Synthesis of O-(2-[18F]fluoroethyl)-l-tyrosine based on a cartridge purification method

Abstract

O-(2-[(18)F]fluoroethyl)-L-tyrosine (FET) is widely used as a positron emission tomography tracer for brain tumors. Usually, a high-performance liquid chromatography (HPLC) purification at the end of the two-step synthesis is applied. In this work, we report an automatic radiosynthesis of FET with a purification procedure based on standard cartridges. O-(2-[(18)F]fluoroethyl)-L-tyrosine was prepared by [(18)F]fluoroethylation of L-tyrosine by a two-step synthesis using a modified [(11)C]methionine module (Nuclear Interface). In the first reaction step, we synthesized [(18)F]fluoroethyltosylate starting from [(18)F]fluoride. After a purification step, L-tyrosine was [(18)F]fluoroethylated with [(18)F]fluoroethyltosylate. The final reaction mixture was purified by means of solid phase extraction. The FET was trapped on an SCX cartridge, eluted with saline solution and trapped again on an HRX cartridge. For a second purification step, the FET was eluted from the HRX cartridge with ammonium acetate buffer and collected on two SCX cartridges followed by a washing step with water. The final product was eluted with saline solution and neutralised with 450 μl NaHCO(3) solution (8.4%). The synthesis was finished after 50 min and delivered the FET in a range of 3-16 GBq. The synthesis typically yielded 41% (21 experiments) of FET (d.c.) without an HPLC purification step. The radiochemical purity ranged between 97% and 100%. We present a radiosynthesis of FET where the usually used HPLC purification procedure has been substituted by a purification step based on standard cartridges. This method is useful for automatic modules without an expensive HPLC purification unit and for the routine production of FET.