Quantitative PET Imaging of Tissue Factor Expression Using 18F-Labeled Active Site-Inhibited Factor VII

J Nucl Med. 2016 Jan;57(1):89-95. doi: 10.2967/jnumed.115.154849. Epub 2015 Sep 17.

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.

Conclusion: (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.

Keywords: 18F; PET/CT; active site inhibited factor VII (FVIIai); cancer; molecular imaging; oncology; pancreatic cancer; positron emission tomography (PET); tissue factor (TF).

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Binding, Competitive
  • Catalytic Domain*
  • Cell Line, Tumor
  • Factor VII / metabolism*
  • Female
  • Fluorine Radioisotopes*
  • Gene Expression Regulation, Neoplastic*
  • Humans
  • Ligands
  • Mice
  • Positron-Emission Tomography / methods*
  • Radiochemistry
  • Thromboplastin / metabolism*
  • Tissue Distribution
  • Tomography, X-Ray Computed

Substances

  • Fluorine Radioisotopes
  • Ligands
  • Factor VII
  • Thromboplastin