CGMP -compliant automated 18F-FSPG production for clinical examination

Introduction: (S)-4-(3-¹⁸F-Fluoropropyl)-L-Glutamic Acid (¹⁸F-FSPG) is a positron emission tomography (PET) tracer that specifically targets cystine/glutamate antiporter (xc^-) – a biomarker that is frequently overexpressed in cancer and several neurological disorders. Pilot studies examining dosimetry and biodistribution in healthy volunteers and tumor detection in patients with non-small cell lung cancer, hepatocellular carcinoma, and brain tumors showed promising results. In particular, low background uptake in the brain, lung, and bowel was observed that further leads to excellent imaging contrasts of ¹⁸F-FSPG PET. However, reliable and cGMP-compliant automated procedures for ¹⁸F-FSPG production are still lacking to increase the clinical adoption of this radiotracer. Herein we report an optimized automated approach to produce ¹⁸F-FSPG adaptable to two commercially available radiosynthesizers that enable the centralized and large-scale production for clinical use.

Methods: Automated procedures to produce ¹⁸F-FSPG were developed that were compatible with two commercially available radiosynthesizers (GE Tracerlab FXN and Fastlab). After azeotropic drying of ¹⁸F-fluoride (11.1-111 GBq) using krytofix/potassium carbonate solution as base, 6.0-12.0 mg of the precursor in anhydrous acetonitrile was added to the reaction vessel and the reaction proceeded at 105°C for 5 minutes to promote the substitution of the ¹⁸F for the sulfonate leaving group. Next, the 1M sulfuric acid solution was added and the reaction is heated at 105°C for 4 minutes to reveal the carboxylic acid groups. The 4.0M sodium hydroxide solution was added and the reaction was heated to 70°C for a period of 5 minutes. The solution was then allowed to cool and a second batch of the 1M sulfuric acid was added to acidify the mixture. The reaction mixture was transferred onto the two MCX Oasis cartridges and the cartridges were washed with water to remove impurities. The ¹⁸F-FSPG was desorbed from the cartridges using the phosphate buffered saline (PBS), and further purified through an alumina Sep-Pak cartridge and an ENVI-carb column. After sterile filtration, quality control for ¹⁸F-FSPG was performed under the USP<823> guidelines.

Results: The reported fully-automated process to produce ¹⁸F-FSPG required less than 40 minutes with radiochemical yields ranging from 13-35% (non-decay corrected) and radiochemical purities of > 90% using either radiosynthesizer. The formulated ¹⁸F-FSPG solution was determined to be sterile and colorless with the pH of 6.5-7.5. No radiolysis of the product was observed up to 6 hours after final batch formulation (3.7-37.0 GBq in 20 mL PBS solution).

Conclusions: CGMP-compliant radiosynthesis and quality control of ¹⁸F-FSPG have been established on two commercially available synthesizers in high activity concentration and radiochemical purity. While the clinical trials using ¹⁸F-FSPG PET are currently underway, these automated approaches reported herein mitigate certain production-related challenges associated with clinical adoption of this radiotracer and support centralized and large-scale production of ¹⁸F-FSPG.