Current evidence and clinical implications of aspirin resistance

The Pre- and Postoperative Prevalence and Risk Factors of ASA Nonresponse in Vascular Surgery

Background

An antiplatelet therapy with acetylsalicylic acid (ASA) is prescribed in the prevention of cardiovascular events, but around 24% of ASA takers are resistant to the treatment.

Aim

In this prospective, observational cohort study, we aimed to identify the prevalence and risk factors of ASA nonresponse in patients who underwent vascular surgery.

Methods

The study was conducted in the University hospital in Frankfurt am Main. In total, 70 patients were pre-treated with 100 mg of ASA per day and underwent either elective carotid thromboendarterectomy, femoral thromboendarterectomy or endovascular aneurysm repair of the abdominal aorta. The platelet function was measured on the first preoperative and the second or fourth postoperative day with the multiple electrode aggregometry by in-vitro stimulation with arachidonic acid (ASPItest) and thrombin receptor activating peptide 6 (TRAPtest). The primary end point was the in-vitro induced platelet aggregation in the ASPItest. If the ASPItest amounted ≥400 AU × min, the patients were categorized as ASA nonresponders.

Results

The total prevalence of ASA nonresponse in our study was 20% preoperatively and 35.7% postoperatively (p = 0.005). As significant predictors for ASA nonresponse, we demonstrated the area under the aggregation curve in the TRAPtest preoperatively (p = 0.04) and postoperatively (p = 0.02), and the two comorbidities arterial hypertension (P < .001; rho 0.44) and diabetes mellitus (p = 0.04; rho 0.39), which are already well known to be associated with ASA nonresponse.

Conclusion

In conclusion, data of the study indicate a high incidence of perioperative, laboratory ASA nonresponse in patients undergoing vascular surgery.

Treatment of blunt cerebrovascular injuries: Anticoagulants or antiplatelet agents?

BACKGROUND

Blunt cerebrovascular injury (BCVI) is associated with cerebrovascular accidents (CVA). Early therapy with antiplatelet agents or anticoagulants is recommended. There are limited data comparing the effectiveness of these treatments. The aim of our study was to compare outcomes between BCVI patients who received anticoagulants versus those who received antiplatelet agents.

METHODS

We performed an (2011-2015) analysis of the Nationwide Readmission Database and included all adult trauma patients 18 years or older who had an isolated BCVI (other body regions Abbreviated Injury Scale [AIS] < 3). Head injury patients or those who developed a CVA during the index admission were excluded. Patients were stratified into anticoagulants and antiplatelet agents. Propensity score matching was performed (1:1 ratio) to control for demographics, comorbidities, BCVI grade, distribution, and severity of injuries. Outcomes were readmission with CVA and mortality within 6 months.

RESULTS

A total of 725 BCVI patients were identified. A matched cohort of 370 patients (antiplatelet agents, 185; anticoagulants, 185) was obtained. Mean age was 50 ± 15 years, neck AIS was 3 (3,4), and Injury Severity Score was 12 (9-17). The majority of the patients (69%) had high-grade BCVI (AIS ≥ 3). Overall, 3.7% were readmitted with CVA and 3% died within 6 months. Patients who received anticoagulants had a lower rate of readmission with CVA (1.8% vs. 5.72%; p = 0.03), and a lower rate of 6-month mortality (1.3% vs. 4.9%; p = 0.03). There was no significant difference between the two groups reading the median time to stroke (9 days vs. 6 days; p = 0.12).

CONCLUSION

The BCVI patients on CVA prophylaxis for BCVI have a 3.7% rate of stroke after discharge. Compared with antiplatelet agents, anticoagulants are associated with lower rates of CVA in the first 6-month postdischarge. Further studies are required to identify the optimal agent to prevent CVA in this high-risk subset of trauma patients.

LEVEL OF EVIDENCE

Therapeutic, level IV.

The dilemma of aspirin resistance in obese patients

Aspirin resistance (AR) commonly refers to the concept of reduced aspirin efficacy in preventing cardiovascular disease and platelet inhibition. Obesity increases the risk of heart disease three- to four-fold and has been associated with AR. Aspirin is used as a tool for both primary and secondary prevention, but recent studies suggest that its lack of efficacy for primary prevention is partly attributable to obesity. Several mechanisms have been described that contribute to AR in obese patients using pharmacokinetics and pharmacodynamics. AR may be attenuated through weight loss, alternative dosing regimens, and different drug formulations. With the global rise of obesity, it is imperative to find preventive therapies that adequately address atherosclerotic cardiovascular disease (ASCVD) risk in this population.

Consensus Document ANMCO/ANCE/ARCA/GICR-IACPR/GISE/SICOA: Long-term Antiplatelet Therapy in Patients with Coronary Artery Disease

Dual antiplatelet therapy (DAPT) with aspirin and a P2Y12 receptor inhibitor is the cornerstone of pharmacologic management of patients with acute coronary syndrome (ACS) and/or those receiving coronary stents. Long-term (>1 year) DAPT may further reduce the risk of stent thrombosis after a percutaneous coronary intervention (PCI) and may decrease the occurrence of non-stent-related ischaemic events in patients with ACS. Nevertheless, compared with aspirin alone, extended use of aspirin plus a P2Y12 receptor inhibitor may increase the risk of bleeding events that have been strongly linked to adverse outcomes including recurrent ischaemia, repeat hospitalisation and death. In the past years, multiple randomised trials have been published comparing the duration of DAPT after PCI and in ACS patients, investigating either a shorter or prolonged DAPT regimen. Although the current European Society of Cardiology guidelines provide a backup to individualised treatment, it appears to be difficult to identify the ideal patient profile which could safely reduce or prolong the DAPT duration in daily clinical practice. The aim of this consensus document is to review contemporary literature on optimal DAPT duration, and to guide clinicians in tailoring antiplatelet strategies in patients undergoing PCI or presenting with ACS.

Review of aspirin and clopidogrel resistance in peripheral arterial disease

OBJECTIVE

Aspirin resistance (AR) and clopidogrel resistance (CR) are terms used to describe a reduction in the medication's efficacy in inhibiting platelet aggregation despite regular dosing. This review gives context to the clinical role and implications of antiplatelet resistance in peripheral arterial disease (PAD).

METHODS

A review of English-language literature on AR and CR in PAD involving human subjects using PubMed and MEDLINE databases was performed in April 2017. A total of 2075 patients in 22 relevant studies were identified. To give this issue context, a review of the larger, more established literature on antiplatelet resistance in coronary disease was undertaken, identifying significant research associating resistance to major adverse cardiovascular events (MACEs).

RESULTS

Studies in the coronary arterial disease literature have strongly associated antiplatelet resistance with increased MACE. Prevalence of AR or CR in coronary disease appears to be >55% for each in some studies. Meta-analyses of >50 studies revealed that AR and CR are significantly associated with MACE (relative risk of 2.09 and 2.8, respectively). This adds further weight to the literature reporting antiplatelet resistance as an independent predictor of and a threefold risk factor for major adverse cardiovascular events. The prevalence of resistance in PAD in this review was comparable to that in the coronary disease literature, with AR and CR prevalence up to 60% and 65%, respectively. There is evidence that the adverse effects of antiplatelet resistance are significant in PAD. In fact, research directly studying stent thrombosis populations with either coronary arterial disease or PAD revealed more significantly impaired platelet responsiveness to clopidogrel and aspirin in PAD compared with similar individuals with coronary disease. AR in PAD was found in studies to be a significant risk factor for iliofemoral stent reocclusion (P = .0093) and stroke in patients with symptomatic carotid disease (P = .018). CR was found to be a significant, independent risk factor in predicting ischemic outcomes in several recent PAD studies (P < .0001). Loss-of-function carriers of enzyme CYP2C19, important in clopidogrel metabolism, have a 30% greater risk of ischemic events (P < .001). Importantly, less antiplatelet drug resistance may be encountered with newer antiplatelet agents, including ticagrelor and prasugrel, because of reduced enzymatic polymorphisms.

CONCLUSIONS

The limited research addressing AR and CR in PAD suggests that further research is required to clarify the role of platelet assays and potential for individualized antiplatelet therapy.

High prevalence of aspirin resistance in elderly patients with cardiovascular disease and metabolic syndrome

Background

Metabolic syndrome is known to be a prothrombotic state. We undertook this study to examine a hypothesis that aspirin resistance may be associated with metabolic syndrome, and to assess other potential determinants of aspirin resistance in patients with cardiovascular disease (CVD).

Methods

A total of 469 elderly patients with CVD were recruited. One hundred and seventy-two patients with metabolic syndrome and 297 without metabolic syndrome (control group) received daily aspirin therapy (≥ 75 mg) over one month. Platelet aggregation was measured by light transmission aggregometry (LTA). Aspirin resistance was defined as ≥ 20% arachidonic acid (AA)- and ≥ 70% adenosine diphosphate (ADP)-induced aggregation according to LTA. Aspirin semi-responders were defined as meeting one (but not both) of these criteria.

Results

By LTA, 38 of 469 (8.1%) patients were aspirin resistant. The prevalence of aspirin resistance was higher in the metabolic syndrome group compared with the control group [11.6 % vs. 6.6%, odds ratio (OR) = 2.039; 95% confidence interval (CI): 1.047–3.973]. In the multivariate logistic regression analysis, metabolic syndrome (OR = 4.951, 95% CI: 1.440–17.019, P = 0.011) was a significant risk factor for aspirin resistance.

Conclusions

A significant number of patients with CVD and metabolic syndrome are resistant to aspirin therapy. This might further increase the risk of cardiovascular morbidity and mortality in these patients.

Optimal duration of dual antiplatelet therapy after coronary stent implantation

Dual antiplatelet pharmacotherapy reduces ischemic events at the cost of excess bleeding in patients who underwent coronary stenting. The currently recommended treatment period is based on trials held some 20 years ago and not relevant to current clinical practice. In recent years, numerous clinical trials have tried to answer the question of what is the optimal duration of therapy to maximize clinical benefit. These trials showed 2 seemingly conflicting answers-on one hand, shorter treatment duration seems to be safer in reducing bleeding while not increasing ischemic events, and on the other hand, longer duration is superior in terms of preventing ischemic events albeit at the cost of increased bleeding rates. In this review, we summarize the evidence favoring each approach, highlight the limitations of the various pivotal clinical trials in this field, review future directions of research and changes in practice that may influence the duration of antiplatelet therapy, and attempt to propose a personalized approach to achieve maximal benefit for the individual patient.

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.

Perioperative aspirin management after POISE-2: some answers, but questions remain

Aspirin constitutes important uninterrupted lifelong therapy for many patients with cardiovascular (CV) disease or significant (CV) risk factors. However, whether aspirin should be continued or withheld in patients undergoing noncardiac surgery is a common clinical conundrum that balances the potential of aspirin for decreasing thrombotic risk with its possibility for increasing perioperative blood loss. In this focused review, we describe the role of aspirin in treating and preventing cardiovascular disease, summarize the most important literature on the perioperative use of aspirin (including the recently published PeriOperative ISchemic Evaluation [POISE]-2 trial), and offer current recommendations for managing aspirin during the perioperative period. POISE-2 suggests that aspirin administration during the perioperative period does not change the risk of a cardiovascular event and may result in increased bleeding. However, these findings are tempered by a number of methodological issues related to the study. On the basis of currently available literature, including POISE-2, aspirin should not be administered to patients undergoing surgery unless there is a definitive guideline-based primary or secondary prevention indication. Aside from closed-space procedures, intramedullary spine surgery, or possibly prostate surgery, moderate-risk patients taking lifelong aspirin for a guideline-based primary or secondary indication may warrant continuation of their aspirin throughout the perioperative period.

Predicting the cost and pace of pharmacogenomic advances: an evidence-based study

BACKGROUND

Adverse outcomes associated with prescription drug use are common and costly. Many adverse outcomes can be avoided through pharmacogenomics: choosing and dosing of existing drugs according to a person's genomic variants. Finding and validating associations between outcomes and genomic variants and developing guidelines for avoiding drug-related adverse outcomes will require further research; however, no data-driven estimates yet exist for the time or money required for completing this research.

METHODS

We identified examples of associations between adverse outcomes and genomic variants. We used these examples to estimate the time and money required to identify and confirm other associations, including the cost of failures, and to develop and validate pharmacogenomic dosing guidelines for them. We built a Monte Carlo model to estimate the time and financial costs required to cut the overall rate of drug-related adverse outcomes by meaningful amounts. We analyzed the model's predictions for a broad range of assumptions.

RESULTS AND CONCLUSIONS

Our model projected that the development of guidelines capable of cutting overall drug-related adverse outcomes by 25%-50% with current approaches will require investment of single-digit billions of dollars and take 20 years. The model forecasts a pump-priming phase of 5-7 years, which would require expenditures of hundreds of millions of dollars, with little apparent return on investment. The single most important parameter was the extent to which genomic variants cause adverse outcomes. The size of the labor force was not a limiting factor. A "50 000 Pharmacogenomes Project" could speed progress. Our approach provides a template for other areas of genomic research.

Frequency, risk factors, prognosis, and genetic polymorphism of the cyclooxygenase-1 gene for aspirin resistance in elderly Chinese patients with cardiovascular disease

BACKGROUND

Cardiovascular disease (CVD) is an important cause of mortality in elderly patients worldwide. Aspirin resistance has been well reported in CVD.

OBJECTIVE

The frequency, risk factors, prognosis, and genetic polymorphism of the cyclooxygenase-1 (COX-1) gene for aspirin resistance have not been reported in elderly patients with CVD. We therefore undertook this study to evaluate these associations among elderly Chinese patients with CVD.

METHODS

Four hundred thirty-one elderly Chinese patients with CVD receiving daily aspirin therapy (≥75 mg) over 1 month were enrolled. Platelet aggregation was measured by light transmission aggregometry (LTA) and thromboelastography platelet mapping assay (TEG) using arachidonic acid (AA) as a stimulus. The median follow-up was 1.8 years.

RESULTS

After the median follow-up, aspirin-resistant patients were at an increased risk of the composite endpoint compared to nonresistant patients by LTAAA + TEGAA (23.7 vs. 9.2%, p = 0.025). Additionally, Cox proportional hazards regression modeling demonstrated that aspirin resistance and cerebrovascular disease were associated with major adverse long-term outcomes (HR for aspirin resistance = 2.31, 95% CI 1.11-4.81, p = 0.026). The variant G-allele of COX-1 rs1330344 (-1676 A/G) significantly increased the risk of aspirin resistance defined by LTAAA + TEGAA (OR = 1.82, 95% CI 1.13- 2.92, p = 0.01).

CONCLUSIONS

Aspirin resistance, evaluated by LTAAA + TEGAA, is associated with an increased risk of adverse clinical events in elderly Chinese patients with CVD. The variant G-allele of COX-1 rs1330344 is significantly associated with aspirin resistance defined by LTAAA + TEGAA.

The prevalence, risk factors and prognosis of aspirin resistance in elderly male patients with cardiovascular disease

AIM

Aspirin resistance is recognized in different population. However, the prevalence and clinical events of aspirin resistance in elderly male patients with cardiovascular disease (CVD) have not been reported.

METHODS

We enrolled 304 elderly male patients with CVD receiving daily aspirin therapy (≥ 75 mg) more than 1 month. Platelet aggregation was measured by light transmission aggregometry (LTA) and thrombelastography platelet mapping assay (TEG). The median follow-up time was 1.8 years. The primary outcome was the composite of death, myocardial infarction, unstable angina, stroke and transient ischemic attack.

RESULTS

By LTA, 25 (8.2%) of elderly patients were aspirin resistant and 106 (34.9%) patients were semiresponders. According to TEG, 62 patients (20.4%) were found to be resistant to aspirin therapy. Of the 62 patients with aspirin resistance by TEG, 21 patients were aspirin resistant by LTA. Twenty-two of the 106 semiresponders by LTA were aspirin resistant by TEG. Patients with aspirin resistance or aspirin semiresponders were at increased risk of the composite outcome compared with aspirin-sensitive patients by LTA (18.3% vs 9.8%, Hazard ratio (HR) = 1.864, 95% confidence interval (CI): 1.046-3.324 p = 0.039). However, aspirin resistance was not associated with an increased risk of clinical vascular events compared to aspirin-sensitive patients by TEG (17.7% vs 10.9%, p = 0.452). In addition, Cox proportional hazard regression modeling demonstrated that aspirin resistance or semiresponders (HR = 3.050, 95% CI: 1.464-6.354, p = 0.003) and diabetes (HR = 2.055, 95% CI: 1.060-3.981, p = 0.033) were associated with major adverse long-term outcomes.

CONCLUSIONS

Aspirin resistance or semiresponders, defined by LTA, are associated with an increased risk of adverse clinical events in elderly male patients with CVD.

Dual antiplatelet therapy in patients with aspirin resistance following coronary artery bypass grafting: study protocol for a randomized controlled trial [NCT01159639]

BACKGROUND

Coronary artery disease remains the dominant cause of mortality in developed countries. While platelets have been recognized to play a pivotal role in atherothrombosis, the ideal antiplatelet regime after coronary artery surgery remains elusive. The evolution of CABG has presently moved beyond technical improvements to involve modulation of pharmacologic management designed to improve patient outcomes. The aim of this trial will be to test the hypothesis that the addition of clopidogrel to patients with documented postoperative aspirin resistance will reduce the incidence of major cardiovascular events.

METHODS

Patients scheduled for isolated coronary artery surgery will be eligible for the study. Patients in whom postoperative multiple electrode aggregometry documents aspirin resistance will be randomized into two groups. The control group will receive 300 mg of aspirin. The dual antiplatelet group will receive 75 mg of clopidogrel in addition to 300 mg of aspirin. Patients will be followed for 6 months. Major adverse cardiac and cerebrovascular events (death from any cause, myocardial infarction, stroke, hospitalization due to cardiovascular pathology) as well as bleeding events will be recorded.

DISCUSSION

This will be the first trial that will specifically address the issue of dual antiplatelet therapy in patients undergoing coronary artery surgery who have been found to be aspirin resistant. In the event that the addition of clopidogrel proves to be beneficial in this subset of surgical patients, this study could significantly impact their future antiplatelet management. This randomized controlled trial has been registered at the ClinicalTrials.gov website (Identifier NCT01159639).

Aspirin resistance and ischemic heart disease on Iranian experience

BACKGROUND

Coronary artery disease (CAD) and myocardial infarction are the most common causes of mortality and morbidity all over the world. Aspirin resistance is an important part of therapeutic failure in patients who experience several atherosclerotic events despite aspirin therapy. Different studies have reported aspirin resistance between 5% and 45% all over the world. According to different responses to aspirin therapy in countries and lack of adequate studies on aspirin resistance in Iran, this study was designed for evaluation of aspirin resistance in ischemic patients.

MATERIALS AND METHODS

Total 170 patients with documented coronary artery stenosis with were enrolled in this cross-sectional prospective study. Two cc urine samples were obtained from all the subjects. Then a questionnaire including questions about major risk factors (hypertension, diabetes, hyperlipidemia, obesity and smoking) was completed for each patient. Thromboxane B2 level in urine was measured two times for each patient by one kit of via ELISA method. Gensini modified was usedfor assessment of severity of coronary arteries involvement. Data were analyzed via SPSS 16. with general linear model (univariate).

RESULTS

75.3% of studied patients were aspirin resistant. There was significant relationship between angiography score and aspirin resistance (P<0.001).Our results also showed that aspirin resistance is more common in studied women than men (P=0.003).Significant correlation was observed between diabetes and aspirin resistance in studied subjects (P=0.023).

CONCLUSION

Our study showed aspirin resistance in a sample of Iranian ischemic patients is so prevalent which is higher than other studies in another communities and also aspirin resistance is more common in patients with severe CAD.

Platelet proteomics

Platelets are small cell fragments, produced by megakaryocytes, in the bone marrow. They play an important role in hemostasis and diverse thrombotic disorders. They are therefore primary targets of antithrombotic therapies. They are implicated in several pathophysiological pathways, such as inflammation or wound repair. In blood circulation, platelets are activated by several pathways including subendothelial matrix and thrombin, triggering the formation of the platelet plug. Studying their proteome is a powerful approach to understand their biology and function. However, particular attention must be paid to different experimental parameters, such as platelet quality and purity. Several technologies are involved during the platelet proteome processing, yielding information on protein identification, characterization, localization, and quantification. Recent technical improvements in proteomics combined with inter-disciplinary strategies, such as metabolomic, transcriptomics, and bioinformatics, will help to understand platelets biological mechanisms. Therefore, a comprehensive analysis of the platelet proteome under different environmental conditions may contribute to elucidate complex processes relevant to platelet function regarding bleeding disorders or platelet hyperreactivity and identify new targets for antiplatelet therapy.

[New era in antiplatelet therapy based on results from recent clinical trials]

Antiplatelet therapy has been successful in reducing mortality and morbidity in cardiovascular diseases (e.g. acute coronary syndromes). Recent advances in understanding the molecular basis of the role of platelets in atherothrombosis have enabled the development of new agents with the potential to further reduce mortality and morbidity. Some limitations associated with the use of aspirin and clopidogrel have led to potential alternatives, including more potent ADP antagonists such as prasugrel, ticagrelor, cangrelor, and thrombin receptor antagonists, which have shown additional benefit in large randomized controlled trials. These new agents open a realistic prospect of a personalized choice of the most appropriate antiplatelet therapy tailored for an individual patient and a clinical condition.

Survey of aspirin use in Sturge-Weber syndrome

Sturge-Weber syndrome is a rare congenital disorder. Seizures, stroke-like episodes, glaucoma, headache, and developmental delay are frequently associated features. An Internet-based questionnaire was designed to assess the frequency of use, effectiveness, and safety of aspirin treatment in Sturge-Weber syndrome. Thirty-four of 98 subjects who completed the survey reported having used aspirin. The mean number of reported stroke-like episodes was reduced from 1.1 to 0.3 per month in the year after starting aspirin (n = 26, p = .014). The median number of seizures was significantly reduced from 3 to 1 episodes per month (n = 21, p = .002). Thirty-nine percent of subjects reported a history of complications (predominantly increased bruising or gum/nose bleeding) while on aspirin; however, none reported discontinuing aspirin because of side effects. Our study showed a significant relative reduction in both self-reported seizure frequency and stroke-like episodes after starting aspirin. It also suggests that low-dose aspirin can be safely used in these patients.

Perioperative assessment of platelet function in patients under antiplatelet therapy

Platelets play a central role in primary hemostasis. Analysis of platelet function is therefore a cornerstone in the global assessment of the coagulation status in the perioperative setting, primarily in patients receiving antiplatelet medication, such as cyclooxygenase-1 inhibitors, adenosine diphosphate antagonists and glycoprotein IIb/IIIa inhibitors. In these patients, knowledge of residual platelet function is highly warranted in order to maintain an optimal and individual balance perioperatively between platelet function and inhibition - that is, bleeding and thrombosis. Traditional laboratory-based assays, such as light-transmission aggregometry and flow cytometry, are the clinical standards of platelet function testing today. Light-transmission aggregometry is one of the most widely used tests to identify and diagnose defects in platelet function. The majority of the conventional laboratory-based techniques are labor intensive, costly and time consuming, and require a high degree of experience and expertise to perform and interpret. Therefore, new automated technologies have been developed to measure platelet function more rapidly and easily, and several techniques can be used at the bedside, including whole blood aggregometry, high shear-induced platelet function assessment or viscoelastic measurement techniques. All methods assessing platelet function are summarized and their limitations are discussed in this article, emphasizing their perioperative use.

Serum aspirin esterase activity is lower in end-stage renal disease patients than in healthy control subjects and increases after haemodialysis

Background

Studies regarding aspirin metabolism can be important in patients with renal failure who have an increased risk of cardiovascular diseases. We undertook this study to assess the aspirin esterase (AE) status in end-stage renal disease (ESRD) patients.

Methods

A total of 42 patients on long-term haemodialysis (HD) with a mean dialysis course of 6.1 y were recruited.

Results

Serum AE levels were 44% lower and cholinesterase (ChE) levels were 22% lower in ESRD patients before dialysis as compared with control subjects ( P = 0.0001). A very strong correlation was found between AE and ChE levels. AE levels increased on average 28% after dialysis with adjustments for age, gender, total cholesterol, triglyceride and high-density lipoprotein cholesterol ( P = 0.002). In addition, ChE levels were significantly increased (48%) after dialysis ( P = 0.0001). Changes in AE activity were significantly and positively correlated with those of ChE ( r = 0.427, P = 0.005). When we adjusted for several confounders, we found that the changes in AE activity operated by dialysis are significant independently of age, gender, aspirin (ASA) intake, cholesterol, triglycerides, high-density lipoprotein cholesterol and ChE.

Conclusions

We report that serum AE activity is significantly lower in ESRD and that treatment by HD results in an increase of activity. We confirm that AE is associated with lipid parameters and ChE. Our results show variations in ASA catabolism between the dialysis sessions, suggesting an oscillating pattern in ASA disposal in these patients. The mechanisms for reduced AE activity in uraemia and the effects of HD need further investigation.

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.

Individual variability of response and non-response to acetyl salicylic acid after cardiac surgery

Insufficient inhibition of platelet function by acetyl salicylic acid (ASA) or other platelet inhibitors is a risk factor for arterial thrombosis in cardiovascular patients. We wanted to collect and analyse information on the frequency and probable causes of non-response to ASA in patients prior to and after cardiac surgery. One hundred and one patients (mean age 68 ± 9 years) undergoing cardiac surgery (98 patients with coronary bypass grafting, 18 cases had combined valve replacement, and three patients with only valve replacement) were enrolled. Post-operatively all patients received metamizole for analgesia. Platelet aggregation in platelet-rich plasma was induced by arachidonic acid (AA; 1.6 mM) or ADP (3 mM) in the absence and presence of exogenous ASA (100 µM). ASA non-response was defined as a maximum AA-induced aggregation of >30%. Eighty eight patients had pre-operative medication with ASA (100 mg/d), and ASA non-response was found in 24%. Irrespective of whether or not ASA medication was continued immediately after surgery, incidence of non-response increased to 55% at the first post-operative day. During continuous post-operative ASA medication (100 mg/d), 65% of patients were non-responder at fifth-7th post-operative day. When estimated on the basis of exogenously added ASA, non-response was observed pre-operative in 10%, at first post-operative day in 53% and at fifth-7th post-operative day in 39% of the patients. Twenty six of the 52 patients who did not adequately respond to exogenous ASA at the first post-operative day became responders when tested at the fifth-7th post-operative day, and 13 of the 46 responders became non-responders. The conversions were not due to small changes around the threshold of 30% aggregation but due to a highly significant decrease or increase in the extend of aggregation. Neither extracorporal circulation nor co-medication with clopidogrel had any significant influence on the platelet response to ASA medication or exogenously added ASA. Some non-steroidal analgesics, including metamizole, have been suggested to prevent inhibition of platelet cylcooxygenase by ASA. However, in 10 healthy volunteers we did not observe any interference of metamizole with the response to ASA. In conclusion, platelet response to ASA is markedly decreased after cardiac surgery. The underlying mechanisms and the clinical consequences of the post-operative instability of non-response to ASA need further evaluation.

Aspirin resistance with genetic dyslipidemia: contribution of vascular thromboxane generation

One clinical intervention against the negative outcomes associated with atherothrombotic vascular disease (AVD) is low-dose, chronic aspirin therapy. However, epidemiological studies suggest that recurrence of adverse vascular events with aspirin therapy is growing and associated with therapy duration. The contributors to this outcome are unclear and include poor patient compliance and aspirin-resistant platelet thromboxane A(2) (TxA(2)) production. Based on previous results in hypercholesterolemic mice, we hypothesized that elevated aspirin-insensitive arachidonic acid (AA)-induced TxA(2) production by the vascular endothelium contributes to aspirin resistance in AVD independent of platelet behavior. AA-induced dilation was blunted in aortic rings and in arterioles from apolipoprotein E (ApoE) and low-density lipoprotein receptor (LDLR) gene deletion mice (vs. C57/Bl6/J), partially due to elevated TxA(2) production. Acute inhibition of cyclooxygenases or TxA(2) synthase attenuated the increased TxA(2) production in ApoE and LDLR and improved AA-induced dilation, responses that were mirrored by chronic treatment with low-dose aspirin of 16 wk duration. However, this effect was not temporally stable, and, with longer-duration therapy, the beneficial impact of aspirin on outcomes diminished. A similar, though less robust, pattern to the impact of chronic aspirin therapy on vascular outcomes was identified with chronic antioxidant treatment (TEMPOL). These results suggest that in dyslipidemic mice, the beneficial impact of chronic aspirin therapy on improving vascular outcomes decay with time and that a contributing element to subsequent negative vascular events may be the development of aspirin-resistant TxA(2) production by the vasculature itself.

Laboratory evaluation of aspirin responsiveness

Aspirin is the most commonly used antiplatelet medication. Laboratory monitoring of aspirin response has recently become a topic of interest due to potential impacts on patient management and clinical outcomes. This article summarizes available laboratory testing of aspirin response with focus on technical issues, limitations, and current opinion on the utility of routine patient testing.