Prevention of ischaemic stroke--antiplatelets

Aspirin is the treatment of first choice for long-term secondary prevention of vascular events in patients with confirmed non-cardioembolic ischaemic stroke or TIA. However, there is no good evidence that it is of benefit in primary stroke prevention. If aspirin is contra-indicated, dipyridamole monotherapy is a relatively cheap, but slightly less effective, alternative. Aspirin and dipyridamole have an additive effect in secondary stroke prevention, but there is a high incidence of side effects and subsequent discontinuation of treatment with combination therapy. It is reasonable to consider clopidogrel for secondary prevention of vascular events in patients with ischaemic stroke who are intolerant of aspirin or dipyridamole, or who have a history of ischaemic heart disease. However, its cost is considerable. Over the next decade, oral antiplatelet agents directed against specific platelet receptors, or a combination of antiplatelet drugs inhibiting different aspects of platelet function, may improve secondary prevention of stroke.     

Clopidogrel and aspirin in cardiovascular medicine: responders or not--current best available evidence

Dual antiplatelet therapy represents an important advance for patients with established coronary artery disease. It is an important strategy for patients with acute coronary syndromes and those undergoing percutaneous transcatheter coronary interventions. Clopidogrel effectively inhibits ADP-induced platelet activation and aggregation by selectively and irreversibly blocking the P2Y(12) receptor on the platelet membrane. Aspirin works by irreversibly acetylating the cyclooxygenase (COX-1) enzyme, thus suppressing the production of thromboxane A(2) (TxA(2)) and inhibiting platelet activation and aggregation. Variable platelet response and potential resistance to therapy has emerged with aspirin and clopidogrel. The definitions of antiplatelet agents variability in responsiveness and nonresponsiveness are discussed. Clopidogrel and aspirin responsiveness as they are measured in the laboratory by various techniques (platelet aggregometry and point-of-care assays such as platelet function analyzer [PFA-100] and rapid platelet function assay [RPFA]) are evaluated. The mechanisms responsible for variations in responsiveness to antiplatelet agents such as clinical, cellular and genetic factors are defined. Aspirin and clopidogrel resistance are emerging clinical entities with potentially severe consequences such as myocardial infarction, stroke or death. The therapeutic interventions to deal with nonresponsiveness are reported, although specific recommendations are not clearly established. In the future, routine measurement of platelet function in patients with cardiovascular disease may become the standard of care. Personalized antithrombotic treatment strategies may be determined by ex-vivo measurements that identify critical pathways influencing thrombotic risk in the individual patient.     

The role of antiplatelet therapy in the management of acute coronary syndromes

The benefit of aspirin use in the emergent care of acute coronary syndromes (ACS) has been well-established. Recent studies have further demonstrated the importance of antiplatelet therapy in the acute setting, primarily with the use of intravenous glycoprotein IIb/IIIa receptor inhibitors. Aspirin and the thienopyridines (ticlopidine and clopidogrel) are oral antiplatelet agents that interfere with platelet activation in complementary, but separate pathways. Combination therapy of aspirin with other antiplatelet agents has demonstrated a benefit for the management of ACS. This article reviews the pathophysiology of platelet activation in ACS, landmark trials regarding antiplatelet agents, and the current recommendations for the use of both intravenous and oral antiplatelet agents in the management of patients with ACS.     

The role of antiplatelet therapy in the management of acute coronary syndromes

The benefit of aspirin use in the emergent care of acute coronary syndromes (ACS) has been well-established. Recent studies have further demonstrated the importance of antiplatelet therapy in the acute setting, primarily with the use of intravenous glycoprotein IIb/IIIa receptor inhibitors. Aspirin and the thienopyridines (ticlopidine and clopidogrel) are oral antiplatelet agents that interfere with platelet activation in complementary, but separate pathways. Combination therapy of aspirin with other antiplatelet agents has demonstrated a benefit for the management of ACS. This article reviews the pathophysiology of platelet activation in ACS, landmark trials regarding antiplatelet agents, and the current recommendations for the use of both intravenous and oral antiplatelet agents in the management of patients with ACS.     

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.     

Prevention of coronary heart disease with aspirin and clopidogrel: efficacy, safety, costs and cost-effectiveness

Atherothrombotic coronary artery disease is the single most common cause of death worldwide and a growing public health problem. Platelets play a central role in the pathogenesis of atherothrombosis and are therefore commonly targeted by one or more antiplatelet drugs as part of primary and secondary atherothrombosis prevention strategies. Aspirin reduces the risk of serious vascular events (myocardial infarction, stroke or cardiovascular death) by approximately 20% in a broad range of high-risk patients and remains the first-line antiplatelet drug because of its relative safety, low cost and cost-effectiveness. Compared with aspirin alone, clopidogrel reduces the risk of serious vascular events by approximately 10% and the combination of aspirin and clopidogrel reduces the risk by approximately 20% in patients with non-ST-segment elevation acute coronary syndrome. Clopidogrel has a similar safety profile to aspirin but clopidogrel tablets are substantially more expensive. However, the incremental cost-effectiveness ratio of clopidogrel compared with aspirin is favourable, particularly in high-risk patients and is intermediate compared with a range of other effective therapeutic strategies for the treatment of coronary heart disease. Clopidogrel should be considered as a replacement for aspirin in patients who are allergic to aspirin, cannot tolerate aspirin, have experienced a recurrent atherothrombotic vascular event whilst taking aspirin and are at very high absolute risk of a serious vascular event (e.g., > 20%/year). The combination of clopidogrel and aspirin should be considered in patients with non-ST-segment elevation acute coronary syndrome or undergoing percutaneous coronary intervention.     

Antiplatelet therapy in peripheral arterial disease

Antiplatelet therapy significantly reduces the incidence of vascular death, nonfatal myocardial infarction, and nonfatal stroke in patients with peripheral arterial disease (PAD) and intermittent claudication, in patients undergoing peripheral grafting, in patients undergoing peripheral angioplasty, and in patients with carotid disease. Aspirin, aspirin plus dipyridamole, ticlodipine, and clopidogrel have been shown to be efficacious in the treatment of PAD. Data from the Clopidogrel versus Aspirin in Patients at Risk for Ischaemic Events (CAPRIE) trial demonstrated in 11,592 patients with PAD that patients randomized to clopidogrel 75 mg daily had a 24% significant (p=0.0028) reduction in vascular death, nonfatal myocardial infarction, and nonfatal stroke than patients randomized to aspirin 325 mg daily. These data favor the use of clopidogrel in patients with PAD.     

Failure of Aspirin to Prevent Atherothrombosis

Aspirin (acetylsalicylic acid) reduces the odds of serious atherothrombotic vascular events and death in a broad category of high risk patients by about one-quarter. The mechanism is believed to be inhibition of thromboxane biosynthesis by inactivation of platelet cyclo-oxygenase-1 enzyme. However, aspirin is not that effective; it still fails to prevent the majority of serious vascular events. Mechanisms that may account for the failure of aspirin to prevent vascular events include non-atherothrombotic causes of vascular disease, non-adherence to aspirin therapy, an inadequate dosage, alternative ‘upstream’ pathways of platelet activation (e.g. via stimulation of the ADP, collagen or thrombin receptors on platelets), aspirin-insensitive thromboxane biosynthesis (e.g. via monocyte cyclo-oxygenase-2), or drugs that interfere with the antiplatelet effects of aspirin. Genetic or acquired factors may further modify the inhibitory effects of aspirin on platelets (e.g. polymorphisms involving platelet-associated proteins, increased platelet turnover states). Identification and treatment of the potential causes of aspirin failure could prevent at least another 20% of serious vascular events (i.e. over and above those that are currently prevented by aspirin). There is currently no role for routine laboratory testing to measure the antiplatelet effects of aspirin. Clinicians should ensure that patients at high risk of atherothrombosis (>3% risk over 5 years) are compliant with aspirin therapy and are taking the correct dosage (75–150 mg/day). Patients who cannot tolerate aspirin, are allergic to aspirin, or have experienced recurrent serious atherothrombotic events whilst taking aspirin, should be treated with clopidogrel, and patients with acute coronary syndromes benefit from the combination of clopidogrel plus aspirin. Future research is required to standardize and validate laboratory testing of the antiplatelet effects of aspirin and to identify treatments that can both improve these laboratory measures and reduce the risk of future atherothrombotic events.     

Prasugrel: a novel thienopyridine antiplatelet agent. A review of preclinical and clinical studies and the mechanistic basis for its distinct antiplatelet profile

Prasugrel (CS-747, LY640315) is a novel member of the thienopyridine class of oral antiplatelet agents that includes ticlopidine and clopidogrel. Like other thienopyridines, prasugrel is a prodrug that is inactive in vitro. Prasugrel's distinct chemical structure permits efficient conversion to its active metabolite with a less rigorous dependence on specific cytochrome P-450 enzymes. Prasugrel is rapidly converted in vivo to an active metabolite (R-138727) that binds specifically and irreversibly to the platelet P2Y 12 purinergic receptor inhibiting ADP-mediated platelet activation and aggregation. Preclinical studies indicated that prasugrel is approximately 10- and 100-fold more potent at inhibiting ex vivo platelet aggregation and in vivo thrombus formation than clopidogrel and ticlopidine, respectively. Early clinical data in healthy subjects confirmed the greater platelet inhibition and consistency with prasugrel compared to clopidogrel. While the active metabolites of prasugrel and clopidogrel resulted in similar levels of platelet inhibition in vitro, the amount of each active metabolite generated in vivo was quite different-prasugrel (60 mg) resulting in an approximately 12-fold greater exposure to its active metabolite compared with clopidogrel (300 mg). This observation provides a mechanistic basis for the faster, greater, and more consistent inhibition of platelet aggregation observed with prasugrel. Clinical studies in patients with cardiovascular disease confirmed the potent antiplatelet effect of prasugrel compared with clopidogrel. Collectively, these phase 1/1b studies and a phase 2 study (JUMBO-TIMI 26) aided in dose selection for the recently completed phase 3 trial (TRITON-TIMI 38) in patients with acute coronary syndrome undergoing percutaneous coronary intervention.     

A brief review on high on-aspirin residual platelet reactivity

Although aspirin is effective in secondary prevention in coronary heart disease, new thromboembolic events in patients on aspirin are frequently seen. In trials on aspirin-treated patients, platelet function tests have revealed large variability in platelet aggregation. This phenomenon has been named aspirin resistance, aspirin non-responsiveness or high-on-aspirin residual platelet reactivity.The mechanism of aspirin antiplatelet effect is due to the inhibition of cyclooxygenase-1 enzyme in platelets. In some trials, almost all patients on aspirin have a very low level of serum thromboxane B₂, indicating that the measured platelet reactivity in aspirin-treated patients might be due to platelet activation via other pathways, such as ADP or thrombin. The prevalence of real aspirin resistance seems to be very low, and probably the term “high-on-aspirin residual platelet reactivity” should be preferred to describe this phenomenon.     

Aspirin and Platelet Adenosine Diphosphate Receptor Antagonists in Acute Coronary Syndromes and Percutaneous Coronary Intervention

Platelet activation and aggregation are key components in the cascade of events causing thrombosis following plaque rupture. Antiplatelet therapy is essential in the treatment of patients with acute coronary syndromes (ACS) and for those requiring percutaneous coronary intervention (PCI). Aspirin (acetylsalicylic acid) is a well established antiplatelet therapy and is mandated for secondary prevention of cardiovascular events following ACS. In patients with ACS, the addition of clopidogrel to aspirin is more effective than aspirin alone. For patients undergoing PCI, dual antiplatelet therapy with aspirin and clopidogrel is warranted. Aspirin should be continued indefinitely after PCI. Pretreatment of patients with clopidogrel prior to PCI lowers the incidence of cardiovascular events, yet the optimum timing of drug administration and dose are still being investigated, as is the duration of therapy following PCI. Late-stent thrombosis with drug-eluting stents has pushed the recommendation for duration of clopidogrel therapy up to 1 year and perhaps beyond, in patients without risks for bleeding. The concepts of aspirin and clopidogrel resistance are important clinical questions. No uniform definition exists for aspirin or clopidogrel resistance. Measurements of resistance are often highly variable and do not necessarily correlate with clinical resistance. Noncompliance remains the most prominent mode of resistance. Screening of selected patient populations for resistance or pharmacologic intervention of those patients termed 'resistant' warrants further study.     

Clopidogrel: a selective inhibitor of platelet ADP receptors

Platelets play a key role in the development of ischemic complications in the arterial circulation. Antiplatelet therapy has proven effective in the treatment and prevention of ischemic events. Numerous clinical studies have confirmed the therapeutic efficacy of aspirin to such a point that this antiplatelet agent has become the gold standard in clinical practice. Clopidogrel is a thienopyridine compound that inhibits platelet aggregation by selectively binding to adenylate cyclase-coupled ADP receptors. Results of a large, double-blind, randomized study (CAPRIE) confirm that administration of clopidogrel to patients with atherosclerotic vascular disease is more effective than aspirin in reducing the combined risk of ischemic stroke, myocardial infarction or vascular death. The present article highlights the importance of activation of platelets through ADP receptors and reviews the pharmacology and clinical studies of clopidogrel, a selective inhibitor of these mechanisms.     

Old and New Molecular Mechanisms Associated with Platelet Resistance to Antithrombotics

Current available data show that about 5 to 40% of coronary patients treated with conventional doses of antithrombotic drugs do not display adequate antiplatelet response. Nowadays, aspirin remains the main antiplatelet therapy. However, a significant number of patients show platelet resistance to aspirin therapy, and recurrent thrombotic events occur. Combined antithrombotic therapies with thienopyridines, such as clopidogrel have been used to resolve this problem. However, clopidogrel treatment has been also associated with wide response variability, and non-responsiveness to clopidogrel also occurs in some patients. Therefore, the main question arising about the antithrombotic therapy is why particular patients do not benefit from the therapy and how they might be identified to improve their treatment. Different hypotheses have been suggested, including genetic factors, platelet heterogeneity, non-compliance and others. However, it is probably that many molecular mechanisms involved in platelet resistance to antithrombotic therapies still remains unknown. New technologies, such as proteomics and genetic, are beginning to show new unknown biological biomarkers and molecular mechanisms which may be associated with platelet antithrombotic drug resistance.     

Dual antiplatelet therapy for prevention of recurrent ischemic events.

The advantages of dual antiplatelet therapy over monotherapy in preventing recurrent ischemic events are examined. Atherosclerosis is an insidious systemic process involving multiple vascular beds, including the cerebral, coronary, and peripheral arteries. Atherosclerotic plaque rupture is one of the inciting events in the progression of platelet activation, aggregation, and thrombus formation. Patients with any clinical manifestation of atherosclerosis are vulnerable to others in different vascular beds since the disease develops throughout the vasculature, and different vascular events have common predisposing risk factors. Ischemic coronary heart disease and cerebrovascular disease are two of the three most frequent causes of death in the United States. The efficacy of aspirin in the secondary prevention of myocardial infarction (MI) and stroke has been demonstrated in numerous trials. While dipyridamole has not been linked with a greater odds reduction than aspirin in the development of MI, stroke, and vascular death, ticlopidine and clopidogrel have been associated with a greater reduction in the development of acute MI, stroke, and vascular death than aspirin. Clinical trials evaluating the efficacy and safety of combination antiplatelet therapy in the prevention of recurrent ischemic events are ongoing. The rationale for using a combination of two mechanistically different antiplatelet agents is supported by ex vivo and clinical studies. Inhibition of platelet aggregation and thrombus formation is enhanced with dual antiplatelet therapy. Combination antiplatelet regimens with different mechanisms of action to inhibit multiple sites in the thrombotic pathway may further improve long-term clinical outcomes. Dual antiplatelet therapy may have advantages over monotherapy in the prevention of recurrent ischemic events.     

Effects of combining three different antiplatelet agents on platelets and leukocytes in whole blood in vitro

1. Antiplatelet drugs have been demonstrated to reduce the incidence of recurrent events in patients with symptomatic vascular disease. However, there is no experimental data indicating the effects of these agents when given together on platelets and leukocytes. We investigated the ability of aspirin (an inhibitor of cyclo-oxygenase), dipyridamole (an inhibitor of phospodiesterases and adenosine uptake) and AR-C69931 (a direct acting P(2T) antagonist with effects similar to those of clopidogrel which can be used in vitro) when used alone or in combination to inhibit platelet and leukocyte function. 2. Measurements of platelet and leukocyte function were performed in blood taken from normal volunteers, and the inhibitory effects of aspirin (100 micromol l(-1)), dipyridamole (10 micromol l(-1)) and AR-C66931 (100 nmol l(-1)) were determined. Platelet aggregation was induced by stirring blood with and without adenosine diphosphate (ADP) or platelet activating factor (PAF) and measured by platelet counting. Platelet P-selectin expression, platelet-leukocyte conjugate formation, and leukocyte activation were determined by flow cytometry. 3. Dipyridamole, AR-C69931, dipyridamole and AR-C69931, dipyridamole and aspirin, AR-C69931 and aspirin, and all three agents together inhibited platelet aggregation induced by stirring, ADP and PAF (P<0.01). However, it was only the combination of all three agents inhibited P-selectin expression (P<0.01). Similarly, it was the combination of all three antiplatelet agents that most consistently inhibited platelet-monocyte and platelet-neutrophil conjugate formation and monocyte and neutrophil activation. 4. Since both platelets and leukocytes are thought to contribute to arterial thrombosis and atherosclerosis, it is possible that combinations of different antiplatelet agents with different mechanisms of action may afford better protection than individual or pairs of agents used on their own.     

阿司匹林抵抗：基础与临床

阿司匹林可通过抑制血栓素A2诱导的血小板聚集,发挥抗血小板疗效,是临床常用的抗血小板药物.然而,临床上部分患者规范服用阿司匹林达不到对血小板的显著抑制,仍有动脉血栓事件发生,称为阿司匹林抵抗.     

Use of Aspirin in normalization of recombinant human erythropoietin-mediated hyper-reactivity of platelets in rats

Objectives:

The cytokine erythropoietin is the primary stimulator of erythropoiesis and recombinant human erythropoietin (rHuEPO), which is widely used in the treatment of anemia associated with advanced chronic kidney disease (CKD). Adverse cardiovascular outcomes have been observed during clinical trials of anemia correction with rHuEPO in CKD patients. We investigated the effects of short-term, high-dose treatment with rHuEPO on platelet reactivity and effects of aspirin on platelet reactivity in healthy rats.

Materials and Methods:

Animals received three daily dose of rHuEPO (25 μg/kg s.c.). Platelets were isolated after 48 h of last dose of rHuEPO to study the arachidonic acid-induced platelet aggregation. Aspirin (75 mg/kg p.o.) was given to animals just before 1 h of isolation of platelets.

Results:

In rats, treatment with rHuEPO increased platelet reactivity and platelet count. The increased platelet reactivity was paralleled by decreased time-to-occlusion (TTO) in arterial thrombosis model, and decreased bleeding time after tail transection in rats. Treatment with rHuEPO followed by single dose of aspirin showed significant reduction in TTO and bleeding time as compared with aspirin-treated group.

Conclusions:

These findings suggest that rHuEPO increases platelet reactivity and aspirin normalizes the hyper-reactive platelet and may reduce the cardiovascular events associated with rHuEPO in CKD patients.KEY WORDS: Aspirin, chronic kidney disease, platelet, rat, recombinant human erythropoietin     

Antiplatelet therapy in peripheral artery disease

Peripheral artery disease (PAD) is a term that relates to atherosclerosis and narrowing of the arteries in the lower extremities. The prevalence of PAD is approximately 12% of the adult population. Despite the low rate of peripheral complications and amputation, PAD is complicated by a high rate of cardiovascular events including myocardial infarction, stroke, and vascular death with an annual incidence of about 5%.The detection of PAD is initially based on the appearance of typical symptoms (claudication and critical limb ischemia) related to peripheral arterial insufficiency. However, PAD may also be present in the absence of clinical symptoms (asymptomatic PAD). Accordingly, asymptomatic disease may occur in up to 50% of all patients with PAD. Ankle brachial index (ABI) is a diagnostic test used to evaluate the presence of PAD, defined by an ABI ≤0.90. The ABI is also demonstrated to be useful in the assessment of vascular risk in asymptomatic and symptomatic patients. Antiplatelet therapy remains a key intervention to reduce cardiovascular risk in PAD. Data from Antithrombotic Trialists' Collaboration showed that antiplatelet treatment was associated with a 23% risk reduction of vascular events in overall population with PAD. However, closer scrutiny of these data reveals that nonaspirin antiplatelet drugs, including ticlopidine, clopidogrel, picotamide, and dipyridamole largely drove the benefits in the PAD subgroup. It remains an open issue if PAD represents an atherosclerotic clinical model where aspirin, differently from coronary heart disease, is less effective in reducing atherosclerotic progression. Based on the reported results further trials with aspirin should be done in asymptomatic (ABI ≤0.90) and symptomatic PAD patients. Finally, the role of new antiplatelet drugs such as prasugrel and ticagrelor has not yet been studied in PAD.     

西洛他唑治疗经皮冠脉内介入术后对阿司匹林致上消化道出血患者的随访研究

目的观察经皮冠脉内介入术(percutaneous coronary intervention,PCI)后使用西洛他唑对阿司匹林致上消化道出血患者的外周血血小板聚集率、PGE2及心血管事件发生率的影响。方法 64例确诊冠心病并行PCI术后的患者,服用阿司匹林和氯吡格雷双重抗血小板治疗出现阿司匹林相关上消化道出血,其中32例患者改用西洛他唑加氯吡格雷,而另外32例患者出血治疗后继续原抗血小板治疗方案,随访比较两组患者血小板聚集率、PGE2及心血管事件发生率。结果平均随访(0.9±0.1)年,两组患者血小板聚集率均明显下降,西洛他唑组的血小板聚集率显著低于阿司匹林组(P<0.05),外周血PGE2的浓度高于阿司匹林组(P<0.05),但两组患者临床不良事件发生率差异无统计意义。结论对PCI术后上消化道出血患者,应用西洛他唑替代阿司匹林,联用氯吡格雷进行抗血小板治疗,经过短期的临床观察,其血小板聚集率优于阿司匹林,升高外周血PGE2的浓度,且安全性与阿司匹林相当,可用于预防上消化道出血的复发。     

Acute and long-term antiplatelet therapy

Clinical presentations of atherothrombotic vascular disease, i.e., acute coronary syndromes, cerebrovascular events and events associated with peripheral arterial disease, are the major causes of mortality and morbidity worldwide. Platelet activation and aggregation play an important role in the progression and clinical presentation of atherothrombotic disease, and antiplatelet therapy improves outcome in patients with atherothrombotic vascular disease. Aspirin has been the cornerstone of antiplatelet therapy for many decades, but in recent years, adenosine diphosphate (ADP) receptor antagonists, mainly clopidogrel and ticlopidine, and glycoprotein (GP) IIb/IIIa (integrin alpha IIb beta 3) inhibitors have also shown similar effectiveness. This review briefly summarizes the major clinical trials and recommendations for the efficacy and safety of antiplatelet therapy in patients with established atherothrombotic disease.