title: 18F-Labeled Single-Stranded DNA Aptamer for PET Imaging of Protein Tyrosine Kinase-7 Expression.
abstract: Protein tyrosine kinase-7 (PTK7), a member of receptor tyrosine kinase superfamily initially identified as colon carcinoma kinase-4, is highly expressed in various human malignancies. Its expression was found to correlate with aggressive biologic behaviors such as increased cell proliferation, invasiveness, and migration. Despite the importance and unmet need of imaging PTK7 in vivo, there is currently no clinically relevant method to visualize tumoral PTK7 expression noninvasively such as PET or SPECT. This study aimed to develop a specific, selective, and high-affinity PET radioligand based on single-stranded DNA aptamer to address this challenge.
  
  
  METHODS
  Sgc8, a 41-oligonucleotide that targets to PTK7, was labeled with (18)F using a 2-step radiochemical synthesis, which featured a direct 1-step radiofluorination on the distinctive spirocyclic hypervalent iodine(III) precursor to give (18)F-fluorobenzyl azide followed by copper-mediated click conjugation with Sgc8-alkyne. (18)F-Sgc8 was evaluated in vitro and in vivo in 2 cell lines, HCT116 and U87MG, which express high and low amounts of PTK7, respectively.
  
  
  RESULTS
  Sgc8 was labeled efficiently with (18)F in an isolated radiochemical yield of 62% ± 2%, non-decay-corrected based on (18)F-fluorobenzyl azide. (18)F-Tr-Sgc8 was found to possess high-affinity binding to both cell lines, with binding affinity values of 2.7 ± 0.6 nM for HCT116 and 16.9 ± 2.1 nM for U87MG. In vivo PET imaging clearly visualized PTK7 expression in HCT116 xenografted mice, with tumor uptake of 0.76 ± 0.09 percentage injected dose per gram (%ID/g) at 30 min after injection for the subcutaneous tumor model and greater than 1.5 %ID/g for the liver metastasis model. U87MG xenograft tumors had much lower tracer accumulation (0.13 ± 0.06 %ID/g at 30 min after injection), which was consistent with the lower expression of PTK7 in this tumor model. The labeled aptamer was rapidly cleared from the blood through the kidneys and bladder to give high tumor-to-blood and tumor-to-muscle ratios of 7.29 ± 1.51 and 10.25 ± 2.08, respectively.
  
  
  CONCLUSIONS
  The (18)F-radiolabeling methodology shown here is a robust procedure for labeling aptamers and similar chemical moieties and can be applied to many different targets. Quantification of PTK7 using (18)F-Tr-Sgc8 may be suitable for clinical translation and might help in the future to select and monitor appropriate therapies.
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Orit
authors: O
authors: Jacobson
authors: Laboratory of Molecular Immunology, National Institute of Allergy and Infectious Diseases; National Institutes of Health, Bethesda, MD, USA.
authors: Ido D
authors: ID
authors: Weiss
authors: Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, USA.
authors: Lu
authors: L
authors: Wang
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Zhe
authors: Z
authors: Wang
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Xiangyu
authors: X
authors: Yang
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Andrew
authors: A
authors: Dewhurst
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Ying
authors: Y
authors: Ma
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Guizhi
authors: G
authors: Zhu
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Gang
authors: G
authors: Niu
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Dale O
authors: DO
authors: Kiesewetter
authors: Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, USA.
authors: Neil
authors: N
authors: Vasdev
authors: Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA, USA.
authors: Steven H
authors: SH
authors: Liang
authors: Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering.
authors: Xiaoyuan
authors: X
authors: Chen
mesh_terms: D000818:Animals; D052157:Aptamers, Nucleotide / Q000737:chemistry*; D015815:Cell Adhesion Molecules / Q000096:biosynthesis* / Q000235:genetics; D045744:Cell Line, Tumor; D004277:DNA, Single-Stranded / Q000235:genetics*; D005462:Fluorine Radioisotopes; D015971:Gene Expression Regulation, Enzymologic / Q000235:genetics; D006801:Humans; D007553:Isotope Labeling; D051379:Mice; D009374:Neoplasms, Experimental / Q000000981:diagnostic imaging; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals / Q000737:chemistry*; D020794:Receptor Protein-Tyrosine Kinases / Q000096:biosynthesis* / Q000235:genetics; D014018:Tissue Distribution; D015899:Tomography, Emission-Computed, Single-Photon; D023041:Xenograft Model Antitumor Assays
chemical_list: C000604953:18F-Tr-Sgc8; D052157:Aptamers, Nucleotide; D015815:Cell Adhesion Molecules; D004277:DNA, Single-Stranded; D005462:Fluorine Radioisotopes; D019275:Radiopharmaceuticals; C096905:PTK7 protein, human; D020794:Receptor Protein-Tyrosine Kinases
keywords: 18F; PET imaging; PTK7; click chemistry; ssDNA aptamer