Abstract Number: PB0881
Meeting: ISTH 2021 Congress
Theme: Platelet Disorders, von Willebrand Disease and Thrombotic Microangiopathies » Platelet Antagonists and Novel Therapeutics
Background: Endothelial cell (EC) activation and injury and platelet activation characterize thrombotic thrombocytopenic purpura (TTP) and atypical hemolytic uremic syndrome (aHUS). We found that 5μg/ml defibrotide inhibits TMA plasma-mediated caspase 8 activation of EC, an initial step in apoptotic injury (ASH 2019, Abstract 3676), but defibrotide was reported to inhibit agonist-induced platelet activation only at clinically unachievable doses of 100-1000μg/ml (ASH 2019, Abstract 3614).
Aims: (1) Evaluate biomarkers of platelet activation and EC injury in TMA plasmas;
(2) determine whether clinically relevant defibrotide concentrations block agonist-mediated platelet activation.
Methods: (1) Biomarkers for platelet activation (platelet factor 4 (PF4), β-thromboglobulin (β-TG)) and EC injury (von Willebrand factor (vWF) antigen) were measured in TMA patient plasmas (9 aHUS, 8 TTP) by ELISA.
(2) Washed human platelets were incubated with the PAR-1 agonist peptide RUJL or ADP (2μM), alone or with 5μg/ml defibrotide. Platelet aggregation was quantified by light transmission aggregometry.
Results: 
PF4 and B-thromboglobulin levels in plasmas of acute TMA patients vs. controls(1) A significant increase in PF4 levels was seen in TMA patients (n=15) vs. healthy controls (n=12) (Fig. 1). A significant difference in β-TG levels was not seen in TMA patients (n=15) vs. controls (n=7). The β-TG:PF4 ratio, a marker of in vivo platelet activation (Ann Rheum Dis 2005;64:484), was >2 in TMA and control plasmas, indicating some in vitro activation, but much more highly elevated in TMA (ratio = 19.4) vs. control plasmas (ratio = 5.6) (p = 0.0058). vWF antigen levels were not significantly different in patients vs. controls.
(2) Defibrotide blocked platelet aggregation induced by both RUJL and ADP at 5μg/ml (Fig. 2).
Conclusions: 
Effect of defibrotide on PAR-1 agonist and ADP induced platelet aggregationThe ability of defibrotide to block TMA plasma-mediated EC injury, shown previously, and now platelet activation has implications for TMA treatment as well as in progressive COVID-19, which presents features characteristic of TMAs and vaso-occlusive disease (J Thromb Haemost 2020:18:3106; Lancet Haematol 2020;7:e575).
To cite this abstract in AMA style:
Elhadad S, Subrahmanian S, Ahamed J, Laurence J. Thrombotic Microangiopathies (TMA): In vivo Evidence for Platelet Activation and Endothelial Cell Injury, and Potential for Therapeutic Intervention with Defibrotide [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 2). https://abstracts.isth.org/abstract/thrombotic-microangiopathies-tma-in-vivo-evidence-for-platelet-activation-and-endothelial-cell-injury-and-potential-for-therapeutic-intervention-with-defibrotide/. Accessed November 29, 2023.« Back to ISTH 2021 Congress
ISTH Congress Abstracts - https://abstracts.isth.org/abstract/thrombotic-microangiopathies-tma-in-vivo-evidence-for-platelet-activation-and-endothelial-cell-injury-and-potential-for-therapeutic-intervention-with-defibrotide/