The aptamer ARC1779 blocks von Willebrand factor–dependent platelet function in patients with thrombotic thrombocytopenic purpura ex vivo

BACKGROUND: In thrombotic thrombocytopenic purpura (TTP), ultralarge von Willebrand factor (VWF) multimers bind platelet (PLT) glycoprotein Ib and lead to the formation of disseminated fibrin‐poor, VWF‐rich PLT thrombi. The aptamer ARC1779 blocks binding of the VWF A1 domain to PLT glycoprotein Ib. We evaluated whether ARC1779 inhibits the excessive VWF activity and VWF‐mediated PLT function in patients with TTP. STUDY DESIGN AND METHODS: We studied the ex vivo concentration response curves for ARC1779 on PLT function analyzer (PFA‐100, Dade Behring) and cone‐and‐plate analyzer (CPA, Impact‐R) PLT function tests, agonist‐induced PLT aggregation, and VWF activity of TTP patients (n = 11, three in acute phase and eight in remission) and healthy controls (n = 44). RESULTS: VWF activity and VWF‐dependent PLT plug formation were increased in TTP patients relative to healthy controls, but agonist‐induced PLT aggregation was not. ARC1779 blocked collagen/adenosine 5′‐diphosphate (ADP)‐induced PLT plug formation as measured by PFA‐100 with an inhibitory concentration (IC)₁₀₀ of approximately 1 µg/mL in citrate‐anticoagulated samples and approximately 3 to 4 µg/mL in hirudin‐anticoagulated samples. A similar concentration of ARC1779 was necessary to block shear‐dependent PLT adhesion in both TTP patients and healthy controls using the CPA assay (IC₁₀₀ of approx. 1 µg/mL for both). ARC1779 blocked VWF activity with an IC₉₀ of approximately 3 to 4 µg/mL in all subjects, but did not inhibit PLT aggregation by ADP, collagen, or arachidonic acid even at concentrations much greater than those that fully inhibited VWF‐dependent PLT function. CONCLUSIONS: ARC1779 potently and specifically inhibits VWF activity and VWF‐dependent PLT function. ARC1779 may be a promising novel therapeutic for the treatment of TTP.