Comparative Evaluation of [18F]5-Fluoroaminosuberic Acid and (4S)-4-3-[18F]fluoropropyl)-L-Glutamate as System x2 C -Targeting Radiopharmaceuticals

System x -C is an appealing biomarker for targeting oxidative stress with oncologic PET imaging and can serve as an alternative PET biomarker to other metabolic indicators. In this paper, we report a direct comparison of 2 18F-labeled amino acid radiopharmaceuticals targeting system x -C, [18F]5-fluoroaminosuberic acid ([18F]FASu) and (4S)-4(3-[18F]fluoropropyl)-L-glutamate ([18F]FSPG), in terms of their uptake specificity and ability to image glioma and lung cancer xenografts in vivo. Methods: Both tracers were synthesized according to previously published procedures. In vitro uptake specificity assays were conducted using prostate (PC-3), glioblastoma (U-87), colorectal (HT 29), ovarian (SKOV3), breast (MDA-MB-231), and lung cancer (A549) cell lines. PET/CT imaging and biodistribution studies were conducted in immunocompromised mice bearing U-87 or A549 xenografts. Results: In vitro cell uptake assays showed that the tracers accumulated in cancer cells in a time-dependent manner and that the uptake of [18F]FASu was blocked by the system x -C inhibitor sulfasalazine and rose bengal, but not by system L inhibitor 2-aminobicyclo-(2,2,1)heptane-2-carboxylic acid, system x- AG inhibitor L-trans-pyrrolidine-2, 4-dicarboxylic acid, or L-serine, which is a substrate for transporter systems A, ACS, B0, and B0,+. Conversely, [18F]FSPG uptake decreased significantly in the presence of an excess of L-trans-pyrrolidine-2,4-dicarboxylic acid in 2 of 3 tested cell lines, indicating some reliance on system x -AG in these cells. In an in vivo setting, [18F]FASu and [18F]FSPG generated good-contrast PET images in U-87 and A549 tumor-bearing mice. Tracer accumulation in A549 tumors was 5.0 & PLUSMN; 0.8 percentage injected dose (%ID)/g ([18F]FASu, n >-5) and 6.3 & PLUSMN; 1.3 %ID/g ([18F]FSPG, n >-6, P = 0.7786), whereas U-87 xenografts demonstrated uptake of 6.1 & PLUSMN;2.4 %ID/g ([18F]FASu, n >-4) and 11.2 & PLUSMN; 4.1 %ID/g ([18F]FSPG, n >-4, P = 0.0321) at 1 h after injection. Conclusion: [18F]FSPG had greater in vitro uptake than [18F]FASu in all cell lines tested; however, our results indicate that residual uptake differences exist between [18F]FSPG and [18F]FASu, suggesting alternative transporter activity in the cell lines tested. In vivo studies demonstrated the ability of both [18F]FASu and [18F]FSPG to image glioblastoma (U-87) and non-small cell lung cancer (A549) xenografts.