Development of the First Tritiated Tetrazine: Facilitating Tritiation of Proteins

Head-to-Head Comparison of the in Vivo Performance of Highly Reactive and Polar 18F-Labeled Tetrazines

Pretargeted imaging harnessing tetrazine ligation has gained increased interest over recent years. Targeting vectors with slow pharmacokinetics may be visualized using short-lived radionuclides, such as fluorine-18 (18F) for positron emission tomography (PET), and result in improved target-to-background ratios compared to conventionally radiolabeled slowly accumulating vectors. We recently developed different radiochemical protocols enabling the direct radiofluorination of various tetrazine scaffolds, resulting in the development of various highly reactive and polar 18F-labeled tetrazines as lead candidates for pretargeted imaging. Here, we performed a direct head-to-head-comparison of our lead candidates to evaluate the most promising for future clinical translation. For that, all 18F-labeled tetrazine-scaffolds were synthesized in similar molar activity for improved comparability of their in vivo pretargeting performance. Intriguingly, previously reported dicarboxylic acid lead candidates with a net charge of -1 were outperformed by respective monocarboxylic acid derivatives bearing a net charge of 0, warranting further evaluation of such scaffolds prior to their clinical translation.

Evaluation of F-537-Tetrazine in a model for brain pretargeting imaging. Comparison to N-(3-[18F] fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine

Brain pretargeted nuclear imaging for the diagnosis of various neurodegenerative diseases is a quickly developing field. The tetrazine ligation is currently the most explored approach to achieve this goal due to its remarkable properties. In this work, we evaluated the performance of F-537-Tetrazine, previously developed by Biogen, and N-(3-[18F]fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine, previously developed in our group, thereby allowing for the direct comparison of these two imaging probes. The evaluation included synthesis, radiolabeling and a comparison of the physicochemical properties of the compounds. Furthermore, their performance was evaluated by in vitro and in vivo pretargeting models. This study indicated that N-(3-[18F] fluoro-5-(1,2,4,5-tetrazin-3-yl)benzyl)propan-1-amine might be more suited for brain pretargeted imaging.