In vitro proof of concept studies of radiotoxicity from Auger electron-emitter thallium-201

Auger electron-emitting radionuclides have potential in targeted treatment of small tumors. Thallium-201 (²⁰¹Tl), a gamma-emitting radionuclide used in myocardial perfusion scintigraphy, decays by electron capture, releasing around 37 Auger and Coster-Kronig electrons per decay. However, its therapeutic and toxic effects in cancer cells remain largely unexplored. Here, we assess ²⁰¹Tl in vitro kinetics, radiotoxicity and potential for targeted molecular radionuclide therapy, and aim to test the hypothesis that ²⁰¹Tl is radiotoxic only when internalized.

Breast cancer MDA-MB-231 and prostate cancer DU145 cells were incubated with 200-8000 kBq/mL [²⁰¹Tl]TlCl. Potassium concentration varied between 0 and 25 mM to modulate cellular uptake of ²⁰¹Tl. Cell uptake and efflux rates of ²⁰¹Tl were measured by gamma counting. Clonogenic assays were used to assess cell survival after 90 min incubation with ²⁰¹Tl. Nuclear DNA damage was measured with γH2AX fluorescence imaging. Controls included untreated cells and cells treated with decayed [²⁰¹Tl]TlCl.

²⁰¹Tl uptake in both cell lines reached equilibrium within 90 min and washed out exponentially (t_1/2 15 min) after the radioactive medium was exchanged for fresh medium. Cellular uptake of ²⁰¹Tl in DU145 cells ranged between 1.6 (25 mM potassium) and 25.9% (0 mM potassium). Colony formation by both cell lines decreased significantly as ²⁰¹Tl activity in cells increased, whereas ²⁰¹Tl excluded from cells by use of high potassium buffer caused no significant toxicity. Non-radioactive TlCl at comparable concentrations caused no toxicity. An estimated average ²⁰¹Tl intracellular activity of 0.29 Bq/cell (DU145 cells) and 0.18 Bq/cell (MDA-MB-231 cells) during 90 min exposure time caused 90% reduction in clonogenicity. ²⁰¹Tl at these levels caused on average 3.5-4.6 times more DNA damage per nucleus than control treatments.

²⁰¹Tl reduces clonogenic survival and increases nuclear DNA damage only when internalized. These findings justify further development and evaluation of ²⁰¹Tl therapeutic radiopharmaceuticals.