Potentials and practical challenges of terbium-161 labeled radiopharmaceuticals

Due to promising preclinical studies and clinical case reports, Tb-161 labeled radiopharmaceuticals for targeted radionuclide therapy (TRT) have gained interest. Unlike Lu-177, Tb-161 not only emits β − particles, but also Auger and conversion electrons, which may improve the current therapeutic efficacy of TRT. However, before implementing Tb-161 for clinical use, several steps are required (E.g., development, optimization, validation). Therefore, this study focuses on the purity of Tb-161 as well as the detection and quantification of Tb–Tb-161-labeled radiopharmaceuticals. As multiple studies are currently aiming to determine the therapeutic effect of Tb-161 labeled radiopharmaceuticals, standardizing and evaluating Tb-161 is essential to be able to compare its therapeutic potential against Lu-177 and other radionuclides. Therefore, we established accurate detection methods, impurity measurements, radiolabeling protocols, and quality control for Tb-161 labeled pharmaceuticals. Parameters of Tb-161 labeled radiopharmaceuticals were investigated and exemplified by [ 161 Tb]Tb-DOTA-TATE, in comparison with [ 177 Lu]Lu-DOTA-TATE. Analysis of Tb-161 stock solution demonstrated the presence of metal impurities (ΣFe, Zn, Cu, Gd, and Dy) at the reference day and increased over time. The geometric effect of vial type demonstrated a decrease in activity when a vial was used without point-source. For both Lu-177 and Tb-161 labeled DOTA-TATE, high radiochemical yield and purity (> 95%) were obtained and remained stable (> 90%) up to 24 h in solution. Analysis of Tb-161 stock showed an increase in metal impurities over time, which could interfere with the production of Tb-161 labeled radiopharmaceuticals. The low gamma energy (48.9 keV and 74.6 keV) of Tb-161 needs to be considered in (pre)clinical applications when quantifying activity. For Tb-161 labeled pharmaceuticals, similar radiolabeling conditions as Lu-177 have been shown to be successful and highly stable.