Labeling of Highly Reactive Tetrazines using [ ¹⁸ F]SuFEx

Pretargeted imaging is an emerging technique to study the in vivo biodistribution of nanomedicines. Currently, the tetrazine ligation is considered the most promising bioorthogonal reaction for pretargeting. Recently, Zheng et al. described an ultrafast late-stage radiolabeling of tetrazines based on sulfur F-18-fluoride exchange click chemistry ([F-18]SuFEx). However, bispyridyl and H-tetrazines-the most promising structures for in vivo pretargeted applications-cannot be labeled using the proposed reaction conditions as they lead to decomposition of the tetrazine core. Here, we report improved conditions, exploiting basic preconditioning conditions for the quaternary methyl ammonium (QMA) cartridge and the use of low basic anions that allow F-18-labeling of bispyridyl and H-tetrazines using SuFEx. This strategy resulted in fast and efficient radiolabeling of highly reactive tetrazines with radiochemical conversions of up to 85% and radiochemical purity above 95%. This opens up the possibility to use SuFEx to F-18-label tetrazines, which are suitable for in vivo pretargeted imaging.