Facile method to radiolabel glycol chitosan nanoparticles with ⁶⁴Cu via copper-free click chemistry for MicroPET imaging

An efficient and straightforward method for radiolabeling nanoparticles is urgently needed to understand the in vivo biodistribution of nanoparticles. Herein, we investigated a facile and highly efficient strategy to prepare radiolabeled glycol chitosan nanoparticles with ⁶⁴Cu via a strain-promoted azide-alkyne cycloaddition strategy, which is often referred to as click chemistry. First, the azide (N₃) group, which allows for the preparation of radiolabeled nanoparticles by copper-free click chemistry, was incorporated to glycol chitosan nanoparticles (CNPs). Second, the strained cyclooctyne derivative, dibenzyl cyclooctyne (DBCO) conjugated with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator, was synthesized for preparing the preradiolabeled alkyne complex with ⁶⁴Cu radionuclide. Following incubation with the ⁶⁴Cu- radiolabeled DBCO complex (DBCO-PEG₄-Lys-DOTA-⁶⁴Cu with high specific activity, 18.5 GBq/μmol), the azide-functionalized CNPs were radiolabeled successfully with ⁶⁴Cu, with a high radiolabeling efficiency and a high radiolabeling yield (>98%). Importantly, the radiolabeling of CNPs by copper-free click chemistry was accomplished within 30 min, with great efficiency in aqueous conditions. In addition, we found that the ⁶⁴Cu-radiolabeled CNPs (⁶⁴Cu-CNPs) did not show any significant effect on the physicochemical properties, such as size, zeta potential, or spherical morphology. After ⁶⁴Cu-CNPs were intravenously administered to tumor-bearing mice, the real-time, in vivo biodistribution and tumor-targeting ability of ⁶⁴Cu-CNPs were quantitatively evaluated by microPET images of tumor-bearing mice. These results demonstrate the benefit of copper-free click chemistry as a facile, preradiolabeling approach to conveniently radiolabel nanoparticles for evaluating the real-time in vivo biodistribution of nanoparticles.