Radiosynthesis of 18F-Labeled Peptides

Fluorine-18 was produced as [¹⁸F]fluoride ion by the ¹⁸O(p,n)¹⁸F reaction in [¹⁸O]water using a CTI/Siemens RDS112 11MeV cyclotron. [¹⁸F]fluoride was trapped by a QMA cartridge and eluted off with tetrabutylammonium bicarbonate (TBAB) into a 5-mL V-vial (Type I Borosilicate), followed by azeotropic drying. [¹⁸F]-fluoride was then dissolved in anhydrous MeCN for the labeling reactions.
Radiofluorination of 1⁺: In a typical experimental, 1⁺OTf^- (0.37 µmol) was mixed with SnCl₄ (3.0 µmol) in MeCN (20 µL). The resulting solution was then combined with a MeCN solution (50 µL) of [¹⁸F]fluoride (10 ± 3 mCi). After shaking at room temperature for 10 min, an aliquot of the reaction mixture (50-100 µCi) was collected for HPLC analysis. Integration of the radio-chromatogram indicated a conversion with a RCY > 95%. For animal studies, 3 ± 1 mCi of [¹⁸F]1⁺ was purified by HPLC. The HPLC mobile phase was then removed by rotary evaporation and the activity was reconstituted in 1 mL phosphate-buffered saline (PBS) and passed through a 0.22 µm syringe filter for in vivo animal experiments. Counted from the end of bombardment, the drying step was 25 min. Other steps include the reaction time (10 min), the sample preparation time (approx. 5 min), HPLC purification (20 min), and the rotary evaporation of the solvent (10-15 min).
Radiofluorination of 2: Compound 2 (130 µg, 0.23 µmol) was mixed with SnCl₄ (3.0 µmol) in MeCN (20 µL). The resulting solution was then combined with a MeCN solution (50 µL) of [¹⁸F]fluoride (10 ± 3 mCi). After shaking at room temperature for 10 min, a portion of the reaction mixture (3 ± 1 mCi) was loaded on the HPLC for purification. [¹⁸F]2 was obtained in a 79% labeling yield with an estimated specific activity of 35 ± 10 mCi/µmol.
Synthesis of [¹⁸F]2-RGD: [¹⁸F]2 was azeotropically dried twice at 80 °C using anhydrous MeCN. Then RGD (200 µg, 0.33 µmol) in DMSO (100 µL) was added to [¹⁸F]2 (2 mCi, 0.07 µmol), followed by addition of 2 µL diisopropylethylamine. The reaction remained at 50 °C under shaking for 15 min. After quenching the reaction with a 5% acetic acid solution (1 mL), a portion of the reaction mixture (1 ± 0.2 mCi) was loaded on the HPLC for purification. The HPLC solvents were removed by rotary evaporation and the activity was reconstituted in 1 mL PBS and passed through a 0.22 µm syringe filter for in vivo animal experiments. [¹⁸F]2-RGD was obtained in an 82% yield with an estimated specific activity of 19 ± 4 mCi/µmol.