A ⁶⁴Cu-Labeled Oxopurine Derivative for Translocator Protein-Targeted Inflammation Imaging

The translocator protein (TSPO) is a mitochondrial biomarker overexpressed in activated immune cells and various tumors that is an attractive target for positron emission tomography (PET) imaging of inflammation and the tumor microenvironment. We designed and evaluated the novel TSPO-targeted PET tracer, ⁶⁴Cu-purine ([⁶⁴Cu]5), constructed from a purine scaffold conjugated to a NOTA chelator. Molecular docking analysis indicated that the DOTA analog exhibited slightly higher TSPO binding affinity than did the NOTA derivative; however, the NOTA chelator was selected for ⁶⁴Cu labeling due to its superior radiochemical yield and stability. [⁶⁴Cu]5 demonstrated high radiochemical purity, excellent in vitro and in vivo stability, and moderate lipophilicity, which minimized nonspecific uptake. In vitro, [⁶⁴Cu]5 showed significantly higher uptake in inflammatory macrophages than in tumor cells. In vivo PET imaging and biodistribution studies revealed preferential accumulation in inflamed and tumor tissues, with reduced uptake following PK11195 administration. Autoradiography and immunohistochemistry confirmed tracer accumulation in both inflamed and tumor tissues, reflecting the presence of inflammation within the tumor microenvironment. Overall, these findings suggest that [⁶⁴Cu]5 is a promising TSPO-targeting PET radiotracer for imaging inflammation-associated processes in tumors.