Von Willebrand factor (vWF) is synthesized in megakaryocytes and endothelial cells as a precursor protein that is assembled into large multimers of greater than 20,000,000 MW. Individual vWF molecules (~275 kDa) are composed of multiple domains and attached to one another via disulfide bonds. vWF multimers are processed by the metalloprotease ADAMTS13, which cleaves vWF within the A2 domain. Congenital absence of ADAMTS13 or neutralizing antibodies directed against the metalloprotease result in excess ultra-large von Willebrand factor multimers (ULVWF), which are procoagulant and contribute to the microangiopathy of thrombotic thrombocytopenic purpura (TTP). ULVWF may represent a tractable target for new therapeutic strategies in TTP. Recombinant ADAMTS13 has been proposed as a means to treat TTP, but it has yet to be studied in human trials and may be problematic in patients with high titer anti-ADAMTS13 antibodies. Dr. Chen and colleagues working in the laboratory of Dr. Jose Lopez at the Puget Sound Blood Center in Seattle now demonstrate an alternative strategy for attacking ULVWF, by chemically reducing the disulfide bonds that are required for vWF polymerization.
Noting similarities between the overall polymeric structure of mucins and that of polymerized vWF, Dr. Chen and his colleagues posited that N-acetylcysteine (NAC) could be used to dissolve multimeric vWF in a manner analogous to its role in chronic obstructive lung disease, in which it acts as a mucolytic agent. The investigators found that adding NAC to plasma ex vivo reduced intermolecular disulfide bonds that link vWF subunits, thereby decreasing the abundance of ULVWF. Similarly, infusing NAC into mice reduced ULVWF in plasma. In addition to decreasing the abundance of ULVWF, NAC reduced an intramolecular bond in the A1 domain (Cys1271-Cys1458) of vWF required for its association with the platelet receptor glycoprotein Ib and inhibited the ability of vWF to bind platelets. Incubation with NAC dissolved platelet-vWF strings that form under flow conditions on cultured endothelial cells stimulated with histamine. The authors also performed a series of tests to evaluate the effect of NAC on platelet thrombus formation in mesenteric venules induced to secrete ULVWF by exposure to calcium ionophore. NAC reduced platelet thrombi in both wild-type and ADAMTS13-/- mice.
In Brief
Despite the substantial advances in our understanding of the etiology of thrombotic thrombocytopenic purpura (TTP), the mainstay of treatment since the late 1970s has been plasmapheresis. Although plasmapheresis is effective, acute TTP is associated with a mortality rate of 20 percent and relapse is common, so there is need for new and safer treatments that leverage the considerable progress made in understanding the molecular pathophysiology of the disease. The clinical implications of the observation that NAC reduces vWF multimers and facilitates the dissolution of thrombus in vivo are compelling. The diagnosis of TTP is not always immediately apparent and plasma exchange is not always immediately available. It is easy to envision how a relatively non-toxic, inexpensive agent could be incorporated into TTP treatment algorithms, at least as a temporizing therapy. Long-term treatment at lower doses of NAC could reduce recurrence. Whether NAC will be effective in ameliorating TTP remains to be determined. It is a relatively nonselective therapy, and effects on platelet function and other coagulation proteins have been described.1,2 NAC was not tested in a mouse model of TTP or even in a systemic thrombosis model, so its clinical utility in TTP remains to be proven. Nonetheless, the results of Chen et al. demonstrate a clever new application of an old drug with an established safety profile and provide a strong rationale for further testing of this approach in TTP.
References
Competing Interests
Dr. Flaumenhaft indicated no relevant conflicts of interest.
