Radiotoxicity of 67-gallium on myeloid leukemic blasts

Promising clinical results are obtained with radiolabeled antibodies in leukemia patients. 67Gallium (67Ga) is a radionuclide that accumulates in many malignant tissues without need for a monoclonal antibody. For this reason, the use of 67Ga as a therapeutic agent is appealing. In the present we study, we report data about the radiotoxicity of 67Ga on peripheral blast cells of 23 patients with acute myelogenous leukemia (AML) in vitro. Isolated blast cells were incubated for 4 days with 0.74 MBq/ml (20 microCi/ml), 1.48 MBq/ml (40 microCi/ml) or 2.96 MBq/ml (80 microCi/ml) 67Ga. Compared with non-irradiated control cells proliferation during incubation was almost abolished. Clonogenic survival was measured by a colony forming unit assay (CFU-assay). In 13 of the 23 patients (56%) sufficient colony growth was observed for evaluation. The mean clonogenic survival of blasts after incubation with 0.74 MBq/ml and 2.96 MBq/ml 67Ga was 22.5, 11.3 and 3.5%, respectively. In some cases colony growth was completely abolished after incubation with only 0.74 MBq/ml 67Ga. No correlation was found between cellular 67Ga-uptake, (micro)dosimetry and transferrin receptor density (CD-71) via which 67Ga enters the cell. In vitro the blasts received a dose of > 10 Gy in 9 of the 2.96 MBq/ml, in 3 of the 1.48 MBq/ml and in 2 of the 0.74 MBq/ml incubations. In one patient, even a radiation dose > 40 Gy was reached. Low dose rate irradiation is known to arrest cells in G2/M-phase of the cell cycle, but no such arrest was observed during incubation with 67Ga. Thus, 67Ga induces clonogenic cell death in leukemic blasts. Cellular uptake of 67Ga in vitro varies between patients and can be very high in some patients. The easy availability, low costs and absence of immunological problems warrant further investigation of the therapeutic potential of 67Ga in refractory or relapsed AML patients.