Reconsidering astatine-211 analytics: Retention effects in reversed-phase HPLC

Astatine-211 is a short-lived alpha-emitting radionuclide with high potential for cancer treatment by targeted alpha therapy (TAT). Reversed-phase high-performance liquid chromatography (RP-HPLC) is central to the development and quality control of ²¹¹At-labeled radiopharmaceuticals. However, accurate analysis remains challenging due to the ultra-trace levels of astatine and its complex, unpredictable chemical behaviour. In this study, we systematically evaluated the recovery of representative inorganic astatine formulations under different redox conditions using various chromatographic conditions, which is a prerequisite for reliable quantification. Examination of four different columns with distinct stationary phase chemistry demonstrated that significant discrepancies can arise when radiochemical conversion is assessed solely based on eluted activity, as is common standard for RP-HPLC analysis. Among the chromatographic conditions examined, the highest and most consistent astatine recoveries (88-98%) were achieved using a basic mobile phase containing 0.4% triethylamine in combination with a base-tolerant stationary phase, likely due to a shift in astatine speciation under alkaline conditions, reduced secondary interactions with the stationary phase, and/or potentially beneficial ion pairing effects. In contrast, widely used standard solvent systems based on acetonitrile/water with or without 0.1% trifluoroacetic acid resulted in unsatisfactory and highly variable recoveries (7-71% or 30-79%, respectively) across all columns investigated. These findings highlight the necessity of optimized chromatographic conditions and suggest that inadequate recovery of free astatine could lead to substantial misestimation of the radiochemical conversion and purity of radiopharmaceutical formulations if not explicitly accounted for by auxiliary quantification strategies.