Radiolabeling, quality control, biodistribution, and imaging studies of ¹⁷⁷ Lu-ibandronate

The purposes of this study were as follows: (1) to radiolabel ibandronic acid (IBA, a third-generation bisphosphonate) with ¹⁷⁷ Lu, investigating optimal labeling conditions, and (2) to analyze biodistribution and imaging properties of intravenous ¹⁷⁷ Lu-ibandronate (¹⁷⁷ Lu-IBA) administered in animals. ¹⁷⁷ Lu-labeled methylene diphosphonate (¹⁷⁷ Lu-MDP) served as a comparator agent. Differing proportions of IBA solution and ¹⁷⁷ LuCl₃ solution were combined to determine an optimal ratio for radiolabeling purposes, varying pH, temperature, and time to establish ideal reactivity conditions. Radiochemical purity of the labeled compounds was then assessed by paper chromatography. In vitro and in vivo stabilities were also measured at specific time intervals. In Kunming mice, biodistributions of ¹⁷⁷ Lu-IBA and ¹⁷⁷ Lu-MDP and respective agent activities in various organs were monitored by gamma counter, and we performed single photon computed tomography/computed tomography (SPECT/CT) imaging of ¹⁷⁷ Lu-IBA in normal New Zealand White rabbits. Radiolabeling yields for ¹⁷⁷ Lu-IBA proved to be >97% within 30 minutes at 90°C, and its radiochemical purity ensured stability in vitro and in vivo. Furthermore, we found that ¹⁷⁷ Lu-IBA is readily soluble in water, showing higher skeletal uptake than ¹⁷⁷ Lu-MDP but lower uptake by liver and spleen. The image quality of ¹⁷⁷ Lu-IBA was so clear that even after 6 days, analysis was still feasible.