Inline gamma spectroscopy and liquid scintillation HPLC reveal daughters in ²²⁵Ac-radiopharmaceutical

Analytical data for ²²⁵Ac-labeled radiopharmaceuticals are currently obtained using thin-layer chromatography and high-performance liquid chromatography, the latter followed by offline fraction collection and delayed gamma-counting after secular equilibrium with gamma-emitting daughters has been reached. These procedures are slow and do not depict the distribution of daughter radionuclides. An inline HPLC detection setup combining liquid scintillation detection and LaBr₃-based gamma-spectroscopy is presented to enable faster and more informative analysis of a ²²⁵Ac radiopharmaceutical.

Four chromatographic signals were identified corresponding to uncomplexed ²²¹Fr with at most a minor contribution of free ²¹³Bi, [²¹³Bi]Bi-PSMA I&T, [²²⁵Ac]Ac-PSMA I&T, and [²⁰⁹Pb]Pb-PSMA I&T. The retention times corresponded to those of the non-radioactive analogs determined by UV detection. Assignments were confirmed by fraction collection followed by gamma-spectroscopy and beta liquid scintillation counting. The distribution of these species changed upon heating the reaction mixture or after addition of another ligand.

The combined detection setup enables rapid observation of isotope distribution and daughter behavior during chromatographic analysis and avoids fraction handling by operating as a closed system, thereby reducing contamination risk and radiation exposure. This approach may contribute to improved quality control procedures for ²²⁵Ac radiopharmaceuticals and other alpha-emitting radionuclides. At the present stage, however, it should be regarded as a qualitative to semi-quantitative proof-of-principle.