Novel approach to production of high specific activity [18F]-Sodium TetrafluoroborateAlex Khoshnevisan1, Jennifer D. Young1, Gareth E. Smith2, Alex Jackson2, Antony Gee1 and Philip J. Blower11King’s College London, Division of Imaging Sciences and Biomedical Engineering, St Thomas’ Hospital, London , United Kingdom2GE Healthcare, The Grove Centre, Amersham, United Kingdom

The tetrafluoroborate anion (BF4-) is a well-established substrate for the

sodium/iodide symporter (NIS). Recently, radiolabelling of NaBF4 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 [18F]-NaBF4.

Introduction

Aims

Our aim is to develop a synthetic route to 18F-NaBF4 which does not proceed via isotopic exchange involving:• Nucleophilic addition of 18F-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 18F-NaBF4 uptake

Experimental methodsExperimental Methods

Results

[1] Jauregui-Osoro et al. (2010), Eur. J. Nucl. Med. Mol. Imaging, 37, 11, 2108-2116 [2] Weeks et al. (2011), Nucl. Med. Commun., 32, 2, 98-105

References

Table 1: RCC following reaction step under various conditions

Conclusions

•Synthesis of [18F]-NaBF4 from BF3.OEt2 and [18F]-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 [19F]-NaBF4 indicates that using [18F]-NaBF4 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 [18F]-NaBF4 from anterior (left) and lateral (centre) views. TRβPV/PV thyroid tumour bearing transgenic mouse 30 min post-injection of [18F]-NaBF4 (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 [19F]-NaBF4 (125 μg/mL) as a reference standard. Labelled peaks a = injection peak, b = Cl-, c = BF4

-. [18F]-BF4- is only

radioactive peak. Retention time of 18F- is ~ 2 min.

C1

CE

M

H LV

CP

C2

OB

MEP

TH

[19F]-NaBF4 [18/19F]-NaBF4 SA = ~1 GBq/mol

18F-

pH ~ 0

[18F]-MBF4BF3.OEt2[18F]-MF

M = Na, K or TBA

[18F]-NaBF4BF3.OEt2[18F]-NaF

15C5, MeCNSA = >38 GBq/mol

Radiochemistry•Various [18F]-fluoride salts were prepared from cyclotron-produced [18F]-F- in H2O 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 BF3.OEt2 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 18F-BF4

- 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 18F- 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 [18F]-NaBF4 (0.1 MBq) and varying concentrations of [19F]-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 [18F]-NaBF4 in HCT116-C19 (hNIS) cells under varying concentrations of [19F]-NaBF4. Estimated in vivo [19F]-NaBF4 concentration following patient dose administration based on SA of final product from this approach and the isotopic exchange approach

-15

-10 -5 0

0

5

10

15

Isoto

pic e

xchan

ge

Reported m

ethod

[19F]-NaBF4 Conc. (1 x 10X M)

% U

pta

ke

QMA Eluent 18F Source RCC (%)

NaCl NaF 77

TBANO3 TBAF 81

K2CO3 KF 0

• While similar levels of incorporation were observed in the reaction with NaF as with TBAF, 19F-NMR indicated that residual moisture due to the hygroscopic TBA salt was causing hydrolysis of the precursor, forming [19F]-NaBF4 and then proceeding via isotopic exchange.

Scheme 1: General reaction scheme for proposed radiosynthesis

Scheme 2: Reaction conditions selected for automation

• Reaction with [18F]-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 19F-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 [18F]-NaBF4 using [19F]-NaBF4 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.