Mechanisms of Thrombotic Thrombocytopenic Purpura Therapies. Deficiency of the metalloprotease ADAMTS13, the cause of TTP, results in loss of proteolytic processing of VWF and plasma accumulation of ultra-large (UL)-VWF multimers. UL-VWF multimers are highly platelet-adhesive and induce thrombi in the micro-vasculature of various organ beds, leading to the clinical manifestations of TTP. When the VWF-platelet interaction is blocked by monoclonal antibodies (ALX-0681 or GBR600) that recognize the site on VWF that binds to the GP1b receptor on platelets, VWF-platelet interaction is blocked, preventing formation of microthrombi and the clinical symptoms of TTP.