Direct Aromatic 18F-Labeling of Highly Reactive Tetrazines for Pretargeted Bioorthogonal PET Imaging

Pretargeted bioorthogonal imaging can be used to visualize and quantify slow accumulating targeting vectors with short-lived radionuclides such as fluorine-18 - the most clinically applied Positron Emission Tomography (PET) radionuclide. Pretargeting results in higher target-to-background ratios compared to conventional imaging approaches using long-lived radionuclides. Currently, the tetrazine ligation is the most popular bioorthogonal reaction for pretargeted imaging, but a direct18 F-labeling strategy for highly reactive tetrazines, which would be highly beneficial if not essential for clinical translation, has thus far not been reported. In this work, a simple, scalable and reliable direct 18 F-labeling procedure has been developed and applied to obtain a pretargeting tetrazine-based imaging agent with favorable characteristics (target-to-background ratios and clearance) that may qualify it for future clinical translation.