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

Abstract

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 using 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 radio-HPLC. Purification of [¹⁸F]FSPG was conducted by trapping and washing of the radiotracer on Oasis MCX SPE cartridges, 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 % radiochemical yield and >96 % radiochemical purity with a molar activity of 11.1 ± 7.7 GBq/μmol. Small alterations, including the implementation of a reverse elution and an altered Hypercarb cartridge, led 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, radiochemical yield, and radiochemical purity. 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 a cassette-based radiosynthesis on an automated synthesis module routinely used for clinical production makes the method amenable to rapid and widespread clinical translation.

Keywords: Automated synthesis; Cancer imaging; FASTlab; Positron emission tomography; SPE purification; [18F]FSPG.

Fig. 1.

FASTlab cassette layouts for the radiosynthesis of [¹⁸F]FSPG. a Original cassette layout. b Optimised cassette layout to enable reverse elution. c ‘Normal’ loading, washing, and elution of [¹⁸F]FSPG. d ‘Normal’ loading and washing of [¹⁸F]FSPG, followed by a ‘reverse’ elution. Numbers refer to each individual valve or cassette position (CP). Consumables and reagents added to each position are described in Table 1. Tubing from the reactor to CP 25 is not shown for clarity. WFI, Water for injection.

Fig. 2.

Optimization of radiotracer concentration. a Set-up of switch valve and radio detector outside the ‘lead castle’ enclosing the FASTlab to facilitate remote collection at the point of highest radioactive concentration. b Real-time output of radioactivity detector during the elution process (radioactivity trace is in red). c Hypercarb cartridges Vs1 and Vs2. Dotted lines indicate the dead volume removed from Vs1 to produce Vs2. d Impact of Hypercarb cartridge Vs1 and Vs2 on radiotracer concentration. Concentration was normalized to total [¹⁸F]FSPG radioactivity (MBq/ml per 100 MBq of product) in order to account for any differences in starting activity or RCY. Error bars are SD. ****P < 0.0001.

Fig. 3.

Modeling [¹⁸F]FSPG concentration and preclinical utility. a [¹⁸F]FSPG concentration and utility over time when using Hypercarb Vs1. b [¹⁸F]FSPG concentration and utility over time when using Hypercarb Vs2.

Fig. 4.

In vivo [¹⁸F]FSPG PET/CT imaging of a mouse bearing a subcutaneous A549 tumor. Maximum intensity projection 40–60 min after [¹⁸F]FSPG injection following the manual removal of the bed. White arrowheads indicate the tumor margins. P, pancreas; B, bladder.