Preparation of an astatine-211 aqueous solution by dry distillation and dissolution in pure water

1462 Objectives: Astatine-211 (211At) is a promising radionuclide for targeted alpha therapy of metastatic cancers. The production of 211At is accomplished in the 209Bi(α, 2n)211At nuclear reaction using an accelerator. Typically, 211At is isolated from the irradiated Bi target through a dry distillation method and dissolved in various solutions. In many cases, methanol or chloroform is used as solvent for dry distilled 211At. However, these solutions are toxic to living body, and it is desirable to dissolve dry distilled 211At with non-toxic aqueous solution for clinical use of 211At. Our group have been preparing the clinical trial of [211At]-sodium astatide ([211At]NaAt) as a reagent for targeted alpha therapy of thyroid cancer. This reagent is prepared by adding aqueous ascorbic acid solution to dry distilled 211At dissolved in pure water. Here, we report our recent results of preparation method for dry distilled 211At dissolved in pure water. Methods: The 211At produced in the 209Bi(α, 2n)211At reaction was supplied from RIKEN through Supply Platform of Short-lived Radioisotopes. The produced 211At was separated from the irradiated Bi target using the dry distillation apparatus shown in Fig. 1. The irradiated Bi target was put in a quartz column and heated up to 850°C using an electric tubular furnace. The evaporated 211At was transported to Teflon trap tube, which is cooled by ice water, with mixed N2 and O2 gas flow including trace moisture added by a moisture generator. The trapped 211At was then dissolved in 100 μL of pure water. The separation yield of 211At was calculated from the 211At radioactivity in the eluted pure water divided by that in the irradiated Bi target measured with a Ge detector. Results: Approximately 80% of 211At (after decay correction) was trapped in the Teflon trap tube. However, the separation yield of 211At dissolved in 100 μL of pure water fluctuated from 35% to 70%; most of 211At remained in the Teflon trap tube when the separation yield was low. The remained 211At in the trap tube was recovered by 2-6 times repeated dissolution with 100 μL of pure water resulting in total separation yield of 65% to 80%. A rough correlation was observed between the separation yield of 211At and the amount of moisture in the carrier gas during heating of the Bi target, indicating that the proportion of chemical species of 211At which can be easily dissolved in pure water might be affected by the amount of moisture in the carrier gas. Optimization of separation condition would lead to preparation of 211At dissolved in small volume of pure water. Conclusions: It is expected that, by optimization of separation condition, our preparation method for dry distilled 211At dissolved in pure water would be applied to clinical application of [211At]NaAt for targeted alpha therapy of thyroid cancer.