The present study demonstrated that the combination of FVIIa and FX appeared to have a higher and more sustainable haemostatic potential than FVIIa alone, and less thrombogenicity than FEIBA. A therapeutic approach combining FVIIa and FX could be a promising and novel approach to compensate for the disadvantages of rFVIIa and FEIBA for haemophiliacs with inhibitors.

Plasma von Willebrand factor (VWF) has been identified as an indispensable factor for platelet adhesion and thrombus formation on a collagen surface under flow conditions. VWF binds to collagen and then tethers platelets to the collagen surface through interaction with platelet glycoprotein Ib and also contributes to the thrombus formation on the collagen surface. In the present study, we demonstrated that the addition of VWF/factor VIII complex or purified VWF (> 2 ristocetin cofactor activity units/mL) increased platelet adhesion to the collagen surface in platelet-reduced blood (ϳ 5 ؋ 10 4 platelets/L) to the normal level. VWF had no stimulatory effect when it was allowed to bind to the collagen surface before blood flow was initiated. Addition of an excess of FITC (fluorescein-5-isothiocyanate)-labeled VWF to platelet-reduced blood under these flow conditions demonstrated that the VWF was mainly incorporated into the platelet aggregates. These results indicated that the supplemented VWF stimulates the platelet adhesion onto the collagen surface by enhancing platelet aggregation in the platelet-reduced condition. This also suggests a possibility that supplementation of VWF to individuals with thrombocytopenia might be effective for increasing their hemostatic potential. ( IntroductionProper formation of a platelet plug at sites of vascular injury requires a spatially and temporally coordinated series of events that allow circulating platelets to adhere onto exposed collagen, become activated, and recruit additional platelets, and form multicellular aggregates, including platelets that are stabilized by fibrin. Von Willebrand factor (VWF), an adhesive plasma glycoprotein, has been indicated to play fundamental roles in the platelet plug formation under physiologic conditions. 1-4 VWF is composed of identical disulfide-linked subunits, each comprising 2050 amino acid residues and up to 22 carbohydrate chains for a total mass of 250 kDa. 5 VWF subunits are crosslinked at the COOH-terminal ends to form a dimer, and the dimer is further converted to a multimeric form with molecular mass from 5 ϫ 10 2 kDa to 2 ϫ 10 4 kDa by intersubunit disulfide linkages at the NH 2 -terminal ends of the dimers. 6 VWF is present in the circulation at a plasma concentration of 5 to 10 g/mL. It is also released from ␣-granules of megakaryocytes and platelets and Weibel-Palade bodies of endothelial cells. 7,8 VWF was shown to have a critical role in platelet adhesion to a collagen surface under flow conditions, especially under high shear rate. 4,9,10 Although platelets do not bind human VWF in solution under low shear rate, VWF binds reversibly with platelet glycoprotein (GP) Ib under high shear rate, and shear stress induces a permanent interaction between VWF and integrin ␣ ⌱⌱b ␤ 3 (GPIIb/IIIa) after the initial binding of GPIb with VWF. 1,11 Moroi et al 12 found that the reversible binding of platelets to immobilized VWF and the tethering speed of platelets was related to the density of VWF on the surface. These and many other r...

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