Automated synthesis of N-(2-[18 F]Fluoropropionyl)-l-glutamic acid as an amino acid tracer for tumor imaging on a modified [18 F]FDG synthesis module

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

PURPOSE

(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, xc- 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.

PROCEDURES

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.

RESULTS

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.

CONCLUSION

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.

Biological Evaluation of [18F]AlF-NOTA-NSC-GLU as a Positron Emission Tomography Tracer for Hepatocellular Carcinoma

Purpose: N-(2-[¹⁸F]fluoropropionyl)-L-glutamate ([¹⁸F]FPGLU) for hepatocellular carcinoma (HCC) imaging has been performed in our previous studies, but its radiosynthesis method and stability in vivo need to be improved. Hence, we evaluated the synthesis and biological properties of a simple [¹⁸F]-labeled glutamate analog, [¹⁸F]AlF-1,4,7-triazacyclononane-1,4,7-triacetic-acid-2-S-(4-isothiocyanatobenzyl)-l-glutamate ([¹⁸F]AlF-NOTA-NSC-GLU), for HCC imaging.

Procedures: [¹⁸F]AlF-NOTA-NSC-GLU was synthesized via a one-step reaction sequence from NOTA-NSC-GLU. In order to investigate the imaging value of [¹⁸F]AlF-NOTA-NSC-GLU in HCC, we conducted positron emission tomography/computed tomography (PET/CT) imaging and competitive binding of [¹⁸F]AlF-NOTA-NSC-GLU in human Hep3B tumor-bearing mice. The transport mechanism of [¹⁸F]AlF-NOTA-NSC-GLU was determined by competitive inhibition and protein incorporation experiments in vitro.

Results: [¹⁸F]AlF-NOTA-NSC-GLU was prepared with an overall radiochemical yield of 29.3 ± 5.6% (n = 10) without decay correction within 20 min. In vitro competitive inhibition experiments demonstrated that the Na⁺-dependent systems XAG-, B0⁺, ASC, and minor XC- were involved in the uptake of [¹⁸F]AlF-NOTA-NSC-GLU, with the Na⁺-dependent system XAG- possibly playing a more dominant role. Protein incorporation studies of the Hep3B human hepatoma cell line showed almost no protein incorporation. Micro-PET/CT imaging with [¹⁸F]AlF-NOTA-NSC-GLU showed good tumor-to-background contrast in Hep3B human hepatoma-bearing mouse models. After [¹⁸F]AlF-NOTA-NSC-GLU injection, the tumor-to-liver uptake ratio of [¹⁸F]AlF-NOTA-NSC-GLU was 2.06 ± 0.17 at 30 min post-injection. In vivo competitive binding experiments showed that the tumor-to-liver uptake ratio decreased with the addition of inhibitors to block the X_AG system.

Conclusions: We have successfully synthesized [¹⁸F]AlF-NOTA-NSC-GLU as a novel PET tracer with good radiochemical yield and high radiochemical purity. Our findings indicate that [¹⁸F]AlF-NOTA-NSC-GLU may be a potential candidate for HCC imaging. Also, a further biological evaluation is underway.