Novel approach to production of high specific activity [ 18 F]-Sodium Tetrafluoroborate Alex Khoshnevisan 1, Jennifer D. Young 1, Gareth E. Smith 2, Alex Jackson 2, Antony Gee 1 and Philip J. Blower 1 1 King’s College London, Division of Imaging Sciences and Biomedical Engineering, St Thomas’ Hospital, London, United Kingdom 2 GE Healthcare, The Grove Centre, Amersham, United Kingdom The tetrafluoroborate anion (BF 4 - ) is a well-established substrate for the sodium/iodide symporter (NIS). Recently, radiolabelling of NaBF 4 with fluorine-18 by isotopic exchange was reported [1], along with associated evaluation of biological activity in rodent models by PET/CT. While this highlighted the potential for clinical use in imaging thyroid cancer and other NIS-related disorders, the use of isotopic exchange labelling limits specific activity (SA) and could result in sub-optimal target uptake. To that end, we sought an alternative route via nucleophilic addition to yield high SA [ 18 F]-NaBF 4. Introduction Aims Our aim is to develop a synthetic route to 18 F-NaBF 4 which does not proceed via isotopic exchange involving: Nucleophilic addition of 18 F-fluoride salts to boron trifluoride diethyl etherate Development and optimisation of reaction conditions Determination of specific activity by ion chromatography Automation of radiosynthesis for clinical application In vitro assessment of the effect of reduced specific activity on 18 F-NaBF4 uptake Experimental methods Experimental Methods Results [1] Jauregui-Osoro et al. (2010), Eur. J. Nucl. Med. Mol. Imaging, 37, 11, [2] Weeks et al. (2011), Nucl. Med. Commun., 32, 2, References Table 1: RCC following reaction step under various conditions Conclusions Synthesis of [ 18 F]-NaBF 4 from BF 3.OEt 2 and [ 18 F]-NaF proceeds rapidly and efficiently, with solid phase extraction approach yielding the desired product in high radiochemical purity. Fully automatable using cassette-based approach for FASTLab platform and could easily be adapted to alternative platforms Measurement of specific activity shows significant improvement over previously published method Blocking action of [ 19 F]-NaBF 4 indicates that using [ 18 F]-NaBF 4 produced by the method described here avoids potential for self-inhibition of uptake in vivo. Figure 1: PET/CT maximum intensity projections of normal mouse (Left and centre) 30 min post-injection of [ 18 F]-NaBF 4 from anterior (left) and lateral (centre) views. TRβ PV/PV thyroid tumour bearing transgenic mouse 30 min post- injection of [ 18 F]-NaBF 4 (right), anterior view.[1] Figure 2: RadioTLC of reaction mixture under optimised conditions. AluminaN TLC sheets with MeOH eluent Figure 3: Conductivity (top) and radioactivity (below) traces from IC analysis of purified product co-injected with [ 19 F]-NaBF 4 (125 μg/mL) as a reference standard. Labelled peaks a = injection peak, b = Cl -, c = BF 4 -. [ 18 F]-BF 4 - is only radioactive peak. Retention time of 18 F - is ~ 2 min. C1 CE M H LV CP C2 OB ME P TH Radiochemistry Various [ 18 F]-fluoride salts were prepared from cyclotron-produced [ 18 F]-F - in H 2 O by trapping on a QMA cartridge (Waters, UK) and eluting with the corresponding chloride or nitrate salt followed by azeotropic distillation, followed by treatment with BF 3.OEt 2 under anhydrous conditions in MeCN. Purification Solid phase extraction of the desired product was carried out using a neutral alumina, followed by QMA cartridge. The purified 18 F-BF 4 - could then be eluted from the QMA in 0.9% NaCl solution. Automation Optimal conditions were automated on a FASTLab (GE Healthcare) synthesis platform using a custom cassette layout: Radioanalytical methods Determination of incorporation of 18 F - into the desired product (radiochemical conversion, RCC) was by radioTLC using alumina strips and MeOH (100%) as the mobile phase. Characterisation of the final product and determination of SA was effected using an ion chromatography (IC) apparatus with in-line radio-detector. Cell culture An established human NIS-expressing cell line[2], HCT116-C19, was seeded into 12- well plates (n = 500,000) 24 h prior to incubation for 30 min in the presence of [ 18 F]- NaBF4 (0.1 MBq) and varying concentrations of [ 19 F]-NaBF4. Uptake of the tracer was measured by determining the amount of activity bound to the cells as a percentage of the total activity administered. Figure 4: Uptake of [ 18 F]-NaBF 4 in HCT116-C19 (hNIS) cells under varying concentrations of [ 19 F]- NaBF 4. Estimated in vivo [ 19 F]- NaBF 4 concentration following patient dose administration based on SA of final product from this approach and the isotopic exchange approach QMA Eluent18F SourceRCC (%) NaClNaF77 TBANO3TBAF81 K2CO3KF0 While similar levels of incorporation were observed in the reaction with NaF as with TBAF, 19 F-NMR indicated that residual moisture due to the hygroscopic TBA salt was causing hydrolysis of the precursor, forming [ 19 F]-NaBF4 and then proceeding via isotopic exchange. Scheme 1: General reaction scheme for proposed radiosynthesis Scheme 2: Reaction conditions selected for automation Reaction with [ 18 F]-NaF was selected for automation and successfully implemented on FASTLab. Radiochemical yield (non-decay corrected) was found to be 11 ±1% (n = 3) with radiochemical purity exceeding 99% in all cases. In all product samples analysed, there was no detectable 19 F-NaBF4 signal, and hence the concentration was below the LOD. Using this as a maximal value, the SA of the final product was calculated as >38 GBq/ μmol. In vitro blocking hNIS transport of [ 18 F]-NaBF 4 using [ 19 F]-NaBF 4 gave an IC50 of 8 μM. SA values for the product obtained using the isotopic exchange approach[1] indicate potential inhibition at in vivo concentrations. This is safely avoided at the SA obtained with this method.