title: Noninvasive imaging of tumor integrin expression using (18)F-labeled RGD dimer peptide with PEG (4) linkers.
abstract: OBJECTIVE
  Various radiolabeled Arg-Gly-Asp (RGD) peptides have been previously investigated for tumor integrin alpha(v)beta(3) imaging. To further develop RGD radiotracers with enhanced tumor-targeting efficacy and improved in vivo pharmacokinetics, we designed a new RGD homodimeric peptide with two PEG(4) spacers (PEG(4) = 15-amino-4,7,10,13-tetraoxapentadecanoic acid) between the two monomeric RGD motifs and one PEG(4) linker on the glutamate alpha-amino group ((18)F-labeled PEG(4)-E[PEG(4)-c(RGDfK)](2), P-PRGD2), as a promising agent for noninvasive imaging of integrin expression in mouse models.
  
  
  METHODS
  P-PRGD2 was labeled with (18)F via 4-nitrophenyl 2-(18)F-fluoropropionate ((18)F-FP) prosthetic group. In vitro and in vivo characteristics of the new dimeric RGD peptide tracer (18)F-FP-P-PRGD2 were investigated and compared with those of (18)F-FP-P-RGD2 ((18)F-labeled RGD dimer without two PEG(4) spacers between the two RGD motifs). The ability of (18)F-FP-P-PRGD2 to image tumor vascular integrin expression was evaluated in a 4T1 murine breast tumor model.
  
  
  RESULTS
  With the insertion of two PEG(4) spacers between the two RGD motifs, (18)F-FP-P-PRGD2 showed enhanced integrin alpha(v)beta(3)-binding affinity, increased tumor uptake and tumor-to-nontumor background ratios compared with (18)F-FP-P-RGD2 in U87MG tumors. MicroPET imaging with (18)F-FP-P-PRGD2 revealed high tumor contrast and low background in tumor-bearing nude mice. Biodistribution studies confirmed the in vivo integrin alpha(v)beta(3)-binding specificity of (18)F-FP-P-RGD2. (18)F-FP-P-PRGD2 can specifically image integrin alpha(v)beta(3) on the activated endothelial cells of tumor neovasculature.
  
  
  CONCLUSIONS
  (18)F-FP-P-PRGD2 can provide important information on integrin expression on the tumor vasculature. The high integrin binding affinity and specificity, excellent pharmacokinetic properties and metabolic stability make the new RGD dimeric tracer (18)F-FP-P-PRGD2 a promising agent for PET imaging of tumor angiogenesis and for monitoring the efficacy of antiangiogenic treatment.
authors.affiliation: The Molecular Imaging Program at Stanford (MIPS), Department of Radiology, Biophysics, and Bio-X Program, Stanford University School of Medicine, Stanford, CA 94305, USA.
authors.forename: Zhaofei
authors.initials: Z
authors.lastname: Liu
authors.forename: Shuanglong
authors.initials: S
authors.lastname: Liu
authors.forename: Fan
authors.initials: F
authors.lastname: Wang
authors.forename: Shuang
authors.initials: S
authors.lastname: Liu
authors.forename: Xiaoyuan
authors.initials: X
authors.lastname: Chen
mesh_terms: D000818:Animals; D001386:Azides / Q000737:chemistry; D019281:Dimerization; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D018698:Glutamic Acid / Q000737:chemistry; D006801:Humans; D039302:Integrin alphaVbeta3 / Q000378:metabolism*; D007457:Iodine Radioisotopes; D007553:Isotope Labeling; D051379:Mice; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000235:genetics* / Q000378:metabolism; D009842:Oligopeptides / Q000737:chemistry* / Q000378:metabolism / Q000493:pharmacokinetics; D011092:Polyethylene Glycols / Q000737:chemistry*; D049268:Positron-Emission Tomography; D011854:Radiochemistry; D014018:Tissue Distribution
chemical_list: D001386:Azides; D039302:Integrin alphaVbeta3; D007457:Iodine Radioisotopes; D009842:Oligopeptides; C102277:4-nitrophenyl 2-fluoropropionate; D018698:Glutamic Acid; D011092:Polyethylene Glycols; C047981:arginyl-glycyl-aspartic acid