Effect of a humanized monoclonal antibody to von Willebrand factor in a canine model of coronary arterial thrombosis

Von Willebrand factor in diagnostics and treatment of cardiovascular disease: Recent advances and prospects

Von Willebrand factor (VWF) is a large multimeric glycoprotein involved in hemostasis. It is essential for platelet adhesion to the subendothelium of the damaged endothelial layer at high shear rates. Such shear rates occur in small-diameter arteries, especially at stenotic sites. Moreover, VWF carries coagulation factor VIII and protects it from proteolysis in the bloodstream. Deficiency or dysfunction of VWF predisposes to bleeding. In contrast, an increase in the concentration of high molecular weight multimers (HMWM) of VWF is closely associated with arterial thrombotic events. Severe aortic stenosis (AS) or hypertrophic obstructive cardiomyopathy (HOCM) can deplete HMWM of VWF and lead to cryptogenic, gastrointestinal, subcutaneous, and mucosal bleeding. Considering that VWF facilitates primary hemostasis and a local inflammatory response at high shear rates, its dysfunction may contribute to the development of coronary artery disease (CAD) and its complications. However, current diagnostic methods do not allow for an in-depth analysis of this contribution. The development of novel diagnostic techniques, primarily microfluidic, is underway. Such methods can provide physiologically relevant assessments of VWF function at high shear rates; however, they have not been introduced into clinical practice. The development and use of agents targeting VWF interaction with the vessel wall and/or platelets may be reasonable in prevention of CAD and its complications, given the prominent role of VWF in arterial thrombosis.

Shear Stress-Induced Activation of von Willebrand Factor and Cardiovascular Pathology

The von Willebrand factor (vWF) is a plasma protein that mediates platelet adhesion and leukocyte recruitment to vascular injury sites and carries coagulation factor VIII, a building block of the intrinsic pathway of coagulation. The presence of ultra-large multimers of vWF in the bloodstream is associated with spontaneous thrombosis, whereas its deficiency leads to bleeding. In cardiovascular pathology, the progression of the heart valve disease results in vWF deficiency and cryptogenic gastrointestinal bleeding. The association between higher plasma levels of vWF and thrombotic complications of coronary artery disease was described. Of note, it is not the plasma levels that are crucial for vWF hemostatic activity, but vWF activation, triggered by a rise in shear rates. vWF becomes highly reactive with platelets upon unfolding into a stretched conformation, at shear rates above the critical value (more than 5000 s⁻¹), which might occur at sites of arterial stenosis and injury. The activation of vWF and its counterbalance by ADAMTS-13, the vWF-cleaving protease, might contribute to complications of cardiovascular diseases. In this review, we discuss vWF involvement in complications of cardiovascular diseases and possible diagnostic and treatment approaches.

Therapeutic strategies for thrombosis: new targets and approaches

Antiplatelet agents and anticoagulants are a mainstay for the prevention and treatment of thrombosis. However, despite advances in antithrombotic therapy, a fundamental challenge is the side effect of bleeding. Improved understanding of the mechanisms of haemostasis and thrombosis has revealed new targets for attenuating thrombosis with the potential for less bleeding, including glycoprotein VI on platelets and factor XIa of the coagulation system. The efficacy and safety of new agents are currently being evaluated in phase III trials. This Review provides an overview of haemostasis and thrombosis, details the current landscape of antithrombotic agents, addresses challenges with preventing thromboembolic events in patients at high risk and describes the emerging therapeutic strategies that may break the inexorable link between antithrombotic therapy and bleeding risk.

Shear-Dependent Platelet Aggregation: Mechanisms and Therapeutic Opportunities

Cardiovascular diseases (CVD) are the number one cause of morbidity and death worldwide. As estimated by the WHO, the global death rate from CVD is 31% wherein, a staggering 85% results from stroke and myocardial infarction. Platelets, one of the key components of thrombi, have been well-investigated over decades for their pivotal role in thrombus development in healthy as well as diseased blood vessels. In hemostasis, when a vascular injury occurs, circulating platelets are arrested at the site of damage, where they are activated and aggregate to form hemostatic thrombi, thus preventing further bleeding. However, in thrombosis, pathological activation of platelets occurs, leading to uncontrolled growth of a thrombus, which in turn can occlude the blood vessel or embolize, causing downstream ischemic events. The molecular processes causing pathological thrombus development are in large similar to the processes controlling physiological thrombus formation. The biggest challenge of anti-thrombotics and anti-platelet therapeutics has been to decouple the pathological platelet response from the physiological one. Currently, marketed anti-platelet drugs are associated with major bleeding complications for this exact reason; they are not effective in targeting pathological thrombi without interfering with normal hemostasis. Recent studies have emphasized the importance of shear forces generated from blood flow, that primarily drive platelet activation and aggregation in thrombosis. Local shear stresses in obstructed blood vessels can be higher by up to two orders of magnitude as compared to healthy vessels. Leveraging abnormal shear forces in the thrombus microenvironment may allow to differentiate between thrombosis and hemostasis and develop shear-selective anti-platelet therapies. In this review, we discuss the influence of shear forces on thrombosis and the underlying mechanisms of shear-induced platelet activation. Later, we summarize the therapeutic approaches to target shear-sensitive platelet activation and pathological thrombus growth, with a particular focus on the shear-sensitive protein von Willebrand Factor (VWF). Inhibition of shear-specific platelet aggregation and targeted drug delivery may prove to be much safer and efficacious approaches over current state-of-the-art antithrombotic drugs in the treatment of cardiovascular diseases.

Comparison between manufacturing sites shows differential adhesion, activation, and GPIbα expression of cryopreserved platelets

BACKGROUND

Transfusion of cryopreserved platelets (cryoplatelets) is not common but may replace standard liquid-preserved platelets (PLTs) in specific circumstances. To better understand cryoplatelet function, frozen concentrates from different manufacturing sites were compared.

STUDY DESIGN AND METHODS

Cryoplatelets from Denver, Colorado (DEN); Sydney, Australia (SYD); and Ghent, Belgium (GHE) were compared (n = 6). A paired noncryopreserved control was included in Ghent. Microfluidic-flow chambers were used to study PLT adhesion and fibrin deposition in reconstituted blood. Receptor expression was measured by flow cytometry. Coagulation in static conditions was evaluated by rotational thromboelastometry (ROTEM).

RESULTS

Regardless of the manufacturing site, adhesion of cryoplatelets under shear flow (1000/sec) was significantly (p < 0.05) reduced compared to control. Expression of GPIbα was decreased in a subpopulation of cryoplatelets comprising 45% ± 11% (DEN), 63% ± 9% (GHE), and 94% ± 6% (SYD). That subpopulation displayed increased annexin V binding and decreased integrin activation. PLT adhesion, agglutination, and aggregation were moreover decreased in proportion to that subpopulation. Fibrin deposition under shear flow was normal but initiated faster (546 ± 163 sec GHE) than control PLTs (631 ± 120 sec, p < 0.01), only in the absence of tissue factor. In static conditions, clotting time was faster, but clot firmness decreased compared to control. Coagulation was not different between manufacturing sites.

CONCLUSION

Cryopreservation results in a subset of PLTs with enhanced GPIbα shedding, increased phosphatidylserine expression, reduced integrin response, and reduced adhesion to collagen in microfluidic models of hemostasis. The proportion of this phenotype is different between manufacturing sites. The clinical effects, if any, will need to be verified.

Usefulness of ADAMTS13 to predict response to recanalization therapies in acute ischemic stroke

OBJECTIVE

We aimed to analyze ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) in relation to arterial recanalization in patients treated with IV tissue plasminogen activator (tPA) and in relation to futile recanalization in patients treated with mechanical thrombectomy.

METHODS

Acute ischemic stroke patients (n = 108) with documented arterial occlusions treated with IV-tPA were selected. ADAMTS13 activity was measured by ELISA in samples collected before treatment. Recanalization was assessed at 2 hours by transcranial Doppler. In 78 consecutive patients treated with endovascular thrombectomy, ADAMTS13 antigen was measured by ELISA and futile recanalization was defined as complete recanalization plus modified Rankin Scale score >2 at 3 months. Independent predictors of recanalization and futile recanalization were determined by logistic regression, adjusted by age, NIH Stroke Scale score, and time from stroke onset.

RESULTS

Patients who achieved tPA-induced recanalization had higher baseline ADAMTS13 activity (78.1% [68%-88%] vs 70.1% [61%-79%], p = 0.021). In logistic regression analysis, ADAMTS13 activity >75% was an independent predictor of recanalization (odds ratio = 6.76 [1.52-30.02], p = 0.012), together with absence of early ischemic signs and Oxfordshire Community Stroke Project classification. Regarding endovascular therapies, a reduced ADAMTS13 concentration (<982 ng/mL) was an independent predictor of futile recanalization (odds ratio = 67.4 [1.4-3,282.1], p = 0.034), together with age and diabetes mellitus. The addition of ADAMTS13 to clinical predictors of tPA-induced recanalization and futile recanalization improved discrimination and reclassification (integrated discrimination improvement = 10.06% and 28.4%, net reclassification improvement = 61.0% and 107.4%, respectively).

CONCLUSIONS

A reduced ADAMTS13 was associated with poor response to recanalization therapies. If confirmed in future prospective studies, a panel of blood biomarkers including ADAMTS13 might be a useful tool to guide reperfusion therapies.

Platelet receptors as therapeutic targets: Past, present and future

Anti-platelet drugs reduce arterial thrombosis after plaque rupture and erosion, prevent stent thrombosis and are used to prevent and treat myocardial infarction and ischaemic stroke. Some of them may also be helpful in treating less frequent diseases such as thrombotic thrombocytopenic purpura. The present concise review aims to cover current and future developments of anti-platelet drugs interfering with the interaction of von Willebrand factor (VWF) with glycoprotein (GP) Ibα, and directed against GPVI, GPIIb/IIIa (integrin αIIbβ3), the thrombin receptor PAR-1, and the ADP receptor P2Y12. The high expectations of having novel antiplatelet drugs which selectively inhibit arterial thrombosis without interfering with normal haemostasis could possibly be met in the near future.

Inhibitors of the interaction between von Willebrand factor and platelet GPIb/IX/V

The formation of platelet-rich thrombi, a critical step in the pathogenesis of atherothrombotic events, is a multistep process involving several components, among which von Willebrand Factor (VWF) plays a central role. Ruptured atherosclerotic plaques expose subendothelial matrix proteins which bind VWF that represents a bridge between the injured blood vessel and activated platelets, playing a crucial role in platelet adhesion and aggregation, especially in conditions of high-shear rate. Due to these peculiarities, the binding of VWF to GPIbα is an attractive drug target. Here we summarize the present knowledge on the different classes of drugs targeting the VWF-GPIb interaction and we give an account of their level of clinical development. In particular, the following compounds are discussed: AJW200, an IgG4 humanized monoclonal antibody against VWF-A1; 82D6A3, a monoclonal antibody against VWF-A3; ALX-0081 and ALX-0681, bivalent humanized nanobodies targeting the VWF-A1 domain; ARC1779 and its advanced formulation ARC15105, second-generation aptamers that bind the VWF-A1 domain; h6B4-Fab, a murine monoclonal antibody, and GPG-290, a recombinant chimeric protein, both directed against GPIbα.

A microchip flow-chamber system for quantitative assessment of the platelet thrombus formation process

As the pathogenesis of arterial thrombosis often includes platelet thrombus formation (PTF), antiplatelet agents are commonly used for the prevention of thromboembolic events. Here, using a novel microchip flow-chamber system we developed to quantitatively analyze the PTF process, we evaluated the pharmacological efficacies of antiplatelet agents under different arterial shear rates. Hirudin-anticoagulated whole blood was perfused over a collagen-coated microchip at shear rates of 1000, 1500, and 2000s(-1), and PTF in the absence and presence of various antiplatelet agents was observed microscopically and quantified by measuring flow-pressure changes. The onset of PTF was measured as T(10) (time to reach 10 kPa), and AUC(10) (area under the flow pressure curve for the first 10 min) was calculated to quantify the overall stability of the formed thrombus. Aspirin and AR-C66096 (P2Y(12)-antagonist) at high concentrations (50 μM and 1000 nM, respectively) prolonged T(10) only modestly (AR-C66096>aspirin), but effectively decreased AUC(10), resulting in unstable PTF at all examined shear rates. With dual inhibition using both aspirin (25 μM) and ARC-66096 (250 nM), AUC(10) was drastically reduced. Nearly complete suppression of AUC(10) was also observed with abciximab (2 μg ml(-1)) and beraprost (PGI(2)-analog; 4 nM). Although OS-1 (GPIbα-antagonist; 100 nM) prevented complete capillary occlusion, significant amounts of microscopic thrombi were observed on the collagen surface. In contrast to abciximab and beraprost, OS-1 differentially affected PTF under higher shear conditions. Our novel analytical system is capable of distinguishing the pharmacological effects of various antiplatelet agents under physiological shear rates, suggesting that this system may aid in the determination of the appropriate type and dose of antiplatelet agent in the clinical setting.

Antithrombotic drug candidate ALX-0081 shows superior preclinical efficacy and safety compared with currently marketed antiplatelet drugs

Neutralizing the interaction of the platelet receptor gpIb with VWF is an attractive strategy to treat and prevent thrombotic complications. ALX-0081 is a bivalent Nanobody which specifically targets the gpIb-binding site of VWF and interacts avidly with VWF. Nanobodies are therapeutic proteins derived from naturally occurring heavy-chain-only Abs and combine a small molecular size with a high inherent stability. ALX-0081 exerts potent activity in vitro and in vivo. Perfusion experiments with blood from patients with acute coronary syndrome on standard antithrombotics demonstrated complete inhibition of platelet adhesion after addition of ALX-0081, while in the absence of ALX-0081 residual adhesion was observed. In a baboon efficacy and safety model measuring acute thrombosis and surgical bleeding, ALX-0081 showed a superior therapeutic window compared with marketed antithrombotics. Pharmacokinetic and biodistribution experiments demonstrated target-mediated clearance of ALX-0081, which leads to a self-regulating disposition behavior. In conclusion, these preclinical data demonstrate that ALX-0081 combines a high efficacy with an improved safety profile compared with currently marketed antithrombotics. ALX-0081 has entered clinical development.

Inhibition of von Willebrand factor-mediated platelet activation and thrombosis by the anti-von Willebrand factor A1-domain aptamer ARC1779

BACKGROUND

von Willebrand factor (VWF) has a role in both hemostasis and thrombosis. Platelets adhere to damaged arteries by interactions between the VWF A1-domain and glycoprotein Ib receptors under conditions of high shear. This initial platelet binding event stimulates platelet activation, recruitment, and activation of the clotting cascade, promoting thrombus formation.

OBJECTIVE

To characterize the inhibitory activity of a VWF inhibitory aptamer.

METHODS

Using in vitro selection, aptamer stabilization, and conjugation to a 20-kDa poly(ethylene glycol), we generated a nuclease-resistant aptamer, ARC1779, that binds to the VWF A1-domain with high affinity (K(D) approximately 2 nM). The aptamer was assessed for inhibition of VWF-induced platelet aggregation. In vitro inhibition of platelet adhesion was assessed on collagen-coated slides and injured pig aortic segments. In vivo activity was assessed in a cynomolgus monkey carotid electrical injury thrombosis model.

RESULTS AND CONCLUSION

ARC1779 inhibited botrocetin-induced platelet aggregation (IC90 approximately 300 nM) and shear force-induced platelet aggregation (IC95 approximately 400 nM). It reduced adhesion of platelets to collagen-coated matrices and formation of platelet thrombi on denuded porcine arteries. ARC1779 also inhibited the formation of occlusive thrombi in cynomolgus monkeys. We have discovered a novel anti-VWF aptamer that could have therapeutic use as an anti-VWF agent in the setting of VWF-mediated thrombosis.

Deficiency of von Willebrand factor protects mice from ischemic stroke

We recently demonstrated that blockade of the platelet adhesion receptor glycoprotein (GP) Ibalpha protects mice from ischemic stroke. Although von Willebrand factor (VWF) is the major ligand for GPIbalpha, GPIbalpha can engage other counterreceptors on endothelial cells, platelets, and leukocytes (eg, Mac-1 or P-selectin) potentially involved in stroke outcome. To further analyze whether VWF is of particular relevance for stroke development, VWF(-/-) mice underwent 60 minutes of middle cerebral artery occlusion. After 24 hours, VWF(-/-) mice had significantly smaller infarctions (P< .05) and less severe neurologic deficits (P< .01) compared with controls. This effect was sustained after 1 week, and intracranial bleeding was absent in VWF(-/-) mice as revealed by serial magnetic resonance imaging. Hydrodynamic injection of a VWF-encoding plasmid restored the susceptibility for stroke in VWF(-/-) mice. This study indicates that VWF is critically involved in cerebral ischemia. Hence, targeted inhibition of the GPIbalpha-VWF pathway might become a promising therapeutic option.

Antiplatelet therapy in the era of drug-eluting stents: current and future perspectives

The use of drug-eluting stents (DESs) dramatically reduced in-stent restenosis. However, the increasing use of these stents has raised concern about their potential thrombogenicity. Indeed, the particularity of DES thrombosis compared with bare metal stent thrombosis is a high rate of late thrombosis. Antiplatelet therapy is efficient in preventing DES thrombosis. However, this therapy could be optimized and may be improved in the future. This article will review the mechanisms and the epidemiology of stent thrombosis. Then, we will summarize the antiplatelet therapeutic strategies used to prevent stent thrombosis and especially DES-associated thrombosis. Finally, we will present some data with regard to potential advantages and pitfalls in DES thrombosis prevention using novel antiplatelet agents currently under development, as well as future stent designs with improved healing properties.

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.

Targeting Platelets in Acute Experimental Stroke

Background— Ischemic stroke is a frequent and serious disease with limited treatment options. Platelets can adhere to hypoxic cerebral endothelial cells by binding of their glycoprotein (GP) Ib receptor to von Willebrand factor. Exposure of subendothelial matrix proteins further facilitates firm attachment of platelets to the vessel wall by binding of collagen to their GPVI receptor. In the present study, we addressed the pathogenic role of GPIb, GPVI, and the aggregation receptor GPIIb/IIIa in experimental stroke in mice.

Methods and Results— Complete blockade of GPIbα was achieved by intravenous injection of 100 μg Fab fragments of the monoclonal antibody p0p/B to mice undergoing 1 hour of transient middle cerebral artery occlusion. At 24 hours after transient middle cerebral artery occlusion, cerebral infarct volumes were assessed by 2,3,5-triphenyltetrazolium chloride staining. In mice treated with anti-GPIbα Fab 1 hour before middle cerebral artery occlusion, ischemic lesions were reduced to ≈40% compared with controls (28.5±12.7 versus 73.9±17.4 mm 3 , respectively; P <0.001). Application of anti-GPIbα Fab 1 hour after middle cerebral artery occlusion likewise reduced brain infarct volumes (24.5±7.7 mm 3 ; P <0.001) and improved the neurological status. Similarly, depletion of GPVI significantly diminished the infarct volume but to a lesser extent (49.4±19.1 mm 3 ; P <0.05). Importantly, the disruption of early steps of platelet activation was not accompanied by an increase in bleeding complications as revealed by serial magnetic resonance imaging. In contrast, blockade of the final common pathway of platelet aggregation with anti-GPIIb/IIIa F(ab) 2 fragments had no positive effect on stroke size and functional outcome but increased the incidence of intracerebral hemorrhage and mortality after transient middle cerebral artery occlusion in a dose-dependent manner.

Conclusions— Our data indicate that the selective blockade of key signaling pathways of platelet adhesion and aggregation has a different impact on stroke outcome and bleeding complications. Inhibition of early steps of platelet adhesion to the ischemic endothelium and the subendothelial matrix may offer a novel and safe treatment strategy in acute stroke.

Effects of IgG and its F(ab')2 fragments of some patients with idiopathic thrombocytopenic purpura on platelet aggregation

OBJECTIVES

To make humanized monoclonal antibodies by phage surface display technology, we screened out the specific anti-platelet glycoproteins (GPs) IgG antibody from patients with chronic idiopathic thrombocytopenic purpura (ITP), which can inhibit platelet aggregation.

METHODS

We studied plasmas from 68 patients with ITP for the presence of IgG antibodies specific for GPIIb/IIIa and/or GPIb/IX using modified monoclonal antibody immobilization of platelet antigen assays. The IgG antibody and its F(ab')(2) fragments of the positive plasmas which could inhibit platelet aggregation function were prepared and purified. Their immunoreactivity to platelet GPs and effects on platelet function were further analyzed.

RESULTS

GPIIb/IIIa- and GPIb/IX-specific antibodies were found in 21 and 19 patients, respectively. Six of them had antibodies against both GP complexes. Among the 34 positive plasmas, four with positive anti-GPIIb/IIIa autoantibody showed significant inhibition of platelet aggregation induced by adenosine diphosphate (ADP), whereas one with GPIb/IX-specific antibody inhibited ristocetin-induced platelet aggregation. The purified IgG and its F(ab')(2) fragments from two patients not only retained the ability to bind to platelet GPs but also impaired the in vitro ADP-induced platelet aggregation.

CONCLUSIONS

F(ab')(2) portion of the IgG is a functional fragment, which is responsible for the autoantibody interaction with platelet GPs in ITP, and some of them also affect platelet function, which can be used to develop completely humanized anti-GPIIb/IIIa small molecular phage antibody.

Generation and characterization of a recombinant single chain Fv antibody to von Willebrand factor A1 domain from phage display library

In this study, cDNA of von Willebrand factor domain A1 (VWF-A1) was cloned from human umbilical vein endothelial cells, and expressed in E. coli. The expressed VWF-A1 was used as antigen to immunize Balb/c mice. Reverse transcribed-polymerase chain reaction (RT-PCR) was used to amplify VH and VL gene segment from the RNA isolated from the immunized mouse spleens. Single chain Fv (ScFv) genes were generated by splicing overlap extension and subcloned into a phagemid vector pHEN-1. Phage display library of repertoire single chain antibodies of anti-VWF-A1 was then constructed and screened for ScFvs that interact with VWF-A1. The ScFv gene of phage clone 28 that has best binding activity to VWF-A1 was cloned into pET20b vector for higher expression in E. coli strain BL-21(DE3) pLysS. The recombinant ScFv specifically reacted with VWF, rVWF-A1 and rVWF-A1/A3, but not with rVWF-A3, P-selectin and BSA. It inhibited ristocetin-induced platelet aggregation with an IC50 of 0.75 micromol/l, and its maximal extent of inhibition is 62.5%. The ScFv had no effect on thrombin-induced platelet aggregation. These data show that the ScFv is specific against VWF-A1 and could have a potential to be used as an antithrombotic agent.

BLOOD COAGULATION TESTS IN TOXICOLOGICAL STUDIES-REVIEW OF METHODS AND THEIR SIGNIFICANCE FOR DRUG SAFETY ASSESSMENT-

In general toxicological studies, prothrombin time and activated partial thromboplastin time are routinely measured to assess blood coagulation. Special (problem-driven) tests for blood coagulation are of significance to detect abnormalities and investigate the mechanism of toxicity in detail. In this review, we compiled widely scattered information on blood coagulation testing from different fields in the biological area, and reviewed the methods available and their significance in toxicological studies. The relevant literature cited here reports large species differences in platelet aggregation, coagulation factors or fibrinolysis, and technical limitations. However, the following tests are basically applicable to laboratory animals; (1) assays for individual coagulation factors and protein induced by vitamin K absence or antagonists (PIVKA) to investigate coagulation factor abnormalities; (2) platelet aggregation-, platelet adhesion-, platelet release-tests and von Willebrand factor assay to screen and/or investigate platelet dysfunction; (3) fibrin/fibrinogen degradation products (FDP), D-dimer and thromboelastogram to detect fibrinolitic abnormalities, and assays for plasminogen, plasmin and their activator/inhibitor to investigate fibrinolysis in detail; and (4) bleeding-time to grossly evaluate blood coagulation capability in vivo. An appropriate battery of these tests provides significant information for risk assessment of drugs.

Pharmacology of platelet adhesion and aggregation

At the injured vessel wall, blood platelets become activated and adhere to the subendothelial surface as well as to each other. These cellular adhesion processes are required for primary hemostasis, but can also lead to thrombosis. Considerable progress has been made during recent years in understanding the molecular mechanisms underlying platelet activation and adhesion. This knowledge will drive future efforts towards the development of new antiplatelet drugs for the prevention and treatment of cardiovascular diseases.

Contribution of von Willebrand factor to thrombus formation on neointima of rabbit stenotic iliac artery under high blood-flow velocity

OBJECTIVE

It has become clear that von Willebrand factor (vWF) plays important roles in platelet adhesion and aggregation under high blood-flow velocity conditions observed in stenotic atherosclerotic arteries. However, its roles in thrombus formation in vivo on diseased arteries have not been fully understood. We examined the contribution of vWF to thrombus formation and subsequent intimal growth by using a repeated balloon-injury model in rabbits.

METHODS AND RESULTS

Rabbit iliac arteries 4 weeks after a first balloon injury showed 37% luminal stenosis by neointimal growth, and blood velocity increased by 2.1 times compared with that of uninjured arteries. The second balloon injury induced fibrin-rich thrombus formation on the injured neointima. Intravenous administration of a monoclonal antibody against vWF (AJW200, 1.0 mg/kg body weight) remarkably prevented botrocetin-induced platelet aggregation ex vivo for 2 days; moreover, thrombus formation, cell proliferation, and subsequent neointimal growth were significantly reduced at 30 minutes, 5 days, and 4 weeks, respectively, after the second balloon injury.

CONCLUSIONS

These results indicate that vWF plays a potent role in fibrin-rich thrombus formation on the neointima under high blood-flow velocity conditions. Inhibition of plasma vWF activity might be effective for the reduction of thrombus formation and/or subsequent neointimal development after coronary interventions.

Mutation analysis and clinical implications of von Willebrand factor-cleaving protease deficiency

BACKGROUND

The pentad of thrombocytopenia, hemolytic anemia, mild renal dysfunction, neurologic signs, and fever, classically characterizes the syndrome of thrombotic thrombocytopenic purpura (TTP). TTP usually occurs in adults as an acquired form but a congenital form in children has also been described. In the latter case, the initial presentation is often with neonatal jaundice and thrombocytopenia. The disorder may subsequently take a relapsing course. Deficiency of a recently identified novel metalloprotease, the von Willebrand factor (vWF)-cleaving protease, originating from mutations in the ADAMTS13 gene plays a major role in the development of TTP.

METHODS

Blood for DNA analysis was collected from six unrelated TTP families, consisting of nine patients from four different countries, and was screened for mutations in the ADAMTS13 gene. This gene spans 29 exons encompassing approximately 37 kb. Conventional techniques of DNA extraction, polymerase chain reaction (PCR), and direct cycle sequencing were used.

RESULTS

Eight novel ADAMTS13 mutations are presented. Half of the total number of mutant ADAMTS13 alleles are amino acid substitutions. The disease-causing mutations are spread over the gene. The pathogenicity of the individual mutations is based upon their predicted effect on the ADAMTS13 protein and segregation in family members. Although most of the patients (seven out of nine) had symptoms during the neonatal period, they were in a remarkably good condition. Only one of the nine patients had a decreased glomerular filtration rate (GFR) with proteinuria and hematuria. Another patient had epileptic seizures.

CONCLUSION

We confirm that deficiency of ADAMTS13 is a molecular mechanism responsible for familial TTP. An early diagnosis allows prophylactic treatment with fresh plasma infusions.

New antithrombotic drugs on the horizon

Venous and arterial thromboembolism are a major cause for morbidity and mortality. The list of established drugs for the prevention of thrombus formation and embolisation includes heparins, hirudin and derivatives, aspirin, ADP and glycoprotein IIb/IIIa receptor antagonists, as well as vitamin K antagonists. Several limitations exist for these drugs that have stimulated the search for new and better anticoagulants. A series of selective clotting factor Xa inhibitors and direct factor IIa (thrombin) inhibitors are on the horizon, two of which are getting close to broad clinical application. Additional therapeutics that are still under preclinical and clinical investigation include inhibitors of the tissue factor pathway/factor VII complex, clotting factor VIII and XIII inhibitors and modulators of the protein C pathway or of endogenous fibrinolysis, as well as novel antiplatelet drugs. This review is focused on the current status of development of novel antithrombotics and their clinical potential. Even though only a few of a broad array of antithrombotic agents have reached clinical testing, some hold the potential for significant improvement in efficacy and safety of anticoagulant therapy.