Radiosynthesis and Analysis of (S)-4-(3-[¹⁸F]Fluoropropyl)-L-Glutamic Acid

(S)-4-(3-[¹⁸F]Fluoropropyl)-L-glutamic acid ([¹⁸F]FSPG) is an L-glutamate derivative used as a PET biomarker to assess intracellular redox status in vivo through targeting of the cystine/glutamate antiporter protein, x_c^- transporter. In this report, we describe a radiosynthesis of [¹⁸F]FSPG for use in PET studies that address specific challenges in relation to the radiotracer purity, molar activity, and quality control testing methods.

The radiosynthesis of [¹⁸F]FSPG was performed using a customised RNPlus Research automated radiosynthesis system (Synthra GmbH, Hamburg, Germany). [¹⁸F]FSPG was labelled in the 3-fluoropropylmoiety at the 4-position of the glutamic acid backbone with fluorine-18 via substitution of nucleophilic [¹⁸F]fluoride with a protected naphthylsulfonyloxy-propyl-L-glutamate derivative. Radiochemical purity of the final product was determined by radio HPLC using a new method of direct analysis using a Hypercarb C₁₈ column.

The average radioactivity yield of [¹⁸F]FSPG was 4.2 GBq (range, 3.4-4.8 GBq) at the end of synthesis, starting from 16 GBq of [¹⁸F]fluoride at the end of bombardment (n = 10) in a synthesis time of 50 min. The average molar activity and radioactivity volumetric concentration at the end of synthesis were 66 GBq µmol^-1 (range, 48-73 GBq µmol^-1) and 343-400 MBq mL^-1, respectively.

Stability tests using a 4.6 GBq dose with a radioactivity volumetric concentration of 369 MBq mL^-1 at the end of synthesis showed no observable radiolysis 3 h after production. The formulated product is of high radiochemical purity (> 95%) and higher molar activity compared to previous methods and is safe to inject into mice up to 3 h after production.