Residual platelet reactivity on aspirin therapy and recurrent cardiovascular events — A meta-analysis

Prevalence of risk factors for venous thromboembolism and aspirin resistance in Australian patients undergoing total hip and knee arthroplasty

INTRODUCTION

Aspirin is used for venous thromboembolism (VTE) prophylaxis after total hip and knee arthroplasty (THA/TKA). However, its efficacy is unclear in patients with multiple VTE risk factors and at risk of aspirin resistance (AR).

BACKGROUND AND AIMS

To determine the prevalence of risk factors for VTE and AR in patients after THA/TKA and to determine the relationship between risk factors and drugs prescribed for thromboprophylaxis.

METHODS

A retrospective cohort study of elective-THA/TKA in six Australian hospitals over a 1-year period. Medical records were manually reviewed to determine demographics, thromboprophylaxis regimen and presence of risk factors. The relationship between individual and cumulative risk factors with the thromboprophylaxis regimen was determined.

RESULTS

In total, 1011 patients were included with a mean (SD) age of 65.9 (±11.0) years, and 56.4% were female. The five most prevalent risk factors were obesity (59.1%), age ≥65 years (58.2%), hypertension (45.3%), dyslipidaemia (35.9%) and diabetes (19.7%). Most patients had ≥1 risk factor for VTE (93.6%) and AR (93.6%), with 49.0% and 35.0% having ≥3 concurrent VTE and AR risk factors, respectively. The only significant relationship between risk factors and drugs was diabetes (P < 0.01). Rivaroxaban was more commonly used as the number of concurrent VTE risk factors increased (P < 0.05).

CONCLUSION

Patients had a high prevalence of VTE and AR risk factors, suggesting aspirin may not be beneficial in many patients. Only diabetes was linked to the selection of thromboprophylaxis. Patients who received rivaroxaban had a greater average number of VTE risk factors. Guidelines should promote individualised prescribing in higher-risk patients.

Monitoring antiplatelet therapy: where are we now?

Single antiplatelet therapy represents the cornerstone of thrombosis prevention in atherosclerotic cardiovascular disease. Dual antiplatelet therapy (DAPT), consisting of aspirin plus a P2Y12 inhibitor, is the standard of care for patients with acute coronary syndrome or undergoing both coronary and peripheral percutaneous interventions. Recent data suggest the efficacy of DAPT also after minor stroke. In this setting, a large body of evidence has documented that genetic and acquired patients' characteristics may affect the magnitude of platelet inhibition induced by antiplatelet agents. The implementation of tools allowing the identification and prediction of platelet inhibition has recently been shown to improve outcomes, leading to an optimal balance between antithrombotic efficacy and bleeding risk. We are therefore clearly moving towards tailored antiplatelet therapy. The aim of this paper is to summarize the available evidence on the evaluation of platelet inhibition in patients with coronary, peripheral, or cerebrovascular atherosclerosis. We will here focus on antiplatelet therapy based on both aspirin and P2Y12 inhibitors. In addition, we provide practical insights into the clinical settings in which it appears reasonable to implement antiplatelet therapy monitoring.

Platelet function assays for the diagnosis of aspirin resistance

Aspirin, an antiplatelet drug, is commonly used at low doses for numerous indications, including prophylaxis of cardiovascular, neurovascular, and venous thromboembolic events. Due to review articles suggesting that aspirin resistance may result in poorer outcomes, interest in assessing platelet function is increasing. Despite this, platelet function tests are rarely used as part of routine clinical practice and therefore, a basic understanding of these tests may be lacking. Although aspirin resistance can be categorized as clinical or laboratory resistance, determining laboratory resistance is the only way to determine resistance before treatment failure occurs. Therefore, knowledge of platelet assays to determine aspirin resistance is of importance. The following review aims to provide a framework for clinicians to understand the main principles of platelet function tests. This includes comparison of the most frequently used platelet assays to diagnose aspirin resistance, including the basic mechanism, methodology, reference ranges, inter-assay comparison, and their respective clinical considerations when using.

On-Treatment Platelet Reactivity is a Predictor of Adverse Events in Peripheral Artery Disease Patients Undergoing Percutaneous Angioplasty

OBJECTIVES

Few data are available on the association between a different entity of platelet inhibition on antiplatelet treatment and clinical outcomes in patients with peripheral artery disease (PAD). The aim of this study was to evaluate the degree of on-treatment platelet reactivity, and its association with ischaemic and haemorrhagic adverse events at follow up in PAD patients undergoing percutaneous transluminal angioplasty (PTA).

METHODS

In this observational, prospective, single centre study, 177 consecutive patients with PAD undergoing PTA were enrolled, and treated with dual antiplatelet therapy with aspirin and a P2Y12 inhibitor. Platelet function was assessed on blood samples obtained within 24 h from PTA by light transmission aggregometry (LTA) using arachidonic acid (AA) and adenosine diphosphate (ADP) as agonists of platelet aggregation. High on-treatment platelet reactivity (HPR) was defined by LTA ≥ 20% if induced by AA, and LTA ≥ 70% if induced by ADP. Follow up was performed to record outcomes (death, major amputation, target vessel re-intervention, acute myocardial infarction and/or myocardial revascularisation, stroke/TIA, and bleeding).

RESULTS

HPR by AA and HPR by ADP were found in 45% and 32% of patients, respectively. During follow up (median duration 23 months) 23 deaths (13%) were recorded; 27 patients (17.5%) underwent target limb revascularisation (TLR), two (1.3%) amputation, and six (3.9%) myocardial revascularisation. Twenty-four patients (15.6%) experienced minor bleeding. On multivariable analysis, HPR by AA and HPR by ADP were independent predictors of death [HR 3.8 (1.2-11.7), p = .023 and HR 4.8 (1.6-14.5), p = .006, respectively]. The median value of LTA by ADP was significantly lower in patients with bleeding complications than in those without [26.5% (22-39.2) vs. 62% (44.5-74), p < .001). LTA by ADP ≤ 41% was independently associated with bleeding HR 14.6 (2.6-24.0), p = .001] on multivariable analysis.

CONCLUSIONS

In this study a high prevalence of on-clopidogrel and aspirin high platelet reactivity was found, which was significantly associated with the risk of death. Conversely, a low on-clopidogrel platelet reactivity was associated with a higher risk of bleeding. These results document that the entity of platelet inhibition is associated with both thrombotic and bleeding complications in PAD patients.

The genetic basis of antiplatelet and anticoagulant therapy: A pharmacogenetic review of newer antiplatelets (clopidogrel, prasugrel and ticagrelor) and anticoagulants (dabigatran, rivaroxaban, apixaban and edoxaban)

INTRODUCTION

The study of pharmacogenomics presents the possibility of individualised optimisation of drug therapy tailored to each patients' unique physiological traits. Both antiplatelet and anticoagulant drugs play a key role in the management of cardiovascular disease. Despite their importance, there is a substantial volume of literature to suggest marked person-to-person variability in their effect. Areas covered: This article reviews the data available for the genetic cause for this inter-patient variability of antiplatelet and anticoagulant drugs. The genetic basis for traditional antiplatelets (i.e. aspirin) is compared with the newly available antiplatelet medicines (clopidogrel, prasugrel and ticagrelor). Similarly, the pharmacogenetics of warfarin is compared with the newer direct oral anticoagulants (DOACs) in detail. Expert Opinion: We identify strengths and weaknesses in the research thus far; including shortcomings in trial design and a review of newer analytical techniques. The direction of this research and its real-world implications are discussed.

Platelet Function Testing in Patients with Acute Ischemic Stroke: An Observational Study

BACKGROUND

The measurement of platelet reactivity in patients with stroke undergoing antiplatelet therapies is not commonly performed in clinical practice. We assessed the prevalence of therapy responsiveness in patients with stroke and further investigated differences between patients on prevention therapy at stroke onset and patients naive to antiplatelet medications. We also sought differences in responsiveness between etiological subtypes and correlations between Clopidogrel responsiveness and genetic polymorphisms.

METHODS

A total of 624 stroke patients on antiplatelet therapy were included. Two different groups were identified: "non-naive patients", and "naive patients". Platelet function was measured with multiple electrode aggregometry, and genotyping assays were used to determine CYP2C19 polymorphisms.

RESULTS

Aspirin (ASA) responsiveness was significantly more frequent in naive patients compared with non-naive patients (94.9% versus 82.6%, P < .0010). A better responsiveness to ASA compared with Clopidogrel or combination therapy was found in the entire population (P < .0010), in non-naive patients (P < .0253), and in naive patients (P < .0010). Multivariate analysis revealed a strong effect of Clopidogrel as a possible "risk factor" for unresponsiveness (odds ratio 3.652, P < .0001). No difference between etiological subgroups and no correlations between responsiveness and CYP2C19 polymorphisms were found.

CONCLUSION

In our opinion, platelet function testing could be potentially useful in monitoring the biological effect of antiplatelet agents. A substantial proportion of patients with stroke on ASA were "resistant", and the treatment with Clopidogrel was accompanied by even higher rates of unresponsiveness. Longitudinal studies are needed to assess whether aggregometry might supply individualized prognostic information and whether it can be considered a valid tool for future prevention strategies.

Direct factor IXa inhibition with the RNA-aptamer pegnivacogin reduces platelet reactivity in vitro and residual platelet aggregation in patients with acute coronary syndromes

BACKGROUND

Residual platelet reactivity is a predictor of poor prognosis in patients with acute coronary syndromes (ACSs) undergoing percutaneous coronary intervention. Thrombin is a major platelet activator and upon initiation of the coagulation cascade, it is subsequently produced downstream of factor IXa, which itself is known to be increased in ACS. Pegnivacogin is a novel RNA-aptamer based factor IXa inhibitor featuring a reversal agent, anivamersen. We hypothesized that pegnivacogin could reduce platelet reactivity.

METHODS

Whole blood samples from healthy volunteers were incubated in vitro in the presence and absence of pegnivacogin and platelet reactivity was analysed. In addition, platelet aggregometry was performed in blood samples from ACS patients in the RADAR trial featuring the intravenous administration of pegnivacogin as well as reversal by anivamersen.

RESULTS

In vitro, pegnivacogin significantly reduced adenosine diphosphate-induced CD62P-expression (100% vs. 89.79±4.04%, p=0.027, n=9) and PAC-1 binding (100% vs. 83.02±4.08%, p=0.010, n=11). Platelet aggregation was reduced (97.71±5.30% vs. 66.53±9.92%, p=0.013, n=10) as evaluated by light transmission aggregometry. In the presence of the RNA-aptamer reversal agent anivamersen, neither CD62P-expression nor platelet aggregation was attenuated. In patients with ACS treated with aspirin and clopidogrel, residual platelet aggregation was significantly reduced 20 min after intravenous bolus of 1 mg/kg pegnivacogin (100% versus 43.21±8.23%, p=0.020).

CONCLUSION

Inhibition of factor IXa by pegnivacogin decreases platelet activation and aggregation in vitro. This effect was negated by anivamersen. In ACS patients, platelet aggregation was significantly reduced after intravenous pegnivacogin. An aptamer-based anticoagulant inhibiting factor IXa therefore might be a promising antithrombotic strategy in ACS patients.

The Prevalence and Clinical Relevance of ASA Nonresponse After Cardiac Surgery: A Prospective Bicentric Study

We aimed to identify the prevalence of acetylsalicylic acid (ASA) nonresponse in patients after coronary artery bypass graft (CABG) surgery and the possible consequences for the rate of major cardiovascular events. This prospective, observational, bicentric cohort study was conducted in two German University hospitals. A total of 400 patients (200 in each study center) undergoing elective CABG surgery were enrolled after written informed consent. Platelet function was analyzed on day 3 (d3) and day 5 (d5) postoperatively following stimulation with arachidonic acid (ASPItest) and with thrombin receptor-activating peptide 6 (TRAPtest) using multiple electrode aggregometry (Multiplate). Individuals with an ASPItest ≥40 AU·min were categorized as ASA nonresponders. A 1-year follow-up recorded the combined end point of cardiovascular events, hospital admissions, or deaths related to cardiovascular disease. The prevalence of ASA nonresponse was 51.5% on d3, and it significantly increased to 71.3% on d5 ( P = .0049). The area under the aggregation curve in the TRAPtest ( P < .0001), the platelet count on d5 ( P = .009), and the cardiopulmonary bypass time ( P = .01) were identified as independent predictors of an ASA nonresponse. A 1-year follow-up recorded 54 events fulfilling criteria for the combined end point with no difference between ASA responders and nonresponders. This study indicates a high incidence of perioperative ASA nonresponse in patients following CABG. No effect on the incidence of cardiovascular events was recorded in the 1-year follow-up. Therefore, a randomized dosage adjustment trial should elucidate whether a tailored ASA treatment after CABG surgery represents a useful concept.

Relationship between high on aspirin platelet reactivity and oxidative stress in coronary artery by-pass grafted patients

The aim of the study was to assess the responsiveness of blood platelets to acetylsalicylic acid (ASA) in patients following coronary artery bypass grafting (CABG) surgery with relation to oxidative and antioxidative plasma status. The study included 37 patients treated with the CABG procedure. During the first 24 h after CABG patients were given 300 mg of ASA with the following dose of 150 mg daily. The blood was collected before the procedure and 10 days after. Whole blood platelet aggregation induced with arachidonic acid, collagen and adenosine diphosphate (ADP) was performed together with whole blood generation of thromboxane B2 (TxB2). Oxidative stress was measured before and 10 days after CABG with total oxidative plasma status (TOS) and total antioxidative status of the plasma (TAS). TOS/TAS index was calculated. We observed a significant increase in the TOS and TOS/TAS index and ADP-induced aggregation 10 days after CABG in comparison with its level before operation. There was a significant decrease in the arachidonic acid-induced aggregation and serum TxB2 level. Patients with ADP-induced and collagen-induced aggregation in the upper quartile had significantly higher TOS and TOS/TAS index before (ADP) and after the operation (ADP and collagen). There were 19 patients (51%) with high on aspirin platelet reactivity after CABG who had also higher TOS and TOS/TAS index and lower TAS value in comparison with aspirin responders. Despite ASA use, increased oxidative stress after CABG can overcome its antiplatelet effect and increase platelet activation through other pathways.

Enhanced platelet MRP4 expression and correlation with platelet function in patients under chronic aspirin treatment

Platelet multidrug resistance protein4 (MRP4)-overexpression has a role in reducing aspirin action. Aspirin in vivo treatment enhances platelet MRP4 expression and MRP4 mediated transport inhibition reduces platelet function and delays thrombus formation. The aim of our work was to verify whether MRP4 expression is enhanced in platelets obtained from patients under chronic aspirin treatment and whether it correlates with residual platelet reactivity. We evaluated changes on mRNA and protein-MRP4 expression and platelet aggregation in four populations: healthy volunteers (HV), aspirin-free control population (CTR), patients who started the treatment less than one month ago (ASA<1 month patients) and aspirinated patients who started the treatment more than two months ago (ASA>2 months patients). In platelets obtained from ASA>2 months patients, it was found a statistically significant MRP4 enhancement of both mRNA and protein expression compared to HV, CTR and ASA<1 month patients. Platelets obtained from ASA>2 months patients that present high levels of platelet MRP4, have higher serum TxB₂ levels and collagen-induced platelet aggregation compared to patient with low levels of MRP4 in platelets. In addition collagen induced platelet aggregation is higher in in vitro aspirinated platelets obtained from patients with high levels of MRP4 patients compared to those obtained from patients with low MRP4 levels. We can assert that, in patients under chronic aspirin treatment, platelets that present high MRP4 levels have an increase of residual platelet reactivity, which is due in part to incomplete COX-1 inhibition, and in part to COX-1-independent mechanism.

Equivalent intraperitoneal doses of ibuprofen supplemented in drinking water or in diet: a behavioral and biochemical assay using antinociceptive and thromboxane inhibitory dose-response curves in mice

Background. Ibuprofen is used chronically in different animal models of inflammation by administration in drinking water or in diet due to its short half-life. Though this practice has been used for years, ibuprofen doses were never assayed against parenteral dose–response curves. This study aims at identifying the equivalent intraperitoneal (i.p.) doses of ibuprofen, when it is administered in drinking water or in diet.

Methods. Bioassays were performed using formalin test and incisional pain model for antinociceptive efficacy and serum TXB₂ for eicosanoid inhibitory activity. The dose–response curve of i.p. administered ibuprofen was constructed for each test using 50, 75, 100 and 200 mg/kg body weight (b.w.). The dose–response curves were constructed of phase 2a of the formalin test (the most sensitive phase to COX inhibitory agents), the area under the ‘change in mechanical threshold’-time curve in the incisional pain model and serum TXB₂ levels. The assayed ibuprofen concentrations administered in drinking water were 0.2, 0.35, 0.6 mg/ml and those administered in diet were 82, 263, 375 mg/kg diet.

Results. The 3 concentrations applied in drinking water lay between 73.6 and 85.5 mg/kg b.w., i.p., in case of the formalin test; between 58.9 and 77.8 mg/kg b.w., i.p., in case of the incisional pain model; and between 71.8 and 125.8 mg/kg b.w., i.p., in case of serum TXB₂ levels. The 3 concentrations administered in diet lay between 67.6 and 83.8 mg/kg b.w., i.p., in case of the formalin test; between 52.7 and 68.6 mg/kg b.w., i.p., in case of the incisional pain model; and between 63.6 and 92.5 mg/kg b.w., i.p., in case of serum TXB₂ levels.

Discussion. The increment in pharmacological effects of different doses of continuously administered ibuprofen in drinking water or diet do not parallel those of i.p. administered ibuprofen. It is therefore difficult to assume the equivalent parenteral daily doses based on mathematical calculations.

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.

Dual antiplatelet therapy tailored on platelet function test after coronary stent implantation: a real-world experience

Patients' response to dual antiplatelet therapy (DAPT) is subject to variations and its monitoring allows to individualize this therapy. In this study, we evaluated if a strategy of tailored DAPT after platelet function testing could reduce high on-treatment platelet reactivity (HPR) and improve outcome of patients treated with stent implantation. In 257 patients undergoing percutaneous angioplasty, platelet function was measured by light transmittance aggregometry (LTA) using 10 µM/L adenosine-diphosphate (ADP) and 1 mM arachidonic acid (AA) as agonists. Patients with HPR by ADP (≥70%) were switched to double-dose clopidogrel, ticlopidine, prasugrel or ticagrelor; in patients with HPR by AA (≥20%) acetylsalicylic acid dose was increased if not contraindicated. Platelet function analysis was repeated 48 hours after therapy variation. At 20-month follow-up major adverse cardiovascular events (MACE) and bleedings were assessed. HPR was detected in 97/257 (37.7%) patients: 69/257 (26.8%) had HPR by ADP and 71/257 (27.6%) had HPR by AA. In patients with HPR by ADP or by AA, tailored DAPT determined a significant reduction in residual platelet reactivity. No significant difference in MACE or bleeding occurrence was documented in HPR patients treated with tailored DAPT vs. those without HPR. HPR patients treated with tailored DAPT had significant lower follow-up MACE and deaths vs. 139 HPR patients not switched, even after propensity score analysis. These results suggest that a DAPT tailored on platelet testing can improve antiplatelet response in HPR patients, possibly reducing their thrombotic events to a level similar to non-HPR patients, without increasing the risk of bleeding.

[Clopidogrel resistance in cerebrovascular disease -- results of one-year follow-up]

INTRODUCTION

In an earlier study the authors reported short-term benefits of clopidogrel treatment in patients with cerebrovascular disease.

AIM

The aim of the authors was to analyze findings during a 12-month follow-up period.

METHOD

100 patients with a history of acute stroke or transient ischaemic attack have been included in the study. The efficacy of the therapy was assessed 7 and 28 days as well as 3, 6 and 12 months after initiation of medical therapy. Patients were divided into two parts (clopidogrel responder and resistant) based on their initial laboratory findings. Risk profiles, medical therapy, laboratory parameters and vascular events were compared between the two patient groups.

RESULTS

After 7 days of treatment, clopidogrel seemed to be ineffective in 11 patients (11%). Strong, clinically significant correlations were found among blood pressure values, blood glucose and lipid parameters, serum C-reactive protein levels and platelet aggregation levels. An aggressive secondary preventive therapy resulted in a decrease of clopidogrel resistance and improvement in laboratory parameters. Patients who were clopidogrel resistant at baseline had significantly higher rate of vascular events compared to those who were clopidogrel responders (18.1 vs. 4.5%, p<0.01), although clopidogrel resistance was not an independent risk factor of unfavourable vascular outcome using a multivariate analysis. No adverse events or hemorrhagic complications were observed.

CONCLUSIONS

Clopidogrel resistance and unfavourable outcome seems to be associated with "classical" risk factors underlying the importance of aggressive secondary prevention.

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.

Low angle light scattering analysis: a novel quantitative method for functional characterization of human and murine platelet receptors

BACKGROUND

Determinations of platelet receptor functions are indispensable diagnostic indicators of cardiovascular and hemostatic diseases including hereditary and acquired receptor defects and receptor responses to drugs. However, presently available techniques for assessing platelet function have some disadvantages, such as low sensitivity and the requirement of large sample sizes and unphysiologically high agonist concentrations. Our goal was to develop and initially characterize a new technique designed to quantitatively analyze platelet receptor activation and platelet function on the basis of measuring changes in low angle light scattering.

METHODS

We developed a novel technique based on low angle light scattering registering changes in light scattering at a range of different angles in platelet suspensions during activation.

RESULTS

The method proved to be highly sensitive for simultaneous real time detection of changes in size and shape of platelets during activation. Unlike commonly-used methods, the light scattering method could detect platelet shape change and aggregation in response to nanomolar concentrations of extracellular nucleotides. Furthermore, our results demonstrate that the advantages of the light scattering method make it a choice method for platelet receptor monitoring and for investigation of both murine and human platelets in disease models.

CONCLUSIONS

Our data demonstrate the suitability and superiority of this new low angle light scattering method for comprehensive analyses of platelet receptors and functions. This highly sensitive, quantitative, and online detection of essential physiological, pathophysiological and pharmacological-response properties of human and mouse platelets is a significant improvement over conventional techniques.

Optimal aspirin dose in acute coronary syndromes: an emerging consensus

ABSTRACT: 

Numerous clinical trials testing the efficacy of aspirin for the secondary prevention of cardiovascular disease have been published. We reviewed the literature pertaining to aspirin dose in acute coronary syndrome patients. Clinical trials assessing the comparative efficacy of different doses of aspirin are scarce. This complex antiplatelet therapy landscape makes it difficult to identify the best aspirin dose for optimizing efficacy and minimizing risk of adverse events, while complying with the various guidelines and recommendations. Despite this fact, current evidence suggests that aspirin doses of 75–100 mg/day may offer the optimal benefit:risk ratio in acute coronary syndrome patients.

Variation in thromboxane B2 concentrations in serum and plasma in patients taking regular aspirin before and after clopidogrel therapy

Dual antiplatelet therapy with aspirin and a P2Y12 antagonist is widely prescribed for the prevention of thrombotic events in patients with an acute coronary syndrome or undergoing percutaneous coronary intervention (PCI). It is recognised that there is inter-individual variation in the antiplatelet effects of both drugs. Recent data also suggest that P2Y12 antagonists can affect the response to aspirin. A direct indicator of the effect of aspirin on platelets is their ability to generate thromboxane, which if measured as the difference between the level of thromboxane B2 in serum and plasma ([TxB2]S-P) avoids the confounding effect of endogenous TxB2 production from other cells. We therefore analysed [TxB2]S-P as a measure of aspirin response in a group of 123 patients undergoing elective PCI before and after the introduction of clopidogrel. In a subgroup of 40 patients taking aspirin alone, we compared [TxB2]S-P and VerifyNow Aspirin for the assessment of aspirin response. There was a wide variation in plasma and serum TxB2 concentrations both before and after clopidogrel therapy but only 3.5% of patients had residual serum concentration of TxB2 > 10 ng/ml. There was a strong correlation between the pre and post clopidogrel levels of TxB2 (r ≥ 0.78; p = 0.001) and no significant difference in [TxB2]S-P. There was no correlation between the magnitude of response to clopidogrel response and the generation of thromboxane B2. Correlation between [TxB2]S-P and VerifyNow Aspirin was poor. We conclude that the use of a P2Y12 antagonist does not influence the effect of aspirin on the ability of platelets to generate thromboxane. Therefore, measurement of TxB2 levels in serum, after subtracting the contribution from plasma, provides a measure of the response to aspirin in patients taking dual antiplatelet therapy.

Long-term clinical efficacy and safety of adding cilostazol to dual antiplatelet therapy for patients undergoing PCI: a meta-analysis of randomized trials with adjusted indirect comparisons

OBJECTIVE

To assess the long-term clinical efficacy and safety of adding cilostazol to aspirin plus clopidogrel (triple antiplatelet therapy, TAT) in patients undergoing percutaneous coronary intervention (PCI) and explore its role in the era of new generation adenosine diphosphate (ADP)-receptor antagonists.

METHODS

PUBMED, EMBASE, and the Cochrane Central Register of Controlled Trials were searched for randomized controlled trials (RCTs) comparing TAT versus dual antiplatelet therapy (DAT), followed by a manual search. Then, a meta-analysis of RCTs comparing TAT versus standard DAT in patients undergoing PCI was performed. Furthermore, indirect comparisons of TAT versus new generation ADP-receptor antagonist based DAT (prasugrel or ticagrelor based DAT) were undertaken, with standard DAT as a common comparator. The included end-points were major adverse cardiovascular event (MACE), target lesion revascularization (TLR), target vessel revascularization (TVR), death, myocardial infarction (MI), stent thrombosis, bleeding and other drug adverse events.

RESULTS

Twelve RCTs with a total of 31,789 patients were included. Compared with standard DAT (n = 2551), TAT (n = 2545) significantly reduced the incidence of MACE (OR: 0.56, 95% CI: 0.47-0.68, P < 0.00001), TLR (OR: 0.51, 95% CI: 0.34-0.75, P = 0.0006) and TVR (OR: 0.59, 95% CI: 0.46-0.75, P < 0.0001), and did not change significantly in death (OR: 0.68, 95% CI: 0.44-1.05, P = 0.08), MI (OR: 0.80, 95% CI: 0.45-1.44, P = 0.46), stent thrombosis (OR: 0.61, 95% CI: 0.27-1.36, P = 0.23), major bleeding (OR: 1.42, 95% CI: 0.52-3.85, P = 0.49) and overall bleeding (OR: 1.16, 95% CI: 0.79-1.69, P = 0.45). Compared with prasugrel (n = 6813) or ticagrelor based DAT (n = 6732), TAT (n = 2545) further reduced the incidence of MACE (OR: 0.80, 95% CI: 0.72-0.90, P = 0.0012; OR: 0.83, 95% CI: 0.75-0.92, P = 0.0003, respectively).

CONCLUSIONS

Compared with standard DAT, the long-term use of TAT in patients with PCI gives more benefits in reducing the incidence of MACE, TLR and TVR without increasing bleeding. Furthermore, it might be superior to prasugrel or ticagrelor based DAT in term of MACE, which needs to be confirmed by future studies with direct comparisons.

Von Willebrand factor antigen predicts response to double dose of aspirin and clopidogrel by PFA-100 in patients undergoing primary angioplasty for ST elevation myocardial infarction

Von Willebrand factor (VWF) is an emerging risk factor in acute coronary syndromes. Platelet Function Analyzer (PFA-100) with Collagen/Epinephrine (CEPI) is sensitive to functional alterations of VWF and also identifies patients with high on-treatment platelet reactivity (HPR). The objective of this study was to verify the effect of double dose (DD) of aspirin and clopidogrel on HPR detected by PFA-100 and its relation to VWF and to its regulatory metalloprotease ADAMTS-13. Between 2009 and 2011 we enrolled 116 consecutive patients with ST elevation myocardial infarction undergoing primary PCI with HPR at day 5 after PCI. Patients recruited were then randomized between a standard dose (SD, n = 58) or DD of aspirin and clopidogrel (DD, n = 58), maintained for 6 months follow-up. Blood samples for PFA-100, light transmittance aggregometry, and VWF/ADAMTS-13 analysis were collected after 5, 30, and 180 days (Times 0, 1, and 2). At Times 1 and 2 we observed a significantly higher CEPI closure times (CT) in DD as compared to SD (P < 0.001). Delta of CEPI-CT (T1 - T0) was significantly related to VWF (P < 0.001) and inversely related to ADAMTS-13 (0.01). Responders had a significantly higher level of VWF at T0. Finally, in a multivariate model analysis, VWF and ADAMTS-13 in resulted significant predictors of CEPI-CT response (P = 0.02). HRP detected by PFA-100 in acute myocardial infarction is reversible by DD of aspirin and clopidogrel; the response is predicted by basal levels of VWF and ADAMTS-13. PFA-100 may be a useful tool to risk stratification in acute coronary syndromes given its sensitivity to VWF.

Tailored antiplatelet therapy and clinical adverse outcomes

OBJECTIVE

The clinical evidence regarding the influence of tailored antiplatelet strategy on adverse outcomes has been controversial. The aim of the study was to evaluate the significance of tailored antiplatelet therapy with respect to clinical adverse events in antiplatelet-resistant patients.

METHODS

Randomised studies that assess clinical relevance of personalised antiplatelet treatment in antiplatelet-resistant patients were identified through a literature search: PubMed, EMBASE, Web of Science and the Cochrane Library. The primary endpoint was the composite of death from any cause and stent thrombosis. All total clinical adverse events and bleeding complications were evaluated.

RESULTS

Data were combined across seven randomised studies comprising 12 048 subjects, of whom 3738 (31.0%) were found to be antiplatelet-resistant. Antiplatelet-resistant patients provided with tailored antiplatelet therapy showed less risk of death or stent thrombosis than those assigned conventional antiplatelet treatment (0.5% vs. 2.2%; OR (95% CI) 0.25 (0.13 to 0.49), p<0.0001). A significant benefit in terms of total adverse event risk reduction was observed during follow-up for tailored vs conventional antiplatelet therapy (5.5% vs. 10.0%; OR (95% CI) 0.40 (0.20 to 0.77), p=0.006). No statistical difference in bleeding complications was observed between these two groups (p=0.08).

CONCLUSIONS

In the study, personalised antiplatelet treatment for antiplatelet resistance was found to be associated with less occurrence of death or stent thrombosis and the less risk of total clinical adverse events than conventional treatment, without increasing the risk of bleeding complications.

High-maintenance-dose clopidogrel in patients undergoing percutaneous coronary intervention: a systematic review and meta-analysis

Background

Despite routine use of clopidogrel, adverse cardiovascular events recur among some patients undergoing percutaneous coronary intervention (PCI). To optimize antiplatelet therapies, we performed a meta-analysis to quantify the efficacy of high versus standard-maintenance-dose clopidogrel in these patients.

Methods

Randomized controlled trials (RCTs) comparing high (>75 mg) and standard maintenance doses of clopidogrel in patients undergoing PCI were included. The primary efficacy and safety end-points were major adverse cardiovascular/cerebrovascular events (MACE/MACCE) and major bleeding. The secondary end-points were other ischemic and bleeding adverse effects. The pooled odds ratio (OR) for each outcome was estimated.

Results

14 RCTs with 4424 patients were included. Compared with standard-maintenance-dose clopidogrel, high-maintenance-dose clopidogrel significantly reduced the incidence of MACE/MACCE (OR 0.60; 95% CI 0.43 to 0.83), stent thrombosis (OR 0.56; 95% CI 0.32 to 0.99) and target vessel revascularization (OR 0.38; 95% CI 0.20 to 0.74), without significant decrease of the risk of cardiovascular death (OR 0.92; 95% CI 0.74 to 1.13) and myocardial infarction (OR 0.83; 95% CI 0.51 to 1.33). For safety outcomes, it did not significantly increase the risk of major bleeding (OR 0.73; 95% CI 0.41 to 1.32), minor bleeding (OR 1.29; 95% CI 1.00 to 1.66) and any bleeding (OR 1.14; 95% CI 0.91 to 1.43).

Conclusion

High-maintenance-dose clopidogrel reduces the recurrence of most ischemic events in patients post-PCI without increasing the risk of bleeding complications.

Platelet biology: the role of shear

Platelets are anucleated fragments produced by megakaryocytes that circulate in the blood. Platelets are involved in the initial cellular response to damaged endothelium and migrate to this area to prevent excessive bleeding. What is becoming more acknowledged over the last few decades is that blood flow (hemodynamics) plays a critical role in platelet function. The purpose of this review is to summarize the current understanding of platelet biology with particular focus on the role of hemodynamics. The emerging concept of shear microgradients, which are challenging the traditional model of platelet function, will also be introduced in the review.

The administration of a loading dose of aspirin to patients presenting with acute myocardial infarction while receiving chronic aspirin treatment reduces thromboxane A2-dependent platelet reactivity

Abstract The optimal dose of aspirin for patients presenting with acute myocardial infarction (AMI) while receiving chronic aspirin therapy has not been clearly established. We evaluated whether continued treatment with 100 mg of aspirin or a loading dose (200-500 mg) influences thromboxane A2 (TX) suppression or platelet reactivity. Sixty-four consecutive patients with AMI and 98 healthy subjects (82 aspirin-free and 16 receiving 100 mg daily for a week) were evaluated. Treatment was at the discretion of the attending physician. Collagen (1 µg/ml)-induced TX synthesis, (14)C-serotonin-release, platelet aggregation, and the PFA-100 assay were evaluated. The platelet TX synthesis of patients receiving a loading dose of aspirin was sixfold lower than that of patients receiving 100 mg of aspirin (p<0.005). This was associated with marked reductions in (14)C-serotonin-release and arachidonic-acid-induced aggregation and an increase in the PFA-100 closure time (p<0.01). Categorization of patients according to their TX synthesis (<95% or ≥ 95% inhibition vs. healthy aspirin-free subjects) revealed that 8% of the patients treated with loading doses had a poor response (<95% inhibition) vs. 53% of those treated with 100 mg (p<0.001). Patients with lower TX inhibition had higher serum NT-Pro-BNP (p<0.005), a marker of poor left ventricular systolic function. Administration of a loading dose of aspirin to patients with AMI during existing chronic aspirin treatment induced greater reductions in platelet TX synthesis and TX-dependent platelet reactivity than the continued treatment alone.

Confirmation of reported aspirin use in community studies: utility of serum thromboxane B2 measurement

Aspirin (ASA) is recommended for the prevention of cardiovascular disease; however, the compliance is low. Reported use may not reflect actual use. Serum thromboxane B2 (STxB2) measurement was evaluated to validate reported ASA use. Males aged 45 to 79 years and females aged 55 to 79 years completed a survey and STxB2 measurement (Thromboxane B2 EIA Kit; Cayman Chemical, Ann Arbor, Michigan). The 107 patients were grouped by use of ASA (56 ASA+ and 51 ASA-) and possible interfering medications (INT) such as nonsteroidal anti-inflammatory drugs. The STxB2 levels (ng/mL) were significantly lower in ASA users: ASA+ INT- 3.0 (0.7, 8.4), ASA+ INT+ 2.0 (0.8, 4.9), ASA- INT+ 176 (75, 390), and ASA- INT- 271 (199, 366). The INT use did not cause a significant difference in STxB2 levels. A STxB2 cut point of 25 ng/mL had high sensitivity (94.1%) and specificity (91.1%) for ASA use. The STxB2 was a reliable marker of ASA use and could be used to confirm ASA exposure in population-based health studies.

Persistent high on-treatment platelet reactivity in acute coronary syndrome

Persistent high on-treatment platelet reactivity in acute coronary syndrome (ACS) patients managed with appropriate antiplatelet therapy has been correlated with increased risk of cardiovascular events; however, the evolution of this phenomenon overtime is not well known. We investigated platelet activity at a three month follow-up after initial presentation with an ACS. We enrolled a total of 124 patients in the study, 65 were diagnosed with ACS and 59 controls who presented with non-cardiac chest pain for baseline comparisons. Of the enrolled patients, we had 25 ACS patients return, in stable condition, three months after their initial presentation for repeat platelet functional testing. Epinephrine (EPI), adenosine diphosphate (ADP), and arachidonic acid induced platelet aggregation were monitored at baseline with repeat measurement of EPI- and ADP-stimulated aggregation at follow-up. In addition, P-selectin and PAC-1 expression were monitored at presentation and at a three month follow-up period. ACS patients were maintained on aspirin therapy during the intervening period. At the three month follow-up visit, ACS patients initiated on aspirin had no significant percentage change in aggregation to submaximal concentrations of EPI and ADP. They also had no significant percentage change in PAC-1 or P-selectin expression. This study demonstrates persistent high on-treatment platelet reactivity in ACS patients at a three month follow-up, which may place these patients at increased risk of recurrent cardiovascular events.

Variability of platelet indices and function: acquired and genetic factors

Each individual has an inherent variable risk of bleeding linked to genetic or acquired abnormal platelet number or platelet dysfunction. In contrast, it is less obvious that the variability of platelet phenotypes (number, mean platelet volume, function) may contribute to the variable individual risk of thrombosis. Interindividual variability of platelet indices or function may be either due to acquired factors, such as age, sex, metabolic variables, smoke, dietary habits, and ongoing inflammation, or due to genetic factors. Acquired variables explain a small portion of the heterogeneity of platelet parameters. Genetic factors, instead, appear to play a major role, although a consistent portion of such a genetic variance has not yet been attributed to any specific genetic factor, possibly due to the high number of DNA loci potentially involved and to the limited effect size of each individual SNP. A portion of variance remains thus unexplained, also due to variability of test performance. A major contradiction in present platelet knowledge is, indeed, the difficulty to reconcile the universally accepted importance of platelet indices or function and the lack of reliable platelet parameters in cardiovascular risk prediction models. Trials on antiplatelet drugs were generally designed to select a homogeneous sample, whose results could be applied to an "average subject," tending to exclude the deviation/extreme values. As the current indications for antiplatelet treatment in primary or secondary prevention of ischemic vascular disease still derive from the results of such clinical trials where platelet function and its variability was not investigated, we cannot at present rely upon any current platelet test to either initiate, or monitor, or modify or stop treatment with any antiplatelet drug. Evidence is, however, increasing that traditional platelet aggregometry and other more recently developed platelet function assays could be useful to optimize antiplatelet therapy and to predict major adverse cardiac events.The observation of interindividual differences in platelet response to antiplatelet drugs has enlarged the spectrum and the possible clinical relevance of the variability of platelet indices or function. The development of "personalized medicine" will benefit from the concepts discussed in this chapter.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

Simvastatin administration reduces thromboxane production in subjects taking aspirin: links between aspirin resistance and thrombin generation

BACKGROUND

Growing evidence indicates that statins may reduce thromboxane A(2) synthesis and thrombin generation. We investigated the relationships between thromboxane production, thrombin generation, and oxidative stress in patients receiving aspirin before and after statin administration.

METHODS

An open-label study was conducted in 112 men, aged 54.4 ± 7.3 years, at an increased cardiovascular risk receiving aspirin (75 mg/d). Prior to and following a 3-month simvastatin treatment (40 mg/d), we evaluated circulating thromboxane B(2) (TXB(2)), inflammatory markers, 8-isoprostane, and prothrombin fragment 1.2 (F1.2), a marker of thrombin generation, which was also measured in blood collected every 60s at the site of standardized skin incisions.

RESULTS

Subjects (n=28) with pretreatment TXB(2) concentrations in the highest quartile ("aspirin-resistant patients") were more frequently current smokers and had elevated C-reactive protein (CRP), interleukin-6, 8-isoprostane, shorter bleeding time, and increased F1.2 production in a model of microvascular injury, when compared with the 3 remaining quartiles (all, p<0.001). Simvastatin decreased serum TXB(2) in the whole group (by 20%, p=0.0008). Patients in the highest quartile of the baseline TXB(2) had still higher posttreatment TXB(2), CRP, interleukin-6, and F1.2 formation following injury (all, p<0.001). Simvastatin-induced change in TXB(2) correlated with the magnitude of changes in maximum levels and the velocity of F1.2 formation (all p<0.001) but not with changes in inflammatory markers or lipid profile.

CONCLUSIONS

The study shows that statins significantly reduce platelet TXA(2) formation in patients taking low-dose aspirin and this effect is associated with attenuated thrombin formation in response to vascular injury.

A critical review of aspirin in the secondary prevention of noncardioembolic ischaemic stroke

Both secondary prevention (such as lifestyle modifications, pharmacotherapy or surgery) and an understanding of the influence of risk factors (including the different aetiologic mechanisms of cerebral ischaemia) play a pivotal role in reducing the burden of recurrent stroke. Regarding the types of preventative treatments available, variations exist across all clinical studies, including differences in target populations (including the type of cerebral ischaemia), risk factors, length of follow-up, drop-out rates and outcomes, which makes translating the results of clinical trials to individual patients difficult. However, with such limitations in mind, this critical albeit nonsystematic review, which compared aspirin with other antiplatelets and in combination with other drugs, showed that the benefit from aspirin treatment is consistently shown in ischaemic stroke, while harms are limited. Furthermore, no definite superiority is apparent across different antiplatelet therapies. Dual antiplatelet regimens may expose to a slight but measurable higher risk of haemorrhagic complications, perhaps in selective groups of patients (i.e. those with severe small-vessel disease or in selective racial groups). Based on our analysis, the indication of aspirin as the first-line choice, also recommended by several acknowledged international or national guidelines, may be confirmed. However, the complex nature of patients at risk of recurrent ischaemic stroke necessitates a comprehensive approach, which should be driven by the primary care physician, whose role is central to successful actions for secondary stroke prevention.

Residual platelet reactivity: predicting short- and long-term clinical outcome in patients undergoing percutaneous coronary revascularization

Adequate platelet inhibition is mandatory in patients undergoing percutaneous coronary intervention in order to prevent recurrent thrombotic events. Dual antiplatelet therapy with aspirin and thienopyridine (e.g., clopidogrel) is the treatment of choice in this setting, providing clear clinical benefit in most of the patients. However, a wide interindividual variability exists in the response to antiplatelet drugs and several factors may contribute to determine fluctuation in platelet reactivity, even within the individual patient. Several methodologies and devices have been developed to monitor individual response to antiplatelet treatment, assessing different pathways of platelet activation and aggregation. Studies performed with the use of these methodologies have clearly demonstrated that patients with high post-treatment residual platelet reactivity present a higher risk of ischemic events both at short (during or soon after percutaneous coronary intervention) and long term. In these patients, more aggressive antithrombotic strategies, based on the results of platelet function tests, may be beneficial in order to reduce ischemic complications after percutaneous coronary intervention.

Point-of-care testing for assessment of adequacy of oral antiplatelet therapy in patients with cardiovascular disease

Studies with recently introduced point-of-care (POC) platelet function tests have shown that individuals are variably responsive to aspirin and clopidogrel therapy, and that hyporesponsiveness to antiplatelet therapy is associated with an increased risk of cardiovascular events. However, the currently available POC tests have undergone only limited clinical evaluation and clinicians are uncertain about the best POC test, the optimal cut-off point to define hyporesponsiveness in different patient populations and clinical settings, the appropriate management of patients demonstrating hyporesponsiveness and the cost effectiveness of adjusting treatment on the basis of the results of POC platelet function testing. Several large randomized controlled trials currently underway are examining whether adjusting antiplatelet therapy on the basis of a POC test result can improve patient-important outcomes. Until these issues are resolved, POC testing to monitor antiplatelet therapy will largely remain a research tool and patients should continue to receive oral antiplatelet therapy without routine monitoring at doses that have been demonstrated to be effective in randomized controlled trials.