Pretargeted Imaging Beyond the Blood-Brain-Barrier

Pretargeting is a powerful nuclear imaging strategy to achieve enhanced imaging contrast for nanomedicines. It reduces the radiation burden to healthy tissue. Pretargeting is based on bioorthogonal chemistry. The most attractive reaction for this purpose is currently the tetrazine ligation, which occurs between trans-cyclooctene (TCO) tags and tetrazines (Tzs). Pretargeted imaging beyond the blood-brain-barrier (BBB) has not been reported thus far. In this study, we developed Tz imaging agents that are capable to ligate in vivo to targets beyond the BBB. We chose to develop 18F-labeled Tzs as they can be applied to positron emission tomography (PET) - the most powerful molecular imaging technology. Fluorine-18 is an ideal radionuclide for PET due to its almost ideal decay properties. Fluorine-18 also allows - as a non-metal radionuclide - to develop Tzs with physicochemical properties enabling passive brain diffusion. In order to develop these imaging agents, we applied a rational drug design approach. This approach was based on estimated and experimental determined parameters such as the BBB score, pretargeted autoradiography contrast, in vivo input and washout curves as well as on metabolism studies. From initially 18 developed structures, five Tzs were selected to be tested on their in vivo click performance. Whereas all selected structures clicked in vivo into the brain, [18F]18 displayed the most favorable characteristics with respect to brain pretargeting. [18F]18 is our lead compound for future pretargeted imaging studies based on BBB-penetrant monoclonal antibodies. Pretargeting beyond the BBB will allow us to image targets beyond the BBB that are currently not imageable. For example, soluble protein isoforms could be imaged. These proteins are valuable drug targets for several neurodegenerative diseases and can currently not be imaged. Imaging would allow for diagnosis of these diseases, identifying responders from non-responders or to monitor treatment. Consequently, imaging will provide valuable information to accelerate drug development and greatly benefit patient care.