Introduction: [ 18 F]Fluorination method with protic solvents showed the possibility of high [ 18 F]fluorination yield and reproducibility. In this experiment, we evaluated various alcohol solvent possibility for [ 18 F]fluorination in the [ 18 F]fluorothymidine ([ 18 F]FLT) synthesis and we also developed an automatic synthesis of [ 18 F]FLT with a protic solvent.
Experimental: We used t-BuOH (2,2-dimethyl-1-propanol), t-amyl alcohol (2-methyl-2-butanol), 2,3-dimethyl-2-butanol, 3,3-dimethyl-2-butanol, 3,3-dimethyl-1-butanol, and 2-trifluoromethyl-2-propanol for [ 18 F]fluorination. After trapping of 185 MBq/0.5 mL of [ 18 F]F -on PS-HCO 3 cartridge, we eluted it with TBAHCO 3 20 ฮผL, 300 ฮผL CH 3 CN and 300 ฮผL H 2 O mixture solution. We have a [ 18 F]fluorination at 100ยฐC for 5-30 min with 20 mg of precursor as (5'-O-DMTr-2'-deoxy-3'-O-nosyl-ฮฒ-D-threo-pentofuranosyl)-3-N-BOC-thymine and evaluated [ 18 F]fluorination yield with radioTLC. Commercially available GE TracerLab MX was used for the automatic synthesis of [ 18 F]FLT. This MX module has blue, red, yellow and green reagent vials. We added 7 mL of CH 3 CN to the blue vial; 20 mg of precursor, 2 mL t-amyl alcohol and 200 ฮผL of CH 3 CN to the red; 1.75 mL 2 N NaOH and 0.7 mL citrate buffer to the yellow; and 3 mL 1 N HCl and 0.25 mL acetonitrile to the green vial. We have 10 minutes of [ 18 F]fluorination at 120ยฐC. For hydrolysis, we have 5 minutes at 85ยฐC. The reaction mixture was moved to HPLC purification system automatically, and we purified [ 18 F]FLT with EtOH:H 2 O=10:90 solution at 5ml/min. Quality control procedures were evaluated according to the previous reported methods.
Results and Discussion: Each alcohol solvent showed different [ 18 F]fluorination yield on radioTLC analysis. The highest [ 18 F]fluorination yield was 94.09ยฑ4.42% with 2,3-dimethyl-2-butanol at 5 min on radioTLC (n=3). Tertiary alcohol solvent except 2-trifluoromethyl-2-propanol showed > 85% of radiochemical yield on the radioTLC analysis after hydrlysis, but primary and secondary alcohol showed 26.27-71.79% of low radiochemical yield. In the automatic synthesis, t-amyl alcohol showed 64.6ยฑ6.1% of radiochemical yield after HPLC purification. (n=43) The highest radiochemical yield was 74.0% and the lowest radiochemical yield was 53.0%. After HPLC purification from automatic preparation, [ 18 F]FLT showed same retention time with cold FLT and no residual alcohol solvent in the gas chromatography analysis.
Conclusion:
We evaluated various alcohol solvents for high radiochemical yield [ 18 F]FLT synthesis and bulky tertiary alcohol showed high radiochemical yield except 2-trifluoromethyl-2-propanol. In the automatic synthesis, t-amyl alcohol showed stable and high radiochemical yield to synthesize [ 18 F]FLT.