title: 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 xc(-)) represents a potentially attractive PET biomarker to enable in vivo quantification of xc(-) activity in response to oxidative stress associated with disease. We have developed a system xc(-) 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 xc(-) 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, (18)F-5-fluoro-aminosuberic acid ((18)F-FASu), as a PET tracer for functional imaging of a cellular response to oxidative stress with remarkable tumor uptake and retention.
  
  
  METHODS
  (18)F-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 xc(-) 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 xc(-) as well as or better than L-Glu. The direct comparison of uptake of tritiated compounds demonstrated more efficient system xc(-) uptake of L-ASu than L-Glu. Radiosynthesis of (18)F-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. (18)F-FASu uptake was approximately 5 times greater than (18)F-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.
  
  
  CONCLUSIONS
  (18)F-FASu is a potentially useful metabolic tracer for PET imaging of a functional cellular response to oxidative stress. (18)F-FASu may provide more sensitive detection than (18)F-FDG in certain tumors.
authors.affiliation: Diagnostics and Biomedical Technologies, GE Global Research, Niskayuna, New York.
authors.forename: Jack M
authors.initials: JM
authors.lastname: Webster
authors.forename: Christine A
authors.initials: CA
authors.lastname: Morton
authors.forename: Bruce F
authors.initials: BF
authors.lastname: Johnson
authors.forename: Hua
authors.initials: H
authors.lastname: Yang
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Rishel
authors.forename: Brian D
authors.initials: BD
authors.lastname: Lee
authors.forename: Qing
authors.initials: Q
authors.lastname: Miao
authors.forename: Chittari
authors.initials: C
authors.lastname: Pabba
authors.forename: Donald T
authors.initials: DT
authors.lastname: Yapp
authors.forename: Paul
authors.initials: P
authors.lastname: Schaffer
mesh_terms: D000600:Amino Acids, Dicarboxylic / Q000138:chemical synthesis / Q000493:pharmacokinetics; D000818:Animals; D045744:Cell Line, Tumor; D006801:Humans; D007553:Isotope Labeling; D051379:Mice; D008819:Mice, Nude; D009367:Neoplasm Staging; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism; D018384:Oxidative Stress / Q000502:physiology*; D049268:Positron-Emission Tomography / Q000379:methods; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D014018:Tissue Distribution; D023041:Xenograft Model Antitumor Assays
chemical_list: C000588667:5-fluoroaminosuberic acid; D000600:Amino Acids, Dicarboxylic; D019275:Radiopharmaceuticals
keywords: 18F; PET; functional imaging; oxidative stress; tumor imaging