title: Evaluation of two novel ⁶⁴Cu-labeled RGD peptide radiotracers for enhanced PET imaging of tumor integrin αvβ₃.
abstract: OBJECTIVE
  Our goal was to demonstrate that suitably derivatized monomeric RGD peptide-based PET tracers, targeting integrin αvβ3, may offer advantages in image contrast, time for imaging, and low uptake in nontarget tissues.
  
  
  METHODS
  Two cyclic RGDfK derivatives, (PEG)2-c(RGDfK) and PEG4-SAA4-c(RGDfK), were constructed and conjugated to NOTA for (64)Cu labeling. Their integrin αvβ3-binding properties were determined via a competitive cell binding assay. Mice bearing U87MG tumors were intravenously injected with each of the (64)Cu-labeled peptides, and PET scans were acquired during the first 30 min, and 2 and 4 h after injection. Blocking and ex vivo biodistribution studies were carried out to validate the PET data and confirm the specificity of the tracers.
  
  
  RESULTS
  The IC50 values of NOTA-(PEG)2-c(RGDfK) and NOTA-PEG4-SAA4-c(RGDfK) were 444 ± 41 nM and 288 ± 66 nM, respectively. Dynamic PET data of (64)Cu-NOTA-(PEG)2-c(RGDfK) and (64)Cu-NOTA-PEG4-SAA4-c(RGDfK) showed similar circulation t 1/2 and peak tumor uptake of about 4 %ID/g for both tracers. Due to its marked hydrophilicity, (64)Cu-NOTA-PEG4-SAA4-c(RGDfK) provided faster clearance from tumor and normal tissues yet maintained excellent tumor-to-background ratios. Static PET scans at later time-points corroborated the enhanced excretion of the tracer, especially from abdominal organs. Ex vivo biodistribution and receptor blocking studies confirmed the accuracy of the PET data and the integrin αvβ3-specificity of the peptides.
  
  
  CONCLUSIONS
  Our two novel RGD-based radiotracers with optimized pharmacokinetic properties allowed fast, high-contrast PET imaging of tumor-associated integrin αvβ3. These tracers may facilitate the imaging of abdominal malignancies, normally precluded by high background uptake.
authors.affiliation: Department of Medical Physics, University of Wisconsin, Madison, WI, 53705, USA.
authors.forename: Reinier
authors.initials: R
authors.lastname: Hernandez
authors.affiliation: Peptides International, Inc., Louisville, KY, 40299, USA.
authors.forename: Andrzej
authors.initials: A
authors.lastname: Czerwinski
authors.affiliation: Department of Radiology, University of Wisconsin, Madison, WI, 53705, USA.
authors.forename: Rubel
authors.initials: R
authors.lastname: Chakravarty
authors.affiliation: Department of Medical Physics, University of Wisconsin, Madison, WI, 53705, USA.
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Graves
authors.affiliation: Department of Radiology, University of Wisconsin, Madison, WI, 53705, USA.
authors.forename: Yunan
authors.initials: Y
authors.lastname: Yang
authors.affiliation: Department of Radiology, University of Wisconsin, Madison, WI, 53705, USA.
authors.forename: Christopher G
authors.initials: CG
authors.lastname: England
authors.affiliation: Department of Medical Physics, University of Wisconsin, Madison, WI, 53705, USA.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Nickles
authors.affiliation: Peptides International, Inc., Louisville, KY, 40299, USA.
authors.forename: Francisco
authors.initials: F
authors.lastname: Valenzuela
authors.affiliation: Department of Medical Physics, University of Wisconsin, Madison, WI, 53705, USA. wcai@uwhealth.org.|Department of Radiology, University of Wisconsin, Madison, WI, 53705, USA. wcai@uwhealth.org.|University of Wisconsin Carbone Cancer Center, Madison, WI, 53705, USA. wcai@uwhealth.org.
authors.forename: Weibo
authors.initials: W
authors.lastname: Cai
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D045744:Cell Line, Tumor; D003301:Copper Radioisotopes; D004355:Drug Stability; D005260:Female; D006801:Humans; D039302:Integrin alphaVbeta3 / Q000378:metabolism*; D007553:Isotope Labeling; D008297:Male; D051379:Mice; D009842:Oligopeptides / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D049268:Positron-Emission Tomography / Q000379:methods*; D011849:Radioactive Tracers; D014018:Tissue Distribution
chemical_list: D003301:Copper Radioisotopes; D039302:Integrin alphaVbeta3; D009842:Oligopeptides; D011849:Radioactive Tracers; C047981:arginyl-glycyl-aspartic acid
keywords: Angiogenesis; Copper-64 (64Cu); Integrin αvβ3; Molecular imaging; Positron emission tomography (PET); RGD peptide