The role of aspirin resistance on outcome in patients with acute coronary syndrome and the effect of clopidogrel therapy in the prevention of major cardiovascular events

Anti-thrombotics and major adverse cardiovascular events in anti-phospholipid syndrome: a cross-sectional study using the 2016-2018 National Inpatient Sample database

OBJECTIVE

This study assessed the relationship between anti-thrombotics and major adverse cardiovascular events (MACE) in patients with anti-phospholipid syndrome (APS).

METHOD

We included 13 947 subjects with APS from the National (Nationwide) Inpatient Sample (NIS) database for 2016-2018, and collected relevant covariates and demographic data using ICD-10 codes. Our two primary outcomes were MACE and death. We performed multivariate logistic regression analysis to assess the impact of various anti-thrombotic regimens on MACE/death in our primary cohort and high-risk subgroups.

RESULTS

Patients on anti-coagulants had significantly reduced odds of MACE [odds ratio (OR) 0.68, 95% confidence interval (CI) 0.62-0.76, p < 0.001] as well as each of its subcomponents. Those not on any anti-coagulants had significantly increased odds of MACE (OR 1.47, 95% CI 1.24-1.72, p < 0.001). No significant association was found between anti-platelet use and the odds of MACE (p > 0.05). Patients on anti-coagulants were the only class that appeared to have a mortality benefit with reduced odds for death (OR 0.64, 95% CI 0.49-0.84, p = 0.001). In the subgroups at higher risk for MACE (those with atrial fibrillation and thrombocytopenia), full anti-coagulation therapy was also the only anti-thrombotic class that significantly affected the odds of MACE, with a protective effect on MACE, but had no mortality benefit.

CONCLUSION

Patients with APS are most likely to benefit from anti-coagulant therapy in reducing MACE. Furthermore, anti-platelets alone or in combination with anti-coagulants are probably not beneficial in MACE reduction and may even increase risk.

PFA-100-measured aspirin resistance is the predominant risk factor for hospitalized cardiovascular events in aspirin-treated patients: A 5-year cohort study

WHAT IS KNOWN AND OBJECTIVE

Aspirin therapy is the clinical gold standard for the prevention of cardiovascular events. However, cardiovascular events still develop in some patients undergoing aspirin therapy. Many laboratory methods exist for measuring aspirin resistance. Using the platelet Function Analyzer (PFA)-100 system, we aimed to determine the effect of aspirin resistance on hospitalized cardiovascular events (hCVE) in a 5-year follow-up cohort. We also sought to determine the impact of aspirin resistance on the relationship between common cardiovascular risk factors and cardiovascular hospitalization.

METHOD

Aspirin resistance was evaluated in aspirin-treated patients from the outpatient department. A total of 465 patients during a 5-year follow-up period were included in this study. The primary endpoint of the study was hospitalization for any acute cardiovascular event. The prevalence and associated risk factors of acute cardiovascular events were evaluated.

RESULTS AND DISCUSSION

Aspirin resistance was prevalent in 91 (20.0%) of 465 patients. Prior hospitalization history of cardiovascular events was highly associated with aspirin resistance (P = .001). At the 5-year follow-up, cardiovascular events were found to have developed in 11 patients (8 stroke and 3 myocardial infarction) who exhibited aspirin resistance (12.1%) and in 9 (4 stroke and 5 myocardial infarction) patients who did not exhibit aspirin resistance (2.4%) (P < .001). At the 5-year follow-up, multivariate logistic regression analysis results showed a strong association between aspirin resistance and cardiovascular events (adjusted odds ratio 4.28; 95% CI: 1.64-11.20; P = .03).

WHAT IS NEW AND CONCLUSION

PFA-100 measurements of aspirin resistance correlate with hCVE, as evidenced by both the past medical history and the 5-year follow-up. The logistic regression analysis results showed that aspirin resistance plays a larger role in hospitalized cardiovascular disease than do other cardiovascular risk factors.

Antiplatelet Therapy in Acute Coronary Syndrome

Antiplatelet agents have for decades been used to improve outcomes in patients with acute coronary syndromes and have become increasingly valued, not only for their antithrombotic properties but also for their anti-inflammatory effects. The drug class continues to evolve as novel agents with increasingly efficacious antiplatelet actions are identified. This review will discuss antiplatelet agents, including aspirin, the P2Y₁₂ receptor antagonists and the glycoprotein IIb/IIIa inhibitors, that are currently used to treat patients with unstable angina and myocardial infarction, focusing on their pharmacological properties and the clinical evidence supporting their use.

The prognostic utility of tests of platelet function for the detection of 'aspirin resistance' in patients with established cardiovascular or cerebrovascular disease: a systematic review and economic evaluation

BACKGROUND

The use of aspirin is well established for secondary prevention of cardiovascular disease. However, a proportion of patients suffer repeat cardiovascular events despite being prescribed aspirin treatment. It is uncertain whether or not this is due to an inherent inability of aspirin to sufficiently modify platelet activity. This report aims to investigate whether or not insufficient platelet function inhibition by aspirin ('aspirin resistance'), as defined using platelet function tests (PFTs), is linked to the occurrence of adverse clinical outcomes, and further, whether or not patients at risk of future adverse clinical events can be identified through PFTs.

OBJECTIVES

To review systematically the clinical effectiveness and cost-effectiveness evidence regarding the association between PFT designation of 'aspirin resistance' and the risk of adverse clinical outcome(s) in patients prescribed aspirin therapy. To undertake exploratory model-based cost-effectiveness analysis on the use of PFTs.

DATA SOURCES

Bibliographic databases (e.g. MEDLINE from inception and EMBASE from 1980), conference proceedings and ongoing trial registries up to April 2012.

METHODS

Standard systematic review methods were used for identifying clinical and cost studies. A risk-of-bias assessment tool was adapted from checklists for prognostic and diagnostic studies. (Un)adjusted odds and hazard ratios for the association between 'aspirin resistance', for different PFTs, and clinical outcomes are presented; however, heterogeneity between studies precluded pooling of results. A speculative economic model of a PFT and change of therapy strategy was developed.

RESULTS

One hundred and eight relevant studies using a variety of PFTs, 58 in patients on aspirin monotherapy, were analysed in detail. Results indicated that some PFTs may have some prognostic utility, i.e. a trend for more clinical events to be associated with groups classified as 'aspirin resistant'. Methodological and clinical heterogeneity prevented a quantitative summary of prognostic effect. Study-level effect sizes were generally small and absolute outcome risk was not substantially different between 'aspirin resistant' and 'aspirin sensitive' designations. No studies on the cost-effectiveness of PFTs for 'aspirin resistance' were identified. Based on assumptions of PFTs being able to accurately identify patients at high risk of clinical events and such patients benefiting from treatment modification, the economic model found that a test-treat strategy was likely to be cost-effective. However, neither assumption is currently evidence based.

LIMITATIONS

Poor or incomplete reporting of studies suggests a potentially large volume of inaccessible data. Analyses were confined to studies on patients prescribed aspirin as sole antiplatelet therapy at the time of PFT. Clinical and methodological heterogeneity across studies precluded meta-analysis. Given the lack of robust data the economic modelling was speculative.

CONCLUSIONS

Although evidence indicates that some PFTs may have some prognostic value, methodological and clinical heterogeneity between studies and different approaches to analyses create confusion and inconsistency in prognostic results, and prevented a quantitative summary of their prognostic effect. Protocol-driven and adequately powered primary studies are needed, using standardised methods of measurements to evaluate the prognostic ability of each test in the same population(s), and ideally presenting individual patient data. For any PFT to inform individual risk prediction, it will likely need to be considered in combination with other prognostic factors, within a prognostic model.

STUDY REGISTRATION

This study is registered as PROSPERO 2012:CRD42012002151.

FUNDING

The National Institute for Health Research Health Technology Assessment programme.

Clinical Significance of Laboratory-determined Aspirin Poor Responsiveness After Primary Percutaneous Coronary Intervention

AIMS

The objective of the present substudy was to examine whether aspirin poor/high responsiveness (APR/AHR) is associated with increased rates of major adverse cardiovascular events (MACE) and serious bleeding after primary percutaneous coronary intervention (PPCI).

METHODS

We analyzed 961 consecutive ST-elevation acute myocardial infarction patients who underwent PPCI between February 2008 and June 2011. Multiplate analyser (Dynabite, Munich, Germany) was used for the assessment of platelet reactivity. APR/AHR were defined as the upper/lower quintiles of ASPI values, determined 24 h after aspirin loading. APR patients were tailored using 300 mg maintenance dose for 30 days. The co-primary end points at 30 days were: MACE (death, non-fatal infarction, ischemia-driven target vessel revascularization and ischemic stroke) and serious bleeding according to the BARC classification.

RESULTS

One hundred and 90 patients were classified as APR, and 193 patients as AHR. At admission, compared with aspirin sensitive patients (ASP), patients with APR had more frequently diabetes, anterior infarction and heart failure, while AHR patients had reduced values of creatine kinase, leukocytes, heart rate and systolic blood pressure. Compared with ASP, the rates of 30-day primary end points did not differ neither in APR group including tailored patients (MACE, adjusted OR 1.02, 95%CI 0.47-2.17; serious bleeding, adjusted OR 1.92, 95%CI 0.79-4.63), nor in patients with AHR (MACE, adjusted OR 1.58, 95%CI 0.71-5.51; serious bleeding, adjusted OR 0.69, 95%CI 0.22-2.12).

CONCLUSIONS

The majority of APR patients were suitable for tailoring. Neither APR including tailored patients nor AHR were associated with adverse 30-day efficacy or safety clinical outcomes.

Circulating primers enhance platelet function and induce resistance to antiplatelet therapy

BACKGROUND

Aspirin and P2Y12 antagonists are antiplatelet compounds that are used clinically in patients with thrombosis. However, some patients are 'resistant' to antiplatelet therapy, which increases their risk of developing acute coronary syndromes. These patients often present with an underlying condition that is associated with altered levels of circulating platelet primers and platelet hyperactivity. Platelet primers cannot stimulate platelet activation, but, in combination with physiologic stimuli, significantly enhance platelet function.

OBJECTIVES

To explore the role of platelet primers in resistance to antiplatelet therapy, and to evaluate whether phosphoinositide 3-kinase (PI3K) contributes to this process.

METHODS AND RESULTS

We used platelet aggregation, thromboxane A2 production and ex vivo thrombus formation as functional readouts of platelet activity. Platelets were treated with the potent P2Y12 inhibitor AR-C66096, aspirin, or a combination of both, in the presence or absence of the platelet primers insulin-like growth factor-1 (IGF-1) and thrombopoietin (TPO), or the Gz-coupled receptor ligand epinephrine. We found that platelet primers largely overcame the inhibitory effects of antiplatelet compounds on platelet functional responses. IGF-1-mediated and TPO-mediated, but not epinephrine-mediated, enhancements in the presence of antiplatelet drugs were blocked by the PI3K inhibitors wortmannin and LY294002.

CONCLUSIONS

These results demonstrate that platelet primers can contribute to antiplatelet resistance. Furthermore, our data demonstrate that there are PI3K-dependent and PI3K-independent mechanisms driving primer-mediated resistance to antiplatelet therapy.

Methodological issues and recommendations for systematic reviews of prognostic studies: an example from cardiovascular disease

BACKGROUND

Prognostic factors are associated with the risk of future health outcomes in individuals with a particular health condition. The prognostic ability of such factors is increasingly being assessed in both primary research and systematic reviews. Systematic review methodology in this area is continuing to evolve, reflected in variable approaches to key methodological aspects. The aim of this article was to (i) explore and compare the methodology of systematic reviews of prognostic factors undertaken for the same clinical question, (ii) to discuss implications for review findings, and (iii) to present recommendations on what might be considered to be 'good practice' approaches.

METHODS

The sample was comprised of eight systematic reviews addressing the same clinical question, namely whether 'aspirin resistance' (a potential prognostic factor) has prognostic utility relative to future vascular events in patients on aspirin therapy for secondary prevention. A detailed comparison of methods around study identification, study selection, quality assessment, approaches to analysis, and reporting of findings was undertaken and the implications discussed. These were summarised into key considerations that may be transferable to future systematic reviews of prognostic factors.

RESULTS

Across systematic reviews addressing the same clinical question, there were considerable differences in the numbers of studies identified and overlap between included studies, which could only partially be explained by different study eligibility criteria. Incomplete reporting and differences in terminology within primary studies hampered study identification and selection process across reviews. Quality assessment was highly variable and only one systematic review considered a checklist for studies of prognostic questions. There was inconsistency between reviews in approaches towards analysis, synthesis, addressing heterogeneity and reporting of results.

CONCLUSIONS

Different methodological approaches may ultimately affect the findings and interpretation of systematic reviews of prognostic research, with implications for clinical decision-making.

Evaluation of Aggregometery Parameters and Efficacy of Plavix versus Clopidex in Patients Suffering from Ischemic Heart Disease: A Randomized Double Blind Clinical Trial

BACKGROUND

Ischemic heart disease is the leading cause of death in most societies. In a pathophysiologic point of view, it chiefly results from the formation of thrombus in coronary arteries which could not be only prevented by aspirin. Many of clinical trials have shown the long-term benefits of antiplatelet drugs in reducing the risk of thrombotic accidents.

OBJECTIVES

Clopidogrel is a thienopyridine derivative used to prevent platelets from adhering together by direct inhibition of Adenosine diphosphate (ADP), the major factor behind platelets aggregation. Sanofi-Aventis and Bristol-Myers are companies that produce Clopidogrel by the name of Clopidogrel bisulfate. Its trade name is Plavix, nonetheless in Iran it is distributed under the name of Clopidex by Exir Company. In this study we are to compare Plavix and Clopidex in terms of efficacy as well as aggregometry parameters like ADP and PRP (Platelet Rich Plasma).

PATIENTS AND METHODS

This is a double blind clinical trial in which we had two groups of patients suffering from Ischemic heart disease who were selected by inclusion criteria. Group A (36 patients) took Plavix (75 mg/d) and group B (36 patients) used clopidex (75 mg/d) both for 30 days. The aggregometry parameters also consisted of PRP and ADP that were run on the patients before and after the study. Finally, a comparison of aforementioned tests, quality of life, lab parameters and compliance in both groups was provided.

RESULTS

In groups A and B, the mean levels of PRP before the study were 348000 and 340000/µL respectively. The ADPs were also 73/76 and 68/07 µM that showed no significant difference (P > 0.05).The Means of ADP5 in group A before and after the study were 66.40 and 43.84 µM respectively that there was significant difference (P = 0.001). The Means of ADP5 in group B before and after the study were 58.04 and 40.16 µM respectively that there was significant difference (P < 0.001).The Means of ADP20 in group A before and after the study were 73.76 and 54.97 µM respectively which showed significant difference (P < 0.001). The Means of ADP20 in group B before and after the study were 68.07 and 52.49 µM respectively which showed significant difference (P = 0.001). Difference of ADP5 between group A and B was not significant (P = 0.495). Difference of ADP20 between group A and B was not significant (P = 0.721). The Means of PRP in group A before and after the study were 348000 and 335000/ µL respectively that there was no significant difference (P = 0.66). The Means of PRP in group B before and after the study were 340000 and 336000/ µL respectively that indicated no significant difference (P = 0.81). Difference of PRP between group A and B was not significant (P = 0.563).

CONCLUSIONS

Our findings suggested that both drugs significantly lessen the ADP level; even so there was no significant difference between two groups in PRP and ADP factors.

Monitoring the effectiveness of antiplatelet therapy: opportunities and limitations

Previous clinical studies have shown heterogeneity in individual patient responses to antiplatelet therapy and high residual platelet reactivity is associated with increased risk of adverse clinical events. Monitoring response to antiplatelet therapy and tailoring treatment accordingly is currently not recommended in routine clinical practice largely due to the lack of a standardized definition of antiplatelet therapy hyporesponse and the need for a widely accepted point-of-care platelet function test that can be reliably utilized in frontline clinical practice. Recent data have shown that titrating the dose of clopidogrel in patients undergoing percutaneous coronary intervention significantly reduces the incidence of major adverse cardiovascular events and large-scale clinical trials are currently underway to investigate whether individually tailored treatment based on results of platelet function testing leads to improved clinical outcome. Furthermore, genetic testing has demonstrated a link between CYP2C19 genetic polymorphisms, altered clopidogrel metabolite concentrations and adverse clinical events. Clinical studies are currently underway to investigate the potential clinical benefit associated with genotype-guided tailoring of antiplatelet therapy. With the advent of newer, more potent antiplatelet agents and their associated increased bleeding risks, it will become imperative in the future to select the most appropriate, safe and effective drug.

Clinical and prognostic implications of the initial response to aspirin in patients with acute coronary syndrome

Increased platelet reactivity and decreased response to antiplatelet drugs may result in recurrent ischemic events after acute coronary syndrome (ACS). We evaluated laboratory response to aspirin in patients with ACS before and after percutaneous coronary intervention (PCI) and assessed its effect on major adverse clinical events. Sixty-three consecutive patients with ACS were tested for response to aspirin by light transmittance aggregometry (LTA) and the IMPACT-R test (with arachidonic acid) before and 2 to 4 days after PCI and clopidogrel loading. Patients were followed for clinical events up to 15 months from PCI. Response to aspirin improved significantly after PCI and clopidogrel treatment (mean arachidonic acid-induced LTA decreased from 34.9 ± 3.35% before PCI to 15.2 ± 2.2% and surface coverage increased from 2.2 ± 0.27% to 6.2 ± 0.6%, p <0.0001 for the 2 methods). Improved response to aspirin after PCI correlated with response to clopidogrel (LTA and IMPACT-R, p <0.01). Patients with good laboratory response to aspirin before but not after PCI had a significantly lower major cardiovascular event rate during 15-month follow-up in multivariate analysis. In conclusion, laboratory response to aspirin is highly dynamic in patients with ACS. Improved response to aspirin after PCI may result from stabilization of coronary artery disease and/or clopidogrel treatment. Laboratory response to aspirin before PCI and clopidogrel loading is a sensitive marker for platelet reactivity that correlates with clinical outcome in patients with ACS.

Antiplatelet drug therapy: role of pharmacodynamic and genetic testing

Antiplatelet therapy represents the cornerstone of treatment for the short- and long-term prevention of atherothrombotic disease processes, in particular in high-risk settings such as in patients with acute coronary syndrome and those undergoing percutaneous coronary intervention. Currently, dual antiplatelet therapy with aspirin and clopidogrel represents the most commonly used treatment regimen in these settings. However, a considerable number of patients continue to experience adverse outcomes, including both bleeding and recurrent ischemic events. Numerous investigations have demonstrated that this phenomenon may be, in part, attributed to the broad variability in individual response profiles to this standard antiplatelet treatment regimen, as identified by various assays of platelet function testing. In addition, recent studies have demonstrated that genetic polymorphisms may also have an important role in determining levels of platelet inhibition and may be considered as a tool to identify patients at risk of adverse events. This article provides an overview on antiplatelet drug response variability, an update on definitions, including the role of pharmacodynamic testing, underlying mechanisms – with emphasis on recent understandings on pharmacogenetics and drug–drug interactions – and current and future perspectives on individualized antiplatelet therapy.

[Antiplatelet agents in acute coronary syndromes. Data from the main clinical trials]

Cardiovascular disease is the leading cause of early death and morbidity in developed countries and is becoming a growing problem in many developing countries. Currently, anti-coagulants play a major role in the management of the acute phase in association or not with reperfusion strategies. The combination of clopidogrel and aspirin, in accordance with the results of large randomized trials, is the treatment of reference. However two new drugs, a thienopyridine (prasugrel), and a cyclo-pentyl-triazolo-pyridine (ticagrelor) have shown their interest in major studies in comparison with clopidogrel. As a result, several effective P2Y12 receptor antagonist anti-platelet agents are now available in the therapeutic arsenal, and the cardiologist have to tailored the best drug scheme according the balance between the risk of hemorrhage and the benefits with regard to the ischemia to determine the optimal prescription for every patient.

Dose-related effect of aspirin on laboratory-defined platelet aggregation and clinical outcome after coronary stenting

BACKGROUND

Acetylsalicylic acid (aspirin) is widely used in the secondary prevention of coronary artery disease. There is controversy regarding the prevalence of aspirin resistance in patients with coronary artery disease and the effect of dose on resistance. Our primary aims were to determine the degree of platelet responsiveness to aspirin, and to study the influence of dose on platelet inhibition and clinical outcomes after coronary stenting.

METHODS AND RESULTS

We prospectively studied the effect of aspirin on platelet function in 106 stable outpatients 6 months after successful percutaneous coronary angioplasty. Participants were randomized in a double-blind, double-crossover study (80 or 500 mg per day for 6 months). The platelet response to aspirin was determined by 10 micromol/L adenosine-5-diphosphate-induced aggregation with light transmission aggregometry. The clinical outcome was determined by single photon emission computed tomography with Tc-99m, and major adverse cardiac events were recorded (myocardial infarction, death, unstable angina or need for revascularization). In both groups 30.2% of the participants were resistant to aspirin. There was no significant difference between the dose of 80 mg compared to 500 mg aspirin in the incidence of aspirin resistance (P= 0.3). No correlation was found between aspirin resistance and clinical outcome (P= 0.4). Female sex and smoking were strongly associated with aspirin resistance.

CONCLUSION

The frequency of aspirin resistance is not dependent on the dose of aspirin. Female sex and smoking were the strongest predictors of aspirin resistance. Aspirin resistance is not a predictor of poor clinical outcome in patients who received double antiplatelet therapy.

Significance of mean platelet volume on prognosis of patients with and without aspirin resistance in settings of non-ST-segment elevated acute coronary syndromes

Platelet volume is a marker of platelet function and activation. An elevated mean platelet volume (MPV) is associated with acute coronary syndromes (ACS). Recurrent cardiovascular events were found to be higher in patients with aspirin resistance. In this study, we investigated the effect of MPV on prognosis of patients with and without aspirin resistance by PFA-100 in settings of non-ST-segment elevated ACS. Two hundred and twenty patients with ACS were followed for an average of 14.86 +/- 5.93 months for the occurrence of death, myocardial infarction (MI) and revascularization. Aspirin effect on platelet function was assessed by PFA-100. According to MPV value and aspirin resistance status, patients were divided into four groups. Group 4 (with an elevated MPV and aspirin resistance) was significantly associated with worse prognosis for composite endpoint (death, MI and revascularization), death and MI (for all, log-rank P < 0.0001). Multivariate analysis showed that presence of an elevated MPV and aspirin resistance was an independent predictor of composite endpoint [hazard ratio 8.21, 95% confidence interval (CI) 3.48-19.35, P < 0.0001], death (hazard ratio 5.48, 95% CI 1.62-18.53, P = 0.006) and MI (hazard ratio 4.44, 95% CI 1.57-12.58, P = 0.005). Presence of an elevated MPV and aspirin resistance was significantly associated with death, MI and the composite endpoint, due to the lack of beneficial effect of aspirin on activated platelets. Patients with ACS, especially in the presence of an elevated MPV may benefit from the evaluation of aspirin resistance for risk stratification.

Clinical importance of aspirin and clopidogrel resistance

Aspirin and clopidogrel are important components of medical therapy for patients with acute coronary syndromes, for those who received coronary artery stents and in the secondary prevention of ischaemic stroke. Despite their use, a significant number of patients experience recurrent adverse ischaemic events. Interindividual variability of platelet aggregation in response to these antiplatelet agents may be an explanation for some of these recurrent events, and small trials have linked "aspirin and/or clopidogrel resistance", as measured by platelet function tests, to adverse events. We systematically reviewed all available evidence on the prevalence of aspirin/clopidogrel resistance, their possible risk factors and their association with clinical outcomes. We also identified articles showing possible treatments. After analyzing the data on different laboratory methods, we found that aspirin/clopidogrel resistance seems to be associated with poor clinical outcomes and there is currently no standardized or widely accepted definition of clopidogrel resistance. Therefore, we conclude that specific treatment recommendations are not established for patients who exhibit high platelet reactivity during aspirin/clopidogrel therapy or who have poor platelet inhibition by clopidogrel.

Aspirin resistance determined from a bed-side test in patients suspected to have acute coronary syndrome portends a worse 6 months outcome

BACKGROUND

Coronary patients resistant to aspirin may have increased risk for ischemic events. Little data were available for patients presenting acutely with chest pain.

METHODS AND RESULTS

We used the VerifyNow Aspirin to determine aspirin responsiveness of 314 patients regularly taking aspirin 75-300 mg daily for >or=4 weeks who presented with suspected acute coronary syndrome in Emergency Department. Aspirin resistance was defined as an aspirin reaction unit (ARU) >or=550, and the clinical team was blinded to the ARU reading. The pre-specified study endpoints were the diagnosis of acute myocardial infarction (AMI) for the index admission and major adverse cardiac events including cardiovascular death or recurrent acute coronary syndrome requiring hospitalization within 6 months. Aspirin resistance was noted in 30 (9.6%) patients. There was no difference in the diagnosis of AMI for the index presentation (3/30, 10% vs. 25/284, 8.8%, P = 0.91). Among the 312 hospital survivors, aspirin resistant patients had increased adverse events over 6 months with an overall hazard ratio of 10.0 [95% confidence interval (CI) 4.6-22.0]. After adjusted for elevated Troponin-T, the only confounder in the model, the hazard ratio was 11.1 (95% CI 4.7-26.0). Results were similar in patients treated only medically without revascularization (adjusted hazard ratio 12.1, 95% CI 4.7-26.4). The increased events were observed both from discharge to 30 days and from 30 days to 6 months.

CONCLUSION

Aspirin resistance occurs in approximately 10% of patients presenting with suspected acute coronary syndrome and is associated with adverse cardiac events.

The clinical importance of laboratory-defined aspirin resistance in patients presenting with non-ST elevation acute coronary syndromes

In this study, we aimed to assess the factors associated with laboratory-defined aspirin resistance and the relationship of this laboratory-defined aspirin resistance with thrombolysis in myocardial infarction risk score, markers of cardiac necrosis, and inflammatory and thrombotic risk factors in patients with unstable angina or non-ST elevation myocardial infarction. Ninety-seven patients who were under aspirin therapy and hospitalized with unstable angina/non-ST elevation myocardial infarction were included in the study. Laboratory-defined aspirin sensitive and resistant groups were determined by platelet function analyzer; aspirin resistance was defined as collagen/epinephrine closure time less than 165 s. Laboratory-defined aspirin resistance was noted in 29 patients (29.9%), and non-ST elevation myocardial infarction was observed in 46 patients (47.4%). Patients in the group with laboratory-defined aspirin resistance had significantly higher thrombolysis in myocardial infarction risk scores (P < 0.001). When the details of cardiac myonecrosis markers were compared, baseline and follow-up creatine kinase-myocardial band and troponin I values were higher in laboratory-defined aspirin-resistant group. Multivariate analyses revealed that laboratory-defined aspirin resistance was an independent predictor of non-ST elevation myocardial infarction (P = 0.022). Laboratory-defined aspirin resistance is associated with non-ST elevation myocardial infarction, higher markers of cardiac necrosis and thrombolysis in myocardial infarction risk score in patients hospitalized with unstable angina/non-ST elevation myocardial infarction.

Association of cyclooxygenase-1-dependent and -independent platelet function assays with adverse clinical outcomes in aspirin-treated patients presenting for cardiac catheterization

BACKGROUND

Poor clinical outcome in aspirin-treated patients has been termed aspirin resistance and may result from inadequate inhibition of platelet cyclooxygenase-1 (COX-1) by aspirin. The objectives of this study were to determine prospectively whether COX-1-dependent and other platelet function assays correlate with clinical outcomes in aspirin-treated patients.

METHODS AND RESULTS

Blood was collected before percutaneous coronary intervention from 700 consecutive aspirin-treated (81 or 325 mg for > or =3 days) patients. Platelet function was tested by (1) serum thromboxane B(2); (2) arachidonic acid-stimulated platelet surface P-selectin and activated glycoprotein IIb/IIIa and leukocyte-platelet aggregates; and (3) platelet function analyzer (PFA)-100 collagen-epinephrine and collagen-ADP closure time (CT). Adverse clinical outcomes of all-cause death, cardiovascular death, and major adverse cardiovascular events (cardiovascular death, myocardial infarction, hospitalization for revascularization, or acute coronary syndrome) were assessed by telephone interview and/or medical record review. Clinical outcomes information was obtained at 24.8+/-0.3 months after platelet function testing. By univariate analysis, COX-1-dependent assays, including serum thromboxane B(2) level, were not associated with adverse clinical outcomes, whereas the COX-1-independent assay, PFA-100 collagen-ADP CT <65 seconds, was associated with major adverse cardiovascular events (P=0.0149). After adjustment for covariables (including sex, aspirin dose, Thrombolysis in Myocardial Infarction risk score, clopidogrel use), both serum thromboxane B(2) >3.1 ng/mL and PFA-100 collagen-ADP CT <65 seconds were associated with major adverse cardiovascular events. In contrast, indirect measures of platelet COX-1 (arachidonic acid-stimulated platelet markers, shortened PFA-100 collagen-epinephrine CT) were not significantly associated with adverse clinical outcomes even after adjustment for covariables.

CONCLUSIONS

In this prospective study of 700 aspirin-treated patients presenting for angiographic evaluation of coronary artery disease, residual platelet COX-1 function measured by serum thromboxane B(2) and COX-1-independent platelet function measured by PFA-100 collagen-ADP CT, but not indirect COX-1-dependent assays (arachidonic acid-stimulated platelet markers, shortened PFA-100 collagen-epinephrine CT), correlate with subsequent major adverse cardiovascular events. This study suggests that multiple mechanisms, including but not confined to inadequate inhibition of COX-1, are responsible for poor clinical outcomes in aspirin-treated patients, and therefore the term aspirin resistance is inappropriate.

Effect of exercise on platelet activation during aspirin or clopidogrel intake in healthy men

Sudden strenuous exercise increases the risk of ischemic cardiac events in patients with coronary artery disease. The exact mechanism behind this observation is unknown, but platelet activation induced by exercise may be of importance. We hypothesized that brief strenuous exercise would activate platelets in healthy men, assessed by the Platelet Function Analyzer 100 and light transmittance aggregometry. Nearly all participants exhibited increased platelet reactivity after exercise measured by the Platelet Function Analyzer 100, whereas only minor changes were detected by light transmittance aggregometry. A significant increase in plasma von Willebrand Factor was also found in response to exercise. In conclusion, platelet activation occurs during exercise in healthy individuals. This activation is not prevented by use of aspirin or clopidogrel, and may partly be explained by an increase in plasma von Willebrand Factor.

Antiplatelet Resistance—Does it Exist and how to Measure it?

Aspirin and clopidogrel are the most commonly used antiplatelet agents in patients with coronary artery disease. The existence of resistance to these agents has been a controversial issue and new drugs are being developed to overcome this problem. Laboratory tests, which can identify resistance and correlate this with clinical outcome, are being studied in order to identify patients at risk of future thrombotic events. We discuss the evidence for the existence of antiplatelet resistance—both in the laboratory and in the clinical setting. So far, platelet aggregometry has been considered the gold standard test, but is very operator dependant, time consuming, and has shown little correlation with other available tests of antiplatelet resistance. We discuss the available tests of platelet function, their limitations, and evidence for their use. A simple, rapid, near-patient test, which is affordable and useful in the clinical (not just laboratory) setting, could allow risk stratification of patients and individualization of antiplatelet medication to improve outcome.

Antiplatelet drug 'resistance'. Part 1: mechanisms and clinical measurements

Antiplatelet drug therapy has become one of the cornerstones of treatment for patients with cardiovascular disease. Large clinical trials have shown that antiplatelet medications have important clinical benefits and prevent adverse outcomes in patients with coronary artery disease. Recurrent adverse cardiovascular events still occur in a substantial proportion of patients on standard dual antiplatelet therapy, however, which has been attributed to nonresponsiveness to this treatment. Both pharmacological and pharmacokinetic mechanisms are involved in variability in responsiveness to antiplatelet agents, and include drug bioavailability, medication noncompliance, drug-drug interactions, cytochrome P450 activity, and genetic polymorphisms. Numerous observational studies have consistently shown an association between antiplatelet drug nonresponsiveness and adverse clinical outcomes. However, these studies are limited by varying antiplatelet drug dosing regimens, heterogeneous laboratory assessments for ex vivo platelet function, and wide interindividual variation in platelet responses. Only within the last 2 years have randomized clinical trials indicated that increased dosing with antiplatelet drugs could reduce adverse clinical outcomes. Nonetheless, large clinical trials with standardized laboratory methods and well-defined protocols are needed that will definitively determine the association between antiplatelet drug nonresponsiveness and clinical events, and establish therapeutic strategies to overcome blunted antiplatelet effects.

Aspirin resistance

The development of adverse cardiovascular events despite aspirin use has established an interest in a possible resistance to the drug. Several definitions have been set and various laboratory testing modalities are available. This has led to a wide range of prevalence reports in different clinical entities. The etiologic mechanism has been related to clinical, genetic, and other miscellaneous factors. The clinical implications of this phenomenon are significant and warrant concern. Management strategies are currently limited to dosing alteration and introduction of other anitplatelet agents. However, these measures have not met the expected efficacy or safety.

Responsiveness to aspirin in patients with unstable angina pectoris by whole blood aggregometry

AIMS

To evaluate aspirin responsiveness in patients with unstable angina pectoris (UAP) by whole blood aggregometry. Another goal was to differentiate aspirin-resistant patients into pharmacokinetic or pharmacodynamic type.

METHODS

We measured platelet aggregation by determining impedance values in 70 normal volunteers and 104 UAP patients on aspirin (100 mg/day > or = 7 days) in four inducing conditions [1 microg/ml collagen, 2 microg/ml collagen, 5 micromol/l adenosine diphosphate (ADP) and 10 micromol/l ADP]. We calculated a cut-off value based on data from normal volunteers to define aspirin responsiveness in cases. Then, the correlation and agreement between the results in the four conditions was analysed to choose a preferred inducing condition for identification of aspirin resistance. Aliquots from all samples were incubated with 0.1 mmol/l aspirin and measured again for aspirin-resistant classification.

RESULTS

Aspirin resistance was observed in 38 patients (36.5%), 51 patients (49.0%), 67 patients (64.4%) and 67 patients (64.4%), respectively, for 1 microg/ml collagen, 2 microg/ml collagen, 5 micromol/l ADP and 10 micromol/l ADP among 104 patients. Collagen at low concentration was suggested as a preferred agent for detecting aspirin inhibitory effect according to the coefficient of sensitivity. After incubation, only three among 38 aspirin-resistant patients showed normal platelet aggregation and were classified into pharmacodynamic type.

CONCLUSIONS

In the presence of collagen at low concentration (1 microg/ml), the prevalence of aspirin resistance is about 36.5% in UAP patients, and according to a classification specific for resistant patients, most of the aspirin 'resistance' is just because of pharmacokinetic issues.

Aspirin resistance: focus on clinical endpoints

INTRODUCTION

Via its antiplatelet effect, aspirin reduces the odds of an arterial thrombotic event in high-risk patients by approximately 25%. However, 10% to 20% of patients with an arterial thrombotic event who are treated with aspirin have a recurrent arterial thrombotic event during long-term follow-up. Nevertheless, the effectiveness of aspirin has been questioned by the emergence of the concept of aspirin resistance, which has been introduced as an explanation of the fact that a considerable proportion of patients treated with aspirin exhibit normal platelet function.

OBJECTIVES AND METHODS

We systematically reviewed all available evidence till March 2008 on prevalence of aspirin resistance and its association with clinical outcome. We also collected articles showing the possible way of treatment.

CONCLUSION

Analyzing the data of different laboratory methods aspirin resistance seems to be associated with poor clinical outcome, although currently no standardized or widely accepted definition of aspirin resistance exists. The widely used laboratory methods might not be comparable with each other; therefore, specific treatment recommendations for patients who exhibit high platelet reactivity during aspirin therapy or who have poor platelet inhibition by aspirin are not established.

Variability in responsiveness to oral antiplatelet therapy

Patients who have acute coronary syndromes or are undergoing percutaneous coronary intervention receive antiplatelet therapy to reduce the risk of atherothrombotic complications. Current guidelines favor the use of aspirin in combination with clopidogrel based on the results of a number of large-scale clinical trials. Aspirin alone is a relatively weak antiplatelet agent because it inhibits only one of many paths to platelet activation. By blockade of an adjunctive signaling pathway, the addition of clopidogrel to aspirin leads to synergistic platelet inhibitory effects. Dual antiplatelet therapy reduces the number of patients who experience adverse cardiovascular outcomes by 20% over aspirin alone. Nevertheless, approximately 10% of patients experience further atherothrombotic events, even while receiving dual antiplatelet therapy. Variability in individual responsiveness, including "resistance," has been attributed to the occurrence of these events. This article discusses variability in individual responses to oral antiplatelet therapy and its implications for clinical outcomes.

Aspirin resistance: disparities and clinical implications

Abstract Aspirin is one of the most widely prescribed drugs for the prevention of thrombosis in patients with vascular disease. Yet, aspirin is unable to prevent thrombosis in all patients. The term "aspirin resistance" has been used to broadly define the failure of aspirin to prevent a thrombotic event. Whether this is directly related to aspirin itself through biochemical aspirin resistance or treatment failure, or if it is because of aspirin's inability to overcome the thrombogenic aspects of the disease process itself, has not been elucidated. This can have dramatic clinical implications for a variety of vascular disease subsets and is cause for concern, considering the high prevalence of aspirin use for both primary and secondary prevention. Disparities exist in the rates of aspirin resistance among certain patient populations, such as women, patients with diabetes mellitus, and those with heart failure, and across clinical conditions, such as cardiovascular and cerebrovascular disease. Clinical trial data from studies observing resistance have revealed that regardless of study size, dose of aspirin, control for drug interactions and adherence, or assay used to measure platelet function, aspirin resistance is associated with an increased risk for adverse events. Although the evidence is mounting, there has yet to be a consensus on the appropriate clinical response to aspirin resistance.

Non-compliance is the predominant cause of aspirin resistance in chronic coronary arterial disease patients

Background

Our previous publication showed that 9% of patients with a history of myocardial infarction MI. could be labeled as aspirin resistant; all of these patients were aspirin resistant because of non-compliance. This report compares the relative frequency of aspirin resistance between known compliant and non-compliance subjects to demonstrate that non-compliance is the predominant cause of aspirin resistance.

Methods

The difference in the slopes of the platelet prostaglandin agonist (PPA) light aggregation curves off aspirin and 2 hours after observed aspirin ingestion was defined as net aspirin inhibition.

Results

After supposedly refraining from aspirin for 7 days, 46 subjects were judged non-compliant with the protocol. Of the remaining 184 compliant subjects 39 were normals and 145 had a past history of MI. In known compliant subjects there was no difference in net aspirin inhibition between normal and MI subjects. Net aspirin inhibition in known compliant patients was statistically normally distributed. Only 3% of compliant subjects (2 normals and 5 MI) had a net aspirin inhibitory response of less than one standard deviation which could qualify as a conservative designation of aspirin resistance. A maximum of 35% of the 191 post MI subjects could be classified as aspirin resistant and/or non-compliant: 9% aspirin resistant because of non-compliance, 23% non-compliant with the protocol and possibly 3% because of a decreased net aspirin inhibitory response in known compliant patients.

Conclusion

Our data supports the thesis that the predominant cause of aspirin resistance is noncompliance.

Aspirin resistance: biological and clinical implications

Cardiovascular events are the leading causes of mortality and morbidity in the United States. This development has prompted the rise of aspirin therapy in the prevention of atherothrombotic events. However, not all patients benefit to the same extent from aspirin therapy and many continue experiencing atherothrombotic complications. Researchers have labeled this phenomenon aspirin resistance, and despite drawing much attention from both researchers and lay people the cause remains unknown. Much needs to be clarified and standardized regarding the phenomenon of aspirin resistance, including the prevalence, definition, appropriate measurement methods, mechanisms, and, most important, linking low response to aspirin with worsened clinical outcomes.

Aspirin resistance in atherosclerosis

Clinically, aspirin resistance is defined as the failure of aspirin therapy to prevent an acute vascular thrombotic event despite regular intake of appropriate doses. In the laboratory, aspirin resistance encompasses the drug's failure to attain a particular level of platelet inhibition. From a clinical standpoint, the inability of aspirin to prevent a thrombotic event, despite appropriate cyclooxygenase-1 inhibition, implies the involvement of other factors. Evidence is emerging that aspirin resistance, as defined by residual platelet activity, merely reflects an individual's enhanced basal platelet function and suggests a hereditary component. Due to the multifactorial nature of cardiovascular disease, it is likely that a single therapy like aspirin cannot fully treat and prevent all thrombotic complications in the setting of atherosclerosis.

Use of the PFA-100 closure time to predict cardiovascular events in aspirin-treated cardiovascular patients: a systematic review and meta-analysis

BACKGROUND

PFA-100 is a point-of-care assay that evaluates platelet reactivity in high-shear-stress conditions by measuring the closure time (CT) of a membrane aperture. When determined with a collagen/epinephrine cartridge (CEPI), the CT is usually prolonged by aspirin. Studies of the predictive value of a short PFA-100CT(CEPI) for ischemic events in aspirin-treated patients have given variable results.

OBJECTIVES

To conduct a systematic review and meta-analysis of studies on the clinical predictive value of a short PFA-100CT(CEPI) in aspirin-treated cardiovascular patients.

PATIENTS AND METHODS

Relevant studies were identified by scanning electronic databases. Studies were selected if they included aspirin-treated patients with symptomatic atherosclerosis, measured the PFA-100CT(CEPI), used a CT cut-off value to define aspirin 'responders' and 'non-responders', and reported ischemic events.

RESULTS

We selected seven non-prospective studies (1466 patients) and eight prospective studies (1227 patients). In non-prospective studies, the PFA-100CT(CEPI) was performed after the ischemic clinical endpoint, and a publication bias was identified. In prospective studies, the global odds ratio (OR) for the recurrence of an ischemic event in 'aspirin non-responders' relative to 'aspirin responders' was 2.1 [95% confidence interval (CI) 1.4-3.4, P < 0.001]. Pooled analysis with a random effect model revealed no heterogeneity (Q Cochran P = 0.36 and I(2) = 9.4%).

CONCLUSIONS

A short PFA-100CT(CEPI) is associated with increased recurrence of ischemic events in aspirin-treated cardiovascular patients. This finding needs to be confirmed in stable ischemic patients, and the PFA-100CT(CEPI) cut-off needs to be refined in these patients.

Failure of aspirin to prevent myocardial infarction and adverse outcome during follow-up - a large series of all-comers

BACKGROUND

Aspirin has been used for secondary prevention of myocardial infarction (MI) in individuals with coronary disease. Although supported by several large controlled trials, aspirin prevents only a portion of recurrent events.

AIM AND METHOD

We set out to study the prevalence of long-term aspirin use prior to admission for MI and its significance for medium-term event-free survival; 998 consecutive patients with acute MI admitted to a tertiary center were included in the study.

RESULTS

Nearly half (42.4%) of all patients reported long-term use of low-dose aspirin prior to the index event. Prior aspirin use was associated with a 50% increase in the risk of both the combined end point of recurrent unstable angina, recurrent myocardial infarction, stroke, or death (OR 1.49; 95% CI 1.12-2.00, P=0.006) and mortality (OR 1.50; 95% CI 1.03-2.17, P=0.03) during 10-month follow-up. Prior aspirin use was not associated with an increased frequency of added antithrombotic therapy at discharge.

CONCLUSIONS

We have found that prior aspirin use is common in patients hospitalized for acute MI. Individuals already on aspirin had increased risk of recurrent ischemic events and all-cause mortality during the 10-month follow-up after their index MI.

Resistance to antiplatelet drugs: molecular mechanisms and laboratory detection

The definition 'resistance to antiplatelet drugs' should be limited to situations in which failure of the drug to hit its pharmacological target has been documented by specific laboratory tests. Aspirin resistance, as determined by specific tests (e.g. serum thromboxane B(2)), appears to be rare (1-2%) and, in most instances, is caused by poor compliance. In contrast to aspirin, studies that used specific tests to measure the pharmacological effect of thienopyridines [e.g. vasodilator-stimulated phosphoprotein (VASP)] showed a wide variability of responses to these drugs, with significant proportions of subjects (15-30%) who are very poor responders. Inter-individual differences in the extent of metabolism of thienopyridines to their active metabolites is the most plausible mechanism for the observed inter-individual variability in platelet inhibition. The demonstration that some patients may be 'resistant' or 'poor responders' to the pharmacological effect of antiplatelet drugs, has prompted the need of laboratory monitoring of antiplatelet therapy. However, many published studies have been performed using unspecific tests of platelet function, which identify patients on antiplatelet treatment with high residual platelet reactivity, which is not necessarily because of resistance to antiplatelet drugs. Despite this drawback, identification of patients with high residual platelet reactivity may be useful to predict their risk of atherothrombotic events. However, many studies still need to be carried out to identify the ideal laboratory test and to answer basic questions on its clinical utility and cost-effectiveness, before monitoring antiplatelet therapy can be recommended in the clinical practise. Until then, monitoring of antiplatelet therapy should be considered for investigational purposes only.

Abnormal PFA-100 closure time is associated with increased platelet aggregation in patients presenting with chest pain

BACKGROUND

Antiplatelet therapy has been proven to be effective for both primary and secondary prevention of myocardial infarction, stroke, and cardiovascular death. However, a significant proportion of patients treated with aspirin experience ischemic events. A number of prospective studies have demonstrated that decreased responsiveness to antiplatelet therapy as measured by various methods, is strongly associated with an increase in clinical events. Our objective was to characterize platelet function in patients presenting with chest pain using a point-of-care assay, PFA-100 and correlating results to traditional platelet aggregometry to determine if patients with aspirin non-responsiveness have increased clinical sequelae.

METHODS

Platelet function was assessed using PFA-100, flow cytometry, and optical aggregometry in 94 patients presenting to the emergency department with chest pain. All patients were on aspirin 81-325 mg daily. Clinical events occurring during the index hospitalization were documented.

RESULTS

Forty-seven patients (50%) were defined as aspirin non-responders by PFA-100 (collagen-epinephrine closure time <or= 193). Compared to aspirin responders, aspirin non-responders had higher levels of mean platelet aggregation to adenosine diphosphate (ADP) (P = 0.004) and high dose epinephrine (P = 0.03). Furthermore, expression of PAC-1 was significantly increased in patients with aspirin nonresponse as compared to aspirin responders (P = 0.003 and P = 0.0006 respectively). No significant difference in clinical events during the index hospitalization was noted between aspirin non-responders and aspirin responders.

CONCLUSION

Patients presenting with chest pain who have abnormal PFA-100 closure times have increased platelet aggregation and activation however this aspirin non-responsiveness does not correlate with increased clinical events in the index hospitalization.