GA-67 IN THE HUMAN LYMPHOID-CELL LINE U-715 - UPTAKE, CYTOTOXICITY AND INTRACELLULAR-LOCALIZATION

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