Functional Imaging of Oxidative Stress with a Novel PET Imaging Agent, ¹⁸F-5-Fluoro-l-Aminosuberic Acid

Abstract

Glutathione is the predominant endogenous cellular antioxidant, playing a critical role in the cellular defensive response to oxidative stress by neutralizing free radicals and reactive oxygen species. With cysteine as the rate-limiting substrate in glutathione biosynthesis, the cystine/glutamate transporter (system x_c^-) represents a potentially attractive PET biomarker to enable in vivo quantification of x_c^- activity in response to oxidative stress associated with disease. We have developed a system x_c^- substrate that incorporates characteristics of both natural substrates, l-cystine and l-glutamate (L-Glu). l-aminosuberic acid (L-ASu) has been identified as a more efficient system x_c^- substrate than L-Glu, leading to an assessment of a series of anionic amino acids as prospective PET tracers. Herein, we report the synthesis and in vitro and in vivo validation of a lead candidate, ¹⁸F-5-fluoro-aminosuberic acid (¹⁸F-FASu), as a PET tracer for functional imaging of a cellular response to oxidative stress with remarkable tumor uptake and retention. Methods: ¹⁸F-FASu was identified as a potential PET tracer based on an in vitro screening of compounds similar to l-cystine and L-Glu. Affinity toward system x_c^- was determined via in vitro uptake and inhibition studies using oxidative stress–induced EL4 and SKOV-3 cells. In vivo biodistribution and PET imaging studies were performed in mice bearing xenograft tumors (EL4 and SKOV-3). Results: In vitro assay results determined that L-ASu inhibited system x_c^- as well as or better than L-Glu. The direct comparison of uptake of tritiated compounds demonstrated more efficient system x_c^- uptake of L-ASu than L-Glu. Radiosynthesis of ¹⁸F-FASu allowed the validation of uptake for the fluorine-bearing derivative in vitro. Evaluation in vivo demonstrated primarily renal clearance and uptake of approximately 8 percentage injected dose per gram in SKOV-3 tumors, with tumor-to-blood and tumor-to-muscle ratios of approximately 12 and approximately 28, respectively. ¹⁸F-FASu uptake was approximately 5 times greater than ¹⁸F-FDG uptake in SKOV-3 tumors. Dynamic PET imaging demonstrated uptake in EL4 tumor xenografts of approximately 6 percentage injected dose per gram and good tumor retention for at least 2 h after injection. Conclusion: ¹⁸F-FASu is a potentially useful metabolic tracer for PET imaging of a functional cellular response to oxidative stress. ¹⁸F-FASu may provide more sensitive detection than ¹⁸F-FDG in certain tumors.