James C. Gilbert

Von Willebrand Factor in Cardiovascular Disease Focus on Acute Coronary Syndromes

von Willebrand factor (VWF) plays a pivotal role in platelet adhesion and aggregation at sites of high shear rates (eg, in coronary arteries that have stenotic or ruptured atherosclerotic plaque lesions). Numerous studies have investigated the relationship between VWF plasma levels and thromboembolic cardiovascular events. In contrast to the rather weak association in the general population, in patients with preexisting vascular disease, VWF is significantly predictive for adverse cardiac events, including death. Likewise, VWF typically rises during the course of acute coronary syndrome, and the extent of this VWF release is an independent predictor of adverse clinical outcome in these patients. Various lines of evidence indicate that VWF is not only a marker but also actually an important effector in the pathogenesis of myocardial infarction. This central role of VWF in thrombogenesis has made it a promising target for research into new antiplatelet therapies that specifically inhibit VWF. This review focuses on the role of VWF in acute coronary syndrome and further outlines the relevance of therapeutic interventions targeting VWF for acute coronary syndrome patients.

First-in-Human Evaluation of Anti–von Willebrand Factor Therapeutic Aptamer ARC1779 in Healthy Volunteers

Background— ARC1779 is a therapeutic aptamer antagonist of the A1 domain of von Willebrand Factor (vWF), the ligand for receptor glycoprotein 1b on platelets. ARC1779 is being developed as a novel antithrombotic agent for use in patients with acute coronary syndromes. Methods and Results— This was a randomized, double-blind, placebo-controlled study in 47 healthy volunteers of doses of ARC1779 from 0.05 to 1.0 mg/kg. Pharmacodynamic effects were measured by an ELISA for free vWF A1 binding sites and by a platelet function analyzer. In terms of pharmacokinetics, the concentration-time profile of ARC1779 appeared monophasic. The observed concentration and area under the curve were dose proportional. The mean apparent elimination half-life was ≈2 hours, and mean residence time was ≈3 hours. The mean apparent volumes of distribution (at steady state and during terminal phase) were approximately one half the blood volume, suggesting that ARC1779 distribution is in the central compartment. The mean clearance ranged from ≈10% to ≈21% of the glomerular filtration rate, suggesting that renal filtration may not be a major mechanism of clearance of ARC1779. Inhibition of vWF A1 binding activity was achieved with an EC90 value of 2.0 μg/mL (151 nmol/L) and of platelet function with an EC90 value of 2.6 μg/mL (196 nmol/L). ARC1779 was generally well tolerated, and no bleeding was observed. Adverse events tended to be minor and not dose related. Conclusions— This is the first-in-human evaluation of a novel aptamer antagonist of vWF. ARC1779 produced dose- and concentration-dependent inhibition of vWF activity and platelet function with duration of effect suitable for the intended clinical use in acute coronary syndromes.

Inhibition of von Willebrand factor by ARC1779 in patients with acute thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) can cause severe organ damage due to enhanced platelet aggregation by ultra-large von Willebrand factor (VWF) multimers. Thus inhibition of VWF by the anti-VWF ARC1779 might potentially be beneficial for TTP patients. This prospective trial tested the safety, pharmacokinetics and pharmacodynamics of the anti-VWF aptamer ARC1779 added to plasma exchange therapy (PEX) in patients with acute TTP. Seven patients received bolus primed continuous i.v. infusions of ARC1779 (1-2 μg/kg/min) in addition to PEX until remission of TTP was induced or for 14 days. Mean steady state ARC1779 plasma concentrations of 9.9 μg/ml reduced VWF activity to 5% (mean baseline activity was 125% in TTP patients compared to a reference plasma). PEX reduced ARC1779 levels by 50%, but steady state concentrations were restored rapidly with a mini-bolus. After discontinuation of PEX, ARC1779 alone further increased platelet counts in one patient. Stopping ARC1779 was associated with an immediate drop of platelet counts in this patient. This suggests that ARC1779 can block the progression of TTP in patients with severe ADAMTS13 is deficiency. ARC1779 was generally well tolerated without any signs of bleeding. Pharmacokinetics and pharmacodynamics of ARC1779 were well predictable and in agreement with those observed in a previous trial with healthy volunteers. Based on its mechanism of action and the observed effect on platelet counts, ARC1779 used as an adjunctive to PEX may help accelerate recovery from organ dysfunction.

Improvement of spatial fibrin formation by the anti-TFPI aptamer BAX499: changing clot size by targeting extrinsic pathway initiation

Summary.  Background: Tissue factor pathway inhibitor (TFPI) is a major regulator of clotting initiation and a promising target for pro‐ and anticoagulation therapy. The aptamer BAX499 (formerly ARC19499) is a high‐affinity specific TFPI antagonist designed to improve hemostasis. However, it is not clear how stimulation of coagulation onset by inactivating TFPI will affect spatial and temporal clot propagation. Objective: To examine the BAX499 effect on clotting in a spatial, reaction‐diffusion experimental system in comparison with that of recombinant activated factor VII (rVIIa). Methods: Clotting in plasma activated by immobilized tissue factor (TF) was monitored by videomicroscopy. Results: BAX499 dose‐dependently improved coagulation in normal and hemophilia A plasma activated with TF at 2 pmole m−2 by shortening lag time and increasing clot size by up to ∼2‐fold. The effect was TFPI specific as confirmed by experiments in TFPI‐depleted plasma with or without TFPI supplementation. Clotting improvement was half‐maximal at 0.7 nm of BAX499 and reached a plateau at 10 nm, remaining there at concentrations up to 1000 nm. The BAX499 effect decreased with TF surface density increase. RVIIa improved clotting in hemophilia A plasma activated with TF at 2 or 20 pmole m−2, both by shortening lag time and increasing spatial velocity of clot propagation; its effects were strongly concentration dependent. Conclusions: BAX499 significantly improves spatial coagulation by inhibiting TFPI in a spatially localized manner that is different to that observed with rVIIa.

Initial experience from a double-blind, placebo-controlled, clinical outcome study of ARC1779 in patients with thrombotic thrombocytopenic purpura.

quency counts and percentages. Continuous variables were summarized using one or more of the following: mean, standard deviation, median, minimum, and maximum, 75th and 90th percentile, ignoring missing data when applicable. Age at survey, age at start of chronic transfusion therapy, average weight, average pretransfusion Hb and %HbS, average transfusion volume, duration of transfusion therapy and predominant transfusion type were summarized with each patient contributing equally. Weight, pretransfusion Hb and %HbS, transfusion volume, and days late were also summarized with each transfusion contributing equally. The probability of pretransfusion %HbS less than 30% was modeled using generalized estimating equations to account for the lack of independence induced by multiple transfusions per subject. The models were used to generate estimates of the odds ratio (OR) and 95% confidence intervals. Results were considered statistically significant if the confidence interval excluded one.

Inhibition of tissue factor pathway inhibitor by the aptamer BAX499 improves clotting of hemophilic blood and plasma

Summary.  Background:  Tissue factor pathway inhibitor (TFPI) is the major inhibitor of tissue factor‐initiated coagulation, making it an interesting and novel therapeutic target in hemophilia treatment. The aptamer BAX499 (formerly ARC19499) is designed to improve hemostasis by specifically inhibiting TFPI.

Evidence of persistent neurologic injury following thrombotic thrombocytopenic purpura.

Despite improvements in our understanding of the pathophysiology of thrombotic thrombocytopenic purpura (TTP), little data exist regarding the long-term sequelae following a diagnosis of TTP. We present the results of a comprehensive evaluation of neurologic injury that included a magnetic resonance imaging (MRI), a neurocognitive testing, and an evaluation of health-related quality of life. Twenty-seven patients with a history of idiopathic TTP functioning normally in their activities of daily living were recruited from existing patient cohorts at both the Ohio State University (n 5 12) (Columbus) and the University College London Hospitals (n 5 15) (London, UK). Nine of 23 (39%) of the MRI studies were abnormal; 17/27 (63%) patients demonstrated neurocognitive impairment, particularly in tests of visual learning and memory. Health-related quality of life scores were also significantly lower than age- and gender-matched US norms for both the composite mental component score and physical component score. These data suggest that the prevalence of neurologic findings in TTP patients in remission is quite high and is largely undetected by routine clinical evaluations. Further longitudinal study will be required to define the risk for neurologic injury and the long-term prognosis in patients previously diagnosed with TTP.

Drug-drug interaction of the anti-TFPI aptamer BAX499 and factor VIII: Studies of spatial dynamics of fibrin clot formation in hemophilia A

BACKGROUND In recent years, a number of tissue factor pathway inhibitor (TFPI) antagonists have been developed to serve as bypassing agents to improve hemostasis in hemophilia A. Since TFPI antagonists and FVIII concentrates are procoagulants, their combined effect on spatial clot formation could be potentially pro-thrombotic. OBJECTIVE To investigate the cooperative effect of TFPI inhibition and supplementation of FVIII in hemophilia A in a spatial, reaction-diffusion experiment in vitro. METHODS Plasma was collected at different time points from hemophilia A patients undergoing prophylaxis and was supplemented in vitro with TFPI inhibitor BAX499 (formerly ARC19499) at concentrations from 0 up to 600nM. Clotting propagation in recalcified plasma activated by a surface with immobilized tissue factor (TF) was monitored by videomicroscopy. RESULTS Increasing concentration of BAX499 improved coagulation for all hemophilia A plasma samples activated with TF at 1.6pmole/m(2) by shortening lag time and increasing initial clot growth velocity and clot size. In contrast, plasma concentration of FVIII had little effect on lag time, but increased spatial clot growth velocity. There was a decrease in the BAX499 efficiency as FVIII concentration increased (lag time shortened by 50% if FVIII:C<5%, but the effect was only 25% if FVIII:C>30%). CONCLUSIONS The results indicate that BAX499 has an effect on clotting in hemophilia A plasma at low FVIII concentrations, however has little effect at high FVIII concentrations.