title: Quantitative PET Imaging of Tissue Factor Expression Using 18F-Labeled Active Site-Inhibited Factor VII.
abstract: Tissue factor (TF) is upregulated in many solid tumors, and its expression is linked to tumor angiogenesis, invasion, metastasis, and prognosis. A noninvasive assessment of tumor TF expression status is therefore of obvious clinical relevance. Factor VII is the natural ligand to TF. Here we report the development of a new PET tracer for specific imaging of TF using an (18)F-labeled derivative of factor VII.
  
  
  METHODS
  Active site-inhibited factor VIIa (FVIIai) was obtained by inactivation with phenylalanine-phenylalanine-arginine-chloromethyl ketone. FVIIai was radiolabeled with N-succinimidyl 4-(18)F-fluorobenzoate and purified. The corresponding product, (18)F-FVIIai, was injected into nude mice with subcutaneous human pancreatic xenograft tumors (BxPC-3) and investigated using small-animal PET/CT imaging 1, 2, and 4 h after injection. Ex vivo biodistribution was performed after the last imaging session, and tumor tissue was preserved for molecular analysis. A blocking experiment was performed in a second set of mice. The expression pattern of TF in the tumors was visualized by immunohistochemistry and the amount of TF in tumor homogenates was measured by enzyme-linked immunosorbent assay and correlated with the uptake of (18)F-FVIIai in the tumors measured in vivo by PET imaging.
  
  
  RESULTS
  The PET images showed high uptake of (18)F-FVIIai in the tumor regions, with a mean uptake of 2.5 ± 0.3 percentage injected dose per gram (%ID/g) (mean ± SEM) 4 h after injection of 7.3-9.3 MBq of (18)F-FVIIai and with an average maximum uptake in the tumors of 7.1 ± 0.7 %ID/g at 4 h. In comparison, the muscle uptake was 0.2 ± 0.01 %ID/g at 4 h. At 4 h, the tumors had the highest uptake of any organ. Blocking with FVIIai significantly reduced the uptake of (18)F-FVIIai from 2.9 ± 0.1 to 1.4 ± 0.1 %ID/g (P < 0.001). The uptake of (18)F-FVIIai measured in vivo by PET imaging correlated (r = 0.72, P < 0.02) with TF protein level measured ex vivo.
  
  
  CONCLUSIONS
  (18)F-FVIIai is a promising PET tracer for specific and noninvasive imaging of tumor TF expression. The tracer merits further development and clinical translation, with potential to become a companion diagnostics for emerging TF-targeted therapies.
authors.affiliation: Minerva Imaging, Copenhagen, Denmark Department of Clinical Physiology, Nuclear Medicine & PET and Cluster for Molecular Imaging, Rigshospitalet and University of Copenhagen, Copenhagen, Denmark; and.
authors.forename: Carsten H
authors.initials: CH
authors.lastname: Nielsen
authors.affiliation: Department of Clinical Physiology, Nuclear Medicine & PET and Cluster for Molecular Imaging, Rigshospitalet and University of Copenhagen, Copenhagen, Denmark; and.
authors.forename: Maria
authors.initials: M
authors.lastname: Erlandsson
authors.affiliation: Department of Clinical Physiology, Nuclear Medicine & PET and Cluster for Molecular Imaging, Rigshospitalet and University of Copenhagen, Copenhagen, Denmark; and.
authors.forename: Troels E
authors.initials: TE
authors.lastname: Jeppesen
authors.affiliation: Minerva Imaging, Copenhagen, Denmark Department of Clinical Physiology, Nuclear Medicine & PET and Cluster for Molecular Imaging, Rigshospitalet and University of Copenhagen, Copenhagen, Denmark; and.
authors.forename: Mette M
authors.initials: MM
authors.lastname: Jensen
authors.affiliation: Minerva Imaging, Copenhagen, Denmark Department of Clinical Physiology, Nuclear Medicine & PET and Cluster for Molecular Imaging, Rigshospitalet and University of Copenhagen, Copenhagen, Denmark; and.
authors.forename: Lotte K
authors.initials: LK
authors.lastname: Kristensen
authors.affiliation: Department of Clinical Physiology, Nuclear Medicine & PET and Cluster for Molecular Imaging, Rigshospitalet and University of Copenhagen, Copenhagen, Denmark; and.
authors.forename: Jacob
authors.initials: J
authors.lastname: Madsen
authors.affiliation: Haemostasis Biology, Novo Nordisk A/S, Maaloev, Denmark.
authors.forename: Lars C
authors.initials: LC
authors.lastname: Petersen
authors.affiliation: Department of Clinical Physiology, Nuclear Medicine & PET and Cluster for Molecular Imaging, Rigshospitalet and University of Copenhagen, Copenhagen, Denmark; and akjaer@sund.ku.dk.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kjaer
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D020134:Catalytic Domain; D045744:Cell Line, Tumor; D005167:Factor VII / Q000378:metabolism*; D005260:Female; D005462:Fluorine Radioisotopes; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008024:Ligands; D051379:Mice; D049268:Positron-Emission Tomography / Q000379:methods*; D011854:Radiochemistry; D013925:Thromboplastin / Q000378:metabolism*; D014018:Tissue Distribution; D014057:Tomography, X-Ray Computed
chemical_list: D005462:Fluorine Radioisotopes; D008024:Ligands; D005167:Factor VII; D013925:Thromboplastin
keywords: 18F; PET/CT; active site inhibited factor VII (FVIIai); cancer; molecular imaging; oncology; pancreatic cancer; positron emission tomography (PET); tissue factor (TF)