The aptamer ARC1779 is a potent and specific inhibitor of von Willebrand Factor mediated ex vivo platelet function in acute myocardial infarction

Factors Associated with Platelet Activation-Recent Pharmaceutical Approaches

Platelets are at the forefront of human health and disease following the advances in their research presented in past decades. Platelet activation, their most crucial function, although beneficial in the case of vascular injury, may represent the initial step for thrombotic complications characterizing various pathologic states, primarily atherosclerotic cardiovascular diseases. In this review, we initially summarize the structural and functional characteristics of platelets. Next, we focus on the process of platelet activation and its associated factors, indicating the potential molecular mechanisms involving inflammation, endothelial dysfunction, and miRs. Finally, an overview of the available antiplatelet agents is being portrayed, together with agents possessing off-set platelet-inhibitory actions, while an extensive presentation of drugs under investigation is being given.

Aptamer grafted nanoparticle as targeted therapeutic tool for the treatment of breast cancer

Breast carcinomas repeat their number and grow exponentially making it extremely frequent malignancy among women. Approximately, 70-80% of early diagnosed or non-metastatic conditions are treatable while the metastatic cases are considered ineffective to treat with current ample amount of therapy. Target based anti-cancer treatment has been in the limelight for decades and is perceived significant consideration of scientists. Aptamers are the 'coming of age' therapeutic approach, selected using an appropriate tool from the library of sequences. Aptamers are non-immunogenic, stable, and high-affinity ligand which are poised to reach the clinical benchmark. With the heed in nanoparticle application, the delivery of aptamer to the specific site could be enhanced which also protects them from nuclease degradation. Moreover, nanoparticles due to robust structure, high drug entrapment, and modifiable release of cargo could serve as a successful candidate in the treatment of breast carcinoma. This review would showcase the method and modified method of selection of aptamers, aptamers that were able to make its way towards clinical trial and their targetability and selectivity towards breast cancers. The appropriate usage of aptamer-based biosensor in breast cancer diagnosis have also been discussed.

Identification and Engineering of Aptamers for Theranostic Application in Human Health and Disorders

An aptamer is a short sequence of synthetic oligonucleotides which bind to their cognate target, specifically while maintaining similar or higher sensitivity compared to an antibody. The in-vitro selection of an aptamer, applying a conjoining approach of chemistry and molecular biology, is referred as Systematic Evolution of Ligands by Exponential enrichment (SELEX). These initial products of SELEX are further modified chemically in an attempt to make them stable in biofluid, avoiding nuclease digestion and renal clearance. While the modification is incorporated, enough care should be taken to maintain its sensitivity and specificity. These modifications and several improvisations have widened the window frame of aptamer applications that are currently not only restricted to in-vitro systems, but have also been used in molecular imaging for disease pathology and treatment. In the food industry, it has been used as sensor for detection of different diseases and fungal infections. In this review, we have discussed a brief history of its journey, along with applications where its role as a therapeutic plus diagnostic (theranostic) tool has been demonstrated. We have also highlighted the potential aptamer-mediated strategies for molecular targeting of COVID-19. Finally, the review focused on its future prospective in immunotherapy, as well as in identification of novel biomarkers in stem cells and also in single cell proteomics (scProteomics) to study intra or inter-tumor heterogeneity at the protein level. Small size, chemical synthesis, low batch variation, cost effectiveness, long shelf life and low immunogenicity provide advantages to the aptamer over the antibody. These physical and chemical properties of aptamers render them as a strong biomedical tool for theranostic purposes over the existing ones. The significance of aptamers in human health was the key finding of this review.

Overview of the Therapeutic Potential of Aptamers Targeting Coagulation Factors

Aptamers are single-stranded DNA or RNA sequences that bind target molecules with high specificity and affinity. Aptamers exhibit several notable advantages over protein-based therapeutics. Aptamers are non-immunogenic, easier to synthesize and modify, and can bind targets with greater affinity. Due to these benefits, aptamers are considered a promising therapeutic candidate to treat various conditions, including hematological disorders and cancer. An active area of research involves developing aptamers to target blood coagulation factors. These aptamers have the potential to treat cardiovascular diseases, blood disorders, and cancers. Although no aptamers targeting blood coagulation factors have been approved for clinical use, several aptamers have been evaluated in clinical trials and many more have demonstrated encouraging preclinical results. This review summarized our knowledge of the aptamers targeting proteins involved in coagulation, anticoagulation, fibrinolysis, their extensive applications as therapeutics and diagnostics tools, and the challenges they face for advancing to clinical use.

Factors Affecting the Formation and Treatment of Thrombosis by Natural and Synthetic Compounds

Venous thromboembolism (VTE) refers to deep vein thrombosis (DVT), whose consequence may be a pulmonary embolism (PE). Thrombosis is associated with significant morbidity and mortality and is the third most common cardiovascular disease after myocardial infarction and stroke. DVT is associated with the formation of a blood clot in a deep vein in the body. Thrombosis promotes slowed blood flow, hypoxia, cell activation, and the associated release of many active substances involved in blood clot formation. All thrombi which adhere to endothelium consist of fibrin, platelets, and trapped red and white blood cells. In this review, we summarise the impact of various factors affecting haemostatic disorders leading to blood clot formation. The paper discusses the causes of thrombosis, the mechanism of blood clot formation, and factors such as hypoxia, the involvement of endothelial cells (ECs), and the activation of platelets and neutrophils along with the effects of bacteria and reactive oxygen species (ROS). Mechanisms related to the action of anticoagulants affecting coagulation factors including antiplatelet drugs have also been discussed. However, many aspects related to the pathogenesis of thrombosis still need to be clarified. A review of the drugs used to treat and prevent thrombosis and natural anticoagulants that occur in the plant world and are traditionally used in Far Eastern medicine has also been carried out.

A critical evaluation of caplacizumab for the treatment of acquired thrombotic thrombocytopenic purpura

Introduction: Acquired thrombotic thrombocytopenic purpura (aTTP) is a thrombotic microangiopathy caused by inhibitory autoantibodies against ADAMTS13 protein. Until recently, the combination of plasma exchange (PEX) and immunosuppression has been the standard front-line treatment in this disorder. However, aTTP-related mortality, refractoriness, and relapse are still a matter of concern. Areas covered: The better understanding of the pathophysiological mechanisms of aTTP has allowed substantial improvements in the diagnosis and treatment of this disease. Recently, the novel anti-VWF nanobody caplacizumab has been approved for acute episodes of aTTP. Caplacizumab is capable to block the adhesion of platelets to VWF, therefore inhibiting microthrombi formation in the ADAMTS13-deficient circulation. In this review, the characteristics of caplacizumab together with the available data of its efficacy and safety in the clinical setting will be analyzed. Besides, the current scenario of aTTP treatment will be provided, including the role of other innovative drugs. Expert opinion: With no doubt, caplacizumab is going to change the way we treat aTTP. In combination with standard treatment, caplacizumab can help to significantly reduce aTTP-related mortality and morbidity and could spare potential long-term consequences by minimizing the risk of exacerbation.

Application and development of aptamer in cancer: from clinical diagnosis to cancer therapy

Traditional anticancer therapies can cause serious side effects in clinical treatment due to their nonspecific of tumor cells. Aptamers, also termed as 'chemical antibodies', are short DNA or RNA oligonucleotides selected from the synthetic large random single-strand oligonucleotide library by systematic evolution of ligands by exponential enrichment (SELEX) to bind to lots of different targets, such as proteins or nucleic acid structures. Aptamers have good affinities and high specificity with target molecules, thus may be able to act as drugs themselves to directly inhibit the proliferation of tumor cells, or own great potentialities in the targeted drug delivery systems which can be used in tumor diagnosis and target specific tumor cells, thereby minimizing the toxicity to normal cells. Here we review the unique properties of aptamer represents a great opportunity when applied to the rapidly developing fields of biotechnology and discuss the recent developments in the use of aptamers as powerful tools for analytic, diagnostic and therapeutic applications for cancer.

von Willebrand Factor Predicts Mortality in ACS Patients Treated with Potent P2Y12 Antagonists and is Inhibited by Aptamer BT200 Ex Vivo

BACKGROUND

 von Willebrand factor (VWF) is crucial for arterial thrombosis and its plasma levels are increased in acute coronary syndromes (ACSs). The effects of conventional platelet inhibitors are compromised by elevated VWF under high shear rates. BT200 is a third-generation aptamer that binds and inhibits the A1 domain of human VWF. This article aims to study whether VWF is a predictor of mortality in ACS patients under potent P2Y12 blocker therapy and to examine the effects of a VWF inhibiting aptamer BT200 and its concentrations required to inhibit VWF in plasma samples of patients with ACS.

METHODS

 VWF activity was measured in 320 patients with ACS, and concentration effect curves of BT200 were established in plasma pools containing different VWF concentrations.

RESULTS

 Median VWF activity in patients was 170% (interquartile range % confidence interval [CI]: 85-255) and 44% of patients had elevated (> 180%) VWF activity. Plasma levels of VWF activity predicted 1-year (hazard ratio [HR]: 2.68; 95% CI: 1.14-6.31; p < 0.024) and long-term (HR: 2.59; 95% CI: 1.10-6.09) mortality despite treatment with potent platelet inhibitors (dual-antiplatelet therapy with aspirin and prasugrel or ticagrelor). Although half-maximal concentrations were 0.1 to 0.2 µg/mL irrespective of baseline VWF levels, increasing concentrations (0.42-2.13 µg/mL) of BT200 were needed to lower VWF activity to < 20% of normal in plasma pools containing increasing VWF activity (p < 0.001).

CONCLUSION

 VWF is a predictor of all-cause mortality in ACS patients under contemporary potent P2Y12 inhibitor therapy. BT200 effectively inhibited VWF activity in a target concentration-dependent manner.

Current Perspectives on Aptamers as Diagnostic Tools and Therapeutic Agents

Aptamers are synthetic single-stranded DNA or RNA sequences selected from combinatorial oligonucleotide libraries through the well-known in vitro selection and iteration process, SELEX. The last three decades have witnessed a sudden boom in aptamer research, owing to their unique characteristics, like high specificity and binding affinity, low immunogenicity and toxicity, and ease in synthesis with negligible batch-to-batch variation. Aptamers can specifically bind to the targets ranging from small molecules to complex structures, making them suitable for a myriad of diagnostic and therapeutic applications. In analytical scenarios, aptamers are used as molecular probes instead of antibodies. They have the potential in the detection of biomarkers, microorganisms, viral agents, environmental pollutants, or pathogens. For therapeutic purposes, aptamers can be further engineered with chemical stabilization and modification techniques, thus expanding their serum half-life and shelf life. A vast number of antagonistic aptamers or aptamer-based conjugates have been discovered so far through the in vitro selection procedure. However, the aptamers face several challenges for its successful clinical translation, and only particular aptamers have reached the marketplace so far. Aptamer research is still in a growing stage, and a deeper understanding of nucleic acid chemistry, target interaction, tissue distribution, and pharmacokinetics is required. In this review, we discussed aptamers in the current diagnostics and theranostics applications, while addressing the challenges associated with them. The report also sheds light on the implementation of aptamer conjugates for diagnostic purposes and, finally, the therapeutic aptamers under clinical investigation, challenges therein, and their future directions.

The aptamer BT200 effectively inhibits von Willebrand factor (VWF) dependent platelet function after stimulated VWF release by desmopressin or endotoxin

Von Willebrand factor (VWF) plays a major role in arterial thrombosis. Antiplatelet drugs induce only a moderate relative risk reduction after atherothrombosis, and their inhibitory effects are compromised under high shear rates when VWF levels are increased. Therefore, we investigated the ex vivo effects of a third-generation anti-VWF aptamer (BT200) before/after stimulated VWF release. We studied the concentration-effect curves BT200 had on VWF activity, platelet plug formation under high shear rates (PFA), and ristocetin-induced platelet aggregation (Multiplate) before and after desmopressin or endotoxin infusions in healthy volunteers. VWF levels increased > 2.5-fold after desmopressin or endotoxin infusion (p < 0.001) and both agents elevated circulating VWF activity. At baseline, 0.51 µg/ml BT200 reduced VWF activity to 20% of normal, but 2.5-fold higher BT200 levels were required after desmopressin administration (p < 0.001). Similarly, twofold higher BT200 concentrations were needed after endotoxin infusion compared to baseline (p < 0.011). BT200 levels of 0.49 µg/ml prolonged collagen-ADP closure times to > 300 s at baseline, whereas 1.35 µg/ml BT200 were needed 2 h after desmopressin infusion. Similarly, twofold higher BT200 concentrations were necessary to inhibit ristocetin induced aggregation after desmopressin infusion compared to baseline (p < 0.001). Both stimuli elevated plasma VWF levels in a manner representative of thrombotic or pro-inflammatory conditions such as arterial thrombosis. Even under these conditions, BT200 potently inhibited VWF activity and VWF-dependent platelet function, but higher BT200 concentrations were required for comparable effects relative to the unstimulated state.

Novel insights into the role of aptamers in the fight against cancer

PURPOSE

Aptamers are a class of single-stranded nucleic acid (DNA or RNA) oligonucleotides that are screened in vitro by a technique called systematic evolution of ligands by exponential enrichment (SELEX). They have stable three-dimensional structures that can bind to various targets with high affinity and specificity. Due to distinct properties such as easy synthesis, high stability, small size, low toxicity and immunogenicity, they have been largely studied as anticancer agents/tools. Consequently, aptamers are starting to play important roles in disease prevention, diagnosis and therapy. This review focuses on studies that evaluated the effect of aptamers on various aspects of cancer therapy. It also provides novel and unique insights into the role of aptamers on the fight against cancer.

METHODS

We reviewed literatures about the role of aptamers against cancer from PUBMED databases in this article.

RESULTS

Here, we summarized the role of aptamers on the fight against cancer in a unique point of view. Meanwhile, we presented novel ideas such as aptamer-pool-drug conjugates for the treatment of refractory cancers.

CONCLUSIONS

Aptamers and antibodies should form a "coalition" against cancers to maximize their advantages and minimize disadvantages.

Pharmacokinetics, pharmacodynamics and safety of aptamers

Aptamers are synthetic molecules structured as single-stranded DNA or RNA oligonucleotides that can be designed to mimic the functional properties of monoclonal antibodies. They bind to the target molecules (typically soluble or cell-bound proteins) with high affinity (with picomolar to low nanomolar range) and specificity, and therefore can be an alternative to therapeutic antibodies or peptide ligands. This paper reviews published data regarding pharmacokinetics, pharmacodynamics and safety of aptamers from preclinical and clinical studies. Aptamers have been developed for the treatment of a variety of diseases, including cancer, macular degeneration,g cardiovascular disease, diabetes and anaemia of chronic diseases. There are several preclinical studies with unmodified aptamers, but the vast majority of aptamer trials in humans have been conducted with modified aptamers, because unmodified aptamers demonstrate metabolic instability, as well as rapid renal filtration and elimination. Various strategies have been developed to improve the pharmacokinetic profile of aptamers. Aside from chemical modification of nucleotides in order to stabilize them against nuclease degradation, the main modification to extend the half-life is pegylation. Therefore, the process of pegylation as well as its benefits and possible shortcomings will briefly be discussed.

TLR4-Binding DNA Aptamers Show a Protective Effect against Acute Stroke in Animal Models

Since Toll-like receptor 4 (TLR4) mediates brain damage after stroke, development of TLR4 antagonists is a promising therapeutic strategy for this disease. Our aim was to generate TLR4-blocking DNA aptamers to be used for stroke treatment. From a random oligonucleotide pool, we identified two aptamers (ApTLR#1R, ApTLR#4F) with high affinity for human TLR4 by systematic evolution of ligands by exponential enrichment (SELEX). Optimized truncated forms (ApTLR#1RT, ApTLR#4FT) were obtained. Our data demonstrate specific binding of both aptamers to human TLR4 as well as a TLR4 antagonistic effect. ApTLR#4F and ApTLR#4FT showed a long-lasting protective effect against brain injury induced by middle cerebral artery occlusion (MCAO), an effect that was absent in TLR4-deficient mice. Similar effects were obtained in other MCAO models, including in rat. Additionally, efficacy of ApTLR#4FT in a model of brain ischemia-reperfusion in rat supports the use of this aptamer in patients undergoing artery recanalization induced by pharmacological or mechanical interventions. The absence of major toxicology aspects and the good safety profile of the aptamers further encourage their future clinical positioning for stroke therapy and possibly other diseases in which TLR4 plays a deleterious role.

The in vitro effect of the new antithrombotic drug candidate ALX-0081 on blood samples of patients undergoing percutaneous coronary intervention

Compound ALX-0081 is a bivalent humanised Nanobody® that binds the A1-domain of von Willebrand factor (VWF) with high affinity. Consequently, it can block the interaction between VWF and its platelet-receptor- glycoprotein Ib, which leads inevitably to formation of arterial thrombi. It was the objective of this study to assess the in vitro effects of ALX-0081 on platelet adhesion and aggregation in coronary artery disease (CAD) patients to determine the optimal concentration of ALX-0081 and the effect of co-medication. We included nine CAD patients, who were scheduled for elective percutaneous coronary intervention (PCI), and 11 healthy volunteers. At admission all patients received aspirin, clopidogrel and heparin. Blood was drawn 24 hours (h) before and 1 h after start of the PCI procedure and was subsequently spiked with different concentrations of ALX-0081 or buffer. The efficacy of ALX-0081 was assessed by in vitro experiments: flow chamber experiments, ristocetin-induced platelet aggregation (RIPA), and the platelet function analyser (PFA-100TM). VWF levels in CAD patients were significantly higher than in healthy controls. During PCI VWF levels did not rise. In all in vitro experiments, ALX-0081 led to complete inhibition of platelet adhesion and aggregation. However, the required effective concentration was higher in patients than in controls and was related to plasma VWF levels. In conclusion, ALX-0081 is able to completely inhibit in vitro platelet adhesion and aggregation in CAD patients scheduled for elective PCI. The efficacy of ALX-0081 is not influenced by PCI or co-medication. However, due to higher VWF levels in CAD patients a higher effective concentration of ALX-0081 was required than in healthy individuals.

The Role of von Willebrand Factor in Vascular Inflammation: From Pathogenesis to Targeted Therapy

Beyond its role in hemostasis, von Willebrand factor (VWF) is an emerging mediator of vascular inflammation. Recent studies highlight the involvement of VWF and its regulator, ADAMTS13, in mechanisms that underlie vascular inflammation and immunothrombosis, like leukocyte rolling, adhesion, and extravasation; vascular permeability; ischemia/reperfusion injury; complements activation; and NETosis. The VWF/ADAMTS13 axis is implicated in the pathogenesis of atherosclerosis, promoting plaque formation and inflammation through macrophage and neutrophil recruitment in inflamed lesions. Moreover, VWF and ADAMTS13 have been recently proposed as prognostic biomarkers in cardiovascular, metabolic, and inflammatory diseases, such as diabetes, stroke, myocardial infarction, and sepsis. All these features make VWF an attractive therapeutic target in thromboinflammation. Several lines of research have recently investigated “tailor-made” inhibitors of VWF. Results from animal models and clinical studies support the potent anti-inflammatory and antithrombotic effect of VWF antagonism, providing reassuring data on its safety profile. This review describes the role of VWF in vascular inflammation “from bench to bedside” and provides an updated overview of the drugs that can directly interfere with the VWF/ADAMTS13 axis.

ATP/pH Dual Responsive Nanoparticle with d-[des-Arg10 ]Kallidin Mediated Efficient In Vivo Targeting Drug Delivery

Inflammation has been reported as one significant hallmark of breast cancer in relation to tumor development, metastasis, and invasion. The bradykinin receptor 1 (B1R) is highly expressed on inflammatory breast tumor cells thus providing a promising targeting site for tumor recognition and sufficient receptor mediated endocytosis. In this study, the authors evaluate the targeting efficiency of l-form and d-form [des-Arg¹⁰ ]kallidin both in vitro and in vivo. To further improve the drug delivery efficiency, the authors establish a dandelion like nanoparticle by combining the polymeric drug conjugates and aptamer complex together. The doxorubicin conjugated polymer is complexed with adenosine-5'-triphosphate (ATP) sensitive hybridized aptamer in self-assembly process by intercalating into the double strand scaffolds. The acid labile conjugating bond and ATP sensitive aptamer endow the nanoparticle with dual responsiveness to intracellular milieu, thus triggering a quick drug release in tumor cells. Remarkable therapeutic effects and tuned in vivo pharmacokinetics profiles are shown by the aptamer complexed drug conjugates nanoparticle with B1R active targeting modification. Therefore the strategies of B1R targeting and ATP/pH dual-responsiveness nanoparticle help achieve enhanced drug accumulation within tumor cells and efficient chemotherapy for breast cancer.

Nucleic acid aptamer-mediated drug delivery for targeted cancer therapy

Aptamers are emerging as promising therapeutic agents and recognition elements. In particular, cell-SELEX (systematic evolution of ligands by exponential enrichment) allows in vitro selection of aptamers selective to whole cells without prior knowledge of the molecular signatures on the cell surface. The advantage of aptamers is their high affinitiy and binding specificity towards the target. This Minireview focuses on single-stranded (ss) oligonucleotide (DNA or RNA)-based aptamers as cancer therapeutics/theranostics. Specifically, aptamer-nanomaterial conjugates, aptamer-drug conjugates, targeted phototherapy and targeted biotherapy are covered in detail. In reviewing the literature, the potential of aptamers as delivery systems for therapeutic and imaging applications in cancer is clear, however, major challenges remain to be resolved, such as the poorly understood pharmacokinetics, toxicity and off-target effects, before they can be fully exploited in a clinical setting.

Targeting von Willebrand factor as a novel anti-platelet therapy; application of ARC1779, an Anti-vWF aptamer, against thrombotic risk

Excessive activation of platelets is a causative factor for thrombotic diseases such as acute coronary syndrome or stroke, and various anti-platelet drugs were developed. Aspirin and clopidogrel have been used as gold standards for anti-platelet therapies, however, their clinical limitations including bleeding problem have increased the demand driving development of novel anti-platelet drugs with new targets. Among several activating pathways leading to platelet aggregation, the interaction between von Willebrand factor (vWF) and glycoprotein Ib, which mainly occurs under high shear stress in arterioles, is recently suggested to be a new promising target. The anti-thrombotic efficacy of anti-vWF agents, such as ARC1779, has been proved in several preclinical and clinical studies. Here, we will discuss the potential benefits of targeting vWF as a novel antiplatelet therapy, providing an insight into the role of vWF in increased thrombotic risk.

Antigen and substrate withdrawal in the management of autoimmune thrombotic disorders

Prevailing approaches to manage autoimmune thrombotic disorders, such as heparin-induced thrombocytopenia, antiphospholipid syndrome and thrombotic thrombocytopenic purpura, include immunosuppression and systemic anticoagulation, though neither provides optimal outcome for many patients. A different approach is suggested by the concurrence of autoantibodies and their antigenic targets in the absence of clinical disease, such as platelet factor 4 in heparin-induced thrombocytopenia and β(2)-glycoprotein-I (β(2)GPI) in antiphospholipid syndrome. The presence of autoantibodies in the absence of disease suggests that conformational changes or other alterations in endogenous protein autoantigens are required for recognition by pathogenic autoantibodies. In thrombotic thrombocytopenic purpura, the clinical impact of ADAMTS13 deficiency caused by autoantibodies likely depends on the balance between residual antigen, that is, enzyme activity, and demand imposed by local genesis of ultralarge multimers of von Willebrand factor. A corollary of these concepts is that disrupting platelet factor 4 and β(2)GPI conformation (or ultralarge multimer of von Willebrand factor oligomerization or function) might provide a disease-targeted approach to prevent thrombosis without systemic anticoagulation or immunosuppression. Validation of this approach requires a deeper understanding of how seemingly normal host proteins become antigenic or undergo changes that increase antibody avidity, and how they can be altered to retain adaptive functions while shedding epitopes prone to elicit harmful autoimmunity.

Markers of endothelial and platelet activation are associated with high on-aspirin platelet reactivity in patients with stable coronary artery disease

INTRODUCTION

Aspirin inhibits the cyclooxygenase-1 (COX-1) mediated thromboxane A2 synthesis. Despite COX-1 inhibition, in patients with coronary artery disease (CAD), platelets can be activated through other mechanisms, like activation by thrombin.

MATERIALS AND METHODS

At baseline in this cross-sectional substudy of the ASCET trial, 1001 stable CAD patients, all on single aspirin treatment, were classified by the PFA100® method, as having high on-aspirin residual platelet reactivity (RPR) or not. Markers of hypercoagulability, endothelial and platelet activation as related to RPR, were evaluated to explore the potential mechanisms behind high on-aspirin RPR.

RESULTS

Altogether, 25.9% (n=259) of the patients were found to have high on-aspirin RPR. S-thromboxane B(2) levels were very low and did not differ between patients having high on-aspirin RPR or not. Patients with high on-aspirin RPR had significantly higher levels of von Willebrand Factor (vWF) (124 vs 100%, p<0.001, platelet count (236 vs 224 × 10(9)/l, p=0.008), total TFPI (68.4 vs 65.5 ng/ml, p=0.005) and ß-thromboglobulin (ß-TG) (33.3 vs 31.3 IU/ml, p=0.041) compared to patients with low on-aspirin RPR. No significant differences between the groups were observed in levels of endogenous thrombin generation (ETP), pro-thrombin fragment 1+2 (F1+2), D-dimer, soluble TF (sTF) or P-selectin (all p>0.05).

CONCLUSIONS

The high on-aspirin RPR as defined by PFA100® seems not to be due to increased thrombin activity as evaluated with ETP, sTF, F1+2 or D-dimer. The elevated levels of platelet count, ß-TG, TFPI and especially vWF might be explained by increased endothelial and platelet activation in these patients.

The anti-von Willebrand factor aptamer ARC1779 increases von Willebrand factor levels and platelet counts in patients with type 2B von Willebrand disease

Blockade of hyperactive von Willebrand factor (VWF) by ARC1779 blunted the platelet drop induced by desmopressin in patients with type 2B von Willebrand disease (VWD). Thus, we hypothesised that ARC1779 may increase VWF levels and correct thrombocytopenia. Three thrombocytopenic patients suffering from type 2B VWD received a loading dose of 0.23 mg/kg ARC1779 followed by 4 μg/kg/min intravenously for 72 hours in a prospective clinical trial. ARC1779 was well tolerated and safe. Plasma concentrations of ARC1779 increased to 76 μg/ml (59-130) leading to an immediate decrease of free VWF A1 domains. VWF/FVIII levels increased as early as 12 h after start of infusion, peaked near the end of infusion, and returned to baseline at follow-up. VWF ristocetin cofactor activity (VWF:RCo) showed a median 10-fold increase 8 hours after end of infusion, while the median VWF-antigen and FVIII increase was less (5-fold and 4-fold, respectively). Most importantly inhibition of hyperactive VWF rapidly increased platelet counts from 40 x 10(9)/l (38-58 x 10(9)//l) to a maximum of 146 x 10(9)//l (107-248 x 10(9)//l). In conclusion, ARC1779 markedly increases VWF/FVIII levels and most importantly improves or even corrects thrombocytopenia in VWD type 2B patients. This underscores the in vivo potency of ARC1779.

Aptamers as therapeutics in cardiovascular diseases

With many advantages over other therapeutic agents such as monoclonal antibodies, aptamers have recently emerged as a novel and powerful class of ligands with excellent potential for diagnostic and therapeutic applications. Typically generated through Systematic Evolution of Ligands by EXponential enrichment (SELEX), aptamers have been selected against a wide range of targets such as proteins, phospholipids, sugars, nucleic acids, as well as whole cells. DNA/RNA aptamers are single-stranded DNA/RNA oligonucleotides (with a molecular weight of 5-40 kDa) that can fold into well-defined 3D structures and bind to their target molecules with high affinity and specificity. A number of strategies have been adopted to synthesize aptamers with enhanced in vitro/in vivo stability, aiming at potential therapeutic/diagnostic applications in the clinic. In cardiovascular diseases, aptamers can be developed into therapeutic agents as anti-thrombotics, anti-coagulants, among others. This review focuses on aptamers that were selected against various molecular targets involved in cardiovascular diseases: von Willebrand factor (vWF), thrombin, factor IX, phospholamban, P-selectin, platelet-derived growth factor, integrin α(v)β(3), CXCL10, vasopressin, among others. With continued effort in the development of aptamer-based therapeutics, aptamers will find their niches in cardiovascular diseases and significantly impact clinical patient management.

ARC15105 is a potent antagonist of von Willebrand factor mediated platelet activation and adhesion

OBJECTIVE

We investigated the stability, pharmacokinetic, and pharmacodynamic profile of the 2(nd) generation anti-von Willeband factor aptamer ARC15105.

METHODS AND RESULTS

Platelet plug formation was measured by collagen/adenosine diphosphate-induced closure time with the platelet function analyzer-100 and platelet aggregation by multiple electrode aggregometry. Platelet adhesion was measured on denuded porcine aortas and in a flow chamber. Aptamer stability was assessed by incubation in nuclease rich human, monkey, and rat serum for up to 72 hours. Pharmacokinetic and pharmacodynamic profiles were tested in cynomolgus monkeys after IV and SC administration. The median IC(100) and IC(50) to prolong collagen/adenosine diphosphate-induced closure timewere 27 nmol/L and 12 nmol/L, respectively. ARC15105 (1.3 μmol/L) completely inhibited ristocetin-induced platelet aggregation in whole blood (P<0.001), but also diminished collagen, ADP, arachidonic acid, and thrombin receptor activating peptide-induced platelet aggregation to some extent (P<0.05). ARC15105 (40 nmol/L) decreased platelet adhesion by >90% on denuded porcine aortas (P<0.001), which was comparable to the degree of inhibition obtained with abciximab. ARC15105 (100 nmol/L) also inhibited platelet adhesion to collagen under arterial shear in a flow chamber by >90% (P<0.001). The IV and SC terminal half-lives in cynomolgus monkeys were 67 and 65 hours, respectively, and the SC bioavailability was ≈98%. Allometric scaling estimates the human T(1/2) would be ≈217 hours. Pharmacodynamic analysis confirms that ARC15105 inhibits von Willeband factor activity >90% in blood samples taken 300 hours after a 20 mg/kg IV or SC dose in monkeys.

CONCLUSIONS

The potency, pharmacokinetic profile, and SC bioavailability of ARC15105 support its clinical investigation for chronic inhibition of von Willeband factor -mediated platelet activation.

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.

Aptamer modules as sensors and detectors

Aptamers comprise a range of molecular recognition scaffolds that can be engineered to bind to a legion of different proteins and other targets with excellent specificity and affinity. Because these non-natural oligonucleotides are accessible entirely synthetically, aptamers can be equipped with all sorts of reporter groups and can be coupled to many different carriers, surfaces, nanoparticles, or other biomolecules. They can be used in a highly modular fashion and often recognize their targets by a mechanism in which the aptamer undergoes considerable structural rearrangement, which can be exploited for transducing a binding event into a signal. As a consequence, aptamers have been adapted to a huge variety of "read-out configurations" and are increasingly used as capture agents in many different bioanalytical methods. But despite considerable success with these applications, many remaining challenges must still be overcome for the more widespread incorporation of aptasensors in clinical and environmental biosensing and diagnostics to take place. Some particularly noteworthy progress on this front is currently being made with aptasensor configurations that can be used for the multiplexed sensing of many analytes in parallel. In this Account, we describe some of the concepts involved in transducing the binding of a ligand into a signal through various physico-chemical interactions. Research in this area usually involves the combination of the molecular biology of proteins and nucleic acids with biotechnology, synthetic chemistry, physical chemistry, and surface physics. We begin with a brief introduction of the properties and characteristics that qualify aptamers as capture agents for many different analytes and their suitability as highly versatile biosensor components. We then address approaches that apply to surface acoustic wave configurations, drawing largely from our own contributions to aptasensor development, before moving on to describe previous and recent progress in multiplexed aptasensors. Obtaining proteome-wide profiles in cells, organs, organisms, or full populations requires the ability to accurately measure many different analytes in small sample volumes over a broad dynamic range. Multiplexed sensing is an invaluable tool in this endeavor. We discuss what we consider the biggest obstacles to the broader clinical use of aptasensor-based diagnostics and our perspective on how they can be surmounted. Finally,we explore the tremendous potential of aptamer-based sensors that can specifically discriminate between diseased and healthy cells. Progress in these areas will greatly expand the range of aptasensor applications, leading to enhanced diagnosis of diseases in clinical practice and, ultimately, improved patient care.

Rapidly regulating platelet activity in vivo with an antidote controlled platelet inhibitor

Millions of individuals are prescribed platelet inhibitors, such as aspirin and clopidogrel, to reduce their risk of thrombosis-related clinical events. Unfortunately many platelet inhibitors are contraindicated in surgical settings because of their inherent bleeding risk complicating the treatment of patients who require surgery. We describe the development of a potent antiplatelet agent, an RNA aptamer-termed Ch-9.14-T10 that binds von Willebrand factor (VWF) with high affinity and inhibits thrombosis in a murine carotid artery damage model. As expected, when this potent antiplatelet agent is administered, it greatly increases bleeding from animals that are surgically challenged. To improve this antiplatelet agent's safety profile, we describe the generation of antidotes that can rapidly reverse the activity of Ch-9.14-T10 and limit blood loss from surgically challenged animals. Our work represents the first antidote controllable antiplatelet agent, which could conceivably lead to improved medical management of patients requiring antiplatelet medication who also need surgery.

Evaluation of the INNOVANCE PFA P2Y test cartridge: sensitivity to P2Y(12) blockade and influence of anticoagulant

Monitoring of platelet ADP receptor P2Y(12) inhibition may be performed by a variety of platelet function assays. Given the lack of sensitivity of the existing PFA-100® cartridge formulations to detect P2Y(12) inhibition, a new cartridge for the PFA-100 (INNOVANCE® PFA P2Y) has recently been developed. The performance of the new PFA-100 test cartridge was compared with standard collagen/ADP (CADP) and collagen/epinephrine (CEPI) cartridges, light transmission aggregometry, vasodilator-stimulated phosphoprotein, the VerifyNow® P2Y(12) assay and multiple electrode aggregometry. In this study, 20 normal blood samples anticoagulated with either citrate or hirudin were spiked with two different clinically relevant concentrations (1 and 10 µM final concentration) of the prasugrel active metabolite (R-138727, Lilly/Daiichi Sankyo) for 30 min at 37°C. Comparison of the platelet function tests demonstrated that all tests (except CADP and CEPI) were substantially inhibited by 10 µM R-138727. Intermediate results were typically obtained with 1 µM R-138727 in citrated blood. However, both MEA ADP and ADPHS tests were highly sensitive to 1 µM R-138727 in hirudin anticoagulated blood. Further comparison of citrate or hirudin blood samples (N = 5) revealed that all platelet tests (except CEPI) became more sensitive to 1 µM R-138727 in hirudinized blood. The INNOVANCE PFA P2Y cartridge proved to be sensitive to P2Y(12) inhibition and was comparable to other currently available platelet function tests. The sensitivity of all platelet function tests for detecting in vitro inhibition of P2Y(12) is markedly different depending on the anticoagulant used.

A dose ranging phase I/II trial of the von Willebrand factor inhibiting aptamer ARC1779 in patients with congenital thrombotic thrombocytopenic purpura

Congenital thrombotic thrombocytopenic purpura (TTP) is a very rare but potentially life-threatening disorder. This phase I/II trial compared the pharmacokinetics and pharmacodynamics and safety of three different administration modes of the anti-von Willebrand factor (VWF) aptamer ARC1779. This was a prospective clinical trial with a partial cross-over design: three periods comprised subcutaneous injections of 50 mg of ARC1779 on seven subsequent days, a low-dose infusion of ARC1779 (0.002 mg/kg/min) for 24-72 hours and a high-dose infusion (0.004-0.006 mg/kg/min) up to 72 hours. ARC1779 concentrations were determined with high performance liquid chromatography, VWF inhibition was measured with enzyme immunoassay and platelet function was determined with the platelet function analyser (PFA-100) and impedance aggregometry. ARC1779 was well tolerated without any bleeding at concentrations spanning over three orders of magnitude. The daily s.c. injection yielded plasma levels (0.5 μg/ml) of the drug that were too low to sufficiently suppress VWF. The low-dose i.v. infusion increased platelet counts in one patient, whereas the high i.v. dose increased plasma concentrations up to 69 μg/ml, completely blocked free A1 domains, VWF-dependent platelet plug formation and enhanced platelet counts in 2/3 patients. In conclusion, infusion of ARC1779 dose-dependently inhibits VWF-dependent platelet function and during infusion ARC1779 increases or stabilises platelet counts in congenital TTP. However, the tested doses, particularly the daily s.c. injections, did not correct all clinical or laboratory features of TTP.

The von Willebrand inhibitor ARC1779 reduces cerebral embolization after carotid endarterectomy: a randomized trial

BACKGROUND AND PURPOSE

Inhibition of von Willebrand factor offers a novel approach to prevention of stroke and myocardial ischemia but has not yet been demonstrated to show efficacy on clinically relevant end points. ARC1779 is an aptamer that inhibits the prothrombotic function of von Willebrand factor by binding to the A1 domain of von Willebrand factor and thereby blocking its interaction with glycoprotein. Phase 1 studies suggest it inhibits platelet aggregation with less increase in bleeding than conventional antiplatelet agents. The effect of ARC 1779 on cerebral emboli immediately after carotid endarterectomy was investigated in a randomized clinical trial.

METHODS

Patients undergoing carotid endarterectomy were randomized double-blind to ARC1779 or placebo administered intravenously. Transcranial Doppler recording, to detect cerebral embolic signals, was performed in the first 3 hours postoperatively. The primary end point was time to first embolic signals.

RESULTS

Thirty-six patients were recruited, 18 in each arm. The Kaplan-Meier median time to first embolic signals was 83.6 minutes for ARC1779 compared with 5.5 minutes for placebo. Using Cox proportional hazards embolic signals occurred statistically significantly later on ARC1779 (P=0.007). Reduced embolic signals counts were correlated with inhibition of von Willebrand factor activity (P=0.03). Increased perioperative bleeding and anemia were seen with ARC1779.

CONCLUSIONS

von Willebrand factor inhibition reduces thromboembolism in humans. It may play a role in treatment of stroke and myocardial ischemia. The extent to which bleeding complications occur in nonoperated patients needs to be assessed in further studies. Clinical Trial Registration- URL: http://clinicaltrials.gov. Unique identifier: NCT00742612.

Patients with severe aortic valve stenosis and impaired platelet function benefit from preoperative desmopressin infusion

BACKGROUND

Patients with severe aortic valve stenosis have a markedly reduced platelet function as measured by a prolonged collagen adenosine diphosphate closure time (CADP-CT) determined by the platelet function analyzer PFA-100. We hypothesized that such patients may benefit from desmopressin when they present with prolonged CADP-CT due to the specific action of desmopressin on von Willebrand factor (VWF) and CADP-CT.

METHODS

In this double-blind, randomized placebo controlled trial, 43 patients undergoing aortic valve replacement (due to severe aortic valve stenosis with CADP-CT>170 seconds) were given desmopressin 0.3 μg/kg or saline intravenously after induction of anesthesia. Measurement of CADP-CT, factor VIII activity, von Willebrand factor antigen, GpIb binding activity, ristocetin cofactor activity, collagen-binding activity, and multimers were performed after induction of anesthesia, one hour after desmopressin infusion, and 24 hours postoperatively.

RESULTS

In the majority of patients, baseline values of von Willebrand factor related indices were normal, but increased one hour after infusion of desmopressin by 73% to 90% as compared with placebo. Selective loss of high molecular weight multimers was seen only in a minority of patients. The CADP-CT was greater than 170 seconds in 92% of screened patients, and desmopressin shortened CADP-CT by 48% versus baseline and reduced postoperative blood loss by 42% (p<0.001).

CONCLUSIONS

Prolonged CADP-CT indicates platelet dysfunction in severe aortic valve stenosis, and can guide the use of desmopressin as an effective prohemostatic agent in patients with severe aortic valve stenosis.

Targeting von Willebrand factor and platelet glycoprotein Ib receptor

Atherothrombotic events, such as acute coronary syndrome or stroke, are the result of platelet activation. Von Willebrand factor (vWF), a multimeric glycoprotein, plays a key role in aggregation of platelets, especially under high-shear conditions. Acting as bridging element or ligand between damaged endothelial sites and the glycoprotein Ib (GPIb) receptor on platelets, vWF is responsible for platelet adhesion and aggregation. This vWF activation and further platelet aggregation mainly occurs under high shear stress present in small arterioles or during deficiency of the vWF-cleaving protease ADAMTS13. There are several substances targeting vWF itself or its binding receptor GPIb on platelets. Two antibodies are directed against vWF: AJW200, an IgG4 humanized monoclonal antibody, and 82D6A3, a monoclonal antibody of the collagen-binding A-3 domain of vWF. ALX-0081 and ALX-0681 are bivalent humanized nanobodies targeting the GPIb binding site of vWF. Aptamers are oligonucleotides with drug-like properties that share some of the attributes of monoclonal antibodies. ARC1779 is a second-generation, nuclease-resistant aptamer, binding to the activated vWF A1 domain and ARC15105 is a chemically advanced follower with an assumed higher affinity to vWF. Antibodies targeting GPIbα are h6B4-Fab, a murine monoclonal antibody; GPG-290, a recombinant, chimeric protein containing the amino-terminal 290 amino acids of GPIbα linked to human IgG1 Fc; and the monoclonal antibody SZ2. There are a number of promising preclinical results and development of some agents (AJW 200, ARC1779 and ALX-0081) has already reached Phase II trials.

Porcine and canine von Willebrand factor and von Willebrand disease: hemostasis, thrombosis, and atherosclerosis studies

Use of animal models of inherited and induced von Willebrand factor (VWF) deficiency continues to advance the knowledge of VWF-related diseases: von Willebrand disease (VWD), thrombotic thrombocytopenic purpura (TTP), and coronary artery thrombosis. First, in humans, pigs, and dogs, VWF is essential for normal hemostasis; without VWF bleeding events are severe and can be fatal. Second, the ADAMTS13 cleavage site is preserved in all three species suggesting all use this mechanism for normal VWF multimer processing and that all are susceptible to TTP when ADAMTS13 function is reduced. Third, while the role of VWF in atherogenesis is debated, arterial thrombosis complicating atherosclerosis appears to be VWF-dependent. The differences in the VWF gene and protein between humans, pigs, and dogs are relatively few but important to consider in the design of VWF-focused experiments. These homologies and differences are reviewed in detail and their implications for research projects are discussed. The current status of porcine and canine VWD are also reviewed as well as their potential role in future studies of VWF-related disorders of hemostasis and thrombosis.

Single dose granulocyte colony-stimulating factor markedly enhances shear-dependent platelet function in humans

Granulocyte colony-stimulating factor (G-CSF) has been associated with the induction of a hypercoagulable state in patients as well as peripheral blood stem donors. Interestingly, sparse data exist on the kinetics of platelet and coagulation activation in response to G-CSF and it is unknown if G-CSF augments shear-dependent platelet function. These two issues are addressed in the current trial. Thirty-six healthy volunteers were enrolled into this study. All subjects received a single-dose of 5 microg/kg filgrastim intravenously. The effects of recombinant G-CSF on platelet and coagulation function were assessed by the platelet function analyzer PFA-100 (collagen/epinephrine (CEPI-CT), collagen/ADP (CADP-CT) closure times), von Willebrand factor activity (vWF : RiCO) ELISA, tissue factor (TF)-mRNA expression on circulating leukocytes and rotation thrombelastography (ROTEM). G-CSF time-dependently enhanced shear dependent platelet function measured by the PFA-100: CEPI-CT declined by 48% and CADP-CT by 31% with nadir values after 24 h (p < 0.001 as compared to baseline) and returned to near-baseline values after 72 hours. In accordance, VWF : RiCO increased by 59% after 24 h (p < 0.001) and returned to baseline 48 h later. TF-mRNA peaked after 4 hours (>6 fold increase p < 0.001) and reached near-baseline values after 24 hours. Nadir closure times were seen after 24 hours (-15%; p < 0.001). Single-dose administration of 5 microg/kg G-CSF significantly enhances shear-dependent platelet function and strongly induces leukocyte TF-mRNA, which translates into shortened clotting times ex vivo.

Clopidogrel in acute coronary syndrome: implications of recent study findings

The platelet ADP receptor antagonist clopidogrel is recommended for the treatment of patients with acute coronary syndrome and/or percutaneous coronary intervention. Patients who received a coronary stent in particular should be protected by sufficient antiplatelet therapy to prevent stent thrombosis. Clopidogrel is a prodrug and has to undergo extensive metabolization before the active metabolite can irreversibly bind to platelets. This makes clopidogrel treatment susceptible to genetic and drug interactions. Recent study findings suggest that initial treatment with a higher dose of clopidogrel may be superior to the currently approved dose. It is not clear whether this approach will be sufficient to entirely overcome clopidogrel hyporesponsiveness, which worsens outcomes in up to one-third of patients. Newer antiplatelet agents are emerging but clopidogrel remains the best established treatment option, with more than 120,000 patients treated in randomized trials and 12 years of clinical postmarketing experience.

An anti-von Willebrand factor aptamer reduces platelet adhesion among patients receiving aspirin and clopidogrel in an ex vivo shear-induced arterial thrombosis

The von Willebrand factor (vWF) aptamer, ARC1779 that blocks the binding of vWF A1-domain to platelet glycoprotein 1b (GPIb) at high shear, may deliver a site-specific antithrombotic effect. We investigated the efficiency of ARC1779 on platelet function in patients with coronary artery disease (CAD) on double antiplatelet therapy. Blood from patients taking aspirin and clopidogrel and from normal volunteers was treated ex vivo with ARC1779 or abciximab, either prior to perfusion (pretherapy) or 10 minutes following the initiation of perfusion (posttherapy) on damaged arteries. Under pre- but not posttherapy, platelet adhesion was significantly reduced by ARC1779 at 83 and 250 nmol/L and by abciximab (100 nmol/L) versus placebo (4.8, 3.8, and 2.9 vs 7.3 platelets × 10(6)/cm(2), P < .05). In contrast to abciximab, ARC1779 did not significantly affect platelet aggregation, P-selectin expression, and platelet-leukocyte binding. These proof-of-concept data may constitute the framework for randomized clinical investigations of this novel antiplatelet therapy among patients with CAD.

A structural explanation for the antithrombotic activity of ARC1172, a DNA aptamer that binds von Willebrand factor domain A1

ARC1172 is a 41-mer DNA aptamer selected to bind the A1 domain of von Willebrand factor (VWF). A derivative of ARC1172 with modifications to increase intravascular survival inhibits carotid artery thrombosis in a Cynomolgus macaque model and inhibits VWF-dependent platelet aggregation in humans, suggesting that such aptamers may be useful to prevent or treat thrombosis. In the crystal structure of a VWF A1-ARC1172 complex, the aptamer adopts a three-stem structure of mainly B-form DNA with three noncanonical base pairs and 9 unpaired residues, 6 of which are stabilized by base-base or base-deoxyribose stacking interactions. The aptamer-protein interface is characterized by cation-pi interactions involving Arg, Lys, and Gln residues, often stabilized by H-bonds with adjacent bases. The ARC1172 binding site on the A1 domain overlaps with that of botrocetin and clashes with glycoprotein Ibalpha binding at an adjacent site, which accounts for the antithrombotic activity of ARC1172 and related aptamers.

The aptamer ARC1779 blocks von Willebrand factor-dependent platelet function in patients with thrombotic thrombocytopenic purpura ex vivo

BACKGROUND

In thrombotic thrombocytopenic purpura (TTP), ultralarge von Willebrand factor (VWF) multimers bind platelet (PLT) glycoprotein Ib and lead to the formation of disseminated fibrin-poor, VWF-rich PLT thrombi. The aptamer ARC1779 blocks binding of the VWF A1 domain to PLT glycoprotein Ib. We evaluated whether ARC1779 inhibits the excessive VWF activity and VWF-mediated PLT function in patients with TTP.

STUDY DESIGN AND METHODS

We studied the ex vivo concentration response curves for ARC1779 on PLT function analyzer (PFA-100, Dade Behring) and cone-and-plate analyzer (CPA, Impact-R) PLT function tests, agonist-induced PLT aggregation, and VWF activity of TTP patients (n = 11, three in acute phase and eight in remission) and healthy controls (n = 44).

RESULTS

VWF activity and VWF-dependent PLT plug formation were increased in TTP patients relative to healthy controls, but agonist-induced PLT aggregation was not. ARC1779 blocked collagen/adenosine 5'-diphosphate (ADP)-induced PLT plug formation as measured by PFA-100 with an inhibitory concentration (IC)(100) of approximately 1 microg/mL in citrate-anticoagulated samples and approximately 3 to 4 microg/mL in hirudin-anticoagulated samples. A similar concentration of ARC1779 was necessary to block shear-dependent PLT adhesion in both TTP patients and healthy controls using the CPA assay (IC(100) of approx. 1 microg/mL for both). ARC1779 blocked VWF activity with an IC(90) of approximately 3 to 4 microg/mL in all subjects, but did not inhibit PLT aggregation by ADP, collagen, or arachidonic acid even at concentrations much greater than those that fully inhibited VWF-dependent PLT function.

CONCLUSIONS

ARC1779 potently and specifically inhibits VWF activity and VWF-dependent PLT function. ARC1779 may be a promising novel therapeutic for the treatment of TTP.

Pharmacokinetic, pharmacodynamic and clinical profile of novel antiplatelet drugs targeting vascular diseases

Platelet inhibitors are the mainstay treatment for patients with vascular diseases. The current 'gold standard' antiplatelet agent clopidogrel has several pharmacological and clinical limitations that have prompted the search for more effective platelet antagonists. The candidates include various blockers of the purinergic P2Y12 receptor such as prasugrel, an oral irreversible thienopyridine; two adenosine triphosphate analogues that bind reversibly to the P2Y12 receptor: ticagrelor (oral) and cangrelor (intravenous); elinogrel, a direct-acting reversible P2Y12 receptor inhibitor (the only antiplatelet compound that can be administered both intravenously and orally); BX 667, an orally active and reversible small-molecule P2Y12 receptor antagonist; SCH 530348, SCH 205831, SCH 602539 and E5555, highly selective and orally active antagonists on the protease-activated receptor 1. A number of drugs also hit new targets: terutroban, an oral, selective and specific inhibitor of the thromboxane receptor; ARC1779, a second-generation, nuclease resistant aptamer which inhibits von Willebrand factor-dependent platelet aggregation; ALX-0081, a bivalent humanized nanobody targeting the GPIb binding site of von Willebrand factor and AJW200, an IgG4 monoclonal antibody of von Willebrand factor. The pharmacology and clinical profiles of new platelet antagonists indicate that they provide more consistent, more rapid and more potent platelet inhibition than agents currently used. Whether these potential advantages will translate into clinical advantages will require additional comparisons in properly powered, randomized, controlled trials.