Development of the First Aliphatic 18F-Labeled Tetrazine Suitable for Pretargeted PET Imaging – Expanding the Bioorthogonal Tool Box

Pretargeting imaging of nanomedicines have attracted considerable interest in nuclear medicine since it has the potential to increase imaging contrast while simultaneously reducing radiation burden to healthy tissue. Currently, the tetrazine ligation is the fastest bioorthogonal reaction available for this strategy and consequently, the state-of-art choice for in vivochemistry. We have recently identified key properties for tetrazines to be applied in pretargeting. We have also developed a method to ¹⁸F-label highly reactive tetrazines using an aliphatic nucleophilic substitution strategy. In this study, we combined this knowledge and developed an ¹⁸F-labeled tetrazine for pretargeted imaging. In order to develop this ligand, a small structure-property study was carried out. The most promising compound - with respect to reactivity, hydrophilicity and ex vivo blocking effect - was selected for labeling and subsequent PET in vivo imaging. Radiolabeling was achieved in satisfying radiochemical yields, molar activities as well as in high radiochemical purities. The tracer displayed favorable pharmacokinetics and remarkable target-to-background ratios in pretargeted experiments - already one hour post injection. We believe that the developed pretargeting imaging agent is a promising candidate for translation into clinical studies.