Oral antiplatelet therapy for atherothrombotic disease: current evidence and new directions

Despite the proven efficacy of dual antiplatelet therapy with aspirin and one of the first-generation P2Y(12) antagonists (clopidogrel, prasugrel) in patients with atherothrombotic disease, residual ischemic risk remains substantial, and bleeding rates are increased. Incomplete protection against ischemic events can be attributed to the fact that these therapies each target a single platelet activation pathway, allowing continued platelet activation via other pathways, including the protease-activated receptor-1 (PAR-1) pathway stimulated by thrombin. Increased bleeding with dual antiplatelet therapy can be attributed to blockade of the thromboxane A(2) (by aspirin) and adenosine diphosphate (by P2Y(12) antagonist) platelet activation pathways that are essential to hemostasis. The second-generation P2Y(12) inhibitor ticagrelor plus aspirin demonstrated superior ischemic outcomes, including reduction in total mortality, versus clopidogrel plus aspirin, but event rates remain high, and major bleeding not related to coronary artery bypass grafting is increased. The novel P2Y(12) antagonist elinogrel, available in intravenous and oral formulations, may have a more favorable benefit-to-risk profile than existing agents in this class because of reversible and competitive binding to the P2Y(12) receptor. Inhibition of PAR-1 is an attractive, novel approach in antiplatelet therapy because it may provide incremental ischemic protection without increasing bleeding. The PAR-1 antagonist vorapaxar (SCH 530348) has been associated with favorable efficacy and safety in phase 2 trials. Two phase 3 trials are evaluating the efficacy and safety of vorapaxar in patients presenting with non-ST-segment elevation acute coronary syndromes and in patients with documented atherothrombotic disease.     

Novel anti-thrombotic therapy in acute coronary syndromes

Despite recent developments in revascularisation, anti-platelet and anti-thrombotic therapies, patients with acute coronary syndromes (ACS) remain at increased risk of recurrent atherothrombotic events. Dual anti-platelet therapy comprising aspirin and platelet P2Y12 receptor inhibition has become the cornerstone of therapy in ACS. However, thrombin-mediated pathways, which contribute to platelet activation and are responsible for the formation of fibrin clot, remain active following initial plaque rupture. Recently, orally administered drugs which directly target thrombin, factor Xa or thrombin-mediated platelet activation have been developed. Efficacy outcomes in trials of these novel anti-thrombotic agents in ACS have yielded mixed results and their adoption in clinical practice is currently hampered due to a penalty of increased bleeding. To date, the direct Xa inhibitor rivaroxaban and the protease-activated receptor-1 antagonist atopaxar have shown most promise and require further evaluation to determine their role in ACS management.     

血小板功能检测与P2Y12受体拮抗剂个体化抗血小板治疗的研究进展

P2Y12受体拮抗剂广泛应用于急性冠脉综合征（ACS）及经皮冠状动脉介入治疗（PCI）后血栓事件的预防。由于抗血小板治疗反应多样性的存在,经典的P2Y12受体拮抗剂氯吡格雷的治疗期间高血小板反应性（HTPR）被证实与患者不良心血管事件的发生密切相关。尽管新药普拉格雷和替格瑞洛的抗栓疗效优于氯吡格雷,但是由于治疗期间低血小板反应性（LTPR）的存在,出血风险明显增加。如何权衡血栓和出血风险,实现个体化抗血小板治疗,已经成为临床的重要挑战。研究证实,血小板反应性（PR）与缺血和出血事件的发生密切有关,基于血小板功能检测（PFT）的治疗窗将有助于个体化抗血小板治疗。本文就PFT与P2Y12受体拮抗剂个体化抗血小板治疗的研究进展作一综述。     

Promises of PAR-1 Inhibition in Acute Coronary Syndrome

Platelet activation is a key process in the pathogenesis of acute coronary syndromes (ACS). Of the many triggers involved in this process, three are presumed to be critical: thromboxane A₂ (TBXA₂) via the TBXA₂ receptor, adenosine diphosphate via the P2Y₁₂ receptor, and thrombin via the protease-activated receptor (PAR)-1. Despite the effective inhibition of the first two pathways with aspirin and an expanding family of P2Y₁₂ inhibitors, the incidence of recurrent ischemic events remains high after ACS. PAR-1 inhibitors are a novel class of antiplatelet agents that inhibit thrombin-mediated platelet activation. Preclinical data and phase 2 clinical trials in patients with stable and unstable coronary disease support the potential of these compounds to improve clinical outcome. In this review we discuss the rationale for developing this novel class of agents with a focus on the two compounds in most advanced clinical development, vorapaxar (SCH 530348) and atopaxar (E5555).     

Dual pathway therapy in acute coronary syndrome

In 10 % of patients, who suffer an acute coronary syndrome (ACS), a major cardiovascular event occurs despite optimal therapy. The occlusion of the vessel is driven by atherothrombosis, which arises from platelet activation and activation of the coagulation cascade. In the last decade the secondary prevention continuously improved by development of dual anti-platelet therapy with new P2Y12-inhibitors such as clopidogrel, prasugrel, and ticagrelor. Until recently, the coagulation cascade was not targeted in secondary prevention. The coagulation factor Xa plays a crucial role in thrombosis and is elevated in patients after acute coronary syndrome, therefore representing an attractive target for novel therapies in ACS. Former studies with vitamin K antagonists showed reduction of cardiovascular events but increased major bleedings. Two phase-3 trials investigated the role of novel oral anticoagulant agents on top of aspirin and clopidogrel in patients with ACS. The APPRAISE-2 study, which tested the oral factor Xa inhibitor apixaban was prematurely terminated because of an increase of major bleedings in the absence of an effect on cardiovascular events. In contrast, the ATLAS ACS2 TIMI-51 trial interrogating the oral factor Xa inhibitor rivaroxaban in a low dose regimen showed significant reduction of cardiovascular events as well as total mortality. Thus, add-on treatment with low dose rivaroxaban emerged as a new option for patients with ACS. This review illustrates recent advances in the development of antithrombotic therapy in acute coronary syndromes, provides guidance on which patients should receive which therapy for secondary prevention of events, and points out potentially fruitful new strategies for the future of antithrombotic treatment in ACS.     

Neue Pl?ttchenhemmstoffe und die Frage der dualen Hemmung

The introduction of clopidogrel for use with aspirin (ASA) as a dual antiplatelet therapy has markedly reduced the risk of atherothrombotic vessel occlusion in patients with acute coronary syndromes (ACS). However, stronger antiplatelet therapy has also been associated with significant increases in severe bleeding, resulting in no change in mortality rates. This raised the question of pharmacological alternatives, specifically new antagonists of the platelet P2Y(12)-ADP receptor, which exhibit better pharmacokinetic and dynamic properties than clopidogrel, as well as improved clinical safety.Prasugrel, the first of these newly developed agents and another thienopyridine, was more potent than clopidogrel in the TRITON-TIMI-38 study in reducing ischemic events in ACS patients, but also increased severe bleeding. Ticagrelor, a structurally different reversible antagonist of the P2Y(12) receptor, was superior to clopidogrel in the PLATO trial on ACS patients, but also increased the risk of severe bleeding in patients not requiring bypass surgery. Interestingly, ticagrelor reduced mortality in PLATO. There have been no satisfying explanations for this phenomenon to date. In addition to different patient populations and treatment protocols, the varying pharmacological properties of these substances are discussed as possible causes. A direct comparison of the two medications in a single study remains to be undertaken.     

State of the art of new P2Y<Subscript>12</Subscript> antagonists

The interaction of ADP with its platelet receptor P2Y12 plays a crucial role in platelet activation and thrombogenesis. This article reviews the pharmacology and clinical trials of specific antagonists of P2Y12. Clopidogrel is a thienopyridine with proven antithrombotic efficacy, but it has some important drawbacks: (a) it is a pro-drug that needs to be metabolized to its active metabolite; (b) it has a delayed onset and offset of action and (c) there is high inter-individual variability in pharmacological response. Prasugrel is also a thienopyridine, with faster onset of action and a more uniform inhibition of platelet function compared to clopidogrel, accounting for lower incidence of ischemic events in patients with acute coronary syndromes (ACS) undergoing percutaneous coronary intervention (PCI) and higher incidence of both non-CABG-related bleeding complications. Two direct and reversible P2Y12 antagonists, Cangrelor and ticagrelor, are characterized by rapid onset and reversal of platelet inhibition. Cangrelor is not superior to clopidogrel in preventing thrombotic events in patients undergoing PCI. Ticagrelor is superior to clopidogrel in preventing major adverse cardiac events in ACS patients, but, like prasugrel, is associated with a higher frequency of non-CABG-related bleeding complications. A shorter period of drug discontinuation before surgery is necessary in ticagrelor-treated patients compared to clopiodgrel-treated patients to limit the severity of post-surgical bleeding.     

The expanding role of antiplatelet agents in coronary artery disease. A current review of aspirin, glycoprotein IIb/IIIa inhibitors, and the thienopyridines

The platelet has assumed an increasingly important role in cardiovascular medicine as our understanding of the pathophysiology of acute coronary syndromes (ACS) has evolved. Plaque rupture, platelet aggregation, and thrombus formation occur as a result of complex interaction between the platelet, the endothelium, and various inflammatory cells and circulating proteins. Aspirin continues to form the foundation of any anti-ischemic regimen, but cardiologists have long recognized the need for newer, more potent antiplatelet agents. Glycoprotein IIb/IIIa receptor antagonists and thienopryidines have been developed over the past decade and now serve as powerful complements to aspirin in the prevention and treatment of coronary events. The paper will begin with a review of aspirin as well as a discussion of the concept of aspirin resistance. The rapidly expanding body of knowledge supporting the use of glycoprotein IIb/IIIa receptor blockers and thienopyridines will then be addressed, with an emphasis on reconciling recent controversies in the literature. Future advances in the treatment of coronary artery disease will likely occur as we further refine the role of these established antiplatelet drugs and develop agents that bind to novel targets in the thrombotic cascade.     

Coronary Calcium: New Insights, Recent Data, and Clinical Role

Antiplatelet therapy with aspirin and a platelet P2Y12 receptor antagonist reduces thrombotic and ischemic events after percutaneous coronary intervention and acute coronary syndrome. The platelet inhibitory effect of the thienopyridine clopidogrel varies widely among individuals, and high on-treatment platelet reactivity has been associated with a substantial hazard for post-PCI cardiovascular events, including stent thrombosis. The clinical availability of ex vivo methods to measure the antiplatelet effect of P2Y12 antagonists raises the possibility that incorporating platelet function testing into clinical practice could facilitate a stratified and efficient approach to antiplatelet therapy following PCI, although data from definitive randomized trials supporting a routine approach are currently lacking.     

The thienopyridines

Platelet adhesion, activation, and aggregation are key processes in the pathogenesis of coronary disease. Inhibition of these processes forms the cornerstone of therapy for coronary artery disease and particularly of acute coronary syndromes (ACS). Aspirin was the only available antiplatelet therapy for over 100 years, and it improves clinical outcome in a wide range of clinical situations. However, aspirin only inhibits platelet activation mediated by thromboxane A2, allowing platelet activation to occur through innumerable other pathways. As a result, adverse ischemic events are common when aspirin alone is used for the treatment of coronary disease, including ACS, during coronary interventions (particularly during stent implantation), and following coronary vascular brachytherapy (VBT). In these clinical situations, the presence of either thrombus, deep injury to the vessel wall, or delayed vascular reendothelialization leads to intense and often prolonged platelet activation, overwhelming the relatively weak effects of aspirin. The development of the thienopyridines, a class of antiplatelet drugs that reduce adenosine diphosphate-(ADP) mediated platelet activation, has significantly improved clinical outcomes in many coronary conditions. Widespread use of ticlopidine, the first available thienopyridine, was limited by frequent side-effects, including life-threatening neutropenia and thrombotic thrombocytopenic purpura. Following the introduction of clopidogrel, a thienopyridine with an excellent safety profile, dual antiplatelet therapy with aspirin and clopidogrel has become standard therapy following coronary stent implantation and coronary VBT. In patients presenting with ACS, the addition of clopidogrel to aspirin has now been proven to reduce ischemic events. The most important limitation of dual antiplatelet therapy is the increased bleeding risk as compared with aspirin alone, particularly in patients undergoing coronary artery bypass grafting during the index hospitalization. However, for many patients with ACS, combination therapy is appropriate.     

PAR-1 genotype influences platelet aggregation and procoagulant responses in patients with coronary artery disease prior to and during clopidogrel therapy

Genetic variations of the protease-activated receptor-1 (PAR-1) have been associated with platelet receptor density and linked to thrombin receptor-activating peptide (TRAP)-induced phenotypes of platelet aggregation and P-selectin expression. We investigated whether the PAR-1 intervening sequence-14 A>T dimorphism influences platelet procoagulant activity. We also determined whether the P2Y12 antagonist clopidogrel could offset any observed functional polymorphism of the PAR-1 receptor by inhibiting P2Y12-mediated amplification of TRAP-induced responses. We studied 54 patients listed for elective percutaneous coronary intervention assessing TRAP-induced platelet aggregation and markers of procoagulant activity. Platelet responses were measured at baseline, 4 h post clopidogrel 300 mg, and 10 and 28 days following clopidogrel 75 mg daily. Each patient was genotyped for the PAR-1 intervening sequence-14 A/T dimorphism. Increased platelet aggregation and procoagulant responses were observed with PAR-1 A allele homozygotes. Clopidogrel significantly inhibited these platelet responses regardless of PAR-1 genotype, but did not offset the hyper-reactivity associated with the A/A homozygotes. We conclude that a common sequence variation within the PAR-1 gene influences TRAP-induced platelet procoagulant activity as well as aggregation. Higher platelet reactivity associated with PAR-1 IVSn-14 A allele homozygotes persists despite clopidogrel therapy. These individuals may be at higher risk of thromboembolic events and may require additional anti-platelet medication.     

Glycoprotein IIb/IIIa and P2Y12 receptor antagonists yield additive inhibition of platelet aggregation, granule secretion, soluble CD40L release and procoagulant responses

Glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists, including abciximab and tirofiban, are administered concurrently with clopidogrel, a P2Y12 antagonist, and aspirin in some patients undergoing percutaneous coronary intervention. We studied the effects of, and interactions between, abciximab, tirofiban, aspirin and the P2Y12 antagonist cangrelor on platelet aggregation, alpha and dense granule secretion and procoagulant responses in vitro. Blood was obtained from healthy volunteers. Platelet aggregation, dense granule secretion, alpha granule secretion (PAI-1 and soluble CD40 ligand levels) and procoagulant responses (annexin-V and microparticle formation) were assessed using collagen and thrombin receptor activating peptide (TRAP) as agonists. All the antagonists used singularly inhibited collagen-induced responses. Combinations of abciximab or tirofiban with aspirin and/or cangrelor gave additive inhibition with the greatest effect seen when abciximab or tirofiban was combined with both aspirin and cangrelor. Cangrelor inhibited TRAP-induced responses and, again, there was additive inhibition of these parameters when abciximab or tirofiban were combined with cangrelor. The GPIIb/IIIa receptor plays an important role in amplification of platelet activation such that there are important interactions between GPIIb/IIIa antagonists and inhibitors of both P2Y12 receptor activation and, to a lesser extent, thromboxane A2 generation. These interactions are likely to have important influences on the safety and efficacy of combination anti-platelet therapies.     

Glycoprotein receptor inhibitors in the management of acute coronary syndromes

Coronary thrombosis and the risk of clinical adverse events remains high despite considerable advances in the management of acute coronary syndromes (ACS) with the combined use of aspirin, heparin, fibrinolytic therapy, and percutaneous coronary intervention (PCI). Platelet aggregation and thrombosis play a key role in the pathogenesis of unstable coronary syndromes. Over the past several years, multiple placebocontrolled trials involving more than 50,000 ACS patients have shown that blockade of the platelet receptor glycoprotein (GP) IIb/IIIa, the final pathway in platelet aggregation, reduces the incidence of ischemic complications among patients with ACS. Three agents (abciximab, eptifibatide, and tirofiban) are currently approved for use with aspirin and heparin in the management of ACS or during percutaneous coronary intervention. They have consistently been shown to reduce the incidence of death or myocardial infarction in the ACS population including the patients not routinely scheduled for early revascularization. They provide an augmented treatment effect among high-risk ACS patients, particularly those who have a baseline troponin-t-positive status. Recently published practice guidelines have recommended their use in high-risk patients with ACS and all those undergoing PCI.     

Antiplatelet treatment of cardiovascular disease: a translational research perspective

Platelet mediated thrombosis is the primary cause of ischemic event occurrence in patients with cardiovascular disease. The P2Y12 receptor plays a central role in thrombus generation and is therefore a major target for pharmacologic therapy. Although various clinical trials have demonstrated the efficacy of dual antiplatelet therapy with aspirin and clopidogrel, recurrent ischemic events occur in approximately 10% of patients with acute coronary artery syndromes. Recent translational research studies have explored the various limitations of dual antiplatelet therapy including wide response variability and resistance. The association of ischemic event occurrence with high on-treatment platelet reactivity to adenosine diphosphate has been reported in recent small studies suggesting that the latter may be a quantifiable and modifiable risk factor. Recent studies have identified a potential therapeutic target for P2Y12 inhibitors that may influence the future development of personalized antiplatelet treatment strategies aimed at the reduction of ischemic event occurrence in high risk patients. Finally, based on the current evidence platelet reactivity may become a standard of care risk factor measured in all patients with cardiovascular disease.     

The P2Y 12 receptor as a therapeutic target in cardiovascular disease

Coronary thrombosis complicating rupture of atherosclerotic plaque is the predominant cause of acute coronary syndromes and platelets play a crucial role in this thrombus formation. Whilst aspirin has been successful in reducing cardiovascular morbidity and mortality, appreciation of its limited antiplatelet effects has stimulated the search for more effective antiplatelet agents. The thienopyridines, ticlopidine and clopidogrel, act, via metabolites, on the platelet ADP receptor subtype now designated P2Y 12 (formerly P 2T , P2T AC , P2Y ADP or P2Y cyc ) and these agents have proven clinical efficacy. Analogues of the natural P2Y 12 receptor antagonist ATP have been developed that act directly on the receptor and have a rapid onset of action. One such antagonist, AR-C69931MX, is being developed for clinical use. AR-C69931MX is a potent antagonist of ADPinduced platelet activation, aggregation and secretion and also antagonises platelet responses, including procoagulant activity, induced by all other agonists in view of the central role of the P2Y 12 receptor in amplifying platelet responses. Phase II studies of intravenous AR-C69931MX in patients with acute coronary syndromes show that this agent has a rapid onset of action, rapidly achieving steady-state inhibition of platelet aggregation, with a half-life of only a few minutes. AR-C69931MX appears to be safe and well tolerated as adjunctive therapy in these patients, and more effective inhibition of platelet function is achieved than with clopidogrel. Orally active ATP analogues are also being developed that may be more effective than clopidogrel. Limitations of platelet glycoprotein IIb/IIIa antagonists leave scope for development of alternative antiplatelet agents.     

Decreasing incidence of stent thrombosis

Stent thrombosis (ST) remains a major pitfall of stent implantation in contemporary percutaneous coronary intervention (PCI) leading to high rates of death and non-fatal myocardial infarction. Many predictors of ST have been reported worldwide but the strongest have to be highlighted regarding the catastrophic prognosis of such an event. Because platelet aggregation has a pivotal role in ST pathogenesis, the new antiplatelet regimens combining aspirin and P2Y12 receptor inhibitors have led to a remarkable decrease in the ST incidence, especially in the setting of acute coronary syndrome (ACS). In this article, our purpose is to review the evolution of ST incidence since first stent use in PCI. We will also overview the main predictors of ST focusing on ACS and clopidogrel low response.     

Antiplatelet therapy in acute coronary syndromes

Acute coronary syndromes (ACS), characterized by unstable angina or a non-ST-segment elevation myocardial infarction, are caused by rupture of an atherosclerotic plaque, leading to platelet activation and aggregation, thrombus formation, and microembolization. Antiplatelet therapy is a cornerstone of therapy. Combined with aspirin, clopidogrel provides significant benefit for patients across the ACS spectrum. However, clopidogrel has limitations given its slow onset and the inconsistent level of inhibition that it achieves. Newer thienopyridine and non-thienopyridine P2Y12 receptor agonists offer the advantages of a rapid onset of action and greater and more consistent platelet inhibition.     

Platelet Glycoprotein IIb/IIIa Integrin Blockade in Coronary Artery Disease: Current State of the Art

Platelets have been shown to play an important role in the pathogenesis of atherosclerosis, acute coronary syndromes, and ischemic complications after percutaneous coronary intervention. Fibrinogen binding via platelet surface glycoprotein (GP) IIb/IIIa receptors constitutes the "final pathway" in platelet aggregation leading to thrombus formation. The GP IIb/IIIa receptor inhibitors, a new class of antiplatelet agents that have emerged in recent years, show great promise in reducing complications of coronary angioplasty and acute coronary syndromes. This review will examine the biology of platelet GP IIb/IIIa receptors, the various classes of GP IIb/IIIa receptor antagonists, the results of the latest clinical trials, and their implications in current clinical practice.     

Platelet adenosine diphosphate receptor antagonists: ticlopidine to ticagrelor-a long continuing journey

Platelet aggregation plays a central role in the pathogenesis of atherothrombosis. Platelet adenosine diphosphate (ADP) receptor antagonists (ticlopidine, clopidogrel, prasugrel, and ticagrelor) are a major advance in the treatment of atherothrombotic diseases, especially acute coronary syndromes (ACS). Ticlopidine was the first thienopyridine introduced into clinical practice, but its potentially serious haematological side-effects limited its use and it was quickly eclipsed by clopidogrel. Clinical trials established aspirin plus clopidogrel as the standard dual anti-platelet therapy in patients with ACS and patients undergoing percutaneous coronary intervention (PCI) with stenting. Clopidogrel was found to have pharmacokinetic and pharmacodynamic limitations. Prasugrel is the next approved thienopyridine that has shown superior efficacy in ACS patients undergoing PCI in comparison to clopidogrel, although at the cost of a higher bleeding risk. Ticagrelor is the latest non-thienopyridine ADP receptor blocker that is potent, effective, reversible, and relatively safer as compared to clopidogrel. Both prasugrel and ticagrelor are more potent than clopidogrel. The data so far suggests that ticagrelor has a wider applicability in usage in patients with ACS as compared to prasugrel. Prasugrel however seems to be better tolerated. Search is on for newer more potent but safer anti-platelet agents.     

P2Y12 platelet inhibition in clinical practice

Platelet adhesion, activation and aggregation play a pivotal role in atherothrombosis. Intracoronary atherothrombosis is the most common cause of the development of acute coronary syndrome (ACS), and plays a central role in complications occurring around percutaneous coronary intervention (PCI) including recurrent ACS, procedure-related myocardial infarction or stent thrombosis. Inhibition of platelet aggregation by medical treatment impairs formation and progression of thrombotic processes and is therefore of great importance in the prevention of complications after an ACS or around PCI. An essential part in the platelet activation process is the interaction of adenosine diphosphate (ADP) with the platelet P2Y12 receptor. The P2Y12 receptor is the predominant receptor involved in the ADP-stimulated activation of the glycoprotein IIb/IIIa receptor. Activation of the glycoprotein IIb/IIIa receptor results in enhanced platelet degranulation and thromboxane production, and prolonged platelet aggregation. The objectives of this review are to discuss the pharmacological limitations of the P2Y12 inhibitor clopidogrel, and describe the novel alternative P2Y12 inhibitors prasugrel and ticagrelor and the clinical implications of the introduction of these new medicines.