Synthesis and characterization of 18F-labeled active site inhibited factor VII (ASIS)

Activated factor VII blocked in the active site with Phe-Phe-Arg-chloromethyl ketone (active site inhibited factor VII (ASIS)) is a 50-kDa protein that binds with high affinity to its receptor, tissue factor (TF). TF is a transmembrane glycoprotein that plays an important role in, for example, thrombosis, metastasis, tumor growth, and tumor angiogenesis. The aim of this study was to develop an F-18-labeled ASIS derivative to assess TF expression in tumors. Active site inhibited factor VII was labeled using N-succinimidyl-4-[F-18] fluorobenzoate, and the [F-18]ASIS was purified on a PD-10 desalting column. The radiochemical yield was 25 +/- 6%, the radiochemical purity was >97%, and the pseudospecific radioactivity was 35 +/- 9 GBq/mu mol. The binding efficacy was evaluated in pull-down experiments, which monitored the binding of unlabeled ASIS and [F-18]ASIS to TF and to a specific anti-factor VII antibody (F1A2-mAb). No significant difference in binding efficacy between [F-18]ASIS and ASIS could be detected. Furthermore, [F-18]ASIS was relatively stable in vitro and in vivo in mice. In conclusion, [F-18]ASIS has for the first time been successfully synthesized as a possible positron emission tomography tracer to image TF expression levels. In vivo positron emission tomography studies to evaluate the full potential of [F-18]ASIS are in progress.