Head-to-HeadComparison of the <i>in Vivo</i> Performance of Highly Reactiveand Polar ¹⁸F‑LabeledTetrazines

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 <i>in vivo</i> 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.