A Double-Blind Randomized Comparison of Combined Aspirin and Ticlopidine Therapy Versus Aspirin or Ticlopidine Alone on Experimental Arterial Thrombogenesis in Humans

Protease-activated receptor antagonists prevent thrombosis when dual antiplatelet therapy is insufficient in an occlusive thrombosis microfluidic model

Background

Platelet activation and arterial thrombosis on a ruptured atherosclerotic plaque is a major cause of myocardial infarction. Dual antiplatelet therapy (DAPT), the combination of platelet aggregation inhibitors, aspirin and a P2Y12 antagonist, is used to prevent arterial thrombosis. However, many people continue to have arterial thrombosis and myocardial infarction despite DAPT, indicating that additional therapies are required where DAPT is insufficient.

Objectives

To determine whether antagonists of protease-activated receptors (PARs) can prevent occlusive thrombosis under conditions where DAPT is insufficient.

Methods

We used human whole blood in a microfluidic model of occlusive thrombosis to compare conditions under which DAPT is effective to those under which DAPT was not. Cangrelor (a P2Y12 antagonist) and aspirin were used to mimic DAPT. We then investigated whether the PAR1 antagonist vorapaxar or the PAR4 antagonist BMS 986120, alone or in combination with DAPT, prevented occlusive thrombosis.

Results and Conclusions

A ruptured plaque exposes collagen fibers and is often rich in tissue factor, triggering activation of platelets and coagulation. Occlusive thrombi formed on type I collagen in the presence or absence of tissue factor (TF). However, although DAPT prevented occlusive thrombosis in the absence of TF, DAPT had little effect when TF was also present. Under these conditions, PAR antagonism was also ineffective. However, occlusive thrombosis was prevented by combining DAPT with PAR antagonism. These data demonstrate that PAR antagonists may be a useful addition to DAPT in some patients and further demonstrate the utility of in vitro models of occlusive thrombosis.

Genetic testing in patients undergoing percutaneous coronary intervention: rationale, evidence and practical recommendations

INTRODUCTION

Clopidogrel is the most frequently utilized P2Y12 inhibitor and is characterized by broad interindividual response variability resulting in impaired platelet inhibition and increased risk of thrombotic complications in a considerable number of patients. The potent P2Y12 inhibitors, prasugrel and ticagrelor, can overcome this limitation but at the expense of an increased risk of bleeding. Genetic variations of the cytochrome P450 (CYP) 2 C19 enzyme, a key determinant in clopidogrel metabolism, have been strongly associated with clopidogrel response profiles prompting investigations of genetic-guided selection of antiplatelet therapy.

AREAS COVERED

The present manuscript focuses on the rationale for the use of genetic testing to guide the selection of platelet P2Y12 inhibitors among patients undergoing percutaneous coronary intervention (PCI). Moreover, a comprehensive appraisal of the available evidence and practical recommendations is provided.

EXPERT COMMENTARY

Implementation of genetic testing as a strategy to guide the selection of therapy can result in escalation (i.e. switching to prasugrel or ticagrelor) or de-escalation (i.e. switching to clopidogrel) of P2Y12 inhibiting therapy. Most recent investigations support the clinical benefit of a genetic guided selection of antiplatelet therapy in patients undergo PCI. Integrating the results of genetic testing with clinical and procedural variables represents a promising strategy for a precision medicine approach for the selection of antiplatelet therapy among patients undergoing PCI.

Tyrosine Kinase Inhibitor Sunitinib Delays Platelet-Induced Coagulation: Additive Effects of Aspirin

BACKGROUND

 Sunitinib is a multitarget tyrosine kinase inhibitor (TKI) used for cancer treatment. In platelets, sunitinib affects collagen-induced activation under noncoagulating conditions. We investigated (1) the effects of sunitinib on thrombus formation induced by other TK-dependent receptors, and (2) the effects under coagulating conditions. Cardiovascular disease is a comorbidity in cancer patients, resulting in possible aspirin treatment. Sunitinib and aspirin are associated with increased bleeding risk, and therefore we also investigated (3) the synergistic effects of these compounds on thrombus and fibrin formation.

METHODS

 Blood or isolated platelets from healthy volunteers or cancer patients were incubated with sunitinib and/or aspirin or vehicle. Platelet activation was determined by TK phosphorylation, flow cytometry, changes in [Ca²⁺]_i, aggregometry, and whole blood perfusion over multiple surfaces, including collagen with(out) tissue factor (TF) was performed.

RESULTS

 Sunitinib reduced thrombus formation and phosphatidylserine (PS) exposure under flow on collagen type I and III. Also, sunitinib inhibited glycoprotein VI-induced TK phosphorylation and Ca²⁺ elevation. Upon TF-triggered coagulation, sunitinib decreased PS exposure and fibrin formation. In blood from cancer patients more pronounced effects of sunitinib were observed in lung and pancreatic as compared to neuroglioblastoma and other cancer types. Compared to sunitinib alone, sunitinib plus aspirin further reduced platelet aggregation, thrombus formation, and PS exposure on collagen under flow with(out) coagulation.

CONCLUSION

 Sunitinib suppresses collagen-induced procoagulant activity and delays fibrin formation, which was aggravated by aspirin. Therefore, we urge for awareness of the combined antiplatelet effects of TKIs with aspirin, as this may result in increased risk of bleeding.

Safety and efficacy of P2Y12 inhibitor monotherapy in patients undergoing percutaneous coronary interventions

Introduction: Antiplatelet therapy represents a key strategy for the prevention of thrombotic complications in patients with both acute and chronic coronary syndromes, particularly those undergoing percutaneous coronary intervention (PCI). Nevertheless, dual antiplatelet therapy (DAPT) is associated with a bleeding risk proportionate to its duration. Ever growing appreciation of the prognostic implications associated with bleeding and the development of safer stent platforms over the past years have led to a number of novel antiplatelet treatment strategies being tested among patients undergoing PCI. Areas covered: P2Y₁₂ inhibitor monotherapy after ashort course DAPT has emerged as ableeding reduction strategy to mitigate such risk while still preventing thrombotic complications. In this review we describe the latest evidence regarding the safety and efficacy of P2Y₁₂ inhibitor monotherapy in patients undergoing PCI in different clinical settings. Expert opinion: P2Y₁₂ inhibitor monotherapy after a brief period of DAPT has emerged as an effective approach to reduce the risk of bleeding without any tradeoff in efficacy (i.e., thrombotic complications). This strategy has shown consistent findings in a number of different clinical settings of patients undergoing PCI. Nevertheless, unanswered questions on the ideal patient and the precise P2Y₁₂ monotherapy regimen warrant further investigation.

Dual antiplatelet therapy is associated with prolonged survival after lower extremity revascularization

BACKGROUND

Dual antiplatelet therapy (DAPT) after coronary stenting prolongs survival by preventing both in-stent thrombosis and other cardiovascular atherothrombotic events. Patients with peripheral artery disease (PAD) typically have a heavy burden of unrevascularized coronary artery disease and also stand to benefit from increased atherothrombotic protection with DAPT. The potential benefit of DAPT compared with aspirin alone in patients with PAD is not well described.

METHODS

We identified all patients undergoing an initial elective lower extremity revascularization (bypass or endovascular) from 2003 to 2016 in the Vascular Quality Initiative registry discharged on aspirin or aspirin plus a thienopyridine antiplatelet agent (DAPT). We first estimated models predicting the likelihood of receiving DAPT and then used inverse probability weighting to account for baseline differences in the likelihood of receiving DAPT and compared late survival. For sensitivity analysis, we also performed Cox proportion hazard modeling on the unweighted cohorts and generated adjusted survival curves.

RESULTS

We identified 57,041 patients undergoing lower extremity revascularization (28% bypass). Of 15,985 bypasses (69% for critical limb ischemia [CLI]), 38% were discharged on DAPT. Of 41,056 endovascular interventions (39% for CLI), 69% were discharged on DAPT. Analyses using inverse probability weighting demonstrated a small survival benefit to DAPT at 1 year for bypass (93% vs 92% [P = .001]) and endovascular interventions (93% vs 92% [P = .005]) that was sustained through 5 years of follow-up (bypass, 80% vs 78% [P = .004]; endovascular, 76% vs 73% [P = .002]). When stratified by severity of PAD, DAPT had a survival benefit for patients with CLI undergoing bypass (5 years, 70% vs 66% [P = .04]) and endovascular intervention (5 years, 71% vs 67% [P = .01]) but not for patients with claudication (bypass, 89% vs 88% [P = .36]; endovascular, 87% vs 85% [P = .46]). The protective effect of DAPT was similar when using Cox proportional hazard models after bypass (hazard ratio, 0.81 [95% confidence interval, 0.72-0.90]) and endovascular intervention (hazard ratio, 0.89 [95% confidence interval, 0.83-0.95]).

CONCLUSIONS

DAPT at time of discharge was associated with prolonged survival for patients with CLI undergoing lower extremity revascularization but not for those with claudication. Further research is needed to quantify the risks associated with DAPT and to identify subgroups at increased risk of thrombotic and bleeding complications to guide medical management of patients with PAD.

The impact of blood shear rate on arterial thrombus formation

The shear rate and corresponding shear stress have impacts on arterial thrombus formation. In particular, the effects of increasing concentration of platelets at the vessel wall and activation of platelets at this site increase the growth and stability of the thrombi which may result in a fatal narrowing of the arterial lumen. The efficacy of many antithrombotic agents is shear dependent as well. It is apparent that there is a need for a point-of-care device to rapidly monitor the risk for arterial thrombosis and to optimize antithrombotic therapy in vitro. The present review focuses on the essential role of shear rate on arterial thrombus formation in native human blood drawn directly from an antecubital vein.

Measurement of whole blood thrombus formation using parallel-plate flow chambers - a practical guide

Custom-made and commercial parallel-plate flow chambers are widely used for studies of platelet activation and thrombus formation in whole blood at defined shear rates. When used in a reproducible way, such flow chamber devices give valuable information on the thrombogenic potential of human, mouse, or rat blood. This article aims to provide a practical guide for the use of parallel-plate flow chambers in combination with routine microscopic imaging techniques. The following methodological aspects are addressed: preparation of surface coatings, calculation of blood flow and shear rate, control of pre-analytical variables, protocols for routine performing of flow chamber tests with non-coagulating or coagulating blood, and procedures for real-time and end-point analysis of thrombus formation. Frequently encountered experimental problems and artifacts are discussed, as well as possibilities for using flow chamber devices as a diagnostic tool to test antithrombotic medication.

The impact of factor Xa inhibition on axial dependent arterial thrombus formation triggered by a tissue factor rich surface

This study was designed to assess the effect of Factor Xa antagonists on thrombus formation at various axial positions on a tissue factor rich surface under arterial blood flow conditions. Non-anticoagulated, flowing human blood, drawn directly from an antecubital vein, was perfused over a tissue factor coated cover slip in a parallel-plate perfusion chamber. Thrombus surface coverage, thrombus mean height and fibrin surface coverage were measured at six different axial positions by confocal microscopy. Both thrombus surface coverage and mean height decreased along the cover slip axis whereas the fibrin surface coverage increased. Pre-chamber treatment of blood with the direct Factor Xa inhibitors Razaxaban and 813893 resulted in significantly reduced thrombus and fibrin formation at all axial positions investigated (P < 0.05). Thrombus and fibrin deposition in a laminar flow chamber changed with axial position with surface coverage measurements being more reproducible than thrombus mean height. Data were more reproducible towards the centre of the flow chamber than at the extremities. Razaxaban and 813893 inhibited thrombus and fibrin formation at the highest concentrations tested. No difference in drug effect was apparent at different axial positions. In conclusion, axial position influences the degree of thrombus and fibrin deposition with measurements being less reproducible at the extremities of the flow chamber. This technique may prove useful for analysing anti-thrombotic drug effects before progression to clinical trials.

Clopidogrel response variability and the advent of personalised antiplatelet therapy. A bench to bedside journey

Platelet-mediated thrombosis is a dreaded clinical event and is the primary cause of acute coronary syndromes and post-percutaneous intervention (PCI) ischaemic events. There has been a long standing interest in the ex vivo quantification of platelet reactivity to assess the risk of thrombosis. Early studies demonstrated platelet activation and heightened platelet reactivity in acute coronary syndromes and after PCI. However, a demonstration that heightened reactivity actually precipitated the ischaemic event was lacking. Our knowledge of platelet receptor physiology and the advent of novel inhibitors have significantly advanced the field. The P2Y12 receptor has been shown to play a pivotal role in the amplification of platelet activation by multiple agonists and its inhibition has resulted in improved clinical outcomes. The most widely used drug to block P2Y12 receptor, clopidogrel is associated with resistance in selected patients and these patients have been shown to be at increased risk for post-PCI ischaemic event occurrence in multiple studies. Importantly, a threshold of high platelet reactivity has been demonstrated, and beyond this threshold ischaemic events occur precipitously. Based on the current evidence, it is rational to quantify the intensity of the ADP-P2Y12 interaction in the patient at the greatest risk for thrombosis-the PCI patient. However, there is only evidence from small clinical trials demonstrating the clinical efficacy of changing an antiplatelet regimen based on an ex vivo platelet function measurement. Moreover, there are numerous patients with vulnerable coronary anatomy that have not yet experienced plaque rupture; the prognostic role of a measurement of platelet reactivity in the latter group has never been studied. Large-scale trials are ongoing that will investigate the role of personalised antiplatelet therapy in the PCI patient.

Aspirin "failures," clopidogrel added to aspirin, and secondary stroke prevention in veterans presenting with TIA or mild-to-moderate ischemic stroke

Our objective was to investigate whether clopidogrel added to low-dose aspirin reduced vascular events in male patients at our VA hospital who had "failed" aspirin therapy because of a mild-to-moderate stroke or a transient ischemic attack. Of 179 consecutive patients who both reported daily aspirin usage at the time of their newest ischemic event as well as were then operationally defined as aspirin "failures," 134 (group A) were treated with combined aspirin-clopidogrel, 15 (group B) underwent an early arterial procedure, 25 (group C) were anticoagulated, and 5 were not entered or continued because of either non-compliance or a refusal to participate. Study therapies were modified because of a vascular event in 4.5% of group A (one non-fatal ischemic stroke, one non-fatal myocardial infarction, and four transient ischemic attacks) and 33% of group B (one non-fatal ischemic stroke, one non-fatal myocardial infarction, and three transient ischemic attacks). No major or fatal bleeding events occurred in any of those on combined aspirin-clopidogrel therapy, with minor bleeding in 10 of 134 (7.5%) and 2 of 15 (13.3%) of group A and B patients, respectively. Patients were followed for 18 +/- 9.7 months. Combined aspirin-clopidogrel therapy appears both safe as well as effective in this single-center, observational study.

Antithrombotic properties of aspirin and resistance to aspirin: beyond strictly antiplatelet actions

Aspirin is effective in the prevention of cardiovascular events in high-risk patients. The primary established effect of aspirin on hemostasis is to impair platelet aggregation via inhibition of platelet thromboxane A(2) synthesis, thus reducing thrombus formation on the surface of the damaged arterial wall. Growing evidence also indicates that aspirin exerts additional antithrombotic effects, which appear to some extent unrelated to platelet thromboxane A(2) production. Aspirin can reduce thrombin generation with the subsequent attenuation of thrombin-mediated coagulant reactions such as factor XIII activation. Aspirin also acetylates lysine residues in fibrinogen resulting in increased fibrin clot permeability and enhanced clot lysis as well as directly promoting fibrinolysis with high-dose aspirin. The variable effectiveness of aspirin in terms of clinical outcomes and laboratory findings, which has been termed aspirin resistance, may be related to these additional antithrombotic effects that are altered when associated with common genetic polymorphisms such as the Leu33Pro beta(3)-integrin or Val34Leu factor XIII mutations. However, the clinical relevance of these observations is still unclear. Elucidation of the actual impacts of aspirin other than antiaggregation effects could be important in view of the widespread use of this drug in the prevention of thrombotic manifestations of atherosclerosis.

Blood flow devices in medical research and clinical testing in humans: are we approaching personalized medicine?

This review focuses on studies of blood flow devices employed in man to unravel the mechanisms of bleeding and thrombotic disorders, and on the characterization of novel experimental antithrombotic entities and drug candidates in biopharmaceutical research and development. Clinical studies with drug candidates and new therapeutic strategies have also been performed, and the predictability of these experimental approaches to clinical situations is excellent. Based on the solid validation of these flow devices, miniature flow devices employing nonanticoagulated blood drawn directly from an antecubital vein should be developed for diagnostic purposes. It is anticipated that such a diagnostic flow device could develop into a personalized medicine approach.

Antiplatelet therapy: current strategies and future trends

Pharmacological management of thrombotic complications is strongly influenced by antiplatelet treatment strategies. Recent clinical trials have clearly indicated that current antiplatelet strategies may not inhibit recurrent thrombotic events in selected patients and improvement is necessary. Recently, there has been a gradual modification in the guidelines for clopidogrel dosing. In addition, newly developed P2Y12 receptor inhibitors and thrombin inhibitors are undergoing Phase II and III clinical trials. Moreover, research related to novel agents that interfere with other steps in coagulation and platelet adhesion, and platelet thromboxane and thrombin receptor blockers, show promise. An important future step will probably be the development of personalized therapy based on defining the individual patient’s propensity for thrombosis through investigation of platelet–thrombin–fibrin interactions. Such an approach will enhance the targeting of specific therapy based on the pathophysiology of the individual patient.

Antiplatelet Therapy in Neuroendovascular Therapeutics

Our understanding of the pharmacology of antiplatelet therapy continues to evolve rapidly. Although the existing data are primarily generated in the setting of interventional and preventative cardiology studies, these data may be extrapolated to guide the rational application of these agents in neuroendovascular procedures. Platelet function testing represents an increasingly available and practical method by which to verify the adequacy of therapy and guide clinical decision making. The optimal application of these agents will undoubtedly improve the risk profile of neuroendovascular procedures, increase the success rate of acute stroke intervention, and facilitate more effective secondary stroke prevention.

Superior efficacy of clopidogrel plus acetylsalicylic acid compared with extended-release dipyridamole plus acetylsalicylic acid in preventing arterial thrombogenesis in healthy volunteers

INTRODUCTION

Recent ex vivo platelet aggregometry data indicate that clopidogrel 75 mg/day plus acetylsalicylic acid (ASA) 75 mg/day is a more potent antiplatelet regimen than the marketed combination of dipyridamole+ASA. The present study was designed to assess the antithrombotic effect of both dual antiplatelet regimens using a human ex vivo model of arterial thrombosis.

MATERIALS AND METHODS

This was a randomized, double-blind, placebo-controlled, crossover study. During two 10-day treatment periods separated by a 14-day washout period, 23 healthy male volunteers received once-daily clopidogrel 75 mg plus acetylsalicylic acid 75 mg, or twice-daily extended-release dipyridamole 200 mg plus acetylsalicylic acid 25 mg. Assessments were made at baseline and on Day 10 of each period. Arterial thrombus formation was induced ex vivo by exposing a collagen-coated surface in a parallel-plate perfusion chamber to native blood for 3 min (arterial wall shear rate 2600 s(-1)). Total platelet and fibrin deposition was determined by immunoenzymatic methods.

RESULTS

Compared with baseline values, the mean inhibition of total platelet deposition was 63.9+/-5.9% with clopidogrel plus acetylsalicylic acid, compared with 18.4+/-5.6% for extended-release dipyridamole plus acetylsalicylic acid (67% reduction; 95% CI, 49-79%; p<0.0001). Corresponding figures for fibrin deposition were 64.9+/-4.8% and 18.3+/-9.7%, respectively (58% reduction; 95% CI, 45-67%; p<0.0001). Both treatments were well tolerated.

CONCLUSIONS

Clopidogrel plus acetylsalicylic acid showed significantly superior antithrombotic efficacy compared with extended-release dipyridamole plus acetylsalicylic acid in preventing arterial thrombogenesis in humans.

Advances in antiplatelet therapy

Cardiovascular diseases (CVDs) are the leading cause of morbidity and mortality in the western world. These disorders share a common pathophysiology -- atherosclerosis, which affects various arterial beds, leading to protean manifestations (coronary artery disease [CAD], stroke, peripheral arterial disease [PAD]). The platelet plays a pivotal role in the perpetuation and clinical expression of these disorders. The platelet, once believed to have a role confined to modulation of thrombosis and haemostasis, also plays an active role in vascular inflammation. Antiplatelet agents have become first-line therapy for CVD, and their unequivocal benefits are demonstrated in various basic and experimental models and supported by overwhelming evidence from clinical trials. Search is underway for more effective and safer antiplatelet therapy. Novel therapies are emerging to target the redundant pathways of platelet adhesion, activation and aggregation. Efforts are also ongoing to enhance implementation of existent therapy, target therapy selectively to high-risk patients and to those likely to respond (pharmacogenomics), and study the incremental benefits and safety of various antiplatelet combinations and their interaction with other medications in patients with CVD treated with polypharmacy.

Clopidogrel: How good is it and how does it work?

Until recently, long-term antiplatelet therapy for the treatment and prevention of the complications of atherothrombotic disease was limited to aspirin. Although an incredibly cost-effective therapy, in placebo-controlled clinical trials approximately 75% of patients at risk continue to experience thrombotic events despite chronic aspirin therapy. The availability of the thienopyridines, in particular clopidogrel, represents an important addition to the physician's armamentarium. A number of clinical trials have confirmed the efficacy of the combination of clopidogrel and aspirin therapy compared with aspirin alone, with multiple other important large-scale clinical trials currently ongoing. The exact mechanism of this benefit is still being elucidated but is clearly related to the inhibition of the many consequences of platelet activation--vascular inflammation, endothelial dysfunction, and localized angiogenesis/mitogenesis--and not just aggregation.

Integrating antithrombin and antiplatelet therapies with early invasive management for non-ST-segment elevation acute coronary syndromes

Non-ST-segment elevation acute coronary syndromes are a dramatic manifestation of coronary artery disease. Multiple clinical trials have shown that early cardiac catheterization improves clinical outcomes in patients with non-ST-segment elevation acute coronary syndromes. Many antithrombotic agents effectively manage unstable coronary syndromes and serve as adjuncts to percutaneous coronary intervention. Yet, the growing number of pharmacologic agents makes early management of non-ST-segment elevation acute coronary syndromes increasingly complex. We review the current evidence regarding the optimal integration of early antithrombotic and antiplatelet therapies with early coronary angiography and subsequent revascularization.

In Vitro Effects of Clopidogrel on the Platelet-Subendothelium Interaction, Platelet Thromboxane and Endothelial Prostacyclin Production, and Nitric Oxide Synthesis

Clopidogrel is an antiplatelet drug that belongs to the group of thienopyridines. Because of its main mechanism of action most studies of clopidogrel have centered on the platelet ADP pathway. The aim of the present study was to compare the effects of clopidogrel, ticlopidine, and aspirin, on platelet activation by collagen (the main inducer of platelet activation in vivo), prostanoid, and NO production, and the effects on blood perfusion experiments. Clopidogrel inhibited platelet aggregation induced in whole blood by collagen and TxB2 production to a greater extent than did ticlopidine. Prostacyclin synthesis did not change after incubation with thienopyridines, whereas aspirin inhibited synthesis in a dose-dependent manner. Thienopyridines increased NO production to a greater extent than did aspirin. All three drugs impaired the platelet-subendothelium interaction under flow conditions. With thienopyridines, the presence of endothelium did not modify the percentage of the surface coated by platelets.

Anticoagulants (Thrombin Inhibitors) and Aspirin Synergize With P2Y 12 Receptor Antagonism in Thrombosis

Background— This study was designed to determine whether (1) P2Y 12 antagonism synergizes with other antithrombotics and (2) anticoagulants (thrombin inhibitors) affect the antithrombotic activity elicited by P2Y 12 antagonism.

Methods and Results— Thrombosis was achieved by perfusion of human and murine blood through type III collagen–coated capillaries at arterial shear rate. CT50547, a direct-acting P2Y 12 antagonist, inhibited thrombosis in PPACK- but not heparin-anticoagulated human blood. In contrast, CT50547 inhibited thrombosis in aspirin-treated individuals independently of the anticoagulant. Thrombin and TXA 2 also synergized with P2Y 12 in the absence of anticoagulation, because combined treatment of aspirin or C921-78 (a factor Xa inhibitor) with CT50547 or 2-MeSAMP (a P2Y 12 antagonist) inhibited the thrombotic process, whereas all treatments failed to inhibit thrombosis when used individually. Synergism was also observed ex vivo when P2Y 12 -deficient (P2Y 12 −/− ) mice were administered aspirin or coagulation inhibitors (C921-78 and bivalirudin). Finally, using intravital microscopy, we found that both C921-78 and bivalirudin abrogated the thrombotic process in P2Y 12 +/ − mice, whereas each showed only partial efficacy in P2Y 12 +/+ animals.

Conclusions— Our study indicates that (1) thrombin inhibitors and aspirin have a demonstrable synergy of antithrombotic activity with P2Y 12 antagonism and (2) the in vitro analysis of the antithrombotic activity of P2Y 12 antagonists is affected by the anticoagulant used for blood collection. This suggests that the antithrombotic potential of P2Y 12 antagonists in vitro may be overestimated in anticoagulated samples of blood and best achieved in vivo by the inclusion of aspirin and/or a thrombin inhibitor.

Clopidogrel’s role in the management of atherosclerotic disease: a focus on acute coronary syndromes

Recent advances in our understanding of the role of the platelet in the atherosclerotic process beyond the acute formation of arterial blood clots, such as inflammation, have highlighted the role of antiplatelet agents as being much more than just 'blood thinners.' Some of the most important cardiovascular trials performed in the last 20 years have studied antiplatelet therapies. However, despite their long history, current global health implications and proven benefit, there remain substantial gaps in our understanding as to how to best utilize the limited number of antiplatelet agents available. This article will discuss the mechanism of action of the antiplatelet class known as thienopyridines, the pharmacodynamics and pharmacokinetics of the thienopyridine agent clopidogrel (Plavix, Bristol-Myers Squibb and Sanofi Pharmaceuticals) as well as the literature supporting its clinical benefits and areas of ongoing research that will help clarify the optimal utilization of clopidogrel for the treatment of cardiovascular disease.

P2Y12 regulates platelet adhesion/activation, thrombus growth, and thrombus stability in injured arteries

The critical role for ADP in arterial thrombogenesis was established by the clinical success of P2Y12 antagonists, currently used at doses that block 40-50% of the P2Y12 on platelets. This study was designed to determine the role of P2Y12 in platelet thrombosis and how its complete absence affects the thrombotic process. P2Y12-null mice were generated by a gene-targeting strategy. Using an in vivo mesenteric artery injury model and real-time continuous analysis of the thrombotic process, we observed that the time for appearance of first thrombus was delayed and that only small, unstable thrombi formed in P2Y12-/- mice without reaching occlusive size, in the absence of aspirin. Platelet adhesion to vWF was impaired in P2Y12-/- platelets. While adhesion to fibrinogen and collagen appeared normal, the platelets in thrombi from P2Y12-/- mice on collagen were less dense and less activated than their WT counterparts. P2Y12-/- platelet activation was also reduced in response to ADP or a PAR-4-activating peptide. Thus, P2Y12 is involved in several key steps of thrombosis: platelet adhesion/activation, thrombus growth, and stability. The data suggest that more aggressive strategies of P2Y12 antagonism will be antithrombotic without the requirement of aspirin cotherapy and may provide benefits even to the aspirin-nonresponder population.

What is the role for improved long-term antiplatelet therapy after percutaneous coronary intervention?

BACKGROUND

Coronary stent placement has replaced balloon angioplasty as the percutaneous coronary intervention (PCI) method of choice, primarily because of its lower restenosis rate. Compared with aspirin (ASA) monotherapy or ASA plus warfarin, the ticlopidine and ASA combination is superior in reducing thrombotic events after stenting. Clopidogrel plus ASA appears to be at least as effective as ticlopidine and ASA. Intravenous glycoprotein IIb/IIIa inhibitors effectively prevent periprocedural thrombotic complications, but their short duration of action and parenteral dosing don't allow for long-term protection. This review aimed to answer how long after PCI with a stent patients are at risk for recurrent thrombotic events and what the optimal way to prevent them is.

RESULTS

Classically, ASA has been prescribed indefinitely, whereas adenosine diphosphate receptor antagonists have been discontinued after 2 to 4 weeks. However, the Clopidogrel in Unstable Angina to Prevent Recurrent Events (CURE) trial found that long-term dual antiplatelet therapy with clopidogrel and ASA was more effective than ASA alone in preventing major cardiovascular events in patients with acute coronary syndrome, including those treated with PCI.

CONCLUSION

Results from additional ongoing studies are needed to clarify the role of long-term dual oral antiplatelet therapy in preventing ischemic events in patients who have undergone PCI.

Short- and long-term oral antiplatelet therapy in acute coronary syndromes and percutaneous coronary intervention

Platelets play a central role in both the short- and long-term manifestations of atherothrombosis. In acute coronary syndrome (ACS), there is a steep rise in cardiovascular events early, followed by an incremental rise in cardiovascular events over the long term. This long-term event rate is related to persistent platelet activation and thrombin generation. There is therefore a need to optimize both short- and long-term oral antiplatelet and antithrombotic strategies. The benefits of aspirin therapy, when administered early and continued over the long term, were demonstrated in several early randomized trials. The Antithrombotic Trialists' Collaboration found a 46% reduction in vascular events with antiplatelet therapy (mostly aspirin). However, despite treatment with aspirin and proven therapies, recurrent events remain high. The adenosine diphosphate receptor antagonists, ticlopidine and clopidogrel, inhibit the early steps of platelet activation, degranulation, and release of prothrombotic and inflammatory mediators, while also preventing activation of the glycoprotein IIb/IIIa receptor. The Clopidogrel in Unstable angina to prevent Recurrent Events (CURE) trial demonstrated the benefits of aspirin plus clopidogrel in reducing major cardiovascular events (cardiovascular death, myocardial infarction [MI], and stroke reduced by 20%, p = 0.00009) in a broad range of patients with ACS when administered early and continued over the long term. The benefits emerge very rapidly after a 300 mg loading dose. For the large number of patients undergoing percutaneous coronary intervention in the CURE trial, there was a substantial risk reduction with clopidogrel pretreatment followed by long-term therapy (p < 0.002). This benefit was present, regardless of whether intervention was performed early or late. The significant benefits of aspirin and clopidogrel persist for the combined efficacy-safety end point of cardiovascular death, MI, stroke, or life-threatening bleeding when clopidogrel is started early, combined with aspirin and other standard therapies, and continued for up to one year.

Strenuous but not moderate exercise increases the thrombotic tendency in healthy sedentary male volunteers

We have investigated the effect of moderate and strenuous exercise on experimental arterial thrombus formation in men. Thrombogenesis was measured in 15 sedentary healthy male volunteers at rest or immediately after two standardized exercise tests performed for 30 min on a bicycle ergometer. The exercises were performed at a constant load corresponding to either 50 or 70% maximal oxygen uptake. Thrombus formation was induced ex vivo by exposing a collagen-coated coverslip in a parallel plate perfusion chamber to native nonanticoagulated blood for 3 min. The shear rate at the collagen surface was 2,600 s(-1). Platelet and fibrin deposition was quantified by immunoenzymatic methods. The results show that moderate exercise did not affect arterial thrombus formation. In contrast, platelet thrombus formation on collagen was increased on the average by 20% after 30 min at 70% maximal oxygen uptake (P = 0.03). Fibrin deposition on collagen remained unchanged with exercise, regardless of its intensity. Thus, with the use of a clinically relevant human experimental model of thrombosis, the present study suggests that exercise of heavy intensity may increase the risk for arterial thrombogenesis in sedentary young healthy male volunteers.

Platelet aggregation inhibitors for use in peripheral vascular interventions: what can we learn from the experience in the coronary arteries?

During the last decade, an enormous amount of information has been gathered about the function of the platelet and its impact on percutaneous vascular interventions. With the discovery of the GP IIb/IIIa receptor, which is responsible for platelet aggregation, new drug antagonists have been developed to prevent platelet aggregation that may result in arterial thrombosis or platelet microembolization. These drugs include the three GP IIb/IIIa receptor antagonists approved by the Food and Drug Administration: abciximab (ReoPro), eptifibatide (Integrilin), and tirofiban (Aggrastat). These drugs have been used in several large studies to improve the outcome of coronary interventions and in conjunction with plasminogen activators to accelerate thrombolysis. In addition, because no oral GP IIb/IIIa inhibitor exists, other oral regimens have been developed with use of the thienopyridines, ticlopidine (Ticlid) and clopidogrel (Plavix), in combination with aspirin to prevent platelet aggregation and thrombosis. Because the majority of investigations have been performed in patients undergoing coronary interventions, knowledge of these data is necessary to attempt to translate the use of these antiplatelet drugs to peripheral vascular interventions. The goal of this article is to review the use of these agents in the percutaneous treatment of coronary artery disease and give insight to their potential utility in noncoronary interventions.

Role of 4 platelet membrane glycoprotein polymorphisms on experimental arterial thrombus formation in men

This study investigates whether the polymorphisms of 3 important platelet receptors affected experimental thrombus formation in men. Forty healthy male volunteers randomly recruited were genotyped for the variable number of tandem repeat (VNTR) of GPIbalpha, the -5T/C polymorphism in the Kozak sequence of GPIbalpha, the 807C/T polymorphism of GPIa, and the PI(A1)/PI(A2) polymorphism of GPIIb/IIIa. Platelet thrombus formation was induced ex vivo by exposing a collagen-coated coverslip in a parallel plate perfusion chamber to native blood for 4 minutes. The shear rates at the collagen surface were 650 and 2600 x s(-1). At 2600 x s(-1) platelet thrombus formation was significantly related only to the 807C/T polymorphism. In contrast, at 650 x s(-1) thrombus formation was significantly altered only by the Kozak sequence polymorphism. The VNTR and the PI(A1)/PI(A2) polymorphisms did not influence thrombus formation. Thus, platelet thrombus formation is significantly influenced by genetic variations of the GPIbalpha and GPIa receptors. The effect of these polymorphisms was dependent on the blood flow rate.

Methods and models to evaluate shear-dependent and surface reactivity-dependent antithrombotic efficacy

The purpose of the present communication is to evaluate the importance of blood flow and surface reactivity for measurement of antithrombotic drug activity or efficacy in selected model systems of thrombus formation. Such information is essential for proper evaluation of antithrombotic drug profiles. The continuous development of flow-dependent thrombosis models for in vitro (anticoagulated blood) and ex vivo (native blood) studies and their application in in vivo animal models from the early 1970s and onwards are briefly considered. Central to this process was the development of various types of perfusion chambers in which a thrombogenic surface is exposed to flowing blood. Such perfusion chambers have been inserted into arteriovenous (AV) shunts in baboon, pig, dog, and rabbit. These approaches have allowed reproducible testing of traditional and novel experimental antithrombotic drugs, and studies on novel drug strategies under well-defined shear conditions and surface reactivity. Shear-dependent antithrombotic efficacy in these models is observed with anticoagulants such as unfractionated heparin, low-molecular weight heparins, or selective inhibitors of thrombin, Factor Xa, or Factor VIIa. However, the degree of shear dependency depends on the nature of the thrombogenic surface, e.g., the inhibition is more pronounced on a tissue factor (TF)-rich surface than on a collagen-rich surface, particularly at venous or low arterial shear. Platelet antagonists such as the COX-1 inhibitor aspirin, inhibitors of thromboxane A2 (TxA2) synthetase, the TxA2 platelet receptor, and of von Willebrand factor (vWf) are shear dependent also, being more efficient at high arterial shear. In contrast, the platelet ADP antagonist clopidogrel, or antagonists to the active platelet membrane glycoprotein IIb-IIIa complex (GPIIb-IIIa) are shear independent. At extremely high arterial shear, which activates platelets and elicit aggregates of circulating platelets, aspirin looses its antithrombotic effect, whereas ADP and GPIIb-IIIa antagonists still interrupt thrombus formation. In general, results obtained with these models mimic and predict antithrombotic efficacy in man when comparison is possible. Information on antithrombotic efficacy in flow devices with various thrombogenic surfaces is now sufficiently available to suggest recommendations for experimental conditions, particularly with regard to blood flow and reactive surfaces.

Effects of pretreatment with clopidogrel and aspirin followed by long-term therapy in patients undergoing percutaneous coronary intervention: the PCI-CURE study

BACKGROUND

Despite the use of aspirin, there is still a risk of ischaemic events after percutaneous coronary intervention (PCI). We aimed to find out whether, in addition to aspirin, pretreatment with clopidogrel followed by long-term therapy after PCI is superior to a strategy of no pretreatment and short-term therapy for only 4 weeks after PCI.

METHODS

2658 patients with non-ST-elevation acute coronary syndrome undergoing PCI in the CURE study had been randomly assigned double-blind treatment with clopidogrel (n=1313) or placebo (n=1345). Patients were pretreated with aspirin and study drug for a median of 6 days before PCI during the initial hospital admission, and for a median of 10 days overall. After PCI, most patients (>80%) in both groups received open-label thienopyridine for about 4 weeks, after which study drug was restarted for a mean of 8 months. The primary endpoint was a composite of cardiovascular death, myocardial infarction, or urgent target-vessel revascularisation within 30 days of PCI. The main analysis was by intention to treat.

FINDINGS

There were no drop-outs. 59 (4.5%) patients in the clopidogrel group had the primary endpoint, compared with 86 (6.4%) in the placebo group (relative risk 0.70 [95% CI 0.50-0.97], p=0.03). Long-term administration of clopidogrel after PCI was associated with a lower rate of cardiovascular death, myocardial infarction, or any revascularisation (p=0.03), and of cardiovascular death or myocardial infarction (p=0.047). Overall (including events before and after PCI) there was a 31% reduction cardiovascular death or myocardial infarction (p=0.002). There was less use of glycoprotein IIb/IIIa inhibitor in the clopidogrel group (p=0.001). At follow-up, there was no significant difference in major bleeding between the groups (p=0.64).

INTERPRETATION

In patients with acute coronary syndrome receiving aspirin, a strategy of clopidogrel pretreatment followed by long-term therapy is beneficial in reducing major cardiovascular events, compared with placebo.

The role of adenosine 5'-diphosphate receptor blockade in patients with cardiovascular disease

Aspirin, which has been the mainstay of antiplatelet agent for many decades, affects a single pathway in the platelet activation process and provides incomplete protection against cardiovascular events. Aspirin also may blunt the hemodynamic effect of angiotensin-converting enzyme inhibitors. Dipyridamole may provide some additional benefit, but there is little evidence to suggest its superiority alone or in combination with aspirin compared to standard doses of aspirin. Oral platelet glycoprotein IIb/IIIa inhibitors, although initially promising, have had disappointing results in recent clinical studies. A new class of medications, the thienopyridines, blocks the activity of platelet adenosine 5'-diphosphate (ADP) receptors, thereby reducing platelet activation. This review discusses the pharmacology, clinical studies, and potential uses of these agents, which include ticlopidine and clopidogrel. ADP inhibitors, by blocking an alternate pathway of platelet activation, are slightly more effective than aspirin in reducing cardiovascular events.

Ticlopidine pretreatment before coronary stenting is associated with sustained decrease in adverse cardiac events: data from the Evaluation of Platelet IIb/IIIa Inhibitor for Stenting (EPISTENT) Trial

BACKGROUND

Platelet inhibition at the time of a percutaneous coronary intervention has consistently been shown to decrease the risk of thrombotic adverse events but not restenosis. The role of enhanced antiplatelet protection through pretreatment with the platelet ADP-receptor antagonist ticlopidine in preventing both the early and late complications of coronary stenting has not previously been explored.

METHODS AND RESULTS

In the Evaluation of Platelet IIb/IIIa Inhibitor for Stenting (EPISTENT) trial, approximately 1600 patients were randomized to stenting with either placebo or abciximab in addition to aspirin and heparin. All stented patients also received ticlopidine after the procedure, but 58% of these patients were given ticlopidine before stenting at the discretion of the investigating physician. Among patients randomized to placebo, ticlopidine pretreatment was associated with a significant decrease in the incidence of the composite end point of death, myocardial infarction, or target vessel revascularization (TVR) at 1 year (adjusted hazard ratio, 0.73; 95% CI, 0.54 to 0.98; P:=0.036). Ticlopidine pretreatment did not significantly influence the risk of death or myocardial infarction in patients randomized to abciximab. Controlling for patient characteristics and for the propensity of being on ticlopidine, Cox proportional hazards regression identified ticlopidine pretreatment as an independent predictor of the need for TVR at 1 year (hazard ratio, 0.62; 95% CI, 0.43 to 0.89; P:=0.010) in both placebo-treated and abciximab-treated patients.

CONCLUSIONS

In the EPISTENT trial, among patients randomized to stenting, starting ticlopidine before the percutaneous coronary intervention was associated with a significant decrease in the incidence of the 12-month composite end point for patients not receiving abciximab and the need for TVR among all patients.

Superiority of clopidogrel versus aspirin in patients with prior cardiac surgery

BACKGROUND

After coronary artery bypass surgery, patients have a high cumulative rate of graft closure and recurrent ischemic events. We sought to determine whether antiplatelet therapy with clopidogrel would be more effective than aspirin, the accepted standard, in these patients.

METHODS AND RESULTS

The event rates for all-cause mortality, vascular death, myocardial infarction, stroke, and rehospitalization were determined for the 1480 patients with a history of cardiac surgery randomized to either clopidogrel or aspirin in a trial of 19 185 patients. The event rate per year of vascular death, myocardial infarction, stroke, or rehospitalization was 22.3% in the 705 patients randomized to aspirin and 15.9% in the 775 patients randomized to clopidogrel (P:=0.001). A risk reduction was also seen in each of the individual end points examined, including a 42.8% relative risk reduction in vascular death in patients on clopidogrel versus aspirin (P:=0.030). In a multivariate model incorporating baseline clinical characteristics, clopidogrel therapy was independently associated with a decrease in vascular death, myocardial infarction, stroke, or rehospitalization in patients with a history of cardiac surgery, with a 31.2% relative risk reduction (95% CI, 15.8 to 43.8; P:=0.0003). Although clopidogrel therapy was efficacious in the entire Clopidogrel Versus Aspirin in Patients at Risk of Ischemic Events (CAPRIE) population, multivariate analysis demonstrated that patients with previous cardiac surgery derived particular benefit (P:=0.015).

CONCLUSION

Compared with aspirin, clopidogrel therapy results in a striking reduction in the elevated risk for recurrent ischemic events seen in patients with a history of prior cardiac surgery, along with a decreased risk of bleeding.

Acute antithrombotic effect of a front-loaded regimen of clopidogrel in patients with atherosclerosis on aspirin

There is a need for a rapid antithrombotic effect after the administration of antiplatelet drugs in the setting of acute coronary syndromes and percutaneous interventions. Clopidogrel, a new thienopyridine derivative, is an efficient antiplatelet agent. However, the standard regimen of clopidogrel (75 mg/d) requires 2 to 3 days before significant antithrombotic effects. Patients with stable arterial disease on chronic aspirin therapy (n=20) were treated with clopidogrel either with a front-loaded regimen, 300 mg the first day and 75 mg/d the next 7 days, or with a standard regimen, 75 mg/d for 8 days. Blood thrombogenicity was assessed by quantification of platelet-thrombus formation in an ex vivo perfusion chamber, by ADP-induced platelet aggregation, and by ADP-induced fibrinogen binding. At 2 hours, mean total thrombus area with the standard regimen was not significantly reduced. In contrast, at 2 hours, the mean total thrombus area with the front-loaded regimen was significantly decreased by 23.1+/-8.5% versus baseline (P<0.05). ADP-induced platelet aggregation (with 5 and 10 micromol/L) was also significantly (P<0.05) reduced with the front-loaded regimen at 2 hours, with the mean platelet aggregation being 82.2+/-4.4% and 81.8+/-4.5%, respectively, versus baseline. Similarly, flow cytometry demonstrated a significant decrease (P<0. 05) in the ADP-induced fibrinogen binding (with 0.12 and 0.6 micromol/L) at 2 hours in this front-loaded regimen group (36.1+/-2. 0% and 53.2+/-9.3%). With the standard regimen, platelet activity was not significantly reduced at 2 hours. Our data suggest that a front-loaded regimen of clopidogrel added to aspirin achieves a significant antithrombotic effect at 2 hours in patients with known atherosclerotic disease on chronic aspirin therapy. This provides a rationale for using front-loaded clopidogrel in combination with aspirin in percutaneous coronary interventions.

Antiplatelet agents in cardiology: the choice of therapy

BACKGROUND

The platelet-rich, intracoronary thrombus is central to the pathogenesis of acute myocardial infarctions, unstable angina, and the majority of complications of percutaneous coronary interventions. Until recently, aspirin was the only antiplatelet agent available to help prevent or treat these events. Over the past several years, there has been a substantial expansion in our antiplatelet armamentarium as well as in our understanding of the clinical importance of antiplatelet therapy in limiting the complications of intracoronary thrombosis. Because of this, it is likely that over the coming years, the use of antiplatelet therapies will continue to expand, and it may not be unusual for a surgeon to encounter a patient being treated with two or even three platelet inhibitors.

CONCLUSIONS

This review will highlight the benefits and limitations of the currently available antiplatelet regimens: aspirin, thienopyridines (ticlopidine and clopidogrel), and the platelet glycoprotein IIb/IIIa inhibitors.

Early potent antithrombotic effect with combined aspirin and a loading dose of clopidogrel on experimental arterial thrombogenesis in humans

BACKGROUND

We conducted a double-blind, randomized, crossover study to assess the antithrombotic effects of the combination of aspirin (acetylsalicylic acid, ASA) and clopidogrel, with or without a loading dose, versus ASA alone in a model of arterial thrombosis in humans.

METHODS AND RESULTS

Eighteen male volunteers received the following 3 regimens for 10 days separated by a 1-month period: (1) 325 mg ASA daily, (2) 325 mg ASA+75 mg clopidogrel daily, (3) 325 mg ASA daily+300-mg clopidogrel loading dose on day 1 and +75 mg clopidogrel per day on days 2 to 10. The antithrombotic effect was measured 1.5, 6, and 24 hours after drug intake on day 1 and 6 hours after drug intake on day 10. Arterial thrombus formation was induced ex vivo by exposing a collagen-coated coverslip in a parallel-plate perfusion chamber to native blood for 3 minutes at an arterial wall shear rate. Without a loading dose, clopidogrel+ASA developed an antithrombotic effect within 6 hours after the first intake. It was superior to that produced by ASA, but it was moderate (P</=0.03). However, with the loading dose, the antithrombotic effect of clopidogrel+ASA appeared within 90 minutes, and after 6 hours it was comparable to that on day 10. On day 10, clopidogrel+ASA decreased platelet thrombus formation by approximately 70%, and the effect was significantly more potent than that produced by ASA alone (P<0.001).

CONCLUSIONS

This study confirms the synergistic antithrombotic effects of a combined ASA and clopidogrel therapy and shows the early benefit obtained with a loading dose of clopidogrel.

Antithrombotic efficacy of the vitamin K antagonist fluindione in a human Ex vivo model of arterial thrombosis : effect of anticoagulation level and combination therapy with aspirin

Thrombin is a main mediator of arterial thrombus formation, and its inhibition is an important antithrombotic strategy. However, the place of vitamin K antagonists among the different therapeutic strategies for preventing arterial thrombus formation is still debated. We studied the antithrombotic efficacy of the vitamin K antagonist fluindione in a human ex vivo model of arterial thrombosis and determined whether aspirin enhances fluindione efficacy. Ten healthy male volunteers were randomly assigned to receive fluindione, alone or in combination with aspirin (325 mg/d). Fluindione was given at increasing doses to give a stable international normalized ratio (INR) between 1.5 and 2.0 and between 2.1 and 3.0. We induced arterial thrombus formation ex vivo by exposing collagen- or tissue factor (TF)-coated coverslips in a parallel-plate perfusion chamber to native blood for 3 minutes at an arterial wall shear rate of 2600 s(-1). Platelet and fibrin deposition were measured by immunoenzymatic methods. Fluindione inhibited thrombus formation on TF-coated coverslips in a dose-dependent manner by 50% and 80% at INR 1.5 to 2.0 and INR 2.1 to 3.0, respectively (P<0.05). Fluindione in combination with aspirin inhibited TF-induced thrombus formation in a comparable manner. Collagen-induced thrombus formation was not reduced in subjects treated by fluindione. It was reduced by 50% to 60% in those treated with fluindione plus aspirin, regardless of the level of anticoagulation (P<0.05). Thus, the effectiveness of fluindione for preventing arterial thrombosis is dependent on the nature of the thrombogenic trigger. Fluindione is very effective in preventing TF- but not collagen-triggered thrombus formation. Aspirin enhances the antithrombotic effectiveness of fluindione, because combined treatment interrupts both TF- and collagen-induced thrombus formation.

Therapeutic benefit. Aspirin revisited in light of the introduction of clopidogrel

BACKGROUND

Antiplatelet agents are widely recognized for their efficacy in reducing the occurrence of vascular events in patients with atherothrombotic disease. Aspirin is currently considered to be the "reference standard" antiplatelet agent and is recommended by the American Heart Association for use in patients with a wide range of manifestations of cardiovascular disease on the basis of its high benefit-to-risk and benefit-to-cost ratios. Recently, clopidogrel (Plavix, Bristol-Myers Squibb Co), another antiplatelet agent, was approved by the Food and Drug Administration for many of the same indications as aspirin.

SUMMARY OF REVIEW

Because physicians will be faced with deciding whether to switch from the well-established practice of recommending aspirin for use in patients with atherothrombotic disease, both aspirin and clopidogrel are compared with respect to the primary factors that influence such decisions (ie, their relative efficacy, safety, cost, and convenience of use).

CONCLUSIONS

Based on the available evidence, aspirin is preferred for the majority of stroke or myocardial infarction patients at risk of recurrent atherothrombotic events. Clopidogrel may, however, provide valuable therapeutic benefit over aspirin in patients with peripheral arterial disease and in stroke or myocardial infarction patients for whom aspirin treatment is contraindicated or for whom aspirin fails to achieve the desired therapeutic effect.

Comparison of the antithrombotic effect of PEG-hirudin and heparin in a human ex vivo model of arterial thrombosis

Polyethylene glycol (PEG)-hirudin is a derivative of hirudin with a long plasma half-life. We have compared the efficacy of PEG-hirudin with unfractionated heparin (UH) in preventing arterial thrombosis. Arterial thrombus formation was induced ex vivo in 12 healthy human volunteers by exposing a tissue factor-coated coverslip positioned in a parallel-plate perfusion chamber to flowing nonanticoagulated human blood drawn directly from an antecubital vein at an arterial wall shear rate of 2600 s-1 for 3.5 minutes. PEG-hirudin, UH, or saline (as control) were administered ex vivo through a heparin-coated mixing device positioned proximal to the perfusion chamber. Platelet and fibrin deposition was quantified by immunoenzymatic measure of the P-selectin and D-dimer content of dissolved plasmin-digested thrombi, respectively. UH was administered to a plasma concentration of 0.35 IU/mL. This concentration prolonged the activated partial thromboplastin time from 32+/-1 seconds to 79+/-4 seconds (P<0.01). UH did not significantly prevent platelet deposition. However, fibrin deposition was reduced by 39% (P<0.05). PEG-hirudin in plasma concentrations of 0.5, 2.5, and 5 microg/mL prolonged the activated partial thromboplastin time to 48+/-2, 87+/-4, and 118+/-4 seconds, respectively. In contrast to UH, PEG-hirudin prevented both platelet and fibrin deposition in a dose-dependent manner with a >80% reduction at 5 microg/mL (P<0.01). Furthermore, the plasma level of PEG-hirudin required to significantly prevent fibrin deposition (0.5 microg/mL) corresponded to a much shorter prolongation of activated partial thromboplastin time (48+/-2 seconds) than that needed for UH (79+/-4 seconds). Thus, our results are compatible with the view that thrombin is greatly involved in recruitment of platelets in evolving thrombi, and that PEG-hirudin is an effective agent for preventing arterial thrombosis in a human ex vivo experimental model.