title: A thiol-reactive 18F-labeling agent, N-[2-(4-18F-fluorobenzamido)ethyl]maleimide, and synthesis of RGD peptide-based tracer for PET imaging of alpha v beta 3 integrin expression.
abstract: The cell adhesion molecule integrin alpha v beta 3 plays a key role in tumor angiogenesis and metastasis. A series of 18F-labeled RGD peptides have been developed for PET of integrin expression based on primary amine-reactive prosthetic groups. In this study we introduced a new method of labeling RGD peptides through a thiol-reactive synthon, N-[2-(4-18F-fluorobenzamido)ethyl]maleimide (18F-FBEM).
  
  
  METHODS
  18F-FBEM was synthesized by coupling N-succinimidyl 4-18F-fluorobenzoate (18F-SFB) with N-(2-aminoethyl)maleimide. After high-pressure liquid chromatography purification, it was allowed to react with thiolated RGD peptides, and the resulting tracers were subjected to receptor-binding assay, in vivo metabolic stability assessment, biodistribution, and microPET studies in murine xenograft models.
  
  
  RESULTS
  Conjugation of monomeric and dimeric sulfhydryl-RGD peptides with 18F-FBEM was achieved in high yields (85% +/- 5% nondecay-corrected on the basis of 18F-FBEM). The radiochemical purity of the 18F-labeled peptides was >98% and the specific activity was 100 approximately 150 TBq/mmol. Noninvasive microPET and direct tissue sampling experiments demonstrated that both 18F-FBEM-SRGD (RGD monomer) and 18F-FBEM-SRGD2 (RGD dimer) had integrin-specific tumor uptake in subcutaneous U87MG glioma and orthotopic MDA-MB-435 breast cancer xenografts.
  
  
  CONCLUSIONS
  The new tracer 18F-FBEM-SRGD2 was synthesized with high specific activity via 18F-FBEM and the tracer exhibited high receptor-binding affinity, tumor-targeting efficacy, metabolic stability, as well as favorable in vivo pharmacokinetics. The new synthon 18F-FBEM developed in this study will also be useful for radiolabeling of other thiolated biomolecules.
authors.affiliation: Molecular Imaging Program at Stanford (MIPS) and Bio-X Program, Department of Radiology, Stanford University School of Medicine, Stanford, California 94305-5484, USA.
authors.forename: Weibo
authors.initials: W
authors.lastname: Cai
authors.forename: Xianzhong
authors.initials: X
authors.lastname: Zhang
authors.forename: Yun
authors.initials: Y
authors.lastname: Wu
authors.forename: Xiaoyuan
authors.initials: X
authors.lastname: Chen
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D005462:Fluorine Radioisotopes / Q000494:pharmacology*; D006801:Humans; D039302:Integrin alphaVbeta3 / Q000096:biosynthesis* / Q000737:chemistry*; D008301:Maleimides / Q000494:pharmacology*; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D009842:Oligopeptides / Q000737:chemistry*; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000494:pharmacology*; D013438:Sulfhydryl Compounds / Q000737:chemistry*
chemical_list: D005462:Fluorine Radioisotopes; D039302:Integrin alphaVbeta3; D008301:Maleimides; C512447:N-(2-(4-18F-fluorobenzamido)ethyl)maleimide; D009842:Oligopeptides; D019275:Radiopharmaceuticals; D013438:Sulfhydryl Compounds; C043592:maleimide; C047981:arginyl-glycyl-aspartic acid