The development of an efficient, rapid method for [^<18>F] fluorobenzyl iodide as a versatile labeling precursor.

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[^<18>F] Fluorobenzyl iodide and bromide ([^<18>F] fluorobenzyl halide) have been expected to be as versatile a precursor for ^<18>F-fluorobenzylation as ^<11>C-labled methyl iodide for ^<11>C-methylation. In the present study, we have aimed at developing a convenient, rapid method for automated preparation of the target compound from [^<18>F] fluoride. Attempts have been made to adapt the following three steps, constituting the synthesis procedure, to on-column processing.1) Synthesis of [^<18>F] fluorobenzaldehyde from [^<18>F] fluoride : Starting substrates were immobilized on solid-phase-supports and examined for aromatic nucleophilic substitution by [^<18>F] fluoride. However, radiochemical yields of [^<18>F] fluorobenzaldehyde obtained by this method were not high enough for successive reactions. Consequently the on-column method was abandoned.2) Reduction of [^<18>F] fluorobenzaldehyde into [^<18>F] fluorobenzyl alcohol : Sodium borohydride was chosen among many reductive reagents as it was used in aqueous solutions. While NaBH_4 on alumina gave rather lower yields, a simple on-column method of just flowing a NaBH_4 solution through a C18 cartridge retaining [^<18>F] fluorobenzaldehyde yielded [^<18>F] fluorobenzyl alcohol almost quantitatively. Thus after the reduction [^<18>F] fluorobenzyl alcohol was eluted with CH_2Cl_2.3) Synthesis of [^<18>F] fluorobenzyl halide from [^<18>F] fluorobenzyl alcohol : Halogenation reagents immobilized to a resin or adsorbed on silica gel were investigated for the reaction. It was observed that radiochemical yields of [^<18>F] fluorobenzyl halide were very dependent on the dryness of the solvent eluting from the C18 cartridge. A semi-automated system was constructed on the basis of the above results and evaluated by preparing new ^<18>F-labeled radioligands from [^<18>F] fluorobenzyl halide.