Recombinant von Willebrand factor fragment AR545C inhibits platelet aggregation and enhances thrombolysis with rtPA in a rabbit thrombosis model.

Platelet adhesion to exposed subendothelium is mediated by platelet receptor glycoprotein Ib and polymeric von Willebrand factor (vWF). To improve the results of coronary arterial thrombolysis, fragments of vWF with enhanced glycoprotein Ib binding competitive with native vWF have been proposed as adjuvants to recombinant tissue-type plasminogen activator (rtPA). We designed a recombinant vWF fragment spanning Ala444 to Asp730 that contains the Arg545Cys mutation (named AR545C) and analyzed its antiplatelet properties in vitro and in vivo. AR545C-platelet interaction was assessed by ristocetin or botrocetin-induced platelet agglutination, or interaction with extracellular matrix under arterial flow conditions. AR545C showed enhanced reactivity with platelet glycoprotein Ib at low concentrations of ristocetin, and 60% bound spontaneously to platelets. AR545C inhibited ristocetin-induced platelet agglutination in a dose-dependent manner, with a concentration necessary to inhibit 50% of agglutination of 0.16+/-0.04 micromol/L. The inhibitory effect of AR545C on rabbit botrocetin-induced platelet agglutination was also dose dependent, with a concentration necessary to inhibit 50% of agglutination of 0.3 to 0.5 micromol/L. AR545C also completely inhibited aggregate formation and decreased the adhesion of platelets to extracellular matrix by 62.5%. The effect of AR545C on thrombolysis with rtPA was evaluated using a modified rabbit femoral thrombosis model. Local injection of AR545C into the thrombosed segment of rabbit femoral artery significantly shortened the time to reperfusion with rtPA (60+/-17.3 versus 103+/-15.2 minutes, P=.05) and significantly prolonged the total patency time (175 versus 21 minutes, P=.04). No significant difference was found in the reperfusion rate or time to reocclusion. AR545C is a potential antithrombotic agent that enhances the thrombolytic effect of rtPA in the rabbit model.