An intermediate-purity factor VIII concentrate supports platelet adhesion under flow conditions.

Von Willebrand factor (vWF) binds to platelets and mediates platelet adhesion to subendothelium to support haemostasis. Factor VIII concentrates containing vWF have been recommended for treatment of bleeding episodes in von Willebrand disease (vWD) types 2 and 3. Their clinical efficacy in normalizing FVIII coagulant levels, shortening the bleeding time, stopping or preventing clinical bleeding and safety have been tested. However, the basic mechanisms of their effects on haemostasis have not been fully characterized. We have analysed the ability of vWF present in an intermediate-purity factor VIII concentrate (Haemate-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 suspensions, or to vWF-depleted reconstituted healthy anticoagulated blood. Immunoelectron microscopy (IEM) revealed vWF arrangements on platelet surfaces which have been exposed to mixtures of the vWF-rich concentrate plus ristocetin. vWF levels in perfusates were confirmed by determination of ristocetin co-factor and vWF-antigen. Baumgartner's perfusion method and computer-assisted morphometry were used to evaluate platelet adhesion of the perfusates onto everted rabbit aorta subendothelium under standardized conditions (shear rate, 800 s(-1) , 10 min, 37 °C). vWF-depleted perfusates showed 15.8% (SEM 1.7) total covered surface (CS) with platelets. An increase in CS resulted when 0.40 or 0.80 IU vWF mL(-1) from Haemate-P (30.1%, SEM 3.0, P<0.05; 39.4%, SEM 3.1, P<0.008, respectively) were added. A similar increase was observed 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 platelets. Furthermore, perfusion studies revealed that the fractionated vWF supports platelet adhesion, thus providing experimental evidence of the therapeutic benefits exerted by Haemate-P.