INHIBITORY EFFECTS OF TFPI ON THROMBIN AND FACTOR XA GENERATION IN-VITRO - MODULATORY ACTION OF GLYCOSAMINOGLYCANS

The effect of tissue factor pathway inhibitor (TFPI) on thrombin an factor Xa generation was studied in an in vitro system using a prothrombin complex concentrate. It was found that TFPI, via the direct inhibition of factor Xa and the tissue factor/factor VIIa complex, inhibited both the further generation of factor Xa and the generation of thrombin in a concentration-dependent manner. The generation of thrombin (IC50 255 ng/ml) was more pronounced than that of factor Xa (IC50 684 ng/ml). The inhibitory activity of TFPI was significantly enhanced when unfractionated heparin was present in the assay system at a concentration of 10 mu g/ml which did not show any inhibitory effects on protease generation in the same system. Furthermore, the influence of TFPI at subthreshold concentration (100 ng/ml and 200 ng/ml, resp.) on the inhibitory action of unfractionated heparin (UFH), a low molecular weight heparin (LMWH), heparan sulfate (HS) and the synthetic heparin penta-saccharide (PS) was investigated. Whereas in the concentration range used (0.3-4.0 mu/ml) these glycosaminoglycans did not inhibit thrombin and factor Xa generation, after supplementation of the system with TFPI a concentration-dependent inhibition of the generation of the proteases up to 40-50 % was seen for UFH, LMWH and HS. TFPI did not increase the activity of PS.