Residual arachidonic acid-induced platelet activation via an adenosine diphosphate-dependent but cyclooxygenase-1- and cyclooxygenase-2-independent pathway: a 700-patient study of aspirin resistance

Prior aspirin use and outcomes in acute coronary syndromes

OBJECTIVES

The purpose of this study was to determine whether patients taking aspirin before an acute coronary syndrome (ACS) are at higher risk of recurrent events or mortality.

BACKGROUND

Controversy exists whether prior aspirin use is an independent predictor of worse outcomes in patients who experience an ACS.

METHODS

We evaluated 66,443 ACS patients from a merged database of previous Thrombolysis in Myocardial Infarction trials. We evaluated the differences in ACS type, total mortality, and the composite end point of death, myocardial infarction (MI), recurrent ischemia, or stroke between prior aspirin and nonprior aspirin users. We used multivariate analysis to control for differences in baseline characteristics.

RESULTS

Prior aspirin users (n = 17,839) were older (63 years vs. 59 years) and had more coronary risk factors and evidence of coronary artery disease (MI, angina, prior intervention) than nonprior aspirin users (n = 48,604) (all p < 0.0001). Prior aspirin use was associated with less severe types of ACS at presentation (e.g., unstable angina > non-ST-segment elevation MI > ST-segment elevation MI) than their nonaspirin user counterparts (p < 0.0001). After multivariate analysis, there was no difference in total mortality between prior aspirin users and nonaspirin users at day 30 (odds ratio [OR]: 1.01; 95% confidence interval [CI]: 0.90 to 1.13) or by the last follow-up visit (mean 328 days) (hazard ratio: 1.03; 95% CI: 0.95 to 1.11). Prior aspirin use was modestly associated with recurrent MI (OR: 1.26; 95% CI: 1.12 to 1.43) and the composite end point (OR: 1.16; 95% CI: 1.08 to 1.24).

CONCLUSIONS

Prior aspirin use was associated with more comorbidities and coronary disease and a higher risk of recurrent MI, but not mortality. As such, it should best be considered a marker of a patient population at high risk for recurrent adverse events after ACS.

Association of serum levels of arachidonic acid and eicosapentaenoic acid with prevalence of major adverse cardiac events after acute myocardial infarction

We studied the association of serum levels of arachidonic acid (AA) and eicosapentaenoic acid (EPA) with the prevalence of major adverse cardiac events (MACE) after acute myocardial infarction (AMI). We measured serum AA and EPA on admission in 146 consecutive AMI patients. The primary clinical endpoint was occurrence of MACE, defined as cardiac death, occurrence of heart failure, reinfarction, recurrent angina pectoris, and requirement of coronary intervention. Common logarithmic transformed serum levels of AA (logAA) and EPA (logEPA) were used in the analyses. The optimum cutoff point of each fatty acid used to distribute patients into two groups for Kaplan-Meier analysis was determined by receiver operating characteristic curves analysis. MACE occurred in 40 patients (27.4%). Kaplan-Meier analysis disclosed that the group with a logAA above the cutoff point [145.3 μg/mL (logAA 2.162)] showed a higher prevalence of MACE than those with a logAA below the cutoff point (P < 0.01). Conversely, the prevalence of MACE was significantly higher in the group with a logEPA below the cutoff point [52.3 μg/mL (logEPA 1.719)] compared to the group with a logEPA above it (P < 0.01). Similar to logAA, logAA/logEPA showed significant differences in the MACE-free curve between the two groups (cutoff 1.301, P < 0.001). Cox proportional hazards regression analysis suggested that logAA, logEPA, and logAA/logEPA were independently associated with the prevalence of MACE. Although the present study included a limited number of patients with single-time point measurement, the results suggested an association of logAA, logEPA, and logAA/logEPA with the prevalence of MACE after AMI. The present study warrants further studies involving a large number of patients to confirm that the serum levels of these fatty acids and their ratios are predictors of MACE after AMI.

The Platelet Activity After Clopidogrel Termination (PACT) study

BACKGROUND

"Rebound" platelet hyperreactivity after discontinuation of clopidogrel has been proposed to lead to increased thrombotic risk, including late stent thrombosis. However the hypothesis that discontinuation of clopidogrel results in platelet hyperreactivity has never been rigorously tested. We therefore performed a randomized, double-blind, placebo-controlled, crossover study: the Platelet Activity after Clopidogrel Termination (PACT) study.

METHODS AND RESULTS

Platelet reactivity in 15 healthy subjects was measured at baseline, during clopidogrel 75 mg or placebo daily for 14 days, and on days 1, 4, 8, 11, 15, and 45 after discontinuation of clopidogrel or placebo. Platelet reactivity was assessed by (1) platelet surface activated GPIIb-IIIa and surface P-selectin (by whole blood flow cytometry) in response to ADP 0.5, 1, and 20 μmol/L; thrombin receptor activating peptide (TRAP) 1 and 20 μmol/L; and collagen/epinephrine 5 μg/mL/5 μmol/L, (2) light transmission aggregation with ADP 2.5, 5, and 20 μmol/L; TRAP 2 and 20 μmol/L; and collagen 6 μg/mL, (3) whole blood impedance aggregation with ADP 1.6 and 6.5 μmol/L; TRAP 4 and 32 μmol/L; and collagen 3.2 μg/mL, and (4) plasma soluble CD40 ligand (by ELISA). Immature platelet fraction was measured in the Sysmex XE-2100. At no time point after discontinuation of clopidogrel was platelet reactivity, as determined by any assay end point, or the immature platelet fraction significantly greater than after discontinuation of placebo.

CONCLUSIONS

This randomized, double-blind, placebo-controlled, crossover study demonstrates that discontinuation of clopidogrel does not result in "rebound" platelet hyperreactivity, as determined by multiple time points, assays, agonists, and agonist concentrations.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00619073.

Antithrombotic therapy in patients with diabetes mellitus and coronary artery disease

Currently approved antiplatelet treatment strategies have proved successful for reducing cardiovascular adverse events in patients with CAD. However, despite the use of recommended antiplatelet treatment strategies, the presence of DM has been consistently associated with a negative impact on outcomes and a high rate of adverse cardiovascular events continue to occur in patients with DM. The elevated prevalence of low response to standard oral antiplatelet agents contribute to these impaired outcomes. Thus, the search for more potent antiplatelet treatment strategies is warranted in high-risk patients, such as those with DM. The present manuscript provides an overview on the current status of knowledge on currently available antiplatelet agents, focusing on the benefits and limitations of these therapies in DM patients, and evaluating the potential role of new antithrombotic agents and treatment strategies currently under development to overcome these limitations.

Aspirin resistance: Fact or fiction? A point of view

Aspirin is a wonder drug that has been used for well over 100 years for its analgesic and antipyretic effects. For the past three decades, it has increasingly been used for the prevention of primary and secondary cardiovascular events. Lately, it has been suggested that a significant number of individuals taking aspirin have become resistant to this drug. The phenomenon of "aspirin resistance" is based on the observation of clinical events in some patients taking aspirin, and/or a diminished platelet aggregation inhibitory response to aspirin therapy. Unfortunately, laboratory assays used to monitor the efficacy of aspirin are far from accurate and the results are not reproducible. Furthermore, results of different platelet function tests are often not congruent. In addition, platelet aggregation studies show marked inter-individual and intra-individual variability. Patients with coronary heart disease take many drugs that interfere with the effect of aspirin on platelet aggregation. Besides inhibiting formation of thromboxane A(2) from arachidonic acid, aspirin has a host of platelet-independent effects that complement its platelet inhibitory effects. Laboratory assays designed to measure platelet function do not take into account these pleiotropic effects of aspirin. In our view, use of the term "aspirin resistance" based on inadequate knowledge of imperfect laboratory tests does a disservice to physicians and patients.

Relation of platelet and leukocyte inflammatory transcripts to body mass index in the Framingham heart study

BACKGROUND

Although many genetic epidemiology and biomarker studies have been conducted to examine associations of genetic variants and circulating proteins with cardiovascular disease and risk factors, there has been little study of gene expression or transcriptomics. Quantitative differences in the abundance of transcripts has been demonstrated in malignancies, but gene expression from a large community-based cohort examining risk of cardiovascular disease has never been reported.

METHODS AND RESULTS

On the basis of preliminary microarray data and previously suggested genes from the literature, we measured expression of 48 genes by high-throughput quantitative reverse-transcriptase polymerase chain reaction in 1846 participants of the Framingham Offspring cohort from RNA derived from isolated platelets and leukocytes. A multivariable stepwise regression model was used to assess clinical correlates of quantitative RNA expression. For specific inflammatory platelet-derived transcripts, including ICAM1, IFNG, IL1R1, IL6, MPO, COX2, TNF, TLR2, and TLR4, there were significant associations with higher body mass index (BMI). Compared with platelets, fewer leukocyte-derived transcripts were associated with BMI or other cardiovascular risk factors. Select transcripts were found to be highly heritable, including GPIBA and COX1. Almost uniformly, heritable transcripts were not those associated with BMI.

CONCLUSIONS

Inflammatory transcripts derived from platelets, particularly those part of the nuclear factor kappa B pathway, are associated with BMI, whereas others are heritable. This is the first study, using a large community-based cohort, to demonstrate clinical correlates of gene expression and is consistent with the hypothesis that specific peripheral-blood transcripts play a role in the pathogenesis of coronary heart disease and its risk factors.

Boosting platelet inhibition in poor responder to aspirin and clopidogrel undergoing percutaneous coronary intervention: role of tirofiban

Nowadays, aspirin (acetylsalicylic acid) and clopidogrel form the cornerstone in prevention of cardiovascular events and their clinical effectiveness has been well established. The thienopyridine clopidogrel is a prodrug that, after hepatic metabolization, strongly inhibits adenosine diphosphate-induced platelet aggregation. Aspirin is a non-steroidal anti-inflammatory drug that exerts its anti-platelet action through the irreversible acetylation of platelet cyclooxygenase (COX)-1, blocking thromboxane A2 production. However, despite dual-antiplatelet therapy, some patients still develop recurrent cardiovascular ischemic events. Many studies have clearly showed that a marked variability exists in the responsiveness to aspirin and clopidogrel, being the poor responder patients at higher risk of short (peri-procedural) and long-term ischemic complications. In particular, these patients showed a major recurrence of myocardial infarction and, after stent implantation, of stent thrombosis. The mechanisms of aspirin and clopidogrel poor response are numerous and not fully elucidated, and are likely multifactorial (eg, genetic polymorphisms, elevated baseline platelet reactivity, drug interaction). How to improve the short- and long-term outcome of these patients is currently unknown. Recently published and ongoing clinical trials are evaluating different strategies for the acute and chronic treatments (eg, reload of clopidogrel, double clopidogrel maintenance dose, switching to prasugrel). In this paper, we reviewed all available evidence on aspirin and clopidogrel resistance and focused our attention on tirofiban, a glycoprotein IIb/IIIa inhibitor that may be used to obtain a better platelet inhibition in poor responder patients during the acute phase and in particular during percutaneous coronary intervention.

Implementing genotype-guided antithrombotic therapy

Genotyping has the potential to improve the efficacy and safety of major antithrombotic drugs. For warfarin, the stable maintenance dose varies from 1-10 mg/day. The VKORC1 -1639G>A allele and the CYP2C9*2 and *3 alleles (cumulative frequency: 90% in Asians, 65% in Europeans and 20% in Africans), explain 45% of response variability in European and 30% in African populations. The large clinical trials COAG and EU-PACT will define the extent to which pharmacogenetic dosing affects the safety and efficacy of warfarin and coumarin derivatives. The platelet inhibitor clopidogrel requires activation by the CYP2C19 enzyme. CYP2C19*2 and *3 alleles (cumulative frequency: 20-50%) produce null enzyme activity, and their presence attenuates platelet inhibition and increases cardiovascular events. The US FDA-mandated drug labeling recognizes the relevance of genotyping in the selection and dosing of both warfarin and clopidogrel.

Response of blood platelets to beta-glucan from Saccharomyces cerevisiae

The effects of the beta-D-glucan, a polysaccharide derived from the yeast cell walls of species such as Saccharomyces cerevisiae, on blood platelets activation induced by physiological agonists (thrombin, ADP, collagen) in vitro were studied. The aim of our study was to assess in vitro if beta-glucan, a naturally strong biological response modifier, may modify platelet activation, i.e. platelet aggregation and degranulation (release of proteins and adenine nucleotides) induced by thrombin, ADP and collagen. Cytochrome c reduction method was used to test the ability of beta-glucan to change superoxide anion generation in platelets. Moreover, we determined also its effect on enzymatic arachidonic acid cascade. The obtained results indicate that beta-glucan has the inhibitory effects on platelet aggregation and secretion. beta-glucan distinctly reduced the arachidonic acid pathway and superoxide anion radical generation in platelets stimulated by biological agonists. The results of the present study suggest that beta-glucan from Saccharomyces cerevisiae has antiplatelet and antioxidative activities, and therefore may be beneficial in the prevention of the excessive blood platelet activation-related diseases, such as cardiovascular or inflammatory diseases.

Cangrelor: a novel P2Y12 receptor antagonist

Antiplatelet therapy is critical in the prevention of thrombotic complications of acute coronary syndrome and percutaneous coronary interventions. Current antiplatelet agents (aspirin, clopidogrel and glycoprotein IIb/IIIa antagonists) have demonstrated the capacity to reduce major adverse cardiac events. However, these agents have limitations that compromise their clinical utility. The platelet P2Y12 receptor plays a central role in platelet function and is a focus in the development of antiplatelet therapies. Cangrelor is a potent, competitive inhibitor of the P2Y12 receptor that is administered by intravenous infusion and rapidly achieves near complete inhibition of ADP-induced platelet aggregation. This investigational drug has been studied for use during coronary procedures and the management of patients experiencing acute coronary syndrome and is undergoing evaluation for use in the prevention of perioperative stent thrombosis.

Variability in aggregometry response before and after initiation of clopidogrel therapy

BACKGROUND

Evaluation of clopidogrel therapy by in vitro methods has limitations which may be of clinical importance. We wanted to explore the variability in aggregometry response in aspirin sensitive patients before and after initiation of clopidogrel therapy.

METHODS

ADP 9.37 microM, AA 1.2mM and TRAP 25 mM stimulated light transmissions aggregometry (LTA) were performed twice before (Exams 1 and 2; 3 weeks apart)-and within one year after-initiation of clopidogrel therapy (Exam 3) in 79 patients treated with PCI. Repeated ADP aggregometry was also performed in 16 healthy volunteers in order to estimate LTA measurement error.

RESULT

Inter-individual differences in ADP aggregation e.g. at Exam 1 were substantial (range 17-77%, SD 15.8%). Intra-individual changes between Exams 1 and 2 were significant (-27 to +36%, SD 14.6%, p<0.05). Inter-individual differences at Exam 3 (on clopidogrel treatment) were larger than expected from Exams 1 and 2 (p<0.01). AA aggregation was the same before and during clopidogrel treatment. In controls, inter-individual differences were smaller at ADP 10 than at ADP 5 microM.

CONCLUSIONS

Inter-individual differences in ADP aggregation were significant both before and during clopidogrel therapy, and there were significant intra-individual variations over time. Therefore, prediction of aggregometry response before or during clopidogrel therapy based on single tests may be unreliable. Inter-individual differences in healthy controls are smaller at high concentrations of ADP, and comparisons of aggregometry response should be performed with caution unless ADP concentrations are standardized.

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.

Comparison of prasugrel and clopidogrel in patients with acute coronary syndrome undergoing percutaneous coronary intervention

Antiplatelet agents are the cornerstone of treatment for patients with acute coronary syndrome (ACS) undergoing percutaneous coronary intervention (PCI). Clopidogrel, when added to aspirin, has demonstrated considerable success at reducing thrombotic complications of ACS and/or PCI compared to aspirin alone and is standard of care for the management of patients with ACS and in patients undergoing PCI. Prasugrel is a novel thienopyridine antiplatelet agent recently approved for the treatment of patients with ACS undergoing PCI. Prasugrel provides greater and more consistent platelet inhibition than clopidogrel due to earlier and more extensive formation of its active metabolite. The enhanced platelet inhibition with prasugrel led to a reduction in major adverse cardiovascular events in patients with moderate to high risk ACS scheduled for PCI in the phase 3 TRITON-TIMI 38 trial. This benefit was seen more in patients suffering a STEMI and those with diabetes. However, this reduction in events was met with a significant increase in the risk of bleeding which overcame prasugrel’s benefit in certain groups. Future studies with prasugrel are needed to determine its optimal utilization to minimize bleeding risks and evaluate its outcomes in ACS and safety profile in special patient populations.

Pharmacogenetics in cardiovascular antithrombotic therapy

Thrombosis is the most important underlying mechanism of coronary artery disease and embolic stroke. Hence, antithrombotic therapy is widely used in these scenarios. However, not all patients achieve the same degree of benefit from antithrombotic agents, and a considerable number of treated patients will continue to experience a new thrombotic event. Such lack of clinical benefit may be related to a wide variability of responses to antithrombotic treatment among individuals (i.e., interindividual heterogeneity). Several factors have been identified in this interindividual heterogeneity in response to antithrombotic treatment. Pharmacogenetics has emerged as a field that identifies specific gene variants able to explain the variability in patient response to a given drug. Polymorphisms affecting the disposition, metabolism, transporters, or targets of a drug all can be implicated in the modification of an individual's antithrombotic drug response and therefore the safety and efficacy of the aforementioned drug. The present paper reviews the modulating role of different polymorphisms on individuals' responses to antithrombotic drugs commonly used in clinical practice.

Postoperative development of aspirin resistance following coronary artery bypass

BACKGROUND

Aspirin therapy is known to substantially reduce mortality and the rate of ischaemic complications after coronary artery bypass grafting (CABG). Rates of perioperative aspirin resistance cited in the literature are up to 50% and could be influenced by extracorporeal circulation. Thus, aspirin resistance after CABG may have a significant clinical relevance.

MATERIALS AND METHODS

In 59 patients undergoing CABG (on-pump, off-pump and combined procedures) aspirin resistance was investigated by arachidonic acid induced platelet aggregometry. Clinical relevance was assessed with 12-month follow up.

RESULTS

Two types of resistance were observed: A preoperative resistance (despite oral aspirin or in vitro addition) was present in 29% and a postoperative developing type was seen in 49% resulting in only 22% of patients with a 'normal' reaction to aspirin. If patients were already on oral aspirin at admission, the rate of resistance was significantly reduced. Off-pump surgery or pump-times exceeding 120 min had no significant impact on resistance. During the 12-month follow up (98.3%), there were three deaths (one stroke, one intestinal ischaemia, one mediastinitis after postoperative delirium) in patients with the perioperative resistance and none in other patients (P = 0.345). In none of those patients who presented with perioperative aspirin resistance, could this aspirin resistance be demonstrated when tested again after 12 months?

CONCLUSIONS

Aspirin resistance is a transient phenomenon present in the majority of patients undergoing CABG. The three deaths in the resistant group may - although not statistically significant - indicate the possibility of a worse outcome for patients with aspirin resistance.

Measurement of platelet P-selectin for remote testing of platelet function during treatment with clopidogrel and/or aspirin

There is great interest in assessing the efficacy of treatment with clopidogrel and aspirin in patients with cardiovascular disease using procedures that can be used in a remote setting. Here we have established methods to assess the effects of clopidogrel and aspirin on platelets based on measurements of platelet P-selectin. Platelets were stimulated in whole blood by adding the combination of adenosine diphosphate and the TXA(2) mimetic U46619 (ADP/U4, designed to assess P2Y(12) inhibition) or the combination of arachidonic acid and epinephrine (AA/Epi, designed to assess COX-1 inhibition). The stimulated samples were then fixed using a fixative solution that provides stability for at least 9 days, and sent to a central laboratory for analysis of P-selectin by flow cytometry. Measurements were performed in blood from healthy volunteers and patients with cardiovascular disease. The inhibitory effects of clopidogrel and aspirin were assessed ex vivo and the effects of the direct acting P2Y(12) antagonist cangrelor and aspirin were assessed in vitro. Measurements of platelet aggregation were also performed for comparison. In healthy volunteers clopidogrel ex vivo and cangrelor in vitro markedly inhibited P-selectin expression induced by ADP/U4. Aspirin did not inhibit and did not interfere with the effects of clopidogrel or cangrelor using this test. There was very little overlap of results obtained in the absence and presence of clopidogrel or cangrelor. In contrast, over half of 42 patients with cardiovascular disease did not respond well to clopidogrel treatment, although cangrelor was still effective. Aspirin markedly inhibited P-selectin expression induced by AA/Epi. Clopidogrel had much less effect and did not interfere with the effects of aspirin. There was no overlap of results obtained in the absence and presence of aspirin. Aspirin provided near-complete inhibition in 29 of 30 patients with cardiovascular disease. Aggregometry measurements agreed well with the P-selectin data obtained ex vivo following both clopidogrel and aspirin treatment. It is concluded that measurements of P-selectin performed on fixed blood samples following platelet stimulation in whole blood in a remote setting can be used effectively to monitor the effects of clopidogrel and aspirin.

Investigating the mechanisms of hyporesponse to antiplatelet approaches

Hyporesponsiveness, or resistance, to antiplatelet therapy may be a major contributor to poorer outcomes among cardiac patients and may be attributed to an array of mechanisms--both modifiable and unmodifiable. Recent evidence has uncovered clinical, cellular, and genetic factors associated with hyporesponsiveness. Patients with severe acute coronary syndromes (ACS), type 2 diabetes, and increased body mass index appear to be the most at risk for hyporesponsiveness. Addressing modifiable mechanisms may offset hyporesponsiveness, while recognizing unmodifiable mechanisms, such as genetic polymorphisms and diseases that affect response to antiplatelet therapy, may help identify patients who are more likely to be hyporesponsive. Hyporesponsive patients might benefit from different dosing strategies or additional antiplatelet therapies. Trials correlating platelet function test results to clinical outcomes are required. Results from these studies could cause a paradigm shift toward individualized antiplatelet therapy, improving predictability of platelet inhibition, and diminishing the likelihood for hyporesponsiveness.

Static platelet adhesion, flow cytometry and serum TXB2 levels for monitoring platelet inhibiting treatment with ASA and clopidogrel in coronary artery disease: a randomised cross-over study

Background

Despite the use of anti-platelet agents such as acetylsalicylic acid (ASA) and clopidogrel in coronary heart disease, some patients continue to suffer from atherothrombosis. This has stimulated development of platelet function assays to monitor treatment effects. However, it is still not recommended to change treatment based on results from platelet function assays. This study aimed to evaluate the capacity of a static platelet adhesion assay to detect platelet inhibiting effects of ASA and clopidogrel. The adhesion assay measures several aspects of platelet adhesion simultaneously, which increases the probability of finding conditions sensitive for anti-platelet treatment.

Methods

With a randomised cross-over design we evaluated the anti-platelet effects of ASA combined with clopidogrel as well as monotherapy with either drug alone in 29 patients with a recent acute coronary syndrome. Also, 29 matched healthy controls were included to evaluate intra-individual variability over time. Platelet function was measured by flow cytometry, serum thromboxane B₂ (TXB₂)-levels and by static platelet adhesion to different protein surfaces. The results were subjected to Principal Component Analysis followed by ANOVA, t-tests and linear regression analysis.

Results

The majority of platelet adhesion measures were reproducible in controls over time denoting that the assay can monitor platelet activity. Adenosine 5'-diphosphate (ADP)-induced platelet adhesion decreased significantly upon treatment with clopidogrel compared to ASA. Flow cytometric measurements showed the same pattern (r² = 0.49). In opposite, TXB₂-levels decreased with ASA compared to clopidogrel. Serum TXB₂ and ADP-induced platelet activation could both be regarded as direct measures of the pharmacodynamic effects of ASA and clopidogrel respectively. Indirect pharmacodynamic measures such as adhesion to albumin induced by various soluble activators as well as SFLLRN-induced activation measured by flow cytometry were lower for clopidogrel compared to ASA. Furthermore, adhesion to collagen was lower for ASA and clopidogrel combined compared with either drug alone.

Conclusion

The indirect pharmacodynamic measures of the effects of ASA and clopidogrel might be used together with ADP-induced activation and serum TXB₂ for evaluation of anti-platelet treatment. This should be further evaluated in future clinical studies where screening opportunities with the adhesion assay will be optimised towards increased sensitivity to anti-platelet treatment.

Aspirin and clopidogrel: efficacy and resistance in diabetes mellitus

Diabetes mellitus patients are characterized by enhanced platelet reactivity which exposes them to an increased risk of atherothrombotic events in the setting of acute coronary syndromes or percutaneous coronary interventions. Although aspirin and clopidogrel, used either solely or in combination, are associated with improved clinical outcomes in high-risk patients, diabetics patients treated with antiplatelet agents remain at higher risk of recurrent ischemic events. Recent laboratory findings suggest that this observation may be related to a reduced responsiveness or 'resistance' to these agents. In this chapter the efficacy of currently available oral antiplatelet agents in preventing ischemic events is reviewed. In addition, the antiplatelet 'resistance' phenomenon in the diabetic population and its impact on clinical outcomes is summarized. Finally, future developments in the field directed towards individualized treatment strategies and novel antiplatelet agents are examined.

Update on oral antiplatelet therapy: principles, problems and promises

Platelets are major players in arterial thrombosis, and antiplatelet therapy has a clear clinical benefit in the treatment and prevention of cardiovascular events. In particular, aspirin and clopidogrel have become cornerstones in the treatment of patients with atherothrombosis. However, despite the proven efficacy of antiplatelet drugs, cardiovascular events remain an important cause of morbidity and mortality in these patients. Furthermore, a considerable variability in platelet reactivity during treatment with established oral antiplatelet therapy has prompted the search for novel drugs against platelet-dependent thrombosis. Possible benefits of upcoming drugs include a more efficient platelet inhibition and a reversible effect on platelet function. Aspirin, clopidogrel, prasugrel, ticagrelor, terutroban, E5555, SCH 530348 and cilostazol are discussed. This review highlights the rationale for important oral antiplatelet drugs in development and provides clinical perspectives on their pharmacological advantages and challenges.

Methods for the measurement of platelet function

This article discusses the advantages and disadvantages of methods for the measurement of platelet function. The focus is on tests that can be used to monitor antiplatelet activity in the setting of cardiovascular disease and potentially predict thrombosis and bleeding. The tests described are platelet aggregometry; impedance aggregometry; VerifyNow (Accumetrics, San Diego, CA); Plateletworks (Helena Laboratories, Beaumont, TX); platelet surface P-selectin, platelet surface-activated glycoprotein IIb/IIIa, and leukocyte-platelet aggregates; TEG Platelet Mapping system (Haemoscope, Niles, IL); Impact cone and plate(let) analyzer (DiaMed, Cressier, Switzerland); Platelet Function Analyzer-100 (Siemens Healthcare Diagnostics, Inc., Deerfield, IL); phosphorylation of vasodilator-stimulated phosphoprotein; serum thromboxane B(2); and urinary 11-dehydro thromboxane B(2). Some of the factors that differentiate these tests are sample volume requirements, the use of whole blood, the presence of shear, point-of-care status, need for a technician, and expense.

Antiplatelet agents

The introduction of aspirin as an anti-thrombotic agent some 50 years ago has changed the therapeutic approach in cardiovascular medicine. Since platelets play a key role in the development of arterial thrombosis, antiplatelet drugs serve as a cornerstone in the prevention and the treatment of these conditions. After many years of a "monopoly" of aspirin, ADP receptor P2Y12 inhibitors were introduced with a significant improvement in clinical outcome. Nowadays dual antiplatelet therapy is the common practice for both acute events and secondary prevention in selected groups of patients. Another revolution was the development of potent inhibitors of the platelet integrin GPIIbIIIa, which significantly improved the outcome of percutaneous interventions (PCI), in cardiology. The improved efficacy of multiple-drug therapy is associated with an increased risk of bleeding, which raises the issue of the dosing of these drugs. Recently, numerous studies have reported a variable laboratory response to aspirin and clopidogrel, which correlates with clinical outcome. Several mechanisms have been proposed to cause this variable response, including genetic variability, disease burden and others. A major obstacle in this field is the lack of a standardized method for testing these responses, and thus some studies cannot be compared to others. Ongoing studies are currently investigating the potential translation of these observations into clinical practice. Such studies may lead to a change in the paradigm of antiplatelet therapy, where individual dose adjustment may improve efficacy and safety. Finally, a variety of new drugs are currently in different stages of development, including new P2Y12 receptor inhibitors, thromboxane receptor blockers, direct thrombin inhibitors and other signaling pathway inhibitors including oral GPIIbIIIa inhibitors. Thus, antiplatelet therapy is currently under intensive development toward multiple-drug therapy and personal-dose adjustment, which may improve clinical outcome.

Inhibition of platelet GPIIb-IIIa and P-selectin expression by aspirin is impaired by stress hyperglycemia

Increased aspirin resistance may contribute to the increase in thrombotic events observed in patients with type 2 diabetes. In this study, we examined if acute exposure to increased plasma glucose impaired the inhibitory effects of aspirin on platelet activation. Whole-blood samples were incubated with 100 (euglycemia), 200, 300, and 600 mg/dl glucose followed by incubation with aspirin [acetylsalicylic acid (ASA)]. Using flow cytometry, GPIIb-IIIa and P-selectin were analyzed in unstimulated and arachidonic acid (AA)-stimulated platelets. In euglycemic blood, AA caused a significant increase in platelet GPIIb-IIIa expression [unstimulated: 59.5+/-8.2 total fluorescence intensity (TFI), AA stimulated: 319.6+/-42.7 TFI, P=.002] and P-selectin (4.4+/-0.7 and 179.5+/-38.5 TFI, P<.001). In vitro, ASA significantly inhibited both GPIIb-IIIa expression (36.5%) and P-selectin expression (81%; P<.005). However, increased blood glucose (200 mg/dl) significantly impaired the inhibitory effect of ASA (84% for GPIIb-IIIa, P<.005; 48% for P-selectin, P=NS). Increasing glucose to 600 mg/dl completely overwhelmed the inhibitory effect of ASA. A statistically significant interaction between glucose concentration and ASA dose was found (P<.001 for GPIIb-IIIa and P=.004 for P-selectin). In vitro, concentration-dependent stress hyperglycemia significantly impaired the inhibitory effects of aspirin on human platelet GPIIb-IIIa and P-selectin expression. Under acute hyperglycemic conditions, the effectiveness of ASA to inhibit platelets via the AA-activation pathway may be significantly reduced.

Aspirin 'resistance': role of pre-existent platelet reactivity and correlation between tests

BACKGROUND

Aspirin 'resistance' is a widely used term for hyporesponsiveness to aspirin in a platelet function test. Serum thromboxane (TX) B(2) is the most specific test of aspirin's effect on platelets.

OBJECTIVES

(i) To examine the role of pre-existent platelet hyperreactivity in aspirin 'resistance'. (ii) To determine the correlation between aspirin resistance defined by serum TXB(2) and other assays of platelet function.

METHODS

To enable pre-aspirin samples to be drawn, platelet function was measured in normal subjects (n = 165) before and after aspirin 81 mg daily for seven days.

RESULTS

The proportion of the post-aspirin platelet function predicted by the pre-aspirin platelet function was 28.3 +/- 7.5% (mean +/- asymptotic standard error) for serum TXB(2), 39.3 +/- 6.8% for urinary 11-dehydro TXB(2), 4.4 +/- 7.7% for arachidonic acid-induced platelet aggregation, 40.4 +/- 7.1% for adenosine diphosphate-induced platelet aggregation, 26.3 +/- 9.2% for the VerifyNow Aspirin Assay, and 45.0 +/- 10.9% for the TEG PlateletMapping System with arachidonic acid. There was poor agreement between aspirin-resistant subjects identified by serum TXB(2) vs. aspirin-resistant subjects identified by the other five assays, irrespective of whether the analysis was based on categorical or continuous variables. Platelet count correlated with pre-aspirin serum TXB(2) and VerifyNow Aspirin Assay, but not with any post-aspirin platelet function test.

CONCLUSIONS

(i) Aspirin 'resistance' (i.e. hyporesponsiveness to aspirin in a laboratory test) is in part unrelated to aspirin but is the result of underlying platelet hyperreactivity prior to the institution of aspirin therapy. (ii) Aspirin resistance defined by serum TXB(2) shows a poor correlation with aspirin resistance defined by other commonly used assays.

Antiplatelet therapy in acute coronary syndromes

Acute coronary syndromes (ACS), characterized by unstable angina or a non-ST-segment elevation myocardial infarction, are caused by rupture of an atherosclerotic plaque, leading to platelet activation and aggregation, thrombus formation, and microembolization. Antiplatelet therapy is a cornerstone of therapy. Combined with aspirin, clopidogrel provides significant benefit for patients across the ACS spectrum. However, clopidogrel has limitations given its slow onset and the inconsistent level of inhibition that it achieves. Newer thienopyridine and non-thienopyridine P2Y12 receptor agonists offer the advantages of a rapid onset of action and greater and more consistent platelet inhibition.

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.

Safety and efficacy of clopidogrel in children with heart disease

OBJECTIVE

To evaluate the efficiency and safety of clopidogrel treatment in children with heart disease.

STUDY DESIGN

We conducted single center retrospective chart review of children with heart disease at the University Hospital, Leuven, Belgium, in whom clopidogrel was used. The indication, dosage, duration of therapy, and adverse events were examined. Clinical efficacy was defined by an absence of thrombotic events.

RESULTS

46 children were identified. The mean age of first clopidogrel dose was 4.9 +/- 4.1 years. The study dosage ranged from 0.1 to 0.7 mg/kg/day clopidogrel. Almost all patients received concomitant aspirin therapy. No thrombotic events developed. Skin bruising developed in almost every patient, suggesting that clopidogrel has an anti-platelet effect. 2 patients who were treated with concomitant warfarin had bleeding complications (severe epistaxis and gastrointestinal bleeding). Hematological abnormalities were documented in 1 patient who received clopidogrel for 1 year; they reversed with medication cessation.

CONCLUSION

Clopidogrel therapy in a pediatric population appears to be relatively safe and effective; however, randomized, controlled prospective studies are needed to determine the true efficacy and safety of clopidogrel in children.

The role of aspirin in cardiovascular prevention: implications of aspirin resistance

Aspirin is well recognized as an effective antiplatelet drug for secondary prevention in subjects at high risk of cardiovascular events. However, most patients receiving long-term aspirin therapy still remain at substantial risk of thrombotic events due to insufficient inhibition of platelets, specifically via the thromboxane A2 pathway. Although the exact prevalence is unknown, estimates suggest that between 5.5% and 60% of patients using this drug may exhibit a degree of "aspirin resistance," depending upon the definition used and parameters measured. To date, only a limited number of clinical studies have convincingly investigated the importance of aspirin resistance. Of these, few are of a sufficient scale, well designed, and prospective, with aspirin used at standard doses. Also, most studies do not sufficiently address the issue of noncompliance to aspirin as a frequent, yet easily preventable cause of resistance to this antiplatelet drug. This review article provides a comprehensive overview of aspirin resistance, discussing its definition, prevalence, diagnosis, and therapeutic approaches. Moreover, the clinical implications of aspirin resistance are explored in various cardiovascular disease states, including diabetes mellitus, hypertension, heart failure, and other similar disorders where platelet reactivity is enhanced.

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.

Pharmacogenomics of platelet responsiveness to aspirin

Aspirin is the most widely used drug in the world for cardiovascular protection. Aspirin's ability to suppress platelet function varies widely among individuals and lesser suppression of platelet function is associated with increased risk of myocardial infarction, stroke and cardiovascular death. Platelet response to aspirin is a complex phenotype involving multiple genes and molecular pathways. Aspirin response phenotypes can be categorized as directly or indirectly related to cyclooxygenase-1 (COX-1) activity, with phenotypic variation indirectly related to COX-1 being much more prominent. Recent data indicate that variability in platelet response to aspirin is genetically determined, but the specific gene variants that contribute to phenotypic variation are not known. An understanding of the relationship between genotype, aspirin response phenotype and clinical outcome will help to bring about a personalized approach to antiplatelet therapy that maximizes antithrombotic benefit whilst minimizing bleeding risk for individual patients.