STUDIES ON THE THROMBOGENIC EFFECTS OF RECOMBINANT TISSUE FACTOR - IN-VIVO VERSUS EX-VIVO FINDINGS

An exposure of blood to tissue factor (TF) activates the coagulation s ystem by the extrinsic pathway and may cause clot formation. Recombina nt TF (r-TF) has been produced and subsequently reconstituted into pho spholipid vesicles. The aim of these studies was to elucidate the in v itro procoagulant effects and the in vivo thrombogenicity of r-TF usin g a rabbit jugular vein stasis thrombosis model. The in vitro studies exhibited a clear concentration-dependent decrease in the clotting tim e when rabbit brain thromboplastin was replaced by r-TF in the prothro mbin time assay. The in vivo studies revealed a dose-dependent thrombo genicity between 1.6 ng/kg and 50 ng/kg, Electron microscope scanning of the surface of representative clots revealed fibrin-rich structures of heterogeneous density. In comparison, thrombi obtained when FEIBA( R) was utilized as the thrombogenic agent were more homogeneous. The i njection of r-TF caused a slight transient drop in blood pressure with little or no effects on the pulse rate, complete blood count (CBC) pr ofile, clotting and amidolytic assays when compared to sham control an imals. In contrast, the whole blood clotting parameters (activated clo tting time and thrombelastograph) were prolonged dose-dependently afte r r-TF injection. The antithrombotic activity of heparin was assessed in this model and compared to the antithrombotic activity when FEIBA(R ) is used as the thrombogenic agent. The apparent ED(50) of heparin wa s found to be 4 times higher in the r-TF system. In control studies, n o thrombogenic effects vr;ere observed by the phospholipid vesicles al one nor by r-TF not embedded in phospholipid vesicles. These data demo nstrate that lipidated r-TF is a potent thrombogenic challenge that ac tivates the hemostatic system by the extrinsic pathway.