Exploring In Vivo Metal Chelation as an Approach for Pretargeted PET Imaging

Pretargeted PET imaging has emerged as a leading strategy for tracking long-circulating agents such as antibodies and nanoparticle-drug delivery systems with short-lived isotopes. Compared to the conventional direct radiolabeling approach, pretargeting benefits from high sensitivity and spatial resolution of PET while minimizing radiation doses and nonspecific accumulation of radioactivity. In addition, it allows for long-term <i>in vivo</i> tracking possibilities. However, a pretargeting approach that can utilize readily available radionuclides as obtained from the generator/cyclotron without the need of complex radiochemical synthesis is highly desirable. Here, we report a metal chelation pretargeting system based on the ⁶⁸Ga chelator tris-(hydroxypyridinone) (THP). THP can be radiolabeled at low concentrations with as-obtained generator-produced radionuclide gallium-68 (<i>t</i> _1/2 = 68 min) at room temperature and physiological conditions with high efficiency. The bifunctional chelator THP-NCS was conjugated to either PEGylated liposomes or a bone-targeting aminobisphosphonate (pamidronate) to examine the metal chelation pretargeted imaging system in both long-circulating nanomedicines and short-circulating small molecules, respectively. <i>In vivo</i> imaging experiments were performed in healthy BalB/c mice at multiple time points. For liposomal pretargeting, the fraction of liposomes circulating in the blood was efficiently radiolabeled <i>in vivo</i>, but limited <i>in vivo</i> radiolabeling was observed for liposomes that had accumulated in the liver and spleen. The pretargeting of the small-molecule, bone-targeting bisphosphonate THP-Pam showed moderate <i>in vivo</i> radiolabeling in bones. Overall, based on this study, the metal chelation method appears to allow easy pretargeting for agents present in the blood and bones but with limited success in other organs.