Aspirin resistance: truth or dare

Aspirin for Primary Cardiovascular Prevention in Patients with Diabetes: Uncertainties and Opportunities

The use of the antiplatelet agent aspirin (acetylsalicylic acid) was previously routinely recommended for the primary prevention of cardiovascular (CV) events in patients with diabetes, but recent large-scale randomized trials have failed to demonstrate a sizeable net clinical benefit with a once-daily, low-dose (81–100 mg) regimen in this population. Previous pharmacokinetic and pharmacodynamic studies have suggested that the aspirin formulation (enteric-coated) and dosing schedule (once daily) studied in randomized trials for primary prevention of CV events defining contemporary clinical practice may not leverage the full potential of the drug, particularly in patients with diabetes. Indeed, the diabetic platelets bear characteristics that increase their thrombotic potential and alter their pharmacologic response to the drug. Consequently, the appropriateness of studying a uniform aspirin regimen in landmark primary prevention trials needs to be revisited. In this review, we present the evidence showing that diabetes not only increases baseline platelet reactivity, but also alters platelet response to aspirin through different mechanisms including a faster platelet turnover rate. Obesity, which is frequently associated with diabetes, also impacts its pharmacokinetics via an increase in distribution volume. Small-scale pharmacokinetic and pharmacodynamic studies have suggested that the relative aspirin resistance phenotype observed in patients with diabetes may be reversed with a twice-daily dosing schedule, and with nonenteric-coated aspirin formulations. Properly powered randomized controlled trials investigating the efficacy and safety of aspirin dosing schedules and formulations tailored to the population of patients with diabetes are urgently required to optimize patient care.

The Importance of Platelets Response during Antiplatelet Treatment after Ischemic Stroke-Between Benefit and Risk: A Systematic Review

Ischemic stroke is a disease related to abnormal blood flow that leads to brain dysfunction. The early and late phases of the disease are distinguished. A distinction is made between the early and late stages of the disease, and the best effect in treating an ischemic stroke is usually achieved within the first hours after the onset of symptoms. This review looked at studies platelet activity monitoring studies to determine the risks and benefits of various approaches including antiplatelet therapy. A study was conducted on recently published literature based on PRISMA. This review includes 32 research articles directly addressing the importance of monitoring platelet function during antiplatelet therapy (dual or monotherapy) after ischemic stroke. In patients with transient ischemic attack or ischemic stroke, antiplatelet therapy can reduce the risk of stroke by 11–15%, assuming that patients respond well. Secondary prevention results are dependent on platelet reactivity, meaning that patients do not respond equally to antiplatelet therapy. It is very important that aspirin-resistant patients can benefit from the use of dual antiplatelet therapy. The individualized approach to secondary stroke prevention is to administer the most appropriate drug at the correct dose and apply the optimal therapeutic procedure to the individual patient.

A comparison of three platelet function tests in ischemic stroke patients with antiplatelet therapy

Predicting the effectiveness of antiplatelet drugs is critical to precision antiplatelet therapy. However, there is a lack of an acceptable method, although there are a variety of methods for detecting platelet function. In this study, we compared three major platelet function tests to assess their performance and found better methods for platelet function evaluation after aspirin or clopidogrel treatment in ischemic stroke patients by comparative study. A total of 249 ischemic stroke patients were enrolled who were treated with aspirin or clopidogrel or both. Three platelet function tests including light transmittance aggregometry (LTA), thromboelastography (TEG), platelet function analyzer (PFA) were performed as well as CYP2C19 genotype determination. Correlation analyses and kappa statistics were used. All three methods were effective in evaluating aspirin function. However, only LTA and TEG had good correlation and consistency (r = -0.37, kappa = 0.634). TEG-ADP was the least sensitive for clopidogrel, as the platelet inhibition ratio did not differ between the clopidogrel-user group and the control (P = 0.074), while LTA and PFA were sensitive (P ＜ 0.001). Correlations between platelet assays were poor for clopidogrel (the absolute value of r range from 0.13 to 0.35) and so was the agreement (Kappa from 0.232 to 0.314). LTA and PFA have a good correlation with CYP2C19 genotyping (P = 0.034 and 0.014). In conclusion, all three tests were able to evaluate aspirin effect, LTA-AA and TEG-AA had a good correlation. TEG perform badly for clopidogrel effect detection. The fair-to-modest agreement among assays indicated further study was indispensable.

Antiplatelet treatment in the primary prophylaxis of cardiovascular disease in patients with arterial hypertension

The benefits of using acetylsalicylic acid (ASA) in the primary prophylaxis of cardiovascular diseases may only slightly exceed the risk of serious bleeding. This warrants the search for alternative, safer preparations with antiaggregatory properties, which could be used in patients burdened with cardiovascular risk factors. Antiaggregatory compounds identified in water-soluble tomato extract include nucleosides, flavonoids, and phenolic acids. The action of standardized tomato extract is multidirectional, reversible, and weaker in comparison to ASA. The European Food Safety Authority (EFSA) has approved this preparation as a dietary agent with antiaggregatory properties. The use of standardized tomato extract appears beneficial in arterial hypertension patients with low or moderate cardiovascular risk and in patients in whom good pressure control cannot be achieved.

ABCC3 Polymorphisms and mRNA Expression Influence the Concentration of a Carboxylic Acid Metabolite in Patients on Clopidogrel and Aspirin Therapy

Acetylsalicylic acid (ASA) and clopidogrel combined therapy has been reported to be beneficial in patients with acute coronary syndrome (ACS). Antiplatelet drug resistance, especially to clopidogrel, is a multifactorial phenomenon that affects a large number of ACS patients. The genetic contribution to this drug response is not fully elucidated. We investigated the relationship of ABC-type efflux subfamily C member 3 (ABCC3) polymorphisms and mRNA expression with plasma concentrations of clopidogrel, salicylic acid (SA) and a carboxylic acid metabolite (CAM). Clopidogrel, CAM and SA plasma concentrations were measured simultaneously by liquid chromatography-tandem mass spectrometry (LCMS/MS) from 83 ACS patients undergoing percutaneous coronary intervention. ABCC3 (rs757421, rs733392 and rs739923) and CYP2C19*2 (rs4244285) polymorphisms as well as mRNA expression were evaluated. A positive correlation was found between CAM concentrations and ABCC3 mRNA expression (r = 0.494, p < 0.0001). Patients carrying genotype AA (rs757421 variant) had higher CAM concentrations and ABCC3 mRNA expression as compared to those of GG + GA carriers (p = 0.017). A multiple linear regression analysis revealed that ABCC3 mRNA expression (p = 0.017), rs757421 AA genotype (p = 0.001), blood collection time (p = 0.018) and clopidogrel dose (p = 0.001) contributed to the concentration of CAM. No associations were observed for the CYP2C19*2 polymorphism. These results suggest that up-regulation of ABCC3 mRNA expression leads to increased plasma CAM levels through MRP3-mediated cell efflux. The ABCC3 rs757421 polymorphism may contribute to gene expression. Therefore, ABCC3 may be a potential biomarker for the response to clopidogrel.

Platelet function testing as a biomarker for efficacy of antiplatelet drugs

Despite the overwhelming evidence in support of the efficacy of dual antiplatelet therapy with aspirin and clopidogrel, it is also obvious that not all patients benefit from these drugs to the same extent. This interindividual variability in platelet responses may underlie clinical differences in drug efficacy, with potential for optimization of antiplatelet therapy to prevent ischemic events without excessively increasing bleeding risk. This review presents the current evidence regarding platelet function testing for monitoring of antiplatelet therapy, with emphasis on the prognostic value of platelet function testing to predict ischemic and bleeding events. The potential of platelet function testing to provide personalized antiplatelet therapy is also discussed, with an outlook toward the future of platelet function testing in high-risk individuals.

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.

Acetylsalicylic acid resistance risk factors in patients with myocardial infarction

BACKGROUND

Despite its commonly recognized benefits in the cardiovascular disease setting, an issue of resistance to this drug has lately emerged. The aim of this research was assessment of the phenomenon of acetylsalicylic acid (ASA) resistance and its risk factors in patients treated for myocardial infarction.

METHODS

This study is a post-hoc analysis of a previous prospective study with approximately 200 patients treated for myocardial infarction with a coated formulation of ASA. The population was divided into two subgroups according to the response to ASA. ASA responsiveness was assessed using the arachidonic acid-dependent platelet aggregation (ASPI-test). The measurements were performed using the technique of impedance aggregometry.

RESULTS

The prevalence of aspirin resistance among the study population was 6.2%. All analyzed aggregometric parameters (including ASPI-test, adenosine diphosphate dependent platelet aggregation - ADP-test, bleeding time measurement) showed significant differences between both subgroups. ASA resistant patients had higher concentrations of brain natriuretic peptide (BNP), high-sensitivity C-reactive protein (hs-CRP), leukocytes (WBC) and platelets (PLT) but lower concentrations of hemoglobin (HGB). The temporal point analysis for both subgroups showed aspirin resistance incidence peak in patients at 9 months after myocardial infarction.

CONCLUSIONS

The prevalence of aspirin resistance in our study population is comparable with rates reported in literature among patients with cardiovascular diseases. There is a possible relation between aspirin resistance and clopidogrel resistance. Presence did not affect the incidence of the clinical end-points.

Variability in the responsiveness to low-dose aspirin: pharmacological and disease-related mechanisms

The main pharmacological aspects of pharmacodynamics (PD) and pharmacokinetics (PK) of aspirin as antiplatelet agent were unravelled between the late sixties and the eighties, and low-dose aspirin given once daily has been shown to be a mainstay in the current treatment and prevention of cardiovascular disorders. Nevertheless, several PD and PK aspects of aspirin in selected clinical conditions have recently emerged and deserve future clinical attention. In 1994, the term "aspirin resistance" was used for the first time, but, until now, no consensus exists on definition, standardized assay, underlying mechanisms, clinical impact, and possible efficacy of alternative therapeutic interventions. At variance with an undefined aspirin-resistant status, in the last 5 years, the concept of variability in response to aspirin due to specific pathophysiological mechanisms and based on PK and/or PD of the drug has emerged. This growing evidence highlights the existence and possible clinical relevance of an interindividual variability of pharmacological aspirin response and calls for new, large studies to test new low-dose aspirin-based regimens which may ameliorate platelet acetylation, reduce variability in drug responsiveness, and improve clinical efficacy on selected populations.

Determination of the prevalence of aspirin and clopidogrel resistances in patients with coronary artery disease by using various platelet-function tests

BACKGROUND

Dual therapy with aspirin and clopidogrel has emerged as the gold standard therapy for patients treated with drug-eluting stents (DES). However, there is variability in patients' responses to this antiplatelet therapy, and some patients continue to show ischemic recurrences after therapy. The purpose of the study was to compare the simultaneously obtained results of various platelet-function tests for assessing the prevalence of antiplatelet resistance in coronary artery disease patients undergoing DES therapy.

METHODS

A total of 66 patients were administered a loading dose of aspirin, clopidogrel, and cilostazol at least 12 hr before stenting. The results of VerifyNow (Accumetrics, USA), multiplate analyzer (Dynabyte Medical, Germany), and vasodilator-stimulated phosphoprotein/P2Y12 (Biocytex, France) assays were compared with those of light transmission aggregometry (LTA) analysis.

RESULTS

The P2Y12 reaction units and P2Y12% inhibition values obtained using the VerifyNow assay showed strong correlation (r) with the results of the LTA analysis. All tests results showed low concordance in defining the antiplatelet resistance in patients, and the degrees of agreement were as follows: 0 for aspirin reaction units; 0.25, P2Y12% inhibition; 0, aspirin-sensitive patients' identification test; 0.21, ADPtest; and 0.14, platelet reactivity index, expressed as the κ statistics. The prevalence of aspirin and clopidogrel resistances in patients resulted in remarkable variations, from 0% to 22.7% and from 9.1% to 48.5%, respectively.

CONCLUSIONS

The clinical usefulness of the different assays for the correct classification of patients in terms of antiplatelet resistance remains unclear. Further studies are required to determine the best method for correlating the occurrences of adverse ischemic events.

Risk assessment and aspirin use in Asian and Western populations

Objective

The aim of this review was to examine aspirin utilization, cardiovascular risk estimation, and clinical evidence for aspirin prophylaxis in Asian versus Western countries.

Methods

A literature search was performed using PubMed and the key terms “aspirin” and “Asia” or “Western”.

Results

Despite the growing burden of cardiovascular disease (CVD), aspirin is underutilized in high-risk patients in both Asian and Western countries. A number of risk estimation scores are available; however, validation is needed in countries such as Japan, India, and in South Asia. Underutilization of aspirin in Asia may be linked to an overestimation of bleeding risks. It is possible that a higher prevalence of Helicobacter pylori infection and genetic differences may make Asians more susceptible to gastrointestinal bleeding. Very low aspirin doses and even the wider use of gastroprotective agents may be the optimal approach to high-risk patients in Asia.

Conclusions

Based on the current evidence, aspirin should be used for CVD prevention when the risk:benefit ratio is favorable. A number of trials are underway, including the Diabetic Atherosclerosis Prevention by Cilostazol and Japanese Primary Prevention Project, which will provide key data on the benefits of aspirin in Asian patients at risk of CVD, and may improve aspirin utilization and risk estimation.

Stroke pharmacogenomics

Circulatory disease accounts for fifteen million deaths each year, of which stroke accounts for four and a half million- with an estimated nine million stroke survivors annually. The overall incidence rate of stroke is 2 to 2.5 per thousand adults with an approximate prevalence of 5 per thousand and an estimated 5-year risk of stroke recurrence of 15 to 40 percent. Conventional risk factors for stroke include: increasing age, hypertension, diabetes mellitus, smoking, increased body mass index, ischemic heart disease, heart failure, atrial fibrillation and lack of physical activity. Age is the strongest risk factor for both ischemic and haemorrhagic stroke with its incidence doubling for each successive decade after the age of fifty-five years. However, there is a substantial portion of patients with significant cerebrovascular disease who do not have any of these stroke risk-factors, leading to the speculation that there are other factors that have not been identified yet So as to improve diagnosis and treatment strategies, as well as to reduce the related public health burden, it could be helpful to successfully identify its extremely complex genetic determinants (polygenic, multiple genes play a role). Pharmacogenetics is the field of pharmacology that deals with the influence of genetic variation on drug response by correlating gene expression and gene variants with the efficacy or toxicity of drugs. The principle drugs in stroke medicine are antithrombotics. The aim of this paper was to review the most commonly used drugs for stroke such as rtPA in the acute phase as well as antiplatelets and wafarin for secondary prophylaxis.

Effect of glucose or fat challenge on aspirin resistance in diabetes

Aspirin has lower antiplatelet activity in diabetic patients. Our aim is to study the roles of acute hyperglycemia and hyperlipidemia on aspirin function in diabetic subjects with and without cardiovascular disease. Using urine thromboxane (pg/mg creatinine) and VerifyNow (Aspirin Resistance Measures-ARU), we investigated diabetic subjects during a 2-hour glucose challenge (n = 49) or a 4-hour fat challenge (n = 11). All subjects were currently taking aspirin (81 or 325 mg). After fat ingestion, urine thromboxane increased in all subjects (Mean ± SE before: after) (1209 ± 336: 1552 ± 371, P = .01), while we noted a trend increase in VerifyNow measures (408 ± 8: 431 ± 18, P = .1). The response to glucose ingestion was variable. Diabetic subjects with cardiac disease and dyslipidemia increased thromboxane (1693 ± 364: 2799 ± 513, P < .05) and VerifyNow (457.6 ± 22.3: 527.1 ± 25.8, P < .05) measures after glucose. We conclude that saturated fat ingestion increases in vivo thromboxane production despite aspirin therapy.

Prevalence of unresponsiveness to aspirin and/or clopidogrel in patients with stable coronary heart disease

This study sought to assess whether inadequate platelet responses to aspirin and clopidogrel are distinct phenomena caused by different mechanisms or different facets of the same phenomenon (i.e., general platelet hyperactivity). A total of 85 patients with stable coronary artery disease who were taking aspirin and clopidogrel daily for > or =3 months were enrolled in the present study. Platelet aggregation was measured by light transmission aggregometry (LTA) stimulated with 1.6 mM of arachidonic acid and 5, 10 and 20 microM of adenosine diphosphate, and by the VerifyNow Aspirin and VerifyNow P2Y12 point-of-care assays. An inadequate platelet response was defined as aggregation greater than or equal to the mean + 2 SDs. The prevalence of an inadequate platelet response varied greatly among the assays. For aspirin, the prevalence was 2.4% using arachidonic acid-induced LTA and 5.9% using the VerifyNow Aspirin assay. For clopidogrel, the prevalence varied from 1.2% to 3.9% using adenosine diphosphate-induced LTA and was 2.4% using the VerifyNow P2Y12 assay. The point-of-care assays did not select the same patients as LTA. No subject was unresponsive to both aspirin and clopidogrel, regardless of the assay used, suggesting that separate mechanisms govern platelet unresponsiveness to aspirin and clopidogrel. In conclusion, an inadequate platelet response to either aspirin or clopidogrel is rare, and the definition is dependent on the platelet function assay used. Because no subject was found to be unresponsive to both agents, the unresponsiveness is suspected to occur through distinct mechanisms of platelet activation.

Aspirin resistance: an update

Aspirin resistance (AR) still lacks a universally accepted definition, but it may be discussed as either a laboratory phenomenon or a clinical presentation. Laboratory resistance is mainly defined as abnormal platelet response to aspirin, whereas the clinical manifestation is the failure of aspirin to prevent cardiovascular events. Although there is evidence of an association, it appears that a laboratory abnormality in platelet function is not the only risk factor for the clinical manifestation of AR. Therapies for primary and secondary prevention of AR still need to be elucidated, but there are some data to suggest that in an acute episode of aspirin failure because of AR, different therapeutic interventions need to be considered.

Evaluation of the platelet count drop method for assessment of platelet function in comparison with "gold standard" light transmission aggregometry

INTRODUCTION

Hyporesponsiveness to antiplatelet agents has been linked to an increased risk of major adverse cardiovascular events. However, light transmission aggregometry (LTA), the gold standard methodology for assessing platelet function, requires expertise and is labour-intensive, which render its use in clinical settings impractical. We assessed whether platelet count drop (PCD), a technique widely available in any haematology laboratory, could replace LTA in testing for inhibition of platelet aggregation induced by antiplatelet agents.

MATERIALS AND METHODS

One hundred and sixty-one coronary artery disease patients taking aspirin alone and 91 patients taking a combination of aspirin and clopidogrel were enrolled. Platelet aggregation was measured by LTA and PCD stimulated with 1.6 mM of arachidonic acid (AA) for aspirin and 5 and 20 microM of adenosine diphosphate (ADP) for clopidogrel.

RESULTS

Correlation between AA-induced LTA and PCD was nonexistent (r=-0.043, p=0.587), while correlation between ADP-induced LTA and PCD was low (r=0.374, p<0.0001 for ADP 5 microM and r=0.402, p<0001 for ADP 20 microM ). PCD, whether stimulated with AA or ADP, overestimated platelet aggregation as assessed by LTA, by 13-18%. The wide 95% limits of agreement suggest that the assays can disagree significantly in individual patients.

CONCLUSIONS

Although the PCD method is widely available in non-specialized laboratories, our results demonstrate that there is poor correlation with the current gold standard, i.e. LTA. Thus, PCD should not be used in replacement of LTA to assess antiplatelet responsiveness.

Hemorrhagic complications from glaucoma surgery in patients on anticoagulation therapy or antiplatelet therapy

PURPOSE

To determine the incidence of and risk factors for hemorrhagic complications in patients on anticoagulation (ACT) or antiplatelet therapy (APT) having glaucoma surgery.

DESIGN

Retrospective case-control study.

METHODS

Medical records of patients who had glaucoma surgery between July 1, 1998 and March 31, 2005 were reviewed. Patients who either used ACT/APT continuously throughout the perioperative period or discontinued its use prior to surgery were compared to case-matched control patients who were not on such therapies. Patients on ACT/APT who experienced postoperative hemorrhagic complications were compared to those who did not. Outcome measures included hemorrhagic complications and thromboembolic events.

RESULTS

Three hundred and forty-seven patients (eyes) who were on ACT or APT prior to glaucoma surgery had a higher rate of hemorrhagic complications than 347 control patients (10.1% vs 3.7%, respectively, P = .002). Patients on ACT had a higher rate of hemorrhagic complications than patients on APT (22.9% vs 8.0%, respectively, P = .003). Patients who continued ACT during glaucoma surgery had the highest rate of hemorrhagic complications (31.8%) when compared to patients who discontinued ACT prior to surgery or patients who used APT alone (P = .001). Hemorrhagic complications following glaucoma surgery were more frequently associated with preoperative ACT, arrhythmia, and higher preoperative and postoperative intraocular pressures (IOP).

CONCLUSION

Chronic ACT/APT was associated with a statistically significant increase in the rate of hemorrhagic complications, and perioperative ACT and a high preoperative IOP are potential risk factors for hemorrhagic complications in patients undergoing glaucoma surgery.

The genetics of aspirin resistance

Aspirin is widely used for the prophylaxis of cardiovascular events in patients with cardiovascular risk factors or established atherosclerotic disease. However, despite aspirin treatment, a substantial number of patients experience recurrent events. Such 'aspirin resistance' is generally defined as failure of aspirin to produce an expected biological response, for example inhibition of platelet aggregation or of thromboxane A2 synthesis. Whilst its aetiology is multifactorial, genetic factors are also likely to play their part. Here we review the evidence for and against such a genetic contribution, as well as the data suggesting the involvement of specific genes.