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

The purposes of this study were as follows: (1) to radiolabel ibandronic acid (IBA, a third-generation bisphosphonate) with Lu-177, investigating optimal labeling conditions, and (2) to analyze biodistribution and imaging properties of intravenous Lu-177-ibandronate (Lu-177-IBA) administered in animals. Lu-177-labeled methylene diphosphonate (Lu-177-MDP) served as a comparator agent. Differing proportions of IBA solution and (LuCl3)-Lu-177 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 Lu-177-IBA and Lu-177-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 Lu-177-IBA in normal New Zealand White rabbits. Radiolabeling yields for Lu-177-IBA proved to be >97% within 30 minutes at 90 degrees C, and its radiochemical purity ensured stability in vitro and in vivo. Furthermore, we found that Lu-177-IBA is readily soluble in water, showing higher skeletal uptake than Lu-177-MDP but lower uptake by liver and spleen. The image quality of Lu-177-IBA was so clear that even after 6 days, analysis was still feasible.