleus and has a strong cytotoxic effect on clonogenic capacity.