AN INTERMEDIATE-PURITY FACTOR-VIII CONCENTRATE SUPPORTS PLATELET-ADHESION UNDER FLOW CONDITIONS

Von Willebrand factor (vWF) binds to platelets and mediates platelet a dhesion to subendothelium to support haemostasis. Factor VIII concentr ates containing vWF have been recommended for treatment of bleeding ep isodes in von Willebrand disease (vWD) types 2 and 3. Their clinical e fficacy in normalizing FVIII coagulant levels, shortening the bleeding time, stopping or preventing clinical bleeding and safety have been t ested. However, the basic mechanisms of their effects on haemostasis h ave not been fully characterized. We have analysed the ability of VWF present in an intermediate-purity factor VIII concentrate (Haemare-P, Centeon) to bind to platelets (ultrastructural studies) and to support platelet adhesion under flow conditions (perfusion studies). For this purpose, Haemate-P or cryoprecipitate was added to washed platelet su spensions, or to VWF-depleted reconstituted healthy anticoagulated blo od. Immunoelectron microscopy (IEM) revealed VWF arrangements on plate let surfaces which have been exposed to mixtures of the vWF-rich conce ntrate plus ristocetin. VWF levels in perfusates were confirmed by det ermination of ristocetin co-factor and vWF-antigen. Baumgartner's perf usion method and computer-assisted morphometry were used to evaluate p latelet adhesion of the perfusates onto everted rabbit aorta subendoth elium under standardized conditions (shear rate, 800 s(-1), 10 min, 37 degrees C). vWF-depleted perfusates showed 15.8% (SEM 1.7) total cove red surface (CS) with platelets. An increase in CS resulted when 0.40 or 0.80 IU vWF mL(-1) from Haemate-P 130.1%, SEM 3.0, P < 0.05; 39.4, SEM 3.1, P < 0.008, respectively) were added. A similar increase was o bserved when cryoprecipitate was added to perfusates (28.6%, SEM 2.4, P < 0.05 for 0.40 IU vWF mL(-1); 27.2%, SEM 2.9, P < 0.01 for 0.80 IU vWF mL(-1)). IEM confirmed that vWF from concentrates binds to platele ts. Furthermore, perfusion studies revealed that the fractionated VWF supports platelet adhesion, thus providing experimental evidence of th e therapeutic benefits exerted by Haemate-P.