Improved specific radioactivity of the PET radioligand [C-11]FLB 457 by use of the GE Medical Systems PETtrace MeI MicroLab

[C-11]FLB 457 is a high affinity dopamine D-2 receptor radioligand that is used for visualisation and quantitation of extrastriatal dopamine D-2 recep tors with positron emission tomography (PET). In this study, we report a co mparison regarding the specific radioactivity of [C-11]FLB 457 obtained by two different methods of synthesising [C-11]methyl iodide. In addition, the synthesis of unlabelled FLB 457 and the corresponding desmethyl-precursor, starting from commercially available material, is reported. The first meth od used for [C-11]methyl iodide synthesis was reduction of [C-11]CO2 with l ithium aluminium hydride in tetrahydrofuran to [C-11]CH3OH, followed by con version into [C-11]CH3I with hydrogen iodide. The second, recently develope d method uses gas phase halogenation of [C-11]CH4 with iodine. [C-11]FLB 45 7 was labelled with [C-11]methyl triflate produced on-line from [C-11]methy l iodide. With the first method a specific radioactivity for [C-11]FLB 457 of 2100 Ci/mmol (78 GBq/mu mol) (n=13) at 40 min after end of bombardment ( EOB) was achieved. Using the gas phase method a specific radioactivity of 3 400 Ci/mmol (126 GBq/mu mol) (n=7) at 40 min EOB could be obtained. The use of the gas phase method also resulted in shorter time for set-up compared to the regular method since no wet chemistry is involved in the preparation of [C-11]methyl iodide.