Re-assessing gallium-67 as a therapeutic radionuclide.

INTRODUCTION: Despite its desirable half-life and low energy Auger electrons that travel further than for other radionuclides, 67 Ga has been neglected as a therapeutic radionuclide. Here, 67 Ga is compared with Auger electron emitter 111 In as a potential therapeutic radionuclide.

METHODS: Plasmid pBR322 studies allowed direct comparison between 67 Ga and 111 In (1MBq) in causing DNA damage, including the effect of chelators (EDTA and DTPA) and the effects of a free radical scavenger (DMSO). The cytotoxicity of internalized (by means of delivery in the form of oxine complexes) and non-internalized 67 Ga and 111 In was measured in DU145 prostate cancer cells after a one-hour incubation using cell viability (trypan blue) and clonogenic studies. MDA-MB-231 and HCC1954 cells were also used.

RESULTS: Plasmid DNA damage was caused by 67 Ga and was comparable to that caused by 111 In; it was reduced in the presence of EDTA, DTPA and DMSO. The A50 values (internalized activity of oxine complexes per cell required to kill 50% of cells) as determined by trypan blue staining was 1.0Bq/cell for both 67 Ga and 111 In; the A50 values determined by clonogenic assay were 0.7Bq/cell and 0.3Bq/cell for 111 In and 67 Ga respectively. At the concentrations required to achieve these uptake levels, non-internalized 67 Ga and 111 In caused no cellular toxicity. Qualitatively similar results were found for MDA-MB-231 and HCC1954 cells.

CONCLUSION: 67 Ga causes as much damage as 111 In to plasmid DNA in solution and shows similar toxicity as 111 In at equivalent internalized activity per cell. 67 Ga therefore deserves further evaluation for radionuclide therapy.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: The data presented here is at the basic level of science. If future in vivo and clinical studies are successful, 67 Ga could become a useful radionuclide with little healthy tissue toxicity in the arsenal of weapons for treating cancer.