Robust and Facile Automated Radiosynthesis of [18F]FSPG on the GE FASTlab

Purpose: (S)-4-(3-¹⁸F-Fluoropropyl)-ʟ-Glutamic Acid ([¹⁸F]FSPG) is a radiolabeled non-natural amino acid that is used for positron emission tomography (PET) imaging of the glutamate/cystine antiporter, system x_C^-, whose expression is upregulated in many cancer types. To increase the clinical adoption of this radiotracer, reliable and facile automated procedures for [¹⁸F]FSPG production are required. Here, we report a cassette-based method to produce [¹⁸F]FSPG at high radioactivity concentrations from low amounts of starting activity.

Procedures: An automated synthesis and purification of [¹⁸F]FSPG was developed for the GE FASTlab. Optimization of the reaction conditions and automated manipulations were performed by measuring the isolated radiochemical yield of [¹⁸F]FSPG and by assessing radiochemical purity using radioHPLC. Purification of [¹⁸F]FSPG was conducted by trapping and washing of the radiotracer on MCX SepPak catridges, followed by a reverse elution of [¹⁸F]FSPG in phosphate-buffered saline. Subsequently, the [¹⁸F]FSPG obtained from the optimized process was used to image an animal model of non-small cell lung cancer.

Results: The optimized protocol produced [¹⁸F]FSPG in 38.4 ± 2.6% RCY and 96% radiochemical purity. Small alterations, including the implementation of a reverse elution and an altered hypercarb cartridge, lead to significant improvements in radiotracer concentration from <10 MBq/mL to >100 MBq/mL. The improved radiotracer concentration allowed for the imaging of up to 20 mice, starting with just 1.5 GBq of [¹⁸F]fluoride.

Conclusions: We have developed a robust and facile method for [¹⁸F]FSPG radiosynthesis in high radiotracer concentration, RCP and RCY. This cassette-based method enabled the production of [¹⁸F]FSPG at radioactive concentrations sufficient to facilitate large-scale preclinical experiments with a single prep of starting activity. The use of cassettes for an ‘out the box’ synthesis on a synthesis module routinely used for clinical production make the method amenable to rapid and widespread clinical translation.