title: MicroPET imaging of integrin αvβ3 expressing tumors using 89Zr-RGD peptides.
abstract: OBJECTIVE
  The dimeric transmembrane integrin, α(v)β(3), is a well-investigated target by different imaging modalities through suitably labeled arginine-glycine-aspartic acid (RGD) containing peptides. In this study, we labeled four cyclic RGD peptides with or without PEG functional groups: c(RGDfK) (denoted as FK), PEG(3)-c(RGDfK) (denoted as FK-PEG(3)), E[c(RGDfK)](2) (denoted as [FK](2)), and PEG(4)-E[PEG(4)-c(RGDfK)](2) (denoted as [FK](2)-3PEG(4)), with (89)Zr (t(1/2) = 78.4 h), using the chelator desferrioxamine-p-SCN (Df) for imaging tumor integrin α(v)β(3).
  
  
  METHODS
  The Df conjugated RGD peptides were subjected to integrin α(v)β(3) binding assay in vitro using MDA-MB-435 breast cancer cells. The (89)Zr-labeled RGD peptides were then subjected to small animal positron emission tomography (PET) and direct tissue sampling biodistribution studies in an orthotopic MDA-MB-435 breast cancer xenograft model.
  
  
  RESULTS
  All four tracers, (89)Zr-Df-FK, (89)Zr-Df-FK-PEG(3), (89)Zr-Df-[FK](2), and (89)Zr-Df-[FK](2)-3PEG(4), were labeled in high radiochemical yield (89 ± 4%) and high specific activity (4.07-6 MBq/μg). Competitive binding assay with (125)I-echistatin showed that conjugation of the RGD peptides to the Df chelator did not have significant impact on their integrin α(v)β(3) binding affinity and the dimeric peptides were shown to be more potent than the monomers. In agreement with binding results, tumor uptake of (89)Zr-Df-[FK](2) and (89)Zr-Df-[FK](2)-3PEG(4) was significantly higher (4.32 ± 1.73%ID/g and 4.72 ± 0.66%ID/g, respectively, at 2 h post-injection) than the monomers (89)Zr-Df-FK and (89)Zr-Df-FK-PEG(3) (1.97 ± 0.38%ID/g and 1.57 ± 0.49%ID/g, respectively, at 2 h post-injection). Out of the four labeled peptides, (89)Zr-Df-[FK](2)-3PEG(4) gave the highest tumor-to-background ratio (18.21 ± 2.52 at 2 h post-injection and 19.69 ± 3.99 at 4 h post-injection), with the lowest uptake in metabolic organs. Analysis of late time points biodistribution data revealed that the uptake in the tumor was decreased, along with increase in the bone, which implies decomplexation of (89)Zr-Df.
  
  
  CONCLUSIONS
  Efficient radiolabeling of peptides with an appropriate chelator such as Df-RGD with (89)Zr was observed. The (89)Zr radiolabeled peptides provided high-quality and high-resolution microPET images in xenograft models. (89)Zr-Df-[FK](2)-3PEG(4) demonstrated the highest tumor-to-background ratio of the compounds tested. Preparation of (89)Zr peptides to take advantage of the longer half-life is unwarranted due to the relatively rapid clearance from the tumor region of peptide tracers prepared for this study and the increased uptake in the bone of transchelated (89)Zr with time (2.0 ± 0.36%ID/g, 24 h post-injection).
authors.affiliation: Laboratory of Molecular Imaging and Nanomedicine (LOMIN), National Institute of Biomedical Imaging and Bioengineering (NIBIB), National Institutes of Health (NIH), Bethesda, MD 20892, USA.
authors.forename: Orit
authors.initials: O
authors.lastname: Jacobson
authors.forename: Lei
authors.initials: L
authors.lastname: Zhu
authors.forename: Gang
authors.initials: G
authors.lastname: Niu
authors.forename: Ido D
authors.initials: ID
authors.lastname: Weiss
authors.forename: Lawrence P
authors.initials: LP
authors.lastname: Szajek
authors.forename: Ying
authors.initials: Y
authors.lastname: Ma
authors.forename: Xilin
authors.initials: X
authors.lastname: Sun
authors.forename: Yongjun
authors.initials: Y
authors.lastname: Yan
authors.forename: Dale O
authors.initials: DO
authors.lastname: Kiesewetter
authors.forename: Shuang
authors.initials: S
authors.lastname: Liu
authors.forename: Xiaoyuan
authors.initials: X
authors.lastname: Chen
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D045744:Cell Line, Tumor; D006801:Humans; D039302:Integrin alphaVbeta3 / Q000378:metabolism*; D036341:Intercellular Signaling Peptides and Proteins; D007457:Iodine Radioisotopes; D008024:Ligands; D051379:Mice; D008819:Mice, Nude; D009369:Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism*; D009842:Oligopeptides / Q000737:chemistry / Q000378:metabolism*; D010455:Peptides / Q000378:metabolism; D049268:Positron-Emission Tomography / Q000379:methods*; D014018:Tissue Distribution; D015040:Zirconium / Q000737:chemistry / Q000378:metabolism*
chemical_list: D039302:Integrin alphaVbeta3; D036341:Intercellular Signaling Peptides and Proteins; D007457:Iodine Radioisotopes; D008024:Ligands; D009842:Oligopeptides; D010455:Peptides; C057986:echistatin; C047981:arginyl-glycyl-aspartic acid; D015040:Zirconium