Quantitative Pet Imaging Of Tissue Factor Expression Using F-18-Labeled Active Site-Inhibited Factor Vii

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 F-18-labeled derivative of factor VII. Methods: Active site-inhibited factor Vila (FVIlai) was obtained by inactivation with phenylalanine-phenylalanine-arginine-chloromethyl ketone. FVIlai was radiolabeled with N-succinimidyl 4-F-18-fluorobenzoate and purified. The corresponding product, F-18-FVIlai, 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 F-18-FVIlai in the tumors measured in vivo by PET imaging. Results: The PET images showed high uptake of F-18-FVIlai 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 F-18-FVIlai 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 FVIlai significantly reduced the uptake of F-18-FVIlai from 2.9 +/- 0.1 to 1.4 +/- 0.1 %ID/g (P < 0.001). The uptake of F-18-FVIlai measured in vivo by PET imaging correlated (r = 0.72, P < 0.02) with TF protein level measured ex vivo. Conclusion: F-18-FVIlai 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.