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

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.

Optimizing platelet P2Y12 inhibition for patients undergoing PCI

Guidelines recommend that dual antiplatelet therapy using aspirin and clopidogrel should be administered to the majority of patients with acute coronary syndromes, including those undergoing percutaneous coronary intervention (PCI). However, the results of a large randomized, placebo-controlled study suggest that a 300-mg loading dose of clopidogrel must be administered at least 15 h prior to PCI in order to achieve a significant reduction in peri-PCI thrombotic events. Other data suggest that 2 h of pretreatment may be sufficient if a 600-mg loading dose is used. Since it is often difficult to achieve an adequate pretreatment goal with clopidogrel in clinical practice, more rapid achievement of platelet P2Y(12) inhibition may improve patient outcomes. Prasugrel, [6-[2-(3,4-diflurophenyl) cyclopropyl1-1-y1] amino-2-propylthio-9-D-ribofuranosyl-9H-purine (AZD6140), and cangrelor are platelet P2Y(12) receptor antagonists currently in development that offer faster acting inhibition of adenosine diphosphate (ADP)--induced platelet aggregation. These agents act upon the same platelet receptor as clopidogrel, but are distinguished by their routes of administration, reversibility, and pharmacodynamic properties. Prasugrel is an orally administered agent that provides faster, higher, and more consistent inhibition of platelet aggregation than clopidogrel. The results of Phase II testing suggest that the risk of bleeding is similar in prasugrel- and clopidogrel-treated patients. AZD6140 is another orally administered platelet inhibitor with rapid and reversible action. Again, Phase II testing suggests similar bleeding risk for clopidogrel. Preliminary evidence suggests that clinical outcomes may be better in prasugrel- and AZD6140-treated patients than in clopidogrel-treated patients. Cangrelor is an intravenously administered, reversible, short-acting agent with a rapid onset of activity. Bleeding risk and clinical outcomes data are similar in cangrelor- and abciximab-treated patients. The results of ongoing Phase III clinical trials involving more than 40,000 patients will demonstrate whether these agents fulfill their potential to improve outcomes in PCI-treated patients by providing faster, higher, and more consistent inhibition of platelet aggregation.

Influence of Antiplatelet Therapy on Cerebral Micro-Emboli after Carotid Endarterectomy using Postoperative Transcranial Doppler Monitoring

AIM

To study the effect of different antiplatelet regimens (APT) on the rate of postoperative TCD registered micro-embolic signals (MES) following carotid endarterectomy (CEA).

DESIGN

Prospective, randomised, double-blinded, pilot study.

METHODS

The study group of 102 CEA patients (76 men, mean age 66.8 years) was randomised to routine Asasantin (Dipyridamole 200mg/Aspirin 25mg) twice daily (group I; n=39), Asasantin plus 75 mg Clopidogrel once daily (group II; n=33), or Asasantin plus Rheomacrodex (Dextran 40) 100g/L iv; 500 ml (group III; n=30). TCD monitoring of the ipsilateral middle cerebral artery for the occurrence of MES was performed intra-operatively and during the second postoperative hour following CEA. Primary endpoints were the rate of postoperative emboli and the occurrence of cerebrovascular complications. Secondary endpoint was any adverse bleeding.

RESULTS

There were no deaths or major strokes. We observed 2 intraoperative TIA's (group II and III) and 1 postoperative minor stroke (group I). In comparison with placebo, Clopidogrel or Rheomacrodex in addition to Asasantin produced no significant reduction in the number of postoperative MES. There was no significant difference between the number of postoperative MES and different antiplatelet regimens. The incidence of bleeding complications was not significantly different between the 3 APT groups.

CONCLUSION

In the present study, we could not show a significant influence of different antiplatelet regimens on TCD detected postoperative embolization following CEA.

Real-time monitoring of adhesion and aggregation of platelets using thickness shear mode (TSM) sensor

Hemostasis is required to maintain vascular system integrity, but thrombosis, formation of a clot in a blood vessel, is one of the largest causes of morbidity and mortality in the industrialized world. Novel clinical and research tools for characterizing the hemostatic system are of continued interest, and the object of this research is to test the hypothesis that clinically relevant platelet function can be monitored using an electromechanical sensor. A piezoelectric thickness shear mode (TSM) biosensor coated with collagen-I fibers to promote platelet activation and adhesion was developed and tested for sensitivity to detect these primary events. Magnitude and frequency response of the sensor were monitored under static conditions at 37 degrees C, using platelet-rich plasma (PRP), and PRP with adenosine diphosphate (ADP), a clinical aggregation inhibitor (abciximab), or a collagen binding inhibitor. Sensors loaded with PRP exhibited a 3-stage response; no significant change in response for the first 20 min (Stage-1), followed by a larger drop in response (Stage-2) and subsequently, response gradually increased (Stage-3). Exogenous ADP stimulated an immediate Stage-2 response, while abciximab delayed and reduced the magnitude change of Stage-2. In the presence of collagen inhibitor, Stage-2 response was similar to that of control but was delayed by an additional 20 min. The obtained results, supported by epifluorescence and complementary SEM studies, demonstrated the selective sensitivity of TSM electromechanical biosensors to monitor platelet function and inhibition, particularly aggregation.

Non?ST segment elevation acute coronary syndromes: Treatment guidelines for the nurse practitioner

PURPOSE

To increase awareness among nurse practitioners (NPs) of the American College of Cardiology (ACC)/American Heart Association (AHA) 2002 guideline update for the diagnosis and treatment of acute coronary syndrome, and for secondary prevention in patients with unstable angina (UA) and non-ST segment elevation myocardial infarction (NSTEMI).

DATA SOURCES

ACC/AHA 2002 guideline update for the management of patients with UA and NSTEMI, ACC/AHA guidelines for patients with coronary and other atherosclerotic vascular disease, 2006 update, selected research and clinical articles.

CONCLUSIONS

Recent research has shown that patients with UA/NSTEMI benefit from the routine, long-term use of dual antiplatelet therapy with aspirin and clopidogrel. In suitable patients, outcome is also improved by adoption of an early invasive strategy combined with aggressive medical therapy.

IMPLICATIONS FOR PRACTICE

Familiarity with the patient as well as current management recommendations can improve clinical outcomes for patients with UA/NSTEMI. Thus, NPs can play a pivotal role in the management of coronary disease, both during and following an acute ischemic event.

Dose-dependent inhibition of human platelet aggregation by prasugrel and its interaction with aspirin in healthy subjects

The aims of this open-label, randomized, dose-escalation pharmacodynamic study of prasugrel, an orally active antiplatelet agent, were to assess its interaction with aspirin (ASA, 325 mg) in healthy subjects after a loading dose (LD) and subsequent 5 days of once-daily maintenance doses (MD) of prasugrel or the active comparator, clopidogrel. We measured platelet aggregation induced by ADP, collagen, and TRAP and compared effects on maximal and residual platelet aggregation responses. On a background of ASA, subjects were randomly assigned to 1 of 4 prasugrel treatment groups (LD/MD in mg: 20/5, 30/7.5, 40/10, or 60/15; n = 8/group) or to clopidogrel 300 mg LD/75 mg MD (n = 11). Prasugrel dose-dependently inhibited ADP-induced platelet aggregation and exhibited higher levels of platelet inhibition than clopidogrel or ASA alone. Prasugrel plus ASA resulted in additive inhibition of collagen- and TRAP-induced platelet aggregation. Although inhibition of residual aggregation was greater than inhibition of maximal aggregation, values were highly correlated. The safety and tolerability of prasugrel plus ASA were also monitored. Within the limitations of the study, prasugrel was found to be well tolerated when dosed as LD followed by MD in the presence of ASA and provided greater platelet inhibition than ASA alone.

Antiplatelet therapy in type 2 diabetes mellitus

PURPOSE OF REVIEW

Patients with diabetes mellitus are characterized by a prothrombotic status. Since platelet reactivity is key to the development of atherothrombotic complications, antiplatelet therapy has a pivotal role in reducing ischemic risk, especially in diabetes mellitus patients. This review summarizes the current knowledge on how antiplatelet therapy affects this high-risk patient population.

RECENT FINDINGS

Numerous studies have shown the clinical benefit of antiplatelet agents in reducing ischemic events in diabetes mellitus patients. Despite the clinical benefit achieved with antiplatelet agents, however, patients with diabetes mellitus continue to have an increased risk of ischemic events compared to non-diabetes mellitus. Recent observations suggest that this may be in part due to inadequate platelet inhibition achieved in diabetes mellitus patients compared to non-diabetics, exposing these subjects to an enhanced ischemic risk.

SUMMARY

Whilst antiplatelet therapy has been clearly shown to reduce morbidity and mortality in patients with diabetes mellitus, there are accruing data demonstrating that in these patients the degree of platelet inhibition achieved with standard treatment regimens may be inadequate. This supports the need for specific antiplatelet drug regimens, with either different dosages of current medication or development of novel antiplatelet drugs, which are more specific to tackle the hyperreactive diabetic platelet.

Clinical use of clopidogrel in acute coronary syndrome

Several therapeutic approaches have been developed to improve the outcome among patients with acute coronary syndrome (ACS). However, treatment with antithrombotic therapies such as oral glycoprotein IIb/IIIa inhibitors has been limited by the lack of efficacy and excess bleeding complications. As the publication of the landmark study Clopidogrel in Unstable Angina to Prevent Recurrent Events (CURE), the clinical benefit of early and intermediate-term use of combined antiplatelet agents--clopidogrel plus aspirin--in non-ST-segment elevation myocardial infarction (NSTEMI) patients became evident. Pretreatment and intermediate-term therapy with clopidogrel in NSTEMI ACS patients undergoing percutaneous coronary intervention (PCI) was further supported by the PCI-CURE trial. Recently, the results of two major trials Clopidogrel as Adjunctive Reperfusion Therapy-Thrombolysis in Myocardial Infarction 28, Clopidogrel and Metoprolol in Myocardial Infarction Trial established the pivotal role of clopidogrel in the other spectrum of ACS-STEMI. Coupled with the results from previous multicentre trials, these two studies provide a guide for the early and long-term use of clopidogrel in the whole spectrum of ACS. A review summarising the results of the recent clinical trials and a discussion on its implications for the clinical management of ACS is presented.

Antiplatelet drug induced isolated profound thrombocytopenia in interventional cardiology: a review based on individual case reports

A combination antithrombotic and antiplatelet therapy with clopidogrel, aspirin, glycoprotein IIb/IIIa receptor inhibitors and heparins is routinely used as adjunct therapy in patients undergoing percutaneous coronary intervention (PCI). As all substances inhibit platelet function, bleeding and thrombocytopenia may occur. We report on three patients who developed isolated profound thrombocytopenia (platelet count of < 20,000/mm(3)) within 24 h after initiation of combination antiplatelet and antithrombotic therapy during a 1 year observation period in 443 consecutive patients undergoing PCI and stent implantation. The data from our cardiology unit revealed an incidence of an isolated profound thrombocytopenia in 0.7% of all patients on combination antithrombotic therapy and in 1.5% of patients with GPIIb/IIIa-blockers. In all three cases with isolated profound thrombocytopenia GPIIb/IIIa-blockers were found to be the causative agents. Negative results of HIT-assays excluded heparin induced thrombocytopenia type II. Despite the extremely low platelet count no severe bleeding was observed and in all cases platelet counts normalized within 3-4 days without specific interventions except discontinuation of the responsible agent. These findings are discussed in conjunct with an overview of the recent literature.

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.

The platelet inhibiting effect of a clopidogrel bolus dose in patients on long-term acetylsalicylic acid treatment

INTRODUCTION

Addition of clopidogrel to patients treated with ASA has been shown to decrease the incidence of in-stent thrombosis after percutaneous coronary interventions. However, it has also been reported that up to 30% of patients do not achieve adequate platelet inhibition from standard dosages of ASA and clopidogrel. There is a demand for reliable methods to measure the individual platelet inhibiting effect of this combination therapy.

MATERIALS AND METHODS

The primary aim of the present investigation was to compare three methods for evaluation of the platelet inhibiting effect of a clopidogrel bolus dose in patients on long-term acetylsalicylic acid treatment. Thirty patients presenting for coronary angiography/PCI were included. Two patients were excluded due to technical problems. All patients were on 75-100 mg ASA/day for at least 8 days. Blood samples were analysed before and 16 h after a 300 mg clopidogrel bolus dose. The platelet inhibiting effect was measured with (1) Whole blood flow cytometry (17 patients); (2) a bed-side test, Platelet Mapping assay for the thrombelastograph (28 patients); and (3) PFA (Platelet function analyser) -100 (26 patients).

RESULTS

With flow cytometry, the percentage of platelets expressing P-selectin (p=0.03) on their surface decreased significantly after the bolus dose of clopidogrel. There was also a reduction of platelets binding fibrinogen when stimulated with ADP. A significantly (p=0.002) increased platelet inhibition could also be demonstrated with Platelet Mapping. PFA-100 could not measure any significant platelet inhibiting effect of clopidogrel.

CONCLUSION

A significant platelet inhibition could be demonstrated with flow cytometry and the Platelet Mapping assay, but not with PFA-100. However, levels of response for the individual patient with these three methods were inconsistent. Further studies are needed to evaluate how the results correlate to the clinical risk of thrombosis and bleeding.

The safety and efficacy of aspirin and clopidogrel as a combination treatment in patients with coronary heart disease

The use of aspirin and clopidogrel in combination has become part of the standard clinical care of patients with coronary artery disease. The use of this combination provides significant benefits compared with the use of aspirin alone in patients with acute coronary syndromes, and in patients treated with percutaneous coronary intervention with stent placement (both bare metal and drug-eluting stents). Clinical trials have demonstrated significant efficacy of this dual therapy; however, there is the potential for significant bleeding complications from the synergistic antiplatelet effects. In total, it appears that when there is vessel injury (mechanical from perctutaneous coronary intervention or a ruptured plaque), dual antiplatelet therapy with aspirin and clopidogrel results in improved outcomes, albeit with a small but significant inherent risk of increased bleeding.

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.

Effect on platelet function of cilostazol, clopidogrel, and aspirin, each alone or in combination

Management of peripheral arterial disease (PAD) requires standard atherosclerotic risk management interventions. However, PAD is often complicated by walking pain (intermittent claudication [IC]), which requires symptom-specific therapies as well. Thus, all PAD patients are encouraged to take antiplatelet agents to reduce the associated risks of major cardiovascular events, and those with IC may also require treatment with cilostazol, an agent proven to increase exercise capacity and enhance quality of life in these patients. Although it was initially thought that cilostazol's antiplatelet properties might render it unsafe to use in combination with other platelet inhibitors because of possible additive effects, a recent study has dispelled such concerns. There is evidence that in a crossover trial of 21 patients with PAD and IC, aspirin alone, or clopidogrel alone, significantly increased bleeding times, but cilostazol alone did not. The combination of aspirin and clopidogrel had a greater effect on increasing bleeding time than either monotherapy, and no further bleeding time prolongation was observed, when cilostazol was added to any aspirin/clopidogrel regimen. These findings suggest that PAD patients with IC may be safely managed with both cilostazol and standard antiplatelet therapy, without increasing the risk of adverse bleeding effects.

Addition of clopidogrel to aspirin in 45,852 patients with acute myocardial infarction: randomised placebo-controlled trial

BACKGROUND

Despite improvements in the emergency treatment of myocardial infarction (MI), early mortality and morbidity remain high. The antiplatelet agent clopidogrel adds to the benefit of aspirin in acute coronary syndromes without ST-segment elevation, but its effects in patients with ST-elevation MI were unclear.

METHODS

45,852 patients admitted to 1250 hospitals within 24 h of suspected acute MI onset were randomly allocated clopidogrel 75 mg daily (n=22,961) or matching placebo (n=22,891) in addition to aspirin 162 mg daily. 93% had ST-segment elevation or bundle branch block, and 7% had ST-segment depression. Treatment was to continue until discharge or up to 4 weeks in hospital (mean 15 days in survivors) and 93% of patients completed it. The two prespecified co-primary outcomes were: (1) the composite of death, reinfarction, or stroke; and (2) death from any cause during the scheduled treatment period. Comparisons were by intention to treat, and used the log-rank method. This trial is registered with ClinicalTrials.gov, number NCT00222573.

FINDINGS

Allocation to clopidogrel produced a highly significant 9% (95% CI 3-14) proportional reduction in death, reinfarction, or stroke (2121 [9.2%] clopidogrel vs 2310 [10.1%] placebo; p=0.002), corresponding to nine (SE 3) fewer events per 1000 patients treated for about 2 weeks. There was also a significant 7% (1-13) proportional reduction in any death (1726 [7.5%] vs 1845 [8.1%]; p=0.03). These effects on death, reinfarction, and stroke seemed consistent across a wide range of patients and independent of other treatments being used. Considering all fatal, transfused, or cerebral bleeds together, no significant excess risk was noted with clopidogrel, either overall (134 [0.58%] vs 125 [0.55%]; p=0.59), or in patients aged older than 70 years or in those given fibrinolytic therapy.

INTERPRETATION

In a wide range of patients with acute MI, adding clopidogrel 75 mg daily to aspirin and other standard treatments (such as fibrinolytic therapy) safely reduces mortality and major vascular events in hospital, and should be considered routinely.

The clopidogrel after surgery for coronary artery disease (CASCADE) randomized controlled trial: clopidogrel and aspirin versus aspirin alone after coronary bypass surgery [NCT00228423]

Background

Saphenous vein graft disease remains a major limitation of coronary artery bypass graft surgery. The process of saphenous vein intimal hyperplasia begins just days after surgical revascularization, setting the stage for graft atherosclerotic disease and its sequalae. Clopidogrel improves outcomes in patients with atherosclerotic disease, and is effective at reducing intimal hyperplasia in animal models of thrombosis. Therefore, the goal of this study will be to evaluate the efficacy of clopidogrel and aspirin therapy versus aspirin alone in the prevention of saphenous vein graft intimal hyperplasia following coronary artery bypass surgery.

Methods

Patients undergoing multi-vessel coronary artery bypass grafting and in whom at least two saphenous vein grafts will be used are eligible for the study. Patients will be randomized to receive daily clopidogrel 75 mg or placebo, in addition to daily aspirin 162 mg, for a one year duration starting on the day of surgery (as soon as postoperative bleeding has been excluded). At the end of one year, all patients will undergo coronary angiography and intravascular ultrasound assessment of one saphenous vein graft as selected by randomization. The trial will be powered to test the hypothesis that clopidogrel and aspirin will reduce vein graft intimal hyperplasia by 20% compared to aspirin alone at one year following bypass surgery.

Discussion

This trial is the first prospective human study that will address the question of whether clopidogrel therapy improves outcomes and reduces saphenous vein graft intimal hyperplasia following cardiac surgery. Should the combination of clopidogrel and aspirin reduce the process of vein graft intimal hyperplasia, the results of this study will help redefine modern antiplatelet management of coronary artery bypass patients.

Clopidogrel: cardiologists' panacea or neurologists' headache?

Clopidogrel (Plavix) is a potent platelet inhibitor that has been demonstrated to be significantly more effective than aspirin for the prevention of atherothrombotic events in patients with established atherosclerotic disease in the Clopidogrel versus Aspirin in Patients at Risk of Ischemic Events (CAPRIE) study. Furthermore, the benefit of clopidogrel compared with placebo in patients with acute coronary syndromes, as well as those undergoing percutaneous coronary intervention receiving a standard therapy including aspirin, has been largely demonstrated by the Clopidogrel in Unstable angina to prevent Recurrent Events (CURE), Clopidogrel for the Reduction of Events During Observation (CREDO), CLopidogrel as Adjunctive ReperfusIon TherapY-Thrombolysis In Myocardial Infarction (CLARITY-TIMI)28, and Clopidogrel and Metoprolol in Myocardial Infarction Trial/Secon Chinese Cardiac Study (COMMIT/CCS-2) trials. However, in the Management of Atherothrombosis with Clopidogrel in High-risk patients with recent transient ischemic attack or ischemic stroke (MATCH) trial, there was no clinical benefit of adding aspirin versus placebo to a standard therapy of clopidogrel in patients with cerebrovascular disease. The implications of these findings, as well as future perspectives from upcoming trials, shall be discussed herein.

Platelet function profiles in patients with type 2 diabetes and coronary artery disease on combined aspirin and clopidogrel treatment

To assess platelet function profiles in diabetic and nondiabetic patients on aspirin and clopidogrel therapy, two patient populations were included to investigate the 1) acute effects of a 300-mg clopidogrel loading dose (group 1, n = 52) and 2) long-term effects of clopidogrel (group 2, n = 120) on platelet function in diabetic compared with nondiabetic patients already on aspirin treatment. Patients were stratified according to the presence of type 2 diabetes. Platelet aggregation was assessed using light transmittance aggregometry (groups 1 and 2). Platelet activation (P-selectin expression and PAC-1 binding) was determined using whole-blood flow cytometry (group 2). Clopidogrel response was also assessed. In group 1, platelet aggregation was significantly increased in diabetic (n = 16) compared with nondiabetic (n = 36) patients at baseline and up to 24 h following a 300-mg loading dose (P = 0.005). In group 2, platelet aggregation and activation were increased in diabetic (n = 60) compared with nondiabetic (n = 60) subjects (P < 0.05 for all platelet function assays). Diabetic subjects had a higher number of clopidogrel nonresponders (P = 0.04). Diabetic patients have increased platelet reactivity compared with nondiabetic subjects on combined aspirin and clopidogrel treatment. Reduced sensitivity to antiplatelet drugs may contribute to the increased atherothombotic risk in diabetic patients.

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.

Monitoring of Clopidogrel Action: Comparison of Methods

Background: Clopidogrel is a potent drug for prevention of adverse effects during and after coronary intervention. Increasing experience indicates that a significant proportion of patients do not respond adequately to clopidogrel. Because failure of antiplatelet therapy can have severe consequences, there is need for a reliable assay to quantify the effectiveness of clopidogrel treatment.

Methods: Of 24 healthy volunteers admitted to the study, 18 were treated for 1 week with clopidogrel (300-mg loading dose and 75-mg maintenance dose), and 6 with placebo. Platelet function was monitored by 2 assays, based on flow cytometry and enzyme immunoassay, that measure the phosphorylation status of vasodilator-stimulated phosphoprotein (VASP) and by aggregometry, flow cytometry of P-selectin, and the platelet function analyzer at baseline, on days 1–5, and on day 9 of treatment.

Results: Aggregometry and VASP phosphorylation revealed a loss of platelet response to ADP within 12 h after clopidogrel intake. The phosphorylation status of VASP correlated with the inhibition of platelet aggregation. In contrast, neither P-selectin expression nor PFA-100 closure time was a clear indicator of clopidogrel effects on platelets.

Conclusions: VASP phosphorylation assays are reliable for quantifying clopidogrel effects. Because the VASP assay directly measures the function of the clopidogrel target, the P2Y12 receptor, the assay is selective for clopidogrel effects rather than effects of other platelet inhibitors commonly in use.

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.

High-Dose Clopidogrel Loading in Percutaneous Coronary Intervention

OBJECTIVE:

To review the use of a 600-mg clopidogrel loading dose in patients undergoing percutaneous coronary intervention (PCI).

DATA SOURCES:

Human clinical trials and platelet studies available through PubMed (1966–March 2005), bibliographies of pertinent articles, and citations supplied by the drug manufacturer were accessed.

DATA SYNTHESIS:

The administration of a 600-mg loading dose of clopidogrel can decrease the time required for maximum platelet inhibition to 2 hours compared with ⩾6 hours achieved with 300 mg. This higher loading dose has been investigated in multiple platelet studies and one observational report. Several randomized controlled trials have used a 600-mg loading dose; however, these studies were not designed to evaluate the efficacy and safety of this loading regimen. To date, only one randomized trial has compared the 600-mg loading dose with a 300-mg loading dose.

CONCLUSIONS:

When compared with a conventional loading regimen of 300 mg in lower-risk patients, pretreatment with clopidogrel 600 mg was shown to be more effective in reducing periprocedural events and demonstrated similar safety. Studies are needed to clarify the use of a 600-mg loading dose in higher-risk patients, with concomitant glycoprotein IIb/IIIa receptor antagonism, or when administration is delayed until immediately before or after PCI.

Enhanced Antiplatelet Effects of Clopidogrel plus Acetylsalicylic Acid Compared with Acetylsalicylic Acid Alone or Combined with Extended-Release Dipyridamole in Healthy Volunteers

BACKGROUND

Previous studies have shown the potential benefit of using antiplatelet agents with complementary modes of action.

METHODS

Using a crossover design, the ex vivo antiplatelet effects of 10 days' treatment with clopidogrel 75 mg + acetylsalicylic acid (ASA) 75 mg daily, ASA 75 mg/day, or extended-release dipyridamole 200 mg/low-dose ASA 25 mg twice daily were compared, using various platelet agonists.

RESULTS

Clopidogrel + ASA was significantly more effective than dipyridamole + ASA in inhibiting collagen-induced platelet aggregation in whole blood (mean 44.9 +/- 5.6% inhibition vs. 16.5 +/- 6.7%; p = 0.0009). Clopidogrel + ASA was significantly more effective than ASA or dipyridamole + ASA in inhibiting ADP-induced platelet aggregation in whole blood (p < or = 0.0001) and platelet-rich plasma (PRP) (p < or = 0.0001), and in inhibiting collagen-induced aggregation in PRP (p < or = 0.0001). ASA alone and clopidogrel + ASA were significantly more effective than dipyridamole + ASA in inhibiting arachidonic acid-induced platelet aggregation in whole blood (p < or = 0.0001).

CONCLUSIONS

Based on ex vivo platelet aggregometry, clopidogrel + ASA is a more potent antiplatelet regimen than either ASA alone or the marketed combination of dipyridamole + ASA. However, the clinical significance of this finding remains to be confirmed.

Antiplatelet Agents: Current Drugs and Future Trends

Antiplatelet drugs in clinical use are discussed in terms of their mechanisms of action and the relevancy of that to the physiology of platelets and the pathophysiology of arterial thrombosis. Current clinical usage is outlined in detail for each drug. Experimental antiplatelet drugs also are discussed.

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.

Identification of low responders to a 300-mg clopidogrel loading dose in patients undergoing coronary stenting

BACKGROUND

Although patients undergoing coronary stenting routinely receive dual antiplatelet treatment to reduce the risk of stent thrombosis, this undesired event still occurs. A suboptimal response to clopidogrel treatment (low responders) has been suggested to contribute to stent thrombosis. In the present study, platelet function profiles were assessed in patients undergoing coronary stenting receiving a standard 300-mg clopidogrel loading dose with the aim to identify low clopidogrel responders.

MATERIALS AND METHODS

Platelet aggregation was assessed by light transmittance aggregometry following 6 microM ADP stimuli in 48 patients before and 10 min, 4 and 24 h after receiving clopidogrel front-loading. Patients having > or =40% inhibition of platelet aggregation 24 h after clopidogrel administration were defined as normal responders, whereas those having <40% inhibition were low responders. Glycoprotein (GP) IIb/IIIa activation and P-selectin expression were assessed by whole blood flow cytometry following 2 microM ADP stimuli at the same time points. Platelet function profiles were compared between normal and low clopidogrel responders.

RESULTS

Twenty-seven patients (56%) were normal responders and 21 (44%) low responders. Baseline GP IIb/IIIa activation was higher in low responders (74.6+/-16.6% vs. 58.2+/-24.5%, p=0.03). Although GP IIb/IIIa activation reduced following clopidogrel front-loading in both groups, it remained increased among low responders at 24 h (58.6+/-21.3% vs. 40.2+/-28.7%, p=0.05) and during the overall study time course (p=0.02). There were no differences in P-selectin expression.

CONCLUSIONS

A considerable proportion of patients have an early suboptimal response to a 300-mg clopidogrel loading dose. An increased GP IIb/IIIa activation before intervention may identify this group of patients suggesting the use of a more aggressive antithrombotic treatment in these individuals.

Does pre-treatment with aspirin and loading dose clopidogrel obviate the need for glycoprotein IIb/IIIa antagonists during elective coronary stenting? A focus on peri-procedural myonecrosis†

AIMS

Although full platelet inhibition with aspirin and thienopyridines before coronary stenting has significantly reduced the risk of acute stent thrombosis, peri-procedural myonecrosis still occurs frequently and is associated with increased death rate. Whether further inhibition of platelet aggregation by a glycoprotein IIb/IIIa antagonist may provide an additional cardioprotection is unknown.

METHODS AND RESULTS

A total of 200 patients pre-treated with aspirin and a loading dose of clopidogrel (450 mg) were randomized just before coronary intervention (percutaneous coronary intervention, PCI) to treatment with or without abciximab. Platelet aggregation was assessed in samples collected during the procedure and the degree of platelet aggregation inhibition was correlated with cardiac enzyme release post-PCI. Abciximab treatment achieved a more complete inhibition of aggregation than dual oral antiplatelet therapy alone (median value of 1 vs. 50%, normal 100%). Any pathological increase in creatinine kinase-MB (CK-MB) post-PCI was present in 21% of the abciximab group and in 22% of the no-abciximab group (P = 0.9). Also the occurrence of clinically relevant myonecrosis [myocardial infarction (MI) = CK-MB > 3x upper limit of normal] was not significantly influenced by treatment assignment: 9 vs. 10% (P = 0.9). In a multiple logistic regression model including clinical, angiographic, and procedural characteristics, post-PCI myonecrosis was not correlated with the degree of platelet aggregation inhibition but with procedural features (such as long inflation time) and with the presence of multi-vessel disease. There were no cases of acute or subacute stent thrombosis. At 6 months, major adverse cardiac events, including cardiac death, non-fatal MI, or target lesion revascularization occurred in 13% of abciximab patients and in 16% of the control patients (P = 0.6).

CONCLUSIONS

In the studied patients scheduled for elective coronary stenting and pre-treated with aspirin and a loading dose of clopidogrel, further inhibition of platelet aggregation by abciximab does not afford additional cardioprotection. Our data suggest that distal athero-embolization rather than thrombo-embolization is involved in the phenomenon of myonecrosis post-elective stenting.

Effects of cilostazol on platelet activation in coronary stenting patients who already treated with aspirin and clopidogrel

BACKGROUND

A recent study has shown that triple anti-platelet therapy (cilostazol+clopidogrel+aspirin) resulted in a significantly lower restenosis rate after coronary stenting than did conventional therapy (clopidogrel+aspirin). However, the anti-platelet effects of cilostazol, when combined with clopidogrel and aspirin, have not been evaluated.

METHODS

Low dose cilostazol (50 mg/BID) was given to 47 patients who had already been taking clopidogrel (75 mg/day) and aspirin (100 mg/day) for more than 1 month subsequent to coronary stenting due to AMI and unstable angina. Markers of platelet activation, P-selectin and activated GPIIb/IIIa on platelets, were measured at baseline and 2 weeks after cilostazol treatment. We empirically divided patients into tertiles (low, n =16; moderate, n = 14; high group, n = 17), according to the baseline P-selectin expression. We then performed a comparative assessment of the anti-platelet effects of cilostazol at baseline and after 2 weeks of cilosatzol administration.

RESULTS

P-selectin was significantly decreased after 2 weeks of cilostazol treatment in total patients (n = 47, 3.2 +/- 2.4% to 2.0 +/- 1.9%, p = 0.03). This inhibition of P-selectin expression was mainly achieved in the moderate and high P-selectin groups (low group; 1.4 +/- 0.5 to 1.9 +/- 1.3%, p > 0.05, moderate group; 2.5 +/- 0.3 to 1.3 +/- 0.3%, p < 0.05, high group; 5.4 +/- 2.7 to 2.7 +/- 2.8%, p < 0.05). Activated GPIIb/IIIa was not significantly changed (13.5% to 17.6%, p > 0.05). Underying disease, cardiovascular risk factors, concomitant medication including statin, and hsCRP were not related to the degree of P-selectin expression.

CONCLUSION

Our data demonstrated that cilostazol treatment in addition to conventional anti-platelet therapy provides more effective suppression of platelet P-selectin expression in patients with relatively high platelet activity.

Antiplatelet and Antithrombin Therapy for Early Management of Acute Coronary Syndromes

Acute coronary syndromes (ACS) are defined as either unstable angina (UA), non-ST-segment elevation myocardial infarction (NSTEMI), or ST-segment elevation myocardial infarction (STEMI). Their management is continuously evolving in terms of pharmacologic therapy. The usual cause of ACS is the disruption of an atherosclerotic plaque leading to formation of a thrombus within a coronary artery. Initial antiplatelet and antithrombin therapy for patients with ST-segment elevation (STE) ACS includes aspirin and unfractionated heparin (UFH). Patients presenting to the hospital early may undergo percutaneous intervention (PCI) with administration of additional medications such as clopidogrel and abciximab or may receive a fibrinolytic agent. Patients with non-ST-segment elevation (NSTE) ACS receive aspirin, clopidogrel, either a low-molecular-weight heparin or UFH, and, in selected patients, a glycoprotein (GP) IIb/IIIa receptor blocker. All of these agents have shown benefit when administered to patients with ACS, but results vary depending on the type of syndrome, timing of administration, and patient’s overall risk. Their mechanism of action, role in ACS, clinical practice recommendations, adverse effects, and monitoring are reviewed. Some inherent limitations to agents such as aspirin and UFH require the need for newer treatment approaches. Therefore, newer classes of drugs such as the direct thrombin inhibitor bivalirudin and the selective factor Xa inhibitor fondaparinux are being explored as alternatives to heparins for ACS management.

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.

807 C/T Polymorphism of the glycoprotein Ia gene and pharmacogenetic modulation of platelet response to dual antiplatelet treatment

Glycoprotein (GP) Ia/IIa is a major platelet-collagen receptor playing a key role in thrombosis following collagen exposure. The 807 C/T polymorphism of the GP Ia gene (ITGA2) has been associated with platelet GP Ia/IIa receptor expression, having T-allele carriers, increased receptor density and thrombotic risk. The aim of the study was to assess the role of the 807 C/T polymorphism on modulating platelet function in patients undergoing coronary stenting receiving a 300 mg clopidogrel loading dose. Platelet aggregation was assessed in 44 patients by light transmittance aggregometry following adenosine diphosphate and collagen stimuli at baseline, and 10 min, 4 h and 24 h after clopidogrel front loading. The T allele was found in 73% of patients. Clopidogrel reduced adenosine diphosphate-induced platelet aggregation (P < 0.01), which was similar in carriers and non-carriers of the T allele throughout the study (P = 0.73). Clopidogrel reduced collagen-induced platelet aggregation only in non-carriers of the T allele (P = 0.03), which resulted in an increase in T allele carriers during the overall study (P = 0.04). In conclusion, the T allele of the GP Ia gene modulates platelet aggregation and clopidogrel antiplatelet effects, suggesting an enhanced reactivity to fibrillar collagens (exposed during coronary stenting) in T allele carriers and might contribute to an increased thrombotic risk in these patients.

Platelet ADP receptors contribute to the initiation of intravascular coagulation

While the adenosine 5'-diphosphate (ADP) pathway is known to enhance thrombus formation by recruiting platelets and leukocytes to the primary layer of collagen-adhering platelets, its role for the initiation of coagulation has not been revealed. Ex vivo inhibition of the P2Y12 ADP receptor by clopidogrel administration diminished the rapid exposure of tissue factor (TF), the major initiator of coagulation, in conjugates of platelets with leukocytes established by the contact of whole blood with fibrillar collagen. Under in vitro conditions, the P2Y12 and P2Y1 ADP receptors were both found to be implicated in the exposure of TF in collagen-activated whole blood. Immunoelectron-microscopy revealed that collagen elicited the release of TF from its storage pools within the platelets. Functional activation of the intravascular TF was reduced by inhibition of the ADP receptors, partially due to the disruption of the platelet-neutrophil adhesions. Injection of collagen into the venous system of mice increased the number of thrombin-antithrombin complexes, indicative for the formation of thrombin in vivo. In P2Y1-deficient mice, the ability of collagen to enhance the generation of thrombin was impaired. In conclusion, the platelet ADP pathway supports the initiation of intravascular coagulation, which is likely to contribute to the concomitant formation of fibrin at the site of the growing thrombus.

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.

PlA polymorphism and platelet reactivity following clopidogrel loading dose in patients undergoing coronary stent implantation

The PlA polymorphism (Leu33Pro) of the platelet glycoprotein (GP) IIIa gene has been suggested to play an important role in coronary thrombosis. In vitro studies have shown differences for this polymorphism in platelet sensitivity towards antiplatelet drugs (aspirin and abciximab), suggesting a pharmacogenetic modulation. The aim of the study was to assess the modulatory effect of the PlA polymorphism on clopidogrel-induced antiplatelet effects in 38 patients undergoing coronary stent implantation receiving a 300 mg clopidogrel loading-dose. Platelet reactivity was assessed as GPIIb/IIIa activation and P-selectin expression in platelets stimulated with 2 micromol/l adenosine diphosphate using whole blood flow cytometry. The distribution of the homozygous PlA1/A1 and heterozygous PlA1/A2 genotypes were 74 and 26%, respectively. PlA2 carriers had a higher degree of GPIIb/IIIa activation (P = 0.05) and P-selectin expression (P = 0.02) during the overall study time course and a lower antiplatelet effect to a 300 mg clopidogrel loading-dose up to 24 h following intervention (P < 0.05). In conclusion, the Pl polymorphism of the GPIIIa gene modulates platelet reactivity towards clopidogrel front loading in patients undergoing coronary stenting. This suggests the need for individualized antithrombotic regimens to optimally inhibit platelet reactivity.

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.

Triple antiplatelet therapy during percutaneous coronary intervention is associated with improved outcomes including one-year survival: results from the Do Tirofiban and ReoProGive Similar Efficacy Outcome Trial (TARGET)

OBJECTIVE

We sought to examine if clopidogrel treatment initiated before coronary stenting improved clinical outcomes among patients receiving aspirin and a glycoprotein (GP) IIb/IIIa inhibitor.

BACKGROUND

Antiplatelet therapy plays a pivotal role in contemporary percutaneous coronary interventions (PCI).

METHODS

Outcomes among 4,809 patients randomized to tirofiban or abciximab during PCI with stent placement were compared according to whether they received 300 mg of clopidogrel before PCI (93.1%) versus immediately after the procedure.

RESULTS

The 30-day primary composite end point (death, myocardial infarction [MI], or urgent target vessel revascularization [TVR]) was lower among clopidogrel-pretreated patients (6.6% vs. 10.4%, p = 0.009), mainly because of reduction of MI (6.0% vs. 9.5%, p = 0.012). The benefit of clopidogrel pretreatment was sustained at six months (death, MI, any TVR: 14.6% vs. 19.8%, HR = 0.71, p = 0.010), and this was due mainly to lowering of death and MI (7.8% vs. 13.0%, p = 0.001). At one year, clopidogrel pretreatment was associated with a lower mortality rate (1.7% vs. 3.6%, p = 0.011). Because clopidogrel pretreatment was not randomized, multivariable and propensity analyses were performed. After adjusting for baseline heterogeneity, clopidogrel pretreatment was an independent predictor for death or MI at 30 days (HR = 0.63, p = 0.012) and at six months (HR = 0.61, p = 0.003), and survival at one year (HR = 0.53, p = 0.044). No excess in 30-day bleeding events was noted with clopidogrel pretreatment.

CONCLUSIONS

Among patients undergoing coronary stent placement with aspirin and a GP IIb/IIIa inhibitor, clopidogrel pretreatment is associated with a reduction of death and MI irrespective of the type of GP IIb/IIIa inhibitor used.

Relationship of arachidonic acid concentration to cyclooxygenase-dependent human platelet aggregation

Inhibition of ex vivo arachidonic acid (AA)-induced aggregation is a biomarker for the isotype selectivity of cyclooxygenase (COX) inhibitors since platelets express COX-1 but not COX-2. At low concentrations, there is broad inter- and intrasubject variability in AA-induced aggregation of platelets ex vivo. This study defined a concentration that reliably induces aggregation without overcoming inhibition by therapeutic aspirin therapy (ASA, 81-mg) treatment. Logistic regression analysis of ex vivo aggregation, induced with increasing concentrations of AA in platelet-rich plasma (PRP), estimated that platelets from > or = 90% of subjects would aggregate at > or = 1.5 mM AA (95% confidence interval [CI], 1.1, 2.1). A concentration of 1.6 mM AA failed to aggregate platelets from 26 healthy volunteers, who had previously aggregated at this concentration, following six daily oral doses of 81 mg of ASA. These data demonstrate that 1.6 mM AA reproducibly induces platelet aggregation in PRP from healthy volunteers without overcoming the antiplatelet effect of daily low-dose aspirin therapy.

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.

Impact of angina class on inhibition of platelet aggregation following clopidogrel loading in patients undergoing coronary intervention: do we need more aggressive dosing regimens in unstable angina?

Pretreatment with thienopyridines has been shown to improve clinical outcomes in patients undergoing percutaneous coronary intervention (PCI). We determine the impact of angina class on inhibition of platelet aggregation (IPA) following clopidogrel loading. Seventy-two patients (mean age, 64 +/- 11 years; 76% male) were pretreated with 450 mg of clopidogrel at least 3 hr prior to PCI. All patients received ASA 325 mg prior to the procedure. Patients were classified into two groups according to angina class: group 1 = stable angina or Braunwald class 1 unstable angina (UA; n = 33); group 2 = Braunwald class 2 or 3 UA (n = 39). IPA was measured prior to PCI, with the Ichor point-of-care platelet analyzer (Helena Laboratories, Beaumont, TX), using 20 microM of ADP. Group 2 patients were more likely to have prior MI (54% vs. 27%; P = 0.023), prior CABG (33% vs. 5%; P = 0.046), and received IV heparin (64% vs. 27%; P = 0.0018). Mean IPA was significantly lower in group 2 compared to group 1 (19% +/- 22% vs. 32% +/- 22%; P = 0.004). In multivariate analysis, higher angina class was independently associated with lower IPA (P = 0.018). Patients with UA undergoing PCI have a lower IPA following clopidogrel loading with 450 mg. This may indicate the possibility of clopidogrel resistance in such patients.

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.