Synthesis of O-(2-[18F]Fluoroethyl)-L-tyrosine based on a cartridge cleaning method

Objectives O-(2-[18F]fluoroethyl)-L-tyrosine (FET) is widely used as PET tracer for brain tumors. Usually a HPLC purification is used. The aim of this study was to develop a cleaning procedure based on standard cartridges.

Methods FET was prepared by a [18F]-fluoroethylation of L-tyrosine by a two step synthesis. The [18F]-fluoride was collected on an AG1X8 column and eluted with 0.4 mL potassium carbonate into the first reactor (modified C-11 methionine module, Nuclear Interface). 20 mg Kryptofix 2.2.2 in 1 mL acetonitrile were added. The solvent was evaporated at 90 °C (helium; reduced preasure). After addition of 15 mg ethylene glycol-1,2-ditosylate in 1.0 mL acetonitrile the mixture was heated 6 min at 90°C. 1.0 mL water was added, the formed [F18]fluorethyltosylate was collected on an EN cartridge and extracted with hot DMSO (3.0 mL) into the second reactor which contains the L-tyrosine disodium salt in 1 mL DMSO. The mixture was heated at 90°C for 10 minutes cleaned by a SCX and HRX cartridge. The formed FET was collected on the SCX cartridge and eluted with 5 mL NaCl solution (0.9 %). The FET was now collected on the HRX cartridge. After drying the cartridges with helium the FET was eluted with 15.0 mL ammonium acetate buffer and subsequent collected on two SCX cartridges. The SCX cartridges were washed with 1.0 mL of water. The final product was eluted with 10 mL NaCl solution (0.9%). The product was neutralized (450 µL NaHCO₃ solution, 8.4 %) and sterile filtered.

Results The synthesis is finished after 50 min and typically yielded 41 ± 4 % [18F]-FET. The radiochemical purity was higher then 97 %.

Conclusions The HPLC cleaning procedure could be successful substituted by a cleaning step based on standard cartridges