A possible mechanism of action of activated factor VII independent of tissue factor

We have used a cell-based model system to examine some aspects of coagulation. Unactivated platelets and tissue factor (TF)-bearing cells were mixed with plasma levels of zymogen factors IX (FIX), FVIII, FX, FV, and prothrombin, as well as coagulation inhibitors antithrombin III and TF pathway inhibitor. Reactions were initiated with plasma levels (0.2 nmol/l) of activated factor VII (FVIIa). We were able to measure platelet activation and subsequent thrombin generation in this system and have established parameters for the normal amount of thrombin generation and the range of values seen with different individuals. If FIX or FVIII were not added to this system, platelet activation but not thrombin generation was seen. We have used this system to examine the mechanism of action of high-dose FVIIa. If platelets were activated with the thrombin receptor agonist peptide SFLLRN and incubated with inhibitors and zymogen factors X, V, and prothrombin, no thrombin generation was observed. Addition of increasing amounts of FVIIa gave increasing amounts of thrombin generation. At the FVIIa concentrations present in the plasma of patients given 60 microg/kg recombinant FVIIa (NovoSeven, Novo Nordisk, Bagsvaerd, Denmark), 10-40 nmol/l, thrombin generation in the model system approached the normal amount seen in the TF-initiated model system. When FIX and FVIII were included in the above reaction, FVIIa could initiate thrombin generation at levels three to four times the amount seen in the TF-initiated model system. We speculate that this platelet-localized thrombin generation may, in part, account for the clinical efficacy of high-dose FVIIa.