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 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 LuCl 3 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.