Optimisation attempts for the direct nucleophilic 18F-labelling of tetrazines

1101 Introduction: Radiolabelled tetrazines have in recent years received increased attention due their possibility to be used in pretargeted imaging and radiotherapy. Current approaches to radiolabel tetrazines with fluorine-18, the most commonly used radionuclide in positron emission tomography (PET), are limited to indirect labelling approaches. Our group has recently developed a directly 18F-radiolabelling approach resulting in modest radiochemical yield (RCY). The approach is based on Cu-mediated fluorination reactions using tin-precursors. In this work, we aimed to increase the RCY thereof and optimized the 18F-recovery, radiochemical incorporation (RCC) and lowered the glass wall absorption. In this respect, it was decided to radiolabel the model compounds 3-methyl-6-(4-(trimethylstannyl)phenyl)-1,2,4,5-tetrazine and 3-(3-trimethylstannyl)phenyl)-1,2,4,5-tetrazine (Figure 1 and 2), which previously have been 18F-radiolabelled, using KOTf preconditioned QMA cartridges and eluted with a KOTf (aq) solution. The RCCs of 3-methyl-6-(4-(trimethylstannyl)phenyl)-1,2,4,5-tetrazine was 25% and RCY of 20%, respectively. Methods: Optimisation attempts were based on different elution conditions using different anion exchange cartridges (QMAs), varying the reaction solvent and different Cu-sources. 18F-recovery was tested using three different QMAs, which were preconditioned with KOTf (aq), K2CO3 (aq) or K3PO4 (aq). Elution was tested using two different elution mixtures using either 1) KOTf and K2CO3 dissolved in water or 2) Bu4NOTf and K2CO3 dissolved in a water/MeOH mixture. RCC were measured directly from the reaction mixture after 5 min and analysed via radioTLC and radioHPLC. DMA with or without n-butanol was used as solvent. As copper source, copper trifluoromethansesulfonate dissolved in pyridine and DMA was used. Results: The best conditions found were; preconditioning the QMA with K3PO4 (aq), using Bu4NOTf and K2CO3dissolved in a water/methanol mixture as eluent and using Cu(OTf)2Py4 in equivalence 3:3 with the respective precursor as a copper source. RCC for 3-methyl-6-(4-(trimethylstannyl)-phenyl)-1,2,4,5-tetrazine was 56% and RCY 42% (Figure 1). RCC for 3-(3-trimethylstannyl)phenyl)-1,2,4,5-tetrazine was 23% and the RCY 10% (Figure 2). Conclusions: Our optimised radiolabelling procedure resulted in an RCY increase of 51% and an increase in RCC of 55%. Future studies will be directed to apply this strategy on highly reactive tetrazines species.