Development of a halofluorocarbon, chromatography-free radiosynthesis of fluorine-18 difluorocarbene.

Abstract

Background: In recent years, the development of the [¹⁸F]difluoromethyl radical ([¹⁸F]2-((difluoromethyl)sulfonyl)benzo[d]thiazole, [¹⁸F]4), and [¹⁸F]difluorocarbene ([¹⁸F]1-chloro-4-((difluoromethyl)sulfonyl)benzene, [¹⁸F]10) prosthetic groups, has paved the way towards direct ¹⁸F-difluoromethylation in routine PET tracer synthesis with high molar activity. However, limitations in their syntheses may be hindering their widespread adoption by the radiochemistry community. Firstly, the synthesis of the precursors 2-((bromofluoromethyl)thio)benzo[d]thiazole (3) and (bromofluoromethyl)(4-chlorophfenyl)sulfane (8) requires the use of the ozone-depleting dibromofluoromethane, a reagent that is not-commercially available. Secondly, the reported syntheses of [¹⁸F]4 and [¹⁸F]10 are lengthy and require semi-preparative HPLC purification prior to the ¹⁸F-difluoromethylation step. Finally, in the case of [¹⁸F]10, very large amounts of precursor material (200 μmol) are required for difluorocarbene insertion. The aim of this work was to develop a halofluorocarbon-free radiosynthesis of [¹⁸F]4 and [¹⁸F]10 on the GE TRACERlab FX_FN module. Additionally, another aim was to develop a chromatography-free, fully-automated synthesis of [¹⁸F]10 on the GE FX_FN module.

Results: Precursors 3 and 8 were synthesised in 21% and 54% yield via decarboxylative bromination, which circumvented the need for ozone-depleting dibromofluoromethane. Difluoromethyl reagents [¹⁸F]4 and [¹⁸F]10 were synthesised on a GE FX_FN module with semi-prep HPLC purification in 4% and 3% RCY (decay-corrected), respectively. The synthesis of [¹⁸F]10 was further simplified through elimination of the semi-prep HPLC purification in favour of a cartridge-based solid-phase extraction (SPE) trapping and elution approach (on an alumina SPE cartridge loaded in series with a C18 Sep-Pak plus SPE cartridge) to give [¹⁸F]10 in 10.1% ± 1.9% (n = 6, decay-corrected) RCY (97% ± 3% RCP, 1.5-11 GBq/μmol). Finally, a fully automated ¹⁸F-difluoromethylation radiosynthesis with [¹⁸F]10 was developed on two GE FX_FN modules linked together to yield the model ¹⁸F-difluoromethylated compound in adequate amounts for biological studies, in under two hours (99.0 MBq, 0.8% RCY {decay-corrected}, 1.5 GBq/μmol, 103 min total synthesis time). Therefore, we have established a path forward for routine automated synthesis of radiotracers via [¹⁸F]difluorocarbene insertion with [¹⁸F]10.

Conclusions: A halofluorocarbon, chromatography-free synthesis on the GE FX_FN module afforded difluorocarbene reagent [¹⁸F]10 in 10.1% ± 1.9% RCY (decay-corrected). Additionally, a fully-automated three-step [¹⁸F]difluorocarbene insertion radiosynthesis using two tandem FX_FNs is described for the first time, providing a path forward to the full automation of [¹⁸F]difluorocarbene insertion on two-reactor systems.