Quality control for [18F]AlF-FAPI-74

All radioactivity, including [¹⁸F]AlF-FAPI-74, was measured using a dose calibrator. Measurement of the radiochemical purity, non-radioactive AlF-FAPI-74 (AlF-FAPI-74), and chemical impurities were carried out using an SPD-20A ultraviolet (UV) detector (λ = 264 nm) using a Shimadzu HPLC system and a Gabi-Star radioactivity detector (Elysia-Raytest, Straubenhardt, Germany). A Chromolith performance RP-18e (100 mm × 4.6 mm) as an analysis column was selected, and the mobile phase was acetonitrile (solvent A) and 0.1% trifluoroacetic acid (solvent B) in gradient mode; its condition was at 95% of solvent B from 0 to 3 min and then at 95% to 50% from 3 to 15 min. The method was calibrated using AlF-FAPI-74 as the reference standard (Linearity in the range from 0.05 to 20 µg/mL and the coefficient of determination of > 0.995 were confirmed). [¹⁸F]AlF-FAPI-74 was determined based on the retention time (RT) of the reference standard and the amount of carrier AlF-FAPI-74 calculated by calibration; the amount of all other chemical impurities was calculated based on the same calibration assuming similar extinction coefficients as AlF-FAPI-74. The set value of the column oven was 30 °C, and the flow rate was 2.0 mL/min.

Residual [¹⁸F]fluoride and [¹⁸F]AlF were analyzed using a radio-TLC analyzer, mini-Gita (Elysia-Raytest, Straubenhardt, Germany). The TLC plate used was HPTLC Silica gel 60 RP-18 and 1 v/v% phosphoric acid in saline/acetonitrile (50/50) as a developing solvent was filled in the tank for more than an h before the plate was deployed.

The residual DMSO and ethanol contents were measured using a flame ionization detector of a Shimadzu GC system. A DB-624 30 m × 0.32 mm, 1.8 µm (Agilent, Santa Clara, CA, USA) as an analysis column was selected and split injection mode (split ratio = 30:1). The carrier gas (helium) was 2 mL/min, and the column, injector, and detector temperatures were 40 °C for 5 min to 200 °C (20 °C/min) for 3 min, 200 °C, and 250 °C, respectively.

pH adjustment of buffers, etc., was performed using a pH meter with electrodes set for low volume (HORIBA, Kyoto, Japan), calibrated with pH standard buffer solution before use.