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The ability of factor VIIa to initiate thrombin generation and clot formation in blood from healthy donors, blood from patients with hemophilia A, and in anti-factor IX antibody-induced ("acquired") hemophilia B blood was investigated. In normal blood, both factor VIIa-tissue factor (TF) complex and factor VIIa alone initiated thrombin generation. The efficiency of factor VIIa was about 0.0001 that of the factor VIIa-TF complex. In congenital hemophilia A blood and "acquired" hemophilia B blood in vitro, addition of 10 to 50 nM factor VIIa (pharmacologic concentrations) corrected the clotting time at all TF concentrations tested (0-100 pM) but had little effect on thrombin generation. Fibrinopeptide release and insoluble clot formation were only marginally influenced by addition of factor VIIa. TF alone had a more pronounced effect on thrombin generation; an increase in TF from 0 to 100 pM increased the maximum thrombin level in "acquired" hemophilia B blood from 120 to 480 nM. Platelet activation was considerably enhanced by addition of factor VIIa to both hemophilia A blood and "acquired" hemophilia B blood. Thus, pharmacologic concentrations of factor VIIa cannot restore normal thrombin generation in hemophilia A and hemophilia B blood in vitro. The efficacy of factor VIIa (10-50 nM) in hemophilia blood is dependent on TF. IntroductionThe blood-coagulation cascade is initiated when cryptic tissue factor (TF) is expressed and exposed to circulating blood and binds plasma factor VIIa. The resulting factor VIIa-TF complex activates the serine protease zymogens factor IX and factor X. The factor Xa that is initially produced generates picomolar amounts of thrombin, which activates platelets and cleaves procofactors factors V and VIII. Factor VIIIa forms a complex on a membrane surface with serine protease factor IXa and activates factor X at a 50-to 100-fold higher rate than the factor VIIa-TF complex. The factor Xa produced, in complex with its cofactor, factor Va, and an appropriate membrane surface forms the prothrombinase complex, which is the primary activator of prothrombin. The thrombin produced amplifies its own generation by activating factor XI and completing the activation of platelets and procofactors. Thrombin cleaves fibrinogen and activates factor XIII to form the insoluble isopeptide cross-linked fibrin clot. The coagulation cascade is down-regulated by the stoichiometric inhibitors antithrombin III (AT-III) and tissue factor pathway inhibitor (TFPI) and by the dynamic protein C system. 1 Genetic and acquired deficiencies in coagulation proteins lead to hemorrhagic syndromes. [2][3][4][5][6] The most common bleeding disorders result from deficiencies of factor VIII (hemophilia A) or factor IX (hemophilia B) coagulant activity. In the past, the principal treatments for hemophilia relied on partially purified concentrates of coagulation factors. 7,8 These concentrates, however, have been associated with thromboembolic complications and viral infections. 9-11 During the past decade, plasma-derived, ...

Objective-This study was performed to evaluate the influences of phospholipids and recombinant factor VIIa (rFVIIa) on thrombin generation and clot formation in "acquired" hemophilia B. Methods and Results-A synthetic mixture corresponding to hemophilia A (SHA) and "acquired" hemophilia B blood (AHBB) manufactured in vitro by an anti-FIX antibody were used in this study. With 10 pmol/L tissue factor (TF), 10 nmol/L rFVIIa, and saturating phospholipid, established thrombin generation in SHA was similar to that observed in the presence of factor VIII and rFVIIa at physiological concentrations. At lower phospholipid concentrations, thrombin generation was delayed and reduced. With 5 pmol/L TF, contact pathway-inhibited AHBB clotted later than normal blood and showed reduced clot stability and thrombin generation. These parameters of effectiveness were increased by the addition of phospholipids to AHBB, which restored clot stability and increased thrombin generation. No correction of clot formation or thrombin generation was observed when rFVIIa and phospholipids were added to AHBB in the absence of TF. Conclusions-The

Background-Monitoring patients on oral anticoagulation is essential to prevent hemorrhage and recurrent thrombosis. We studied tissue factor-induced whole-blood coagulation in patients on warfarin therapy with similar international normalized ratios (INRs). Methods and Results-Contact pathway-suppressed whole-blood coagulation initiated with tissue factor was studied in 8 male subjects (group W) and in 1 individual multiple times (subject A). Coagulation profiles for group W showed that subjects with similar INRs had widely varying clot times (6.2 to 23 minutes) and thrombin-antithrombin III (TAT) profiles with rates of 25 to 40 nmol · L Ϫ1 · min Ϫ1 and maximum levels varying from 192 to 349 nmol/L. The normal control group exhibited clot times of 5.7Ϯ0.3 minutes and TAT rates of 57Ϯ13 nmol · L Ϫ1 · min Ϫ1 , reaching maximum levels of 742Ϯ91 nmol/L. Subject A, who was stably anticoagulated at an INR of 2.1Ϯ0.4 for 6 months, had widely ranging profiles with clot times of 9.0 to 22.7 minutes, TAT maximums varying from 141 to 345 nmol/L, and TAT formation rates of 10 to 57 nmol · L Ϫ1 · min Ϫ1 . INR did not correlate with TAT formation. Platelet activation was decreased by anticoagulants but also displayed variability. Fibrinopeptide A generation showed threshold variability independent of the INR. Factor VIII levels were increased (Pϭ0.03) in group W (204Ϯ34.4%) compared with normal control subjects (149.4Ϯ37.4%). A significant correlation was identified between increasing factor VIII levels and years on warfarin therapy (rϭ0.78, Pϭ0.01), suggesting a possible factor VIII compensatory mechanism. Conclusions-These results suggest that control of anticoagulation in patients to a set INR therapeutic range may be less secure than anticipated. Patients with similar INRs show significant individual variability in their tissue factor coagulation response, suggesting different risks to anticoagulation when confronted with underlying vascular anomalies.

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