Abstract
The 18F syntheses of tracers for positron emission tomography (PET) typically require several steps, including extraction of [18F]fluoride from H2[18O]O, elution, and drying, prior to nucleophilic substitution reaction, being a laborious and time-consuming process. The elution of [18F]fluoride is commonly achieved by phase transfer catalysts (PTC) in aqueous solution, which makes azeotropic drying indispensable. The ideal PTC is characterized by a slightly basic nature, its capacity to elute [18F]fluoride with anhydrous solvents, and its efficient complex formation with [18F]fluoride during subsequent labeling. Herein, we developed tri-(tert-butanol)-methylammonium iodide (TBMA-I), a quaternary ammonium salt serving as the PTC for 18F-fluorination reactions. The favorable elution efficiency of [18F]fluoride using TBMA-I was demonstrated with aprotic and protic solvents, maintaining high 18F-recoveries of 96–99%. 18F-labeling reactions using TBMA-I as PTC were studied with aliphatic 1,3-ditosylpropane and aryl pinacol boronate esters as precursors, providing 18F-labeled products in moderate-to-high radiochemical yields. TBMA-I revealed adequate properties for application to 18F-fluorination reactions and could be used for elution of [18F]fluoride with MeOH, omitting an additional base and azeotropic drying prior to 18F-labeling. We speculate that the tert-alcohol functionality of TBMA-I promotes intermolecular hydrogen bonding, which enhances the elution efficiency and stability of [18F]fluoride during nucleophilic 18F-fluorination.
Highlights
The diagnosis and quantification of various physiological and pathophysiological processes in vivo by position emission tomography (PET) have become increasingly crucial in medical research [1]
Scheme 1 shows the synthesis of Tert-Butanol Methylamine Iodide Salt (TBMA-I) by neat-treating of tri-(tert-BuOH)A with 1.1 equivalents of MeI under pressure at 70 ◦ C for
With TBMA-I, we studied its property as a phase transfer catalyst (PTC) for [18 F]fluoride elution, as summarized in Table 1, when different parameters were optimized for the elution of [18 F]fluoride from quaternary methylammonium (QMA) cartridges using 1 mL of various elution mixtures of PTC
Summary
The diagnosis and quantification of various physiological and pathophysiological processes in vivo by position emission tomography (PET) have become increasingly crucial in medical research [1]. Radiolabeled biological and pharmaceutical active molecules carrying 18 F are of increasing importance for preclinical, clinical, and nuclear medical research due to the unique properties of 18 F, such as low β+ -energy, long half-life (109.77 min), and the easy accessibility of no-carrier-added [18 F]fluoride [2]. The nuclear reaction of 18 O(p,n) F in a small-scale cyclotron is the commonly applied process for the production of [18 F]fluoride, which involves the bombardment of H2 [18 O]O with highly accelerated protons, and is the basis of the supply of radiopharmaceutical facilities. The extraction of [18 F]fluoride from water uses anionexchange columns, quaternary methylammonium (QMA) cartridges, which allow the removal and recovery of H2 [18 O]O. The water is removed by azeotropic drying
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