N-(4-(di-tert-butyl[F-18]fluorosilyl)benzyl)-2-hydroxy-N, N-dimethylethylammonium bromide ([F-18]SiFAN(+)Br(-)): A novel lead compound for the development of hydrophilic SiFA-based prosthetic groups for F-18-labeling

F-18-labeled compounds play a major role in the development of new in vivo imaging agents for Positron Emission Tomography (PET), a non invasive imaging modality depicting the biodistribution of radioactive compounds in humans. Recently we reported a new method for the introduction of fluorine-18 into a range of organic molecules exploiting the very fast F-18-F-19 isotope exchange of fluorosilanes (termed SiFA compounds). Here, we wish to report the labeling of the first charged SiFA molecule N-(4-(di-tert-butylfluorosilyl)benzyl)-2-hydroxy-N,N-dimethylethylammonium bromide (SiFAN(+)Br(-)) serving as a lead compound in the development of SiFA-based prosthetic groups of reduced lipophilicity for biomolecule labeling. Mild conditions for synthesis of [F-18]SiFAN(+)Br(-) and an easy purification procedure using simple C-18 solid phase cartridge have been developed yielding the [F-18]SiFAN(+)Br(-) in radiochemical yields of 34% (non-decay corrected) within 40 min. A series of kinetic experiments were performed that show high isotopic exchange rate constants. Low activation energy (15.7 kcal/mol) and a large preexponential factor (7.9 x 10(13) M-1 s(-1)) were calculated for the isotopic exchange reaction from a corresponding Arrhenius plot. For comparison, the F-18-fluorination of ethyleneglycol-di-p-tosylate via the formation of a carbon-F-18 bond showed a 1.3 kcal/mol higher activation energy and a much lower preexponential factor of 2.9 x 10(9) M-1 s(-1). Moderate hydrophilicity (log D = 0.44), stability in aqueous media at pH up to 7.4 and a high specific activity of [F-18]SiFAN(+)Br(-) (SA = 20.4 GBq/mu mol, 0.55 Ci/mu mol) make this charged SiFA compound useful for the development of novel SiFA-based F-18-labeling synthons. (c) 2010 Elsevier B.V. All rights reserved.