Resistance to aspirin in vitro is associated with increased platelet sensitivity to adenosine diphosphate

Aspirin Reduces the Potentiating Effect of CD40L on Platelet Aggregation via Inhibition of Myosin Light Chain

Background

Antiplatelet therapy with aspirin (acetylsalicylic acid [ASA]) is less efficient in some coronary patients, which increases their risk of developing thrombosis. Elevated blood levels of thromboinflammatory mediators, like soluble CD40L (sCD40L), may explain such variabilities. We hypothesized that in the presence of elevated levels of sCD40L, the efficacy of ASA may vary and aimed to determine the effects of ASA on CD40L signaling and aggregation of platelets.

Methods and Results

The effects of ASA on CD40L‐treated human platelets, in response to suboptimal concentrations of collagen or thrombin, were assessed at levels of aggregation, thromboxane A₂ secretion, and phosphorylation of p38 mitogen‐activated protein kinase, nuclear factor kappa B, transforming growth factor‐β–activated kinase 1, and myosin light chain. sCD40L significantly elevated thromboxane A₂ secretion in platelets in response to suboptimal doses of collagen and thrombin, which was reversed by ASA. ASA did not inhibit the phosphorylation of p38 mitogen‐activated protein kinase, nuclear factor kappa B, and transforming growth factor‐β–activated kinase 1, with sCD40L stimulation alone or with platelet agonists. sCD40L potentiated platelet aggregation, an effect completely reversed and partially reduced by ASA in response to a suboptimal dose of collagen and thrombin, respectively. The effects of ASA in sCD40L‐treated platelets with collagen were related to inhibition of platelet shape change and myosin light chain phosphorylation.

Conclusions

ASA does not affect platelet sCD40L signaling but prevents its effect on thromboxane A₂ secretion and platelet aggregation in response to collagen, via a mechanism implying inhibition of myosin light chain. Targeting the sCD40L axis in platelets may have a therapeutic potential in patients with elevated levels of sCD40L and who are nonresponsive or less responsive to ASA.

The Role of Chemoprophylactic Agents in Modulating Platelet Aggregability After Traumatic Brain Injury

BACKGROUND

The pathophysiology behind the subacute but persistent hypercoagulable state after traumatic brain injury (TBI) is poorly understood but contributes to morbidity induced by venous thromboembolism. Because platelets and their microvesicles have been hypothesized to play a role in post-traumatic hypercoagulability, administration of commonly used agents may ameliorate this coagulability. We hypothesized that utilization of aspirin, ketorolac, amitriptyline, unfractionated heparin, or enoxaparin would modulate the platelet aggregation response after TBI.

METHODS

Concussive TBI was induced by weight drop. Mice were then randomized to receive aspirin, ketorolac, amitriptyline, heparin, enoxaparin, or saline control at 2 and 8 h after TBI. Mice were sacrificed at 6 or 24 h after injury to determine coagulability by rotational thromboelastometry (ROTEM), platelet function testing with impedance aggregometry, and microvesicle enumeration. Platelet sphingolipid metabolites were analyzed by mass spectrometry.

RESULTS

ROTEM demonstrated increased platelet contribution to maximum clot firmness at 6 h after TBI in mice that received aspirin or amitriptyline, but this did not persist at 24 h. By contrast, adenosine diphosphate- and arachidonic acid-induced platelet aggregation at 6 h was significantly lower in mice receiving ketorolac, aspirin, and amitriptyline compared with mice receiving saline at 6 h after injury and only arachidonic acid-initiated platelet aggregation was decreased by aspirin at 24 h. There were no differences in microvesicle production at either time point. Platelet sphingosine-1-phosphate levels were decreased at 6 h in the group receiving amitriptyline and increased at 24 h along with platelet ceramide levels at 24 h in the amitriptyline group.

CONCLUSION

After TBI, amitriptyline decreased platelet aggregability and increased contribution to clot in a manner similar to aspirin. The amitriptyline effects on platelet function and sphingolipid metabolites may represent a possible role of the acid sphingomyelinase in the hypercoagulability observed after injury. In addition, inhibition of platelet reactivity may be an underappreciated benefit of low molecular weight heparins, such as enoxaparin.

Frequency of development of aspirin resistance in the early postoperative period and inadequate inhibition of thromboxane A2 production after coronary artery bypass surgery

Background

This study aims to investigate the frequency of the development of aspirin resistance, whether or not this resistance was reversible, and to evaluate the efficiency of the mechanism of incomplete inhibition of thromboxane A2 in development of aspirin resistance in the early postoperative period in patients who had undergone coronary artery bypass grafting.

Methods

Eighty patients (55 males, 25 females; mean age 63.1±9.2 years; range 51 to 75 years) who underwent coronary artery bypass grafting between February 2009 and March 2010 at our clinic were prospectively evaluated. Venous blood samples were collected from all patients and evaluated by a platelet function analyzer in the preoperative period and on postoperative days 7 and 15. Aspirin resistance diagnosis was defined as collagen-epinephrine closure time less than 186 seconds. The urine levels of 11-dehidro thromboxane B2 were also measured on postoperative day one.

Results

Aspirin resistance was found in 23 patients (28.75%) in the preoperative period, in 31 patients (38.75%) on the postoperative seventh day and in 25 patients (31.25%) on the postoperative 15th day. The urine levels of 11-dehidro thromboxane B2 in patients with aspirin resistance on the postoperative seventh day were significantly higher than those in patients without aspirin resistance (p<0.001). The mean aortic cross-clamping time (p=0.003) and cardiopulmonary bypass time (p=0.029) in the patients with aspirin resistance on the postoperative seventh day were significantly higher than those in patients without aspirin resistance.

Conclusion

The results of this study suggest that aspirin resistance develops within the first seven days after coronary artery bypass grafting and is highly reversible, and that the mechanism of inadequate inhibition of thromboxane A2 by aspirin has a role in the development of aspirin resistance in the early postoperative period.

Multiplate® evaluation of acetylsalicylic acid efficacy in carotid surgery: routine and genetic influencing factors

Essentials Acetylsalicylic acid (ASA) is prescribed to patients scheduled for carotid endarterectomy (CEA). We measured ASA efficacy during CEA by Multiplate® and searched for influencing factors. Most patients scheduled for CEA and treated by ASA are sensitive to this therapy. Influencing genomic factors are involved in ASA metabolism and in platelet function modulations.

SUMMARY

Background Acetylsalicylic acid (ASA) is recommended before, during and after carotid endarterectomy (CEA). The efficacy of ASA is influenced by numerous biological and genotypic factors. Objectives To determine the biological efficacy of ASA by using the Multiplate® method, and to explore the biological parameters and genomic factors influencing this efficacy. Methods This descriptive cross-sectional study included all patients scheduled for CEA between January 2012 and April 2013. Multiplate® tests were performed at day 0 and day 30. A set of 66 single-nucleotide polymorphisms (SNPs) from 38 genes or DNA regions were selected and studied along with phenotypic parameters by the use of hierarchical clustering (HC) for multidimensional data management. Results Fifty-five patients receiving ASA were analyzed. Of the patients, 95% were found to be sensitive to ASA, with values under the threshold of normality (400 AU min-1 ). However, there were notable differences in residual aggregation among subjects over a wide range. HC revealed four subclusters comprising three categories of parameters: (i) routine and functional parameters - in ASA-treated patients, the ASPItest was highly linked to the ADPtest, to platelet count, and, to a lesser extent, to fibrinogen and hematocrit; (ii) polymorphisms in genes involved in ASA absorption and in the arachidonic acid pathway (ABCB1 and COX-1); and (iii) polymorphisms in genes modulating basal platelet function, i.e. TBXA2R, ADRA2A, PEAR1, ITGA2 and ITGB1. Conclusion Most patients treated with ASA before CEA were sensitive to it, according to Multiplate® ASPItest results. Genomic factors influencing this efficacy are SNPs involved in ASA absorption and metabolic pathway, and in modulations in basal platelet function.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

ASPIRIN RESISTANCE IN NEUROVASCULAR DISEASES

INTRODUCTION

The issue of resistance to antiplatelet therapy has raised many questions in the area of neurovascular diseases. The first objective of this work was to determine the prevalence of aspirin resistance in neurovascular patients with clinical non-responsiveness to aspirin treatment and a high-risk of atherothrombotic complications using two interpretable and independent methods (aggregation and PFA 100). The second objective was to find the correlation between both assays and to evaluate the results in groups at risk for various cerebrovascular diseases.

MATERIAL AND METHODS

Laboratory tests of aspirin resistance were performed in 79 patients with clinical non-responsiveness to aspirin treatment suffering from neurovascular diseases. Patients were divided into the two groups: expected low risk for aspirin resistance due to the first manifestation of a neurovascular disease (n = 34) and expected high risk due to the second clinical manifestation of a neurovascular disease (n = 45).

RESULTS

The prevalence of aspirin resistance in both groups combined as determined by the PFA-100 and CPG techniques were 50.6% and 17.7%, respectively. No correlation was found between the two techniques.

CONCLUSIONS

No significant prevalence of aspirin resistance was demonstrated by either method despite the heterogeneous pathophysiological mechanisms. However, we are presently unable to provide an accurate opinion on the value of laboratory test result or routine monitoring in clinical neurology.

Circulating primers enhance platelet function and induce resistance to antiplatelet therapy

BACKGROUND

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

OBJECTIVES

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Searching for factors associated with resistance to acetylsalicylic acid used for secondary prevention of stroke

INTRODUCTION

The aim of the study was to evaluate the prevalence of resistance to acetylsalicylic acid (ASA), used for secondary prevention of stroke, including the assessment of risk factors associated with the lack of ASA anti-aggregatory action.

MATERIAL AND METHODS

Patients after a transient ischaemic attack (TIA) or ischaemic stroke in the acute (n = 111) and chronic phase (n = 87) were enrolled in the study. The assessment of platelet function was performed by whole blood impedance aggregometry using a multi-channel platelet function analyser (Multiplate).

RESULTS

A proper response to ASA was found in 121 patients (61.1%) (ASA responders), a partial response to ASA in 59 patients (29.8%) (ASA partial responders), and ASA resistance in 18 patients (9.1%) (ASA non-responders). Acetylsalicylic acid resistance was observed more frequently in the chronic phase. The mean low-density lipoprotein (LDL) concentration was higher in ASA non-responders (p = 0.02). The mean heart rate (p = 0.03) and the mean haematocrit (p = 0.03) were higher in the group of ASA partial responders and ASA non-responders. Angiotensin II receptor antagonists were more often used in the group of ASA partial responders and ASA non-responders (p = 0.04). Diuretics were more rarely used by ASA non-responders, whereas fibrates were more rarely used by ASA partial responders.

CONCLUSIONS

The method enabled the detection of ASA resistance in some patients with cerebrovascular disease. The study revealed some possible risk factors of ASA resistance: long ASA therapy, increased heart rate, higher LDL concentration, and higher haematocrit value. The relationship between the effect of ASA and other medications (angiotensin II receptor blockers, fibrates, diuretics) requires further study. Platelet function monitoring should be considered in patients at a greater risk of ASA resistance.

Pharmacometabolomics reveals that serotonin is implicated in aspirin response variability

While aspirin is generally effective for prevention of cardiovascular disease, considerable variation in drug response exists, resulting in some individuals displaying high on-treatment platelet reactivity. We used pharmacometabolomics to define pathways implicated in variation of response to treatment. We profiled serum samples from healthy subjects pre- and postaspirin (14 days, 81 mg/day) using mass spectrometry. We established a strong signature of aspirin exposure independent of response (15/34 metabolites changed). In our discovery (N = 80) and replication (N = 125) cohorts, higher serotonin levels pre- and postaspirin correlated with high, postaspirin, collagen-induced platelet aggregation. In a third cohort, platelets from subjects with the highest levels of serotonin preaspirin retained higher reactivity after incubation with aspirin than platelets from subjects with the lowest serotonin levels preaspirin (72 ± 8 vs. 61 ± 11%, P = 0.02, N = 20). Finally, ex vivo, serotonin strongly increased platelet reactivity after platelet incubation with aspirin (+20%, P = 4.9 × 10⁻⁴, N = 12). These results suggest that serotonin is implicated in aspirin response variability.

High on-treatment platelet reactivity in peripheral endovascular procedures

The use of aspirin is considered the "gold standard" for the decrease of major adverse cardiovascular events in patients with atherosclerosis, including peripheral arterial disease (PAD), whereas a dual-antiplatelet regimen with aspirin and clopidogrel is usually indicated for such patients after angioplasty and stent deployment. However, a substantial number of subsequent adverse events still occur, even in patients who receive double-antiplatelet therapy. The "high on-treatment platelet reactivity" (HTPR) phenomenon has been lately recognized and plays a major role in the management of patients with PAD. Greater and more rapid inhibition of platelet aggregation has become the goal for new antiplatelet agents with the expectation of further improving outcomes for percutaneous intervention for PAD. The purpose of this review article is to highlight current evidence regarding the prevalence, aetiology, and clinical implications of HTPR in PAD as well as to discuss the possibilities of novel alternative antiplatelet regiments.

Resistance to acetylsalicylic acid in patients with type 2 diabetes mellitus is associated with lipid disorders and history of current smoking

BACKGROUND

Diabetes mellitus (DM) is an important risk factor for stroke. Acetylsalicylic acid (ASA) is the most frequently used medication for prevention of cardio-cerebral vascular diseases. However, some patients experience ischaemic vascular events despite the use of ASA. This phenomenon is known as "aspirin resistance" (AR). The aim of this study was to assess the prevalence of AR in diabetic patients and search for factors associated with it.

MATERIALS AND METHODS

The examined group consisted of 96 subjects with diagnosed type 2 DM. Platelet function test was performed by the method of whole blood impedance aggregometry.

RESULTS

Among examined subjects, 51 patients (53.1 %) were sensitive to ASA action (ASA responders) and 45 patients (46.9 %) were resistant to ASA action (ASA non-responders). No association was found between platelet aggregation and gender, age, dose of ASA, known duration of diabetes, BMI, heart rate, mean systolic and diastolic blood pressure, and risk factors except for current smoking (p = 0.030). ASA non-responders were treated shorter with ASA than ASA responders (p = 0.010). The mean total cholesterol (p = 0.020), LDL concentration (p = 0.005), HCT (p = 0.010), WBC (p = 0.030), and PLT (p = 0.050) were significantly higher in ASA non-responders. No association was found between AR and results of other laboratory tests and medications. Multiple logistic regression analysis revealed factors associated with AR: current smoking and LDL concentration higher than 3.5 mmol/l.

CONCLUSIONS

Results of our study did not confirm the association between poor glycaemic control in the diabetic patients and AR. Resistance to ASA in diabetic patients is associated with lipid disorders and history of current smoking.

Purine pathway implicated in mechanism of resistance to aspirin therapy: pharmacometabolomics-informed pharmacogenomics

Although aspirin is a well-established antiplatelet agent, the mechanisms of aspirin resistance remain poorly understood. Metabolomics allows for measurement of hundreds of small molecules in biological samples, enabling detailed mapping of pathways involved in drug response. We defined the metabolic signature of aspirin exposure in subjects from the Heredity and Phenotype Intervention Heart Study. Many metabolites, including known aspirin catabolites, changed on exposure to aspirin, and pathway enrichment analysis identified purine metabolism as significantly affected by drug exposure. Furthermore, purines were associated with aspirin response, and poor responders had higher postaspirin adenosine and inosine levels than did good responders (n = 76; both P < 4 × 10(-3)). Using our established "pharmacometabolomics-informed pharmacogenomics" approach, we identified genetic variants in adenosine kinase associated with aspirin response. Combining metabolomics and genomics allowed for more comprehensive interrogation of mechanisms of variation in aspirin response--an important step toward personalized treatment approaches for cardiovascular disease.

Vascular and platelet responses to aspirin in patients with coronary artery disease

BACKGROUND

Variability in the response to aspirin (sometimes known as aspirin resistance) in modulating platelet activity is a potentially important clinical issue in coronary artery disease (CAD), but may be also be important in other areas of pathophysiology.

MATERIALS AND METHODS

Testing the hypothesis of a relationship between aspirin resistance and vascular function, inflammation and coagulation, we recruited 175 stable CAD outpatients taking 75 mg aspirin daily. Indices were compared to 58 controls not taking aspirin. Platelet activity was assessed by light transmission aggregometry (LTA) to 0·5 mg/mL arachidonic acid (AA), plasma markers soluble P selectin and thromboxane (ELISA), and resting and AA stimulated membrane P selectin and PAC-1 expression (flow cytometry). Vascular function was assessed by arterial stiffness (Sphygmocor system), von Willebrand factor and soluble E selectin (ELISA), inflammation by high sensitivity CRP and interleukin-6, and coagulation by tissue factor and fibrin d-dimers levels (all immunoassay).

RESULTS

The 5-min LTA response AA was superior to flow cytometry in discriminating the response of platelets to aspirin. Using the cut-off of 20% LTA response to AA, 32·6% of patients were aspirin resistant. The latter had higher soluble P selectin (P = 0·03), CRP (P = 0·029) and fibrin d-dimers (P = 0·01) compared to those who were aspirin sensitive. There was no relationship between aspirin response status and any vascular index.

CONCLUSION

We conclude that LTA is a more sensitive marker of aspirin resistance than is flow cytometry for P-selectin and PCA-1, and that aspirin response has no influence on vascular function.

Aspirin and platelet adenosine diphosphate receptor antagonists in acute coronary syndromes and percutaneous coronary intervention: role in therapy and strategies to overcome resistance

Platelet activation and aggregation are key components in the cascade of events causing thrombosis following plaque rupture. Antiplatelet therapy is essential in the treatment of patients with acute coronary syndromes (ACS) and for those requiring percutaneous coronary intervention (PCI). Aspirin (acetylsalicylic acid) is a well established antiplatelet therapy and is mandated for secondary prevention of cardiovascular events following ACS. In patients with ACS, the addition of clopidogrel to aspirin is more effective than aspirin alone. For patients undergoing PCI, dual antiplatelet therapy with aspirin and clopidogrel is warranted. Aspirin should be continued indefinitely after PCI. Pretreatment of patients with clopidogrel prior to PCI lowers the incidence of cardiovascular events, yet the optimum timing of drug administration and dose are still being investigated, as is the duration of therapy following PCI. Late-stent thrombosis with drug-eluting stents has pushed the recommendation for duration of clopidogrel therapy up to 1 year and perhaps beyond, in patients without risks for bleeding. The concepts of aspirin and clopidogrel resistance are important clinical questions. No uniform definition exists for aspirin or clopidogrel resistance. Measurements of resistance are often highly variable and do not necessarily correlate with clinical resistance. Noncompliance remains the most prominent mode of resistance. Screening of selected patient populations for resistance or pharmacologic intervention of those patients termed 'resistant' warrants further study.

Aspirin resistance: current status and role of tailored therapy

Aspirin is integral in the primary and secondary prevention of coronary artery disease and acute coronary syndrome. Given the high clinical importance of aspirin in the management of coronary artery disease, much attention has been directed towards the concept of "aspirin resistance." Unfortunately, the term aspirin resistance is ill-defined in the literature, leading to a large variance in the reported prevalence of this phenomenon. In this review, the current understanding of aspirin resistance is discussed. Commonly used functional and diagnostic tests of platelet function, including their strengths and weakness, are reviewed. We next discuss several proposed mechanisms of aspirin resistance and special high-risk groups at risk for aspirin treatment failure. We then discuss optimal dosing and diagnostic strategies for those populations at risk for aspirin resistance with a focus on tailored aspirin therapy for high-risk groups. Finally, future topics of interest in the field of aspirin resistance are considered.

Aldose reductase, oxidative stress, and diabetic mellitus

Diabetes mellitus (DM) is a complex metabolic disorder arising from lack of insulin production or insulin resistance (Diagnosis and classification of diabetes mellitus, 2007). DM is a leading cause of morbidity and mortality in the developed world, particularly from vascular complications such as atherothrombosis in the coronary vessels. Aldose reductase (AR; ALR2; EC 1.1.1.21), a key enzyme in the polyol pathway, catalyzes nicotinamide adenosine dinucleotide phosphate-dependent reduction of glucose to sorbitol, leading to excessive accumulation of intracellular reactive oxygen species (ROS) in various tissues of DM including the heart, vasculature, neurons, eyes, and kidneys. As an example, hyperglycemia through such polyol pathway induced oxidative stress, may have dual heart actions, on coronary blood vessel (atherothrombosis) and myocardium (heart failure) leading to severe morbidity and mortality (reviewed in Heather and Clarke, 2011). In cells cultured under high glucose conditions, many studies have demonstrated similar AR-dependent increases in ROS production, confirming AR as an important factor for the pathogenesis of many diabetic complications. Moreover, recent studies have shown that AR inhibitors may be able to prevent or delay the onset of cardiovascular complications such as ischemia/reperfusion injury, atherosclerosis, and atherothrombosis. In this review, we will focus on describing pivotal roles of AR in the pathogenesis of cardiovascular diseases as well as other diabetic complications, and the potential use of AR inhibitors as an emerging therapeutic strategy in preventing DM complications.

The pattern of platelet response to clopidogrel in Iranian patients after percutaneous coronary intervention

Despite certain clinical benefit in using clopidogrel in patients undergoing percutaneous coronary intervention (PCI), some patients do not attain adequate antiplatelet effects. In this study, the authors investigated the response to clopidogrel in Iranian patients after PCI. Patients who were candidates for elective PCI were enrolled in this study. All patients had received aspirin 80 to 325 mg daily for ≥1 week before PCI. Blood samples were taken from patients at baseline, 2 hours after taking a 600-mg loading dose of clopidogrel, and 24 hours and 30 days after stenting. Platelet aggregation was measured by light transmittance aggregometry with adenosine diphosphate (5 and 20 μM) and arachidonic acid (500 and 5000 μg/mL). One hundred twelve patients were included (79 men, 33 women). Maximal and minimal clopidogrel nonresponsiveness occurred at 2 hours (26%) and 48 hours (13%) after taking 600 mg clopidogrel, respectively. Pretreatment platelet reactivity had no effects on posttreatment platelet reactivity. Moreover, clopidogrel responsiveness did not correlate with pretreatment reactivity. Patients' demographic and procedural characteristics had no significant effect on clopidogrel responsiveness. The frequency of clopidogrel nonresponsiveness in this study was similar to other studies. However, clopidogrel required more than 2 hours for induction of its maximal antiplatelet effect in this study.

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.

Incomplete inhibition of platelet function as assessed by the platelet function analyzer (PFA-100) identifies a subset of cardiovascular patients with high residual platelet response while on aspirin

Sixty-six patients with a history of ischemic events (myocardial infarction, unstable angina, or stroke) on chronic aspirin therapy were studied by different platelet function tests: 37 patients had suffered a recurrent event while on aspirin and 29 were without recurrences. Based on results from light transmission aggregometry (LTA) induced by arachidonic acid (AA) and serum TxB(2) both COX-1-dependent methods, only one patient could be identified as aspirin "resistant". However, when methods only partially-dependent on platelet COX-1 activity were considered, the prevalence of aspirin non-responders ranged, according to the different tests, from 0 to 52%. No difference was observed between patients with recurrences and those without. Among patients with recurrent events, those with an incomplete inhibition of platelet function, as assessed by the PFA-100, had significantly higher residual serum TxB(2) (2.4 ± 2.4 ng/mL vs 0.4 ± 0.1 ng/mL, p = 0.03), residual LTA-AA (9.2 ± 10.6% vs 2.0 ± 1.6%, p = 0.008), LTA-Coll (49.3 ± 14.6% vs 10.2 ± 8.3%, p = 0.007) and LTA-ADP (50.9 ± 16.2% vs 34.3 ± 11.0%, p = 0.04). In conclusion, laboratory tests solely exploring the AA-mediated pathway of platelet function, while being the most appropriate to detect the effect of aspirin on its pharmacologic target (platelet COX-1), may fail to reveal the functional interactions between minimal residual TxA(2) and additional stimuli or primers potentially leading to aspirin-insensitive platelet aggregation. High residual platelet response in platelet function tests only partially dependent on COX-1 may reveal a condition of persistent platelet reactivity in a subset of aspirin-treated patients characterizing them as a subgroup at higher vascular risk.

Redox-generated isoprostanes are associated with residual platelet activity in aspirin-treated patients with stable coronary heart disease

AIM

Insufficient platelet inhibition by low-dose aspirin is associated with poor prognosis in patients with coronary heart disease (CHD). We sought to investigate the prevalence of this phenomenon in patients with stable CHD and to study whether oxidative stress plays a role in its pathogenesis.

METHODS AND RESULTS

We studied the platelet response to long-term (≥ 6 months) low-dose (100 mg per day) aspirin in 130 consecutive patients with stable CHD (age 66 ± 8 years, 83% male). Among a wide distribution of platelet responses to collagen, ADP, and arachidonic acid, the vast majority of patients in the highest tertile of residual platelet activity (defined as 'aspirin low-responders') were characterized by lack of platelet inhibition by aspirin in vitro, significantly although not completely suppressed platelet TXB₂ production and COX-1 activity, and significantly higher urinary 8-iso-prostaglandin F(2α) excretion [186 (147-230) vs. 230 (188-318) pg per mg creatinine; median (IQR), P < 0.001; measured by GC-MS].

CONCLUSION

A relevant proportion of patients with CHD show insufficient platelet inhibition by low-dose aspirin. Oxidative stress and lipid peroxidation causing isoprostane formation may underlie inadequate platelet inhibition in an aspirin-insensitive manner in patients with cardiovascular disease.

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.

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

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

Plasma salicylate level and aspirin resistance in survivors of myocardial infarction

To investigate the effect of aspirin on the platelets of survivors of myocardial infarction we correlated plasma salicylate level with platelet reactivity in ten patients and ten normal controls. The patients and controls were tested at the end of 2 week periods on 75, 150 and 300 mg aspirin daily by mouth. Platelet reactivity was measured, under high shear stress conditions, using cartridges containing adrenaline and adenosine diphosphate in a PFA-100 platelet function analyser. The time taken by the developing platelet aggregate to close an aperture in the collagen membrane of the cartridge, the closure time, was taken as an index of platelet reactivity. There was no difference in baseline haematocrit, platelet count or plasma vWF antigen level between the groups. There was a dose-dependent increase in closure time of the adrenaline containing cartridge in the controls (P < 0.001), but not in the patients (P = 0.08), compatible with a reduced anti-platelet effect of aspirin in the patients. Furthermore, plasma salicylate level was higher in the patient group (P < 0.05).

Clinical importance of aspirin and clopidogrel resistance

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

Platelet aspirin resistance in ED patients with suspected acute coronary syndrome

BACKGROUND

Platelet aspirin resistance is reported to be as high as 45%. The prevalence of emergency department (ED) platelet aspirin resistance in suspected acute coronary syndrome (ACS) is not described. Our purpose was to determine the prevalence of platelet aspirin resistance.

METHODS

We determined platelet aspirin resistance in a convenience sample of ED suspected ACS patients. Eligible patients had longer than 10 minutes of chest pain or an ischemic equivalent. Two hours after receiving 325 mg of aspirin, blood was assessed for platelet function (Accumetrics, San Diego, CA). Definitions are as follows: aspirin resistance, at least 550 aspirin reaction units; positive troponin T, greater than 0.1 ng/mL; significant coronary lesion, at least 70% stenosis. The composite end point was prospectively defined as a 30-day revisit, positive cardiac catheterization, or hospital length of stay (LOS) longer than 3 days.

RESULTS

Of 200 patients, 50.5% were male, 50.0% were black, troponin T was positive in 7.5%, cardiac catheterization was done in 10.5%, and 33.3% had a significant stenosis. Final diagnoses were noncardiac in 83.4%, stable angina in 8.0%, and unstable angina in 8.5%. Overall, 6.5% were resistant to aspirin; and high-risk patients trended to more aspirin resistance than non-high-risk patients (23.1% [3] vs 9.1% [17]; P value 95% confidence interval [CI], -0.0929 to 0.373). One-month follow-up found ED revisits in 12.5% of aspirin-resistant vs 4.9% of non-aspirin-resistant patients (95% CI, -0.114 to 0.182) and rehospitalization in 12.5% of resistant patients vs 4.3% of nonresistant patients (P value 95% CI, -0.108 to 0.187). Although LOS was similar at index admission, if rehospitalized, LOS was 6.5 for aspirin-resistant patients vs 3.2 days in nonresistant patients (P < .0001).

CONCLUSION

This first report of platelet aspirin resistance in patients presenting to the ED with suggested ACS finds that it is present in 6.5% of patients.

Critical Overview on the Benefits and Harms of Aspirin

Aspirin is widely used internationally for a variety of indications, with the most prominent one being that of cardiovascular disease. However, aspirin has also been proposed as a treatment option in a diverse range of conditions such as diabetes mellitus, cancer prevention, and obstetrics. In our overview, we critically appraise the current evidence from recent systematic reviews and meta-analyses covering the benefits of aspirin across these conditions. We also look at evidence that some patients may not derive benefit due to the concept of aspirin resistance. Aspirin is also associated with the potential for significant harm, principally from haemorrhagic adverse events. We critically appraise the threat of haemorrhagic complications, and weigh up these risks against that of any potential benefit.

Dosing strategies for antiplatelet therapy in percutaneous coronary intervention

Both clopidogrel and aspirin have been shown to decrease the rate of cardiovascular events and especially stent thrombosis in patients undergoing percutaneous coronary intervention (PCI). However, recent studies have suggested that there is large inter-individual response variability to these drugs (especially to clopidogrel) and that improved inhibition of platelet reactivity using higher doses or new, more potent agents would further reduce the occurrence of cardiovascular events, but may also increase the risk of bleeding. Many different protocols of antiplatelet therapy have been studied and have shown benefit in reducing the rate of major adverse cardiovascular events after PCI. Therefore, the choice of an appropriate antiplatelet therapy protocol is sometimes difficult for the clinician and should be individualized as per the particular patient risk, accounting for both the risk of recurrent cardiovascular events and bleeding. We review the recent data on efficacy and safety of dosing strategies for antiplatelet therapy in PCI.

Stroke and aspirin non-responder patients: relation with hypertension and platelet response to adenosine diphosphate

Despite its widespread use, there are many concerns about the efficacy of aspirin in the secondary prevention of cardiovascular events after stroke, leading to the concept of aspirin non-response (ANR). Although the mechanisms of ANR remain uncertain, it is expected to be due to a combination of clinical, biological and genetic characteristics affecting platelet function. In this study, we investigated whether clinical and/or biological factors such as hypertension and platelet response to ADP could contribute to the ANR. As a secondary objective, we determine whether ANR and collagen/ADP closure time (CADP-CT) could be related to platelet glycoprotein single nucleotide polymorphisms (SNPs). One hundred patients on aspirin (160 mg/day) were enrolled. ANR was measured with a platelet function analyzer (PFA-100); genotyping of four SNPs (GP IIIa, GP Ia, P2Y12 and GP VI) was performed using a tetra-primer amplification refractory mutation system. Using a collagen/epinephrine-coated cartridge on the PFA-100, the prevalence of ANR was 15% (n = 15). In the ANR group, (i) CADP-CT was significantly shorter and (ii) hypertension was an independent clinical predictive factor of ANR (OR = 4.25; 95%CI: 1.06-17.11). No clear relation was found between CADT-CT and platelet gene polymorphism as well as ANR status and SNPs. In conclusion our study confirms the independent relationship between hypertension, platelet hypersensitivity to ADP and aspirin (160 mg/day) non-response. The differential sensitivity to aspirin may have potential clinical implications, where adaptation of antiplatelet therapy is necessary according to a patient's clinical and genetic characteristics.

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

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

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.

Aspirin and clopidogrel hyporesponsiveness and nonresponsiveness in patients with coronary artery stenting

Patients undergoing coronary artery stenting receive an antiplatelet regimen to reduce the risk of antithrombotic complications. Current guidelines recommend the use of acetyl salicylic acid (aspirin) and clopidogrel as evidenced by large clinical trials. There has been a concern about variable responses of patients to aspirin and clopidogrel which may predispose them to subacute stent thrombosis or late stent thrombosis. Up to 25% of patients with acute myocardial infarction (AMI) undergoing percutaneous coronary intervention (PCI) were found to have hyporesponsiveness or resistance to clopidogrel which may predispose them to recurrent events. Dual antiplatelet regimen is a standard therapy in these patients and there is always a concern about variable responses to aspirin and clopidogrel predisposing them to acute coronary syndrome (ACS). Prevalence of this hyporesponsiveness or resistance may be due to noncompliance, genetic mutations, co-morbid situations and concomitant use of other drugs. This issue is of considerable importance in the era of coronary drug eluting stents when a long-term dual antiplatelet regimen is needed. This paper is a review for clinicians taking care of such patients with hyporesponsiveness or nonresponsiveness to dual antiplatelet regimen.

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.

Lack of association between gene sequence variations of platelet membrane receptors and aspirin responsiveness detected by the PFA-100 system in patients with coronary artery disease

Platelet membrane receptors play a pivotal role in thrombus formation. Expression of platelet membrane receptors are under genetic control and gene sequence variations of receptors pivotal to thrombotic formation have been hypothesized to contribute to different degrees of individual response to aspirin. The aim of the present study was to investigate the impact of functional genetic polymorphisms of platelet membrane receptors on aspirin sensitivity assessed by means of the PFA-100 system in patients with coronary artery disease. Gene sequence variations of three platelet membrane receptors (GPIa/IIa, P2Y12, GPIIb/IIIa) pivotal to thrombus formation were assessed in 76 patients with coronary artery disease on chronic aspirin treatment. Patients with reduced sensitivity to aspirin were defined when closure-times of collagen/epinephrine cartridges < or =193 seconds and coined as PFA-100 non-responders. PFA-100 non-responders were observed in 33% of patients. Patients with diabetes mellitus were more frequently PFA-100 non-responders. Closure times of collagen/ADP coated cartridges were reduced in PFA-100 non-responders. The genotype distribution was similar in PFA-100 responder and non-responder patients for all three genotypes and did not vary in contemporaneous carriers of allelic variants. In conclusion, in vitro determined sensitivity to aspirin assessed using PFA-100 is not associated with gene sequence variations of platelet membrane receptors key to thrombus formation.

Review article: Diabetes mellitus and heart failure--an overview of epidemiology and management

Heart failure and diabetes mellitus are chronic complex medical conditions that are closely related and commonly coexist.Treatment options have varied over the years, but newer treatment modalities have developed which have improved prognosis and longevity of patients with these conditions. Unfortunately, despite these advances, the evidence base remains insufficient, and larger randomised control trials need to be conducted. Here we discuss the available evidence and treatment and management of these inter-related conditions.

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.

Variability in platelet response to the antiplatelet agents aspirin and clopidogrel: mechanisms, measurement, and clinical relevance

Platelet reactivity (eg, platelet adhesion, activation, aggregation) is the underlying pathology for atherothrombotic processes and subsequent ischemic complications. Antiplatelet drugs, including aspirin, dipyridamole, thienopyridines (clopidogrel and ticlopidine), and glycoprotein IIb/IIIa antagonists, have proven efficacy in atherothrombotic event prevention. However, variability of platelet response measured in the laboratory has been reported and is a subject of keen interest.It is unclear to what extent variability of platelet response to antiplatelet agents is associated with clinical outcomes. A better understanding of this issue requires a general consensus for a standard, preferably point-of-care, ex vivo or in vitro assay to determine the effects of antiplatelet agents on key platelet functions. Currently, results using various methods have not yielded an obvious answer. Small-scale studies have examined the correlation between ex vivo inhibition of platelet aggregation or residual platelet activity and clinical endpoints, and although evidence shows that such correlations may exist, results have not been consistent or definitive. Data from large-scale prospective trials are needed to expand our current understanding of the benefits and limitations of utilizing platelet function tests to effectively manage the balance between protection and risks associated with the antiplatelet therapies, aspirin, and clopidogrel.

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 resistance

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

Diabetes mellitus: a prothrombotic state implications for outcomes after coronary revascularization

Coronary stent thrombosis is a serious problem in the drug-eluting stent era. Despite aggressive antiplatelet therapy during and after percutaneous coronary intervention (PCI), the incidence of sub-acute stent thrombosis remains approximately 0.5%–2%, which may represent a catastrophic clinical situation. Both procedural factors and discontinuation of antiplatelet therapy are normally associated with this event. We report on simultaneous stent thromboses of two drug-eluting stents implanted in two different vessels, which resulted in a life-threatening clinical condition. Possible contributing factors that led to synergistic thrombotic effects are discussed.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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