Aspirin dosage and thromboxane synthesis in patients with vascular disease

Structural modification of natural axinelline A: Achieving reduced colitis side effects through balanced COX inhibition

Marine natural products continue to hold great promise as potential candidates for the discovery of anti-inflammatory drug. In our previous investigation, we successfully synthesized axinelline A, a naturally occurring cyclooxygenase-2 (COX-2) inhibitor, as a promising anti-inflammatory lead compound. This study was to discover novel COX inhibitors with balanced inhibition, aiming to mitigate the severe adverse effects through further structural modification of axinelline A. Of the synthetic derivatives, compound 5e showed highest COX-2 inhibitory activity and balanced COX inhibition (IC50 = 1.74 µM; selectivity ((IC50 (COX-1)/IC50(COX-2) = 16.32). The in vitro anti-inflammatory results indicated that 5e effectively suppressed the expression of pro-inflammatory mediators via the NF-κB signaling pathway rather than the MAPK signaling pathway. The in vivo ulcerative colitis assay demonstrated 5e significantly ameliorated the histological damages and showed strong protection against DSS-induced acute colitis. Therefore, our findings suggest that compound 5e exhibits potential as a promising anti-inflammatory agent with attenuated colitis-related adverse effects.

Design of Balanced Cyclooxygenase Inhibitors Based on Natural Anti-inflammatory Ascidian Metabolites and Celecoxib

The serious adverse effects caused by non-selective and selective cyclooxygenase-2 (COX-2) inhibitors remain significant concerns for current anti-inflammatory drugs. In this study, we present the design and synthesis of a novel series of celecoxib analogs incorporating a hydrazone linker, which were subjected to in silico analysis to compare their binding poses with those of clinically used nonsteroidal anti-inflammatory drugs (NSAIDs) against COX-1 and COX-2. The synthesized analogs were evaluated for their inhibitory activity against both COX enzymes, and compound 6 m, exhibiting potent balanced inhibition, was selected for subsequent in vitro anti-inflammatory assays. Treatment with 6 m effectively suppressed the NF-κB signaling pathway in lipopolysaccharide (LPS)-stimulated murine RAW264.7 macrophages, resulting in reduced expression of pro-inflammatory factors such as inducible nitric oxide synthase (iNOS), COX-2, tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), IL-1β, as well as decreased production of prostaglandin E2 (PGE2 ), nitric oxide (NO), and reactive oxygen species (ROS). However, 6 m has no effect on the MAPK signaling pathway. Therefore, due to its potent in vitro anti-inflammatory activity coupled with lack of cytotoxicity, 6 m represents a promising candidate for further development as a new lead compound targeting inflammation.

Factors Associated with Aspirin Resistance in Hong Kong Chinese Patients with Stable Coronary Heart Disease Using the Multiplate® Analyzer and Serum Thromboxane B2

Introduction: Aspirin resistance may be associated with various conditions. We measured serum thromboxane B2 (TXB2) and platelet function using the Multiplate® Analyzer with arachidonic acid (ASPI) in patients on long term aspirin therapy to identify aspirin resistance and associated factors. Materials and Methods: Chinese patients with stable coronary heart disease had samples for serum TXB2 and ASPI measurement taken before and 1 h after taking a morning dose of 80 mg aspirin. Results: In 266 patients with mean age 66.6 ± 10.7 years, 17% were female and 55% were current or previous smokers. TXB2 and ASPI measurements were significantly higher before the dose than at 1 h post dose, with 46% of subjects having high ASPI values (AUC > 300 AU*min) pre dose compared with 27% at 1 h post dose. TXB2 and ASPI measures of platelet aggregation showed weak correlations, which were only significant before the dose (r = 0.219, p = 0.001). Increased ASPI measurements were associated with white blood cell (WBC) count, haematocrit, platelet count and heart rate at 24 h post dose but only with WBC count, smoking history and heart rate at 1 h post dose. Diabetes was not associated with reduced platelet response to aspirin. The WBC count associated with aspirin resistance was over 6.55 × 109/L by receiver operating characteristic analysis. Conclusions: The antiplatelet response to aspirin was reduced in a large proportion of patients. Patients with higher WBC count within the normal range appear to be at increased risk of aspirin resistance. Higher or more frequent doses of aspirin may be needed in many patients.

Circadian Rhythm of Cardiovascular Disease: The Potential of Chronotherapy With Aspirin

Almost all the systems in our body adhere to a daily 24 h rhythm. The cardiovascular system is also affected by this 24 h rhythm. In the morning there is a change in various cardiovascular processes, including platelet aggregability. These changes may play a role in the relative excess of early morning cardiovascular events. The number of recurrent cardiovascular diseases (CVD) could, in theory, be reduced by responding to this 24 h rhythm with timed medication intake (chronotherapy), which also applies to aspirin. Multiple studies on chronotherapy with low-dose aspirin are promising, showing a decrease in early morning platelet activity with evening intake compared with morning intake. However, in order to further demonstrate its clinical impact, randomized trials with cardiovascular events as a primary outcome are needed. This review discusses the available evidence of the effects of circadian rhythm on CVD and the potential positive effect of chronotherapy with aspirin.

Obesity and laboratory aspirin resistance in high-risk pregnant women treated with low-dose aspirin

BACKGROUND

Low-dose aspirin is used for preeclampsia prevention in high-risk women, but the precise mechanism and optimal dose are unknown. Evidence suggests that an imbalance in prostacyclin and thromboxane A₂ (TXA₂) plays a key role in the pathogenesis of preeclampsia. Aspirin has a dose-dependent effect blocking production of TXA₂, a potent stimulator of platelet aggregation and promoter of vasoconstriction. Incomplete inhibition of platelet aggregation, designated aspirin resistance, can be reduced by increasing the aspirin dose. Evidence in the nonobstetric literature suggests that aspirin resistance may be more common among patients with a high body mass index.

OBJECTIVE

To investigate the association of obesity on platelet-derived thromboxane inhibition in high-risk women treated with low-dose aspirin.

MATERIALS AND METHODS

This was a secondary analysis of a prospective multi-centered study investigating the effect of low-dose aspirin (60-mg) administration in women at high risk for preeclampsia. Maternal serum TXB₂ (an indirect measure of TxA₂) levels were drawn at 3 time points: randomization (13-26 weeks' gestation), second trimester (at least 2 weeks after randomization and 24-28 weeks' gestation), and third trimester (34-38 weeks' gestation). Patients were included in the analysis if a TXB₂ level was recorded at randomization and at least 1 time point thereafter. Patients were stratified by body mass index category and treatment arm. Median TXB₂ levels were calculated at each time point, as well as rates of complete TXB₂ inhibition (<0.01 ng/mL). A multivariate logistic regression analysis was performed to generate odds ratios (OR) for complete TXB₂ inhibition by body mass index category, adjusting for maternal age, race, high-risk group at randomization, nulliparity, and rate of randomization less than 16 weeks' gestation.

RESULTS

A total of 1002 patients were included in the analysis, 496 (49.5%) and 506 (50.5%) in the low-dose aspirin and placebo groups respectively. There were substantial decreases in TXB₂ levels among low-dose aspirin-treated women in all body mass index categories. In contrast, women assigned to placebo did not show a marked decrease in TXB₂ levels after randomization, and obese women had higher median TXB₂ levels in both the second (16.5, interquartile range [IQR] 8.0-31.8 vs 14.0, IQR 6.9-26.7, ng/mL; P = .032) and third (15.7, IQR 7.6-28.5 vs 11.9, IQR 4.6-25.9, ng/mL; P = .043) trimesters. When comparing among stratified body mass index low-dose aspirin groups, women with class III obesity had the lowest odds of undetectable TXB₂ levels in the second trimester (adjusted odds ratio [aOR], 0.33; 95% confidence interval [CI], 0.15-0.72) and third trimester (aOR, 0.30; 95% CI, 0.11-0.78) as well as at both time points (aOR, 0.09; 95% CI, 0.02-0.41).

CONCLUSION

High-risk obese women receiving low-dose aspirin for the prevention of preeclampsia have lower rates of complete inhibition of TXB₂. These data suggest that an increase in aspirin dosing or frequency may be necessary in this population.

Aspirin Resistance and Stroke

Stroke is a leading cause of death and disability worldwide. Aspirin is the most commonly used antiplatelet drug in both primary and secondary prevention of cerebrovascular and cardiovascular diseases. A proportion of patients may have stroke recurrence while they are on treatment with aspirin, giving rise to term aspirin resistance or aspirin failure. Studies have suggested that such recurrence could partly be attributed to biochemical aspirin resistance, with an estimated prevalence ranging between 5% and 65% among patients with ischemic stroke in the published studies. Common methods to evaluate laboratory aspirin resistance include light transmission aggregometry, PFA-100, VerifyNow-Aspirin assay, serum thromboxane B2, and urinary 11-dehydrothromboxane B2. Aspirin resistance is multifactorial in origin and involves diverse environmental and genetic factors, including single-nucleotide polymorphisms, miRNAs, drug interactions, and co-morbid risk factors. The current review overviews the concept of aspirin resistance, its evaluation and relationship with stroke recurrence, its outcome, and its implications on stroke management in the future.

Platelet Aggregometry Testing: Molecular Mechanisms, Techniques and Clinical Implications

Platelets play a fundamental role in normal hemostasis, while their inherited or acquired dysfunctions are involved in a variety of bleeding disorders or thrombotic events. Several laboratory methodologies or point-of-care testing methods are currently available for clinical and experimental settings. These methods describe different aspects of platelet function based on platelet aggregation, platelet adhesion, the viscoelastic properties during clot formation, the evaluation of thromboxane metabolism or certain flow cytometry techniques. Platelet aggregometry is applied in different clinical settings as monitoring response to antiplatelet therapies, the assessment of perioperative bleeding risk, the diagnosis of inherited bleeding disorders or in transfusion medicine. The rationale for platelet function-driven antiplatelet therapy was based on the result of several studies on patients undergoing percutaneous coronary intervention (PCI), where an association between high platelet reactivity despite P2Y12 inhibition and ischemic events as stent thrombosis or cardiovascular death was found. However, recent large scale randomized, controlled trials have consistently failed to demonstrate a benefit of personalised antiplatelet therapy based on platelet function testing.

Urinary 11-Dehydro-Thromboxane B2 and Mortality in Patients With Stable Coronary Artery Disease

Antiplatelet therapy with aspirin has been shown to reduce adverse outcomes in patients with coronary artery disease (CAD). Aspirin irreversibly inhibits platelet cyclooxygenase-1 and attenuates thromboxane A₂ (TXA₂)-mediated platelet aggregation, but there is variable suppression of cyclooxygenase-1. From a cohort of patients with stable CAD, we performed blinded, detailed chart abstraction, and measured urinary 11-dehydro-thromboxane B₂ (11dhTxB₂), an inactive metabolite of TxA₂ from frozen samples. There were 327 men (73%) and 122 women (27%) with a mean age (±SD) of 67 ± 10 and 65 ± 10 years, respectively. A positive linear trend for age was observed among tertiles of 11dhTxB₂ (p trend = 0.01). Higher proportions of women (p = 0.001), chronic obstructive pulmonary disease (p trend = 0.0003), and heart failure (p trend = 0.003) were observed in the upper tertile of 11dhTxB₂. Sixty-seven patients (14.9%) died over a median follow-up of 1,149 days and 87.5% of the deaths were due to cardiovascular causes. Twenty-six nonsurvivors (38.8%) were treated with P2Y12 receptor antagonists versus 161 survivors (42.2%; p = 0.61). By stepwise Cox proportional hazards analysis, we identified that patients in the middle (hazard ratio 7.14; 95% CI 2.46 to 20.68) and upper tertiles (hazard ratio 9.91; 95% CI 3.45 to 28.50) had higher risks for mortality after adjusting for age and co-morbidities. In conclusion, urinary concentration of 11dhTxB₂ was a strong independent risk factor for all-cause mortality among patients with stable CAD on aspirin therapy and may be a marker for patients with CAD who require more intensive secondary prevention measures.

Effects of Firocoxib, Flunixin Meglumine, and Phenylbutazone on Platelet Function and Thromboxane Synthesis in Healthy Horses

OBJECTIVE

Determine the effects of nonsteroidal anti-inflammatory drugs (NSAID) on platelet function and thromboxane synthesis immediately after drug administration and following 5 days of NSAID administration in healthy horses.

STUDY DESIGN

Randomized cross-over study.

ANIMALS

Healthy adult horses (n=9; 6 geldings and 3 mares).

METHODS

Horses received either flunixin meglumine (1.1 mg/kg IV every 12 hours), phenylbutazone (2.2 mg/kg IV every 12 hours), or firocoxib (loading dose of 0.27 mg/kg IV on day 1, then 0.09 mg/kg IV every 24 hours for 4 days) for a total of 5 days. Blood samples were collected prior to drug administration (day 0), 1 hour after initial NSAID administration (day 1), and then 1 hour post-NSAID administration on day 5. Platelet function was assessed using turbidimetric aggregometry and a platelet function analyzer. Serum thromboxane B₂ concentrations were determined by commercial ELISA kit. A minimum 14 day washout period occurred between trials.

RESULTS

At 1 hour and 5 days postadministration of firocoxib, flunixin meglumine, or phenylbutazone, there was no significant effect on platelet aggregation or function using turbidimetric aggregometry or a platelet function analyzer. There was, however, a significant decrease in thromboxane synthesis at 1 hour and 5 days postadministration of flunixin meglumine and phenylbutazone that was not seen with firocoxib.

CONCLUSION

Preoperative administration of flunixin meglumine, phenylbutazone, or firocoxib should not inhibit platelet function based on our model. The clinical implications of decreased thromboxane B₂ synthesis following flunixin meglumine and phenylbutazone administration are undetermined.

Platelet function tests: a comparative review

In physiological hemostasis a prompt recruitment of platelets on the vessel damage prevents the bleeding by the rapid formation of a platelet plug. Qualitative and/or quantitative platelet defects promote bleeding, whereas the high residual reactivity of platelets in patients on antiplatelet therapies moves forward thromboembolic complications. The biochemical mechanisms of the different phases of platelet activation – adhesion, shape change, release reaction, and aggregation – have been well delineated, whereas their complete translation into laboratory assays has not been so fulfilled. Laboratory tests of platelet function, such as bleeding time, light transmission platelet aggregation, lumiaggregometry, impedance aggregometry on whole blood, and platelet activation investigated by flow cytometry, are traditionally utilized for diagnosing hemostatic disorders and managing patients with platelet and hemostatic defects, but their use is still limited to specialized laboratories. To date, a point-of-care testing (POCT) dedicated to platelet function, using pertinent devices much simpler to use, has now become available (ie, PFA-100, VerifyNow System, Multiplate Electrode Aggregometry [MEA]). POCT includes new methodologies which may be used in critical clinical settings and also in general laboratories because they are rapid and easy to use, employing whole blood without the necessity of sample processing. Actually, these different platelet methodologies for the evaluation of inherited and acquired bleeding disorders and/or for monitoring antiplatelet therapies are spreading and the study of platelet function is strengthening. In this review, well-tried and innovative platelet function tests and their methodological features and clinical applications are considered.

Optimal aspirin dose in acute coronary syndromes: an emerging consensus

ABSTRACT: 

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

Clinical consequences of aspirin and clopidogrel resistance: an overview

The aim of this review is to introduce the concept of personalized medicine in secondary stroke prevention with antiplatelet medication. In the last years, many studies have been conducted regarding aspirin resistance and genotyping of clopidogrel metabolism. A review of the currently published data on this issue emphasizes the importance of focusing on the individualizing approach in antiplatelet therapy to achieve maximal therapeutic beneficial effect. However, many authors suggest that, before new information from ongoing trials become available, good clinical practice should dictate the use of low dose of aspirin that was shown to be effective in the prevention of stroke and death in patients with ischemic cerebrovascular disease, because higher doses do not have significantly better efficacy than lower doses in secondary stroke prevention, but lower-dose aspirin is associated with less side effects. On the other hand, many factors are associated with clopidogrel resistance, and recent genetic studies showed that the CYP2C19*2 genotype (loss-of-function allele) is related to poor metabolism of clopidogrel, but larger studies are needed to definitively confirm or rule out the clinical significance of this genetic effect. The aim of personalized approach in secondary stroke prevention is to take the most appropriate medicine in the right dose in accordance with the clinical condition of the patient and associated risk factors.

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

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

Antiplatelet resistance and thromboembolic complications in neurointerventional procedures

Antiplatelet resistance is emerging as a significant factor in effective secondary stroke prevention. Prevalence of aspirin and clopidogrel resistance is dependent upon laboratory test and remains contentious. Large studies in cardiovascular disease populations have demonstrated worse ischemic outcomes in patients with antiplatelet resistance, particularly in patients with coronary stents. Thromboembolism is a complication of neurointerventional procedures that leads to stroke. Stroke rates related to aneurysm coiling range from 2 to 10% and may be higher when considering silent ischemia. Stroke associated with carotid stenting is a major cause of morbidity. Antiplatelet use in the periprocedure setting varies among different centers. No guidelines exist for use of antiplatelet regimens in neurointerventional procedures. Incidence of stroke in patients post procedure may be partly explained by resistance to antiplatelet agents. Further research is required to establish the incidence of stroke in patients with antiplatelet resistance undergoing neurointerventional procedures.

Clinical predictors of dual aspirin and clopidogrel poor responsiveness in stable cardiovascular patients from the ADRIE study

BACKGROUND

Poor response to both aspirin and clopidogrel (dual poor responsiveness [DPR]) is a major risk factor for recurrent ischemic events.

OBJECTIVES

The aim of this study was to identify factors associated with DPR, defined with specific tests, and derive a predictive clinical score.

METHODS

We studied 771 consecutive stable cardiovascular patients treated with aspirin (n = 223), clopidogrel (n = 111), or both drugs (n = 37). Aspirin responsiveness was evaluated by serum thromboxane (Tx)B₂ assay, and clopidogrel responsiveness by calculating the platelet reactivity index (PRI) on the basis of the phosphorylation status of the vasodilator phosphoprotein. The analysis was focused on patients treated with both drugs, and on independent predictors of DPR.

RESULTS

Among patients on dual therapy, there was no relevant correlation between TxB₂ levels and PRI values (r = 0.11). Sixty-seven patients (15.4%) had DPR. Diabetes [odds ratio (OR) 1.89, 95% confidence interval (CI) 1.06-3.39], high body weight (> 86 kg vs. < 77 kg, OR 4.74, 95% CI 2.49-9.73), low aspirin dose (75-81 mg vs. ≥ 160 mg, OR 0.12, 95% CI 0.09-0.93) and high C-reactive protein (CRP) level (> 1.6 mg L⁻¹ vs. < 0.6 mg L⁻¹, OR 3.66, 95% CI 1.74-8.72) were independently associated with DPR, via increased TxB(2) levels, increased PRI, or both. These associations with TxB₂ and PRI were reproduced across the whole population. With use of a factor-weighed score (c-index = 0.74), the predicted prevalence of DPR was 57% in the highest strata of the score as compared with < 4% for the lowest strata.

CONCLUSIONS

Diabetes, body weight, the aspirin dose and CRP levels are readily available independent predictors of DPR, and some are potential targets for reducing its prevalence.

Treatment of aspirin-resistant patients with omega-3 fatty acids versus aspirin dose escalation

OBJECTIVES

The aim of this study was to evaluate whether addition of omega-3 fatty acids or increase in aspirin dose improves response to low-dose aspirin among patients who are aspirin resistant.

BACKGROUND

Low response to aspirin has been associated with adverse cardiovascular events. However, there is no established therapeutic approach to overcome aspirin resistance. Omega-3 fatty acids decrease the availability of platelet arachidonic acid (AA) and indirectly thromboxane A2 formation.

METHODS

Patients (n = 485) with stable coronary artery disease taking low-dose aspirin (75 to 162 mg) for at least 1 week were screened for aspirin response with the VerifyNow Aspirin assay (Accumetrics, San Diego, California). Further testing was performed by platelet aggregation. Aspirin resistance was defined by > or =2 of 3 criteria: VerifyNow score > or =550, 0.5-mg/ml AA-induced aggregation > or =20%, and 10-micromol/l adenosine diphosphate (ADP)-induced aggregation > or =70%. Thirty patients (6.2%) were found to be aspirin resistant and randomized to receive either low-dose aspirin + omega-3 fatty acids (4 capsules daily) or aspirin 325 mg daily. After 30 days of treatment patients were re-tested.

RESULTS

Both groups (n = 15 each) had similar clinical characteristics. After treatment significant reductions in AA- and ADP-induced aggregation and the VerifyNow score were observed in both groups. Plasma levels of thromboxane B2 were also reduced in both groups (56.8% reduction in the omega-3 fatty acids group, and 39.6% decrease in the aspirin group). Twelve patients (80%) who received omega-3 fatty acids and 11 patients (73%) who received aspirin 325 mg were no longer aspirin resistant after treatment.

CONCLUSIONS

Treatment of aspirin-resistant patients by adding omega-3 fatty acids or increasing the aspirin dose seems to improve response to aspirin and effectively reduces platelet reactivity.

The impact of intravenous aspirin administration on platelet aspirin resistance after on-pump coronary artery bypass surgery

Aspirin resistance continues to be a major challenge in patients after coronary artery bypass grafting (CABG). We investigated the impact of intravenous aspirin on platelet function in this clinical setting. Forty-two patients received 100 mg of oral aspirin once daily, beginning on day 1 after the operation. Between day 6 and 8 post operation one oral dose was replaced by an intravenous dose of 300 mg. Platelet function analyzer (PFA-100) closure times (CT), turbidimetric platelet aggregation (TPA) and impedance platelet aggregation (IPA) induced by arachidonic acid (AA), collagen and ADP were measured prior to and 1 h and 24 h after intravenous aspirin. Results obtained prior to the intravenous aspirin were compared with respective values from 120 healthy individuals. Despite the postoperative oral aspirin that was given once daily, ADP-induced TPA (ADPTPA) and IPA values induced by AA, ADP or collagen were significantly greater in patients than in controls, while PFA-100 CT were significantly shorter. Intravenous aspirin induced a significant reduction of platelet aggregability as measured by collagen/epinephrine (CEPI) CT, TPA and IPA induced by AA and collagen 1 h and 24 h after administration. Intravenous aspirin was not found to influence collagen/ADP (CADP) CT and IPA induced by ADP. Concomitantly, the number of patients with laboratory aspirin resistance as measured by CEPI-CT and TPA but not by IPA induced by AA or collagen dropped significantly. Agreement in the detection of aspirin responders and non-responders among platelet function tests was poor. Our findings indicate that the intravenous aspirin appears to be a promising approach for reducing laboratory aspirin resistance during the postoperative phase of CABG.

Resistance to low-dose aspirin therapy among patients with acute coronary syndrome in relation to associated risk factors

BACKGROUND

A substantial proportion of patients have recurrence of vascular events despite daily intake of low-dose aspirin therapy. Therefore, different patients may require different aspirin dosages to achieve complete inhibition of platelet function.

OBJECTIVE

The aim of this work was to measure the response to low-dose aspirin therapy (150 mg/day) among patients with unstable angina or non-ST-segment elevation myocardial infarction and to find out whether titrating aspirin dosage to 300 mg/day, would provide a better therapeutic response in the resistant cases. Moreover, we also aimed to study any association between aspirin non-responsiveness and atherothrombotic risk factors.

METHODS

The antiplatelet effect of 150 mg/day aspirin was studied prospectively in 50 consecutive patients with unstable angina or non-ST-segment elevation myocardial infarction. Platelet aggregation was measured using optical platelet aggregometry and serum thromboxane B(2) level. Aspirin resistance was defined as collagen (1 μg/mL) and adenosine diphosphate (ADP) (5 μmol/L)-induced platelet aggregation of ≥ 40% when compared with control values. Twenty healthy age- and sex-matched individuals were taken as a control group. All patients were subjected to complete medical history (risk factors, medications), thorough clinical examination, ECG, coronary angiography and laboratory investigations including: complete haemogram, coagulation, kidney, liver and lipid profiles, fasting blood glucose and glycated haemoglobin (HbA(1C) ).

RESULTS

Eleven of 50 patients (22%) were found to be aspirin resistant. A highly significant difference was found between the mean values of ADP, collagen-induced platelet aggregation percentage and thromboxane B(2) level after aspirin 150 mg/day when compared with the corresponding mean values after aspirin 300 mg/day among the resistant patients (66 ± 7.01%, 62 ± 4.34% and 620 ± 64.58 pg/mL, respectively, vs. 26.87 ± 2.85%, 16.5 ± 3.8% and 77 ± 11.3 pg/mL) indicating enhanced response to aspirin after escalating the dose. The presence of atherothrombotic risk factors (hypertension, smoking, family history of ischaemic heart disease and previous MI) were not statistically different between aspirin-resistant and aspirin-sensitive patients. However, there was a highly significant difference between the aspirin sensitive and the resistant patients regarding the other risk factors (diabetes mellitus and dyslipidaemia) (P < 0.01).

CONCLUSION

There is inter-individual variability in response to the antiplatelet effect of standard doses of aspirin (150, 300 mg/day). The response to aspirin 300 mg/day is enhanced in resistant patients when compared to 150 mg/day. There was a significant association between aspirin resistance and atherothrombotic risk factors (diabetes, hyperlipidaemia and obesity).

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.

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

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

Aspirin resistance and adverse clinical events in patients with coronary artery disease

PURPOSE

We sought to determine the clinical significance of aspirin resistance measured by a point-of-care assay in stable patients with coronary artery disease (CAD).

METHODS

We used the VerifyNow Aspirin (Accumetrics Inc, San Diego, Calif) to determine aspirin responsiveness of 468 stable CAD patients on aspirin 80 to 325 mg daily for > or =4 weeks. Aspirin resistance was defined as an Aspirin Reaction Unit > or =550. The primary outcome was the composite of cardiovascular death, myocardial infarction (MI), unstable angina requiring hospitalization, stroke, and transient ischemic attack.

RESULTS

Aspirin resistance was noted in 128 (27.4%) patients. After a mean follow-up of 379+/-200 days, patients with aspirin resistance were at increased risk of the composite outcome compared to patients who were aspirin-sensitive (15.6% vs 5.3%, hazard ratio [HR] 3.12, 95% confidence intervals [CI], 1.65-5.91, P < .001). Cox proportional hazard regression modeling identified aspirin resistance, diabetes, prior MI, and a low hemoglobin to be independently associated with major adverse long-term outcomes (HR for aspirin resistance 2.46, 95% CI, 1.27-4.76, P = .007).

CONCLUSIONS

Aspirin resistance, defined by an aggregation-based rapid platelet function assay, is associated with an increased risk of adverse clinical outcomes in stable patients with CAD.

Aspirin resistance

Aspirin resistance refers to less than expected suppression of thromboxane A(2) production by aspirin and has been reported to be independently associated with an increased risk of adverse cardiovascular events. Possible causes of aspirin resistance include poor compliance, drug interaction, inadequate aspirin dose, increase turnover of platelets, genetic polymorphisms of cyclo-oxygenase-1, and upregulation of alternate (non-platelet) pathways of thromboxane production. Laboratory methods used to detect aspirin resistance include those that measure thromboxane A(2) production and thromboxane A(2)-dependent platelet function. However, since there is currently no standardised approach to the diagnosis and there are no proven effective treatments for aspirin resistance that improve outcome, patients with cardiovascular disease receiving aspirin should not be routinely tested for aspirin resistance. Instead physicians should be aware of the factors that may impair aspirin function, ensure that they use an appropriate dose of aspirin and optimise compliance with aspirin therapy. Further research exploring the mechanisms of aspirin resistance is needed in order to better define and develop a standardised test for aspirin resistance that is specific, reliable, can be readily applied in routine laboratories and correlate with an increased risk of cardiovascular events.

Nonsteroidal anti-inflammatory drug use is a significant cause of peptic ulcer disease in a tertiary hospital in Singapore: a prospective study

BACKGROUND

Peptic ulcers due to nonsteroidal anti-inflammatory drug (NSAID) use may have contributed to the static prevalence of ulcer disease in Asia.

GOAL

We aimed to determine the current etiology of peptic ulcer disease in Singapore.

STUDY

Consecutive patients undergoing esophagogastroduodenoscopy who had not been exposed to antibiotics, or antiulcer therapy within the past 6 months, and in whom peptic ulcers were found, were prospectively studied. Before endoscopy, patients were interviewed regarding the use of NSAID or aspirin. During endoscopy, antral biopsies were obtained for urease test and histology. Serum thromboxane B2 levels were compared with those of healthy volunteers.

RESULTS

Peptic ulcers were detected in 600 patients during a 2-year period. The ulcers were negative for Helicobacter pylori in 212 patients (35.3%) and these H. pylori negative ulcers were related to NSAID use in 68.9% of cases. On the basis of serum thromboxane B2 levels, 30.8% of the patients with non-H. pylori non-NSAID were considered to have consumed NSAID.

CONCLUSIONS

H. pylori negative peptic ulcer makes up a significant proportion of peptic ulcer in Singapore. Most of these ulcers were related to NSAID use. Serum thromboxane profile suggested surreptitious NSAID use in many of the non-H. pylori and apparently non-NSAID patients.

Usefulness of whole blood aggregometry and its comparison with thromboxane generation assay in monitoring acetylsalicylic acid effectiveness--a multiparametric study in rats

BACKGROUND

There is a need for consensus concerning universal methodological criteria for detection of suboptimal response to acetylsalicylic acid (ASA) therapy. Therefore, animal models to test for ASA effectiveness remain of interest. Our objective was to verify the usefulness of multiparametric whole-blood impedance aggregometry and thromboxane A(2) generation, which are the most popular techniques used for monitoring of ASA treatment effectiveness.

METHODS

Using multiparametric analysis of whole-blood impedance aggregometry, we examined which parameters of platelet aggregation or disaggregation allow for the best discrimination between ASA-treated (4 or 40 mg/kg for 60 days) and non-treated male rats. The effectiveness of ASA-mediated inhibition of platelet cyclooxygenase-1 was verified by determination of plasma thromboxane B(2) and urine 11-dehydro-thromboxane B(2), accepted as reference assays for monitoring of ASA-mediated platelet cyclooxygenase-1 inhibition.

RESULTS

Two of the platelet agonists used, collagen (1 mg/L) and arachidonic acid (0.5 mmol/L), allowed discrimination of control and ASA-treated animals, whereas adenosine diphosphate (5 micromol/L) was not effective. It is noteworthy that only ASA-mediated changes in duration of the rising phase for platelet aggregation and the area under the curve for collagen-induced aggregation allowed significant discrimination between low and high ASA dose and remained correlated with the reference parameter, plasma thromboxane B(2).

CONCLUSIONS

Analysis of aggregation curves, routinely based only on the amplitude and rate of platelet aggregation, may not be enough discriminative to distinguish between varying ASA doses and treatment schedules.

Aspirin and clopidogrel resistance

Despite aspirin's and clopidogrel's proven benefit in reducing cardiovascular (CV) events, recurrent CV events still occur in patients receiving antiplatelet therapy. Many of these patients are resistant or only partially responsive to the antiplatelet effects of aspirin and clopidogrel, as determined by standard platelet assays. However, current clinical guidelines do not support routine screening for aspirin or clopidogrel resistance, in part because determination of the most appropriate screening test has not been established. This review attempts to (1) describe the phenomena of clinical aspirin and clopidogrel resistance (ie, treatment failure), (2) discuss the complexity of defining and identifying aspirin and clopidogrel resistance, (3) identify factors that may be responsible for aspirin and clopidogrel resistance, (4) outline several standard platelet function assays and their limitations, and (5) describe potential new antiplatelet therapies that may benefit aspirin- or clopidogrel-resistant patients.

Aspirin resistance

Aspirin resistance is the inability of aspirin to reduce platelet production of thromboxane A2 and thereby platelet activation and aggregation. Increasing degrees of aspirin resistance may correlate independently with increasing risk of cardiovascular events. Aspirin resistance can be detected by laboratory tests of platelet thromboxane A2 production or platelet function that depend on platelet thromboxane production. Potential causes of aspirin resistance include inadequate dose, drug interactions, genetic polymorphisms of COX-1 and other genes involved in thromboxane biosynthesis, upregulation of non-platelet sources of thromboxane biosynthesis, and increased platelet turnover. Aspirin resistance can be overcome by treating the cause or causes, and reduced by minimising thromboxane production and activity, and blocking other pathways of platelet activation. Future research is aimed at defining aspirin resistance, developing reliable tests for it, and establishing the risk of associated cardiovascular events. Potential mechanisms of aspirin resistance can then be explored and treatments assessed.

Off-Pump Coronary Artery Bypass Leads to a Regional Hypercoagulable State Not Detectable Using Systemic Markers

OBJECTIVE: It is believed that off-pump coronary artery bypass grafting (OPCAB) leads to hypercoagulability, but efforts to document such a state have been unrevealing. We hypothesized that OPCAB increases the risk of developing a regional hypercoagulable state. METHODS: Blood was obtained from the aorta and coronary sinus (CS) after CABG performed off- (N=69) or on-pump (N=35) to determine the transcardiac gradients of F1.2 (thrombin production), XIIa (coagulation activation), myoglobin (ischemia) and IL-6, IL-8 using ELISA and platelet-derived microparticles using FACS. Platelet function was measured using aggregometry. Regional myocardial pH and SVG flow were recorded intraoperatively. SVG biopsies were analyzed for endothelial integrity (EI) using immunohistochemistry and graft patency was determined by predischarge CT angiography. RESULTS: Compared with on-pump, OPCAB provoked significantly higher transcardiac F1.2 (117±200 v. 31±38%), FXII-a (14±29 v. 2±4%), microparticles (14±-9.5% v. 6.4±-4.1%), IL-6 (119±183 v. 28±39%), and a trend toward increased IL-8 (67±94 v. 24±46%, P = 0.077). Myoglobin release after OPCAB, also greater than on-pump CABG (54±89 v. 8±14%, P < 0.01), correlated with regional pH change (R=-0.96, P < 0.0001), and F1.2 release (R=0.55, P = 0.0002). In contrast, systemic changes in these markers were all less after OPCAB. SVG flow was significantly reduced in OPCAB (39.4 versus 66.5 mL/min, P = 0.0002), but EI and graft patency rates were the same. CONCLUSIONS: Through the use of transcardiac assays, we illustrated that regional coagulation was enhanced after off- compared with on-pump CABG. If the findings of this pilot study are confirmed, OPCAB may require additional antithrombotic therapies to respond to this local hypercoagulable state.

A Critical Appraisal of the Phenomenon of Aspirin Resistance

Aspirin is the mainstay antiplatelet treatment in patients with high risk of cardiovascular atherothrombotic events, and its beneficial effect is documented in several clinical trials. Nevertheless, the effectiveness of aspirin has been questioned by the emergence of the concept of 'aspirin resistance' (AR). This phenomenon, although lacking a precise definition, covers the fact that some patients do not exhibit the expected platelet inhibition by use of various techniques for measuring platelet function. In this critical review, we evaluate the methods used for measuring AR. We will discuss the available data regarding the prevalence and the clinical importance of the phenomenon. Finally, the potential mechanisms underlying AR are considered.

Contribution of cyclooxygenase-2 to elevated biosynthesis of thromboxane A2 and prostacyclin in cigarette smokers

BACKGROUND

Cigarette smoking is highly pathogenic to the vasculature. In smokers, the biosynthesis of both thromboxane (Tx) A2 and prostacyclin is increased. We hypothesized that the excess in prostacyclin biosynthesis in smokers was derived from the inducible cyclooxygenase-2 (COX-2). We further hypothesized that if the overproduction of prostacyclin in smokers were restraining platelet activation, then inhibition of COX-2 would lead to an increase in the activation of platelets, with a corresponding increase in the biosynthesis of TxA2.

METHODS AND RESULTS

Smokers and nonsmokers received rofecoxib 25 mg twice daily or placebo for 1 week each in random sequence. The systemic biosynthesis of TxA2 and prostacyclin was assessed by analysis of their respective urinary metabolites, 11-dehydrothromboxane B2 (Tx-M) and 2'3-donor-6-keto-PGF(1alpha) (PGI-M). Serum TxB2 was measured as an indicator of platelet COX-1 activity. Results are expressed as mean+/-SE with median and range. The elevated PGI-M in smokers (189+/-25, median 174, range 85 to 390 pg/mg creatinine) was reduced by rofecoxib to 78+/-27, median 71.5, range 50 to 135 pg/mg creatinine (P=0.002), and in nonsmokers, PGI-M at baseline (115+/-10, median 107, range 67 to 198 pg/mg creatinine) fell to 56+/-15, median 50, range 34 to 125 pg/mg creatinine (P=0.001) with rofecoxib. The increased excretion of Tx-M in smokers (284+/-26, median 252, range 200 to 569 pg/mg creatinine) was reduced by 21% to 223+/-16, median 206, range 154 to 383 pg/mg creatinine by rofecoxib (P=0.04) but was not changed in nonsmokers. Levels of serum TxB2 were not different in smokers and nonsmokers and were unaffected by rofecoxib.

CONCLUSIONS

The increased prostacyclin biosynthesis in smokers is derived largely from the inducible COX-2. COX-2 also contributes to the increased biosynthesis of TxA2 in smokers, most likely from inflammatory cells.

Aspirin Resistance: An Evaluation of Current Evidence and Measurement Methods

Aspirin resistance is a poorly characterized phenomenon, whereby certain patients do not benefit from the antithrombotic effect of aspirin. The frequency of aspirin resistance is unknown, but estimates range from 5-60%. The mechanism of aspirin resistance also is unknown; proposed mechanisms are poor patient compliance, poor aspirin absorption, increased isoprostane activity, platelet hypersensitivity to agonists, increased cyclooxygenase-2 activity, a cyclooxygenase-1 polymorphism, and the platelet alloantigen 2 polymorphism of platelet glycoprotein IIIa. Aspirin resistance appears to be dose related in some patients and therefore may be overcome with high doses. Evidence indicates that aspirin resistance is a dynamic state, with significant intrapatient variability in aspirin sensitivity with time. To date, a sensitive and specific assay of aspirin effect that reliably predicts treatment failure has not emerged. However, several commercially available products are being marketed for this purpose without convincing clinical data. Despite a wealth of literature on the topic, aspirin resistance remains an enigma. Further investigation is needed regarding strategies to identify and treat patients resistant to aspirin.

Low-dose aspirin increases aspirin resistance in patients with coronary artery disease

PURPOSE

We sought to investigate the association of aspirin dose and aspirin resistance in stable coronary artery disease patients measured by a point-of-care assay.

METHODS

We studied 468 consecutive stable coronary artery disease patients in a referral cardiac center who were taking aspirin 80 to 325 mg daily for > or =4 weeks. The VerifyNow Aspirin (Ultegra RPFA-ASA, Accumetrics Inc, San Diego, Calif) was used to determine aspirin responsiveness. An aspirin reaction unit (ARU) > or =550 indicates the absence of aspirin-induced platelet dysfunction, based on correlation with epinephrine-induced light transmission aggregometry. Demographic and clinical data were collected to analyze the predictors of aspirin resistance.

RESULTS

Aspirin resistance was noted in 128 (27.4%) patients. Univariate predictors of aspirin resistance include elderly (P = 0.002), women (P <0.001), anemia (P <0.001), renal insufficiency (P = 0.009) and aspirin dose < or =100 mg (P = 0.004). Multivariate analysis revealed hemoglobin (odds ratio [OR] 0.6; 95% confidence interval [CI] 0.51 to 0.69; P <0.001) and aspirin dose < or =100 mg (OR 2.23; 95% CI 1.12 to 4.44; P = 0.022) to be independent predictors of aspirin resistance. Daily aspirin dose < or = 100 mg was associated with increased prevalence of aspirin resistance compared with 150 mg and 300 mg daily (30.2% vs 16.7% vs 0%, P = 0.0062).

CONCLUSION

A 100 mg or less daily dose of aspirin, which may have lower side effects, is associated with a higher incidence of aspirin resistance in patients with coronary artery disease. Prospective randomized studies are warranted to elucidate the optimal aspirin dosage for preventing ischemic complications of atherothrombotic disease.

Effect of increasing doses of aspirin on platelet function as measured by PFA-100 in patients with diabetes

INTRODUCTION

Platelets of diabetic patients have been reported to be less sensitive to aspirin. The aim of this study is to compare a medium (300 mg) and low (100 mg) dose of aspirin on platelet function in diabetic patients.

METHODS

We have included one hundred and two patients with type 2 diabetes mellitus. Platelet function was measured as closure time (CT) with the Platelet Function Analyzer (PFA)-100 before the administration of aspirin. Initially the patients were given 100 mg aspirin once daily for seven days, and then the measurements were repeated. If the CT exceeded the upper limit of 300 s, the study was terminated. If not, the patients continued the aspirin therapy with a dose of 300 mg daily for another seven days, and the CTs were measured again.

RESULTS

After taking 100 mg aspirin, the CT significantly increased from 126+/-29 s to 256+/-66 s (p<0.001). In 68 of 102 (67%) patients, the CT increased to 300 s. In the remaining 34 patients, the baseline CT was 113+/-29, and increased to 170+/-45 s after 100 mg aspirin (p<0.001). In these patients, there was a further increase in the CT from 170+/-45 to 229+/-75 s following 300 mg aspirin (p<0.001). On average, the CT was increased by 60% and 39% following ingestion of 100 and 300 mg aspirin, respectively. CT>300 s were obtained in 15 (44%) of 34 patients after 300 mg aspirin.

CONCLUSIONS

Although, a daily dose of 100 mg aspirin effectively inhibited platelet function in a majority of diabetics, a considerable proportion of patients showed a greater platelet inhibition with the use of 300 mg aspirin. The PFA-100 closure time may be used to separate those patients who require a higher dose of aspirin to achieve desired antiplatelet effect.

Aspirin Resistance in Cardiovascular Disease: A Review

BACKGROUND

Aspirin is effective at reducing the cardiovascular event rate in defined patient groups. The introduction of antiplatelet therapies other than aspirin and the concept of aspirin resistance have led to critical reappraisal of current treatment. This review aims to clarify the evidence for aspirin resistance in patients with atherosclerosis.

METHODS

Medline search was performed to identify publications concerned with antiplatelet effects of aspirin and failure of aspirin therapy. Manual cross referencing was also performed.

RESULTS AND CONCLUSION

Wide variations in the rate of aspirin resistance (5.5-75%) have been reported. The lack of consensus on an appropriate definition and the number of different tests used to investigate aspirin resistance needs to be addressed. There are few studies where the primary aim was to document aspirin resistance or aspirin non-response. Further work should aim to investigate if aspirin resistance is clinically important and, if it is, what treatments may be beneficial to the at risk patient.

What is the lowest dose of aspirin for maximum suppression of in vivo thromboxane production after a transient ischemic attack or ischemic stroke?

BACKGROUND

There is still worldwide disagreement about the optimal lowest dose of aspirin to be used in patients after a transient ischemic attack (TIA) or nondisabling stroke. We measured the urinary 11-dehydro-thromboxane-B(2) (uTXB(2)) excretion to compare the degree of suppression of in vivo platelet activation by various low doses of aspirin.

METHODS

60 patients were randomly allocated to treatment with either 30, 50, 75 or 325 mg of aspirin. All patients received a 413-mg loading dose of carbasalate calcium (equivalent to 325 mg of aspirin) on day 0. The study population was stratified into a subgroup with acute ischemic stroke (AIS; n = 20; onset of symptoms <48 h) and a subgroup with a recent TIA or minor stroke (TIA/mS; n = 40) with onset of symptoms beyond 30 days, but less than a year previously. Urine samples were collected on day 0, 1, 5, 11 and 28 in patients with AIS, and on day 0, 11 and 28 in the patients with a TIA/mS.

RESULTS

On day 28, mean uTXB(2) levels were 241, 130, 217 and 187 pmol/mmol creatinine in the four treatment groups (ANOVA, p = 0.43). In the AIS subgroup, uTXB(2) remained suppressed on days 5 and 11 in all except the patients with the lowest dose (mean uTXB(2) on days 5 and 11: 475 and 392 pmol/mmol creatinine; log-transformed ANOVA, p = 0.05).

CONCLUSION

In patients with a TIA or nondisabling stroke, a daily dose of 30 mg of aspirin provides sufficient suppression of thromboxane synthesis. No indication of a dose-effect relationship was found. However, whether such a low dose adequately suppresses thromboxane synthesis in patients with acute stroke is uncertain.

Serial urinary 11-dehydrothromboxane B2, aspirin dose, and vascular events in blacks after recent cerebral infarction

BACKGROUND AND PURPOSE

Incomplete platelet inhibition by aspirin (aspirin resistance) may be a reason for stroke recurrence in some patients. 11-dehydrothromboxane B2 (11-DTB2) is a stable thromboxane A2 metabolite that reflects in vivo platelet activation. This pilot study was intended to evaluate the reproducibility of urinary 11-DTB2 over time and to look for evidence of aspirin resistance.

METHODS

All subjects were screened for the African American Antiplatelet Stroke Prevention Study (AAASPS) 7 to 90 days after noncardioembolic cerebral infarction. Of 83 subjects with at least 1 urine sample, 52 were enrolled in AAASPS (randomized to blinded treatment with aspirin 650 mg/d or ticlopidine 500 mg/d), and 31 were enrolled in an open-label antiplatelet therapy cohort. Subjects were followed up for 2 years, with 11-DTB2 measurements scheduled at baseline and 6, 12, and 24 months. Vascular events were cerebral infarction, myocardial infarction, or vascular death.

RESULTS

Despite considerable individual up or down fluctuations, the median 11-DTB2 change did not significantly differ from zero in any of the subgroups. However, in 6 subjects with a 4-fold decrease in aspirin dose from 1300 to 325 or 81 mg/d, the 11-DTB2 level increased from 611 to 1881 pg/mg creatinine (P=0.06). Vascular events occurred in 7 of 61 aspirin-treated subjects, and 11-DTB2 levels did not correlate with the events.

CONCLUSIONS

Fluctuations in urinary 11-DTB2 after cerebral infarction in blacks do not correlate with changes in aspirin doses, except perhaps when the dose changes by a factor of 4 or more. A larger study is needed to look further for aspirin resistance.

Antiplatelet Effect of Aspirin in Patients With Cerebrovascular Disease

BACKGROUND AND PURPOSE

Aspirin is used commonly to prevent ischemic strokes and other vascular events. Although aspirin is considered safe and effective, it has limited efficacy with a relative risk reduction of 20% to 25% for ischemic stroke. We sought to determine if aspirin as currently used is having its desired antiplatelet effects.

METHODS

We ascertained patients with cerebrovascular disease who were taking only aspirin as an antiplatelet agent. Platelet function was evaluated using a platelet function analyzer (PFA-100). PFA test results were correlated with aspirin dose, formulation, and basic demographic factors.

RESULTS

We ascertained 129 patients, of whom 32% were taking an enteric-coated aspirin preparation and 32% were taking low-dose (< or =162 mg/d) aspirin. For the entire cohort, 37% of patients had normal PFA-100 results, indicating normal platelet function. For the patients taking low-dose aspirin, 56% had normal PFAs compared with 28% of those taking > or =325 mg/d of aspirin, while 65% of patients taking enteric-coated aspirin had normal PFAs compared with 25% taking an uncoated preparation (P<0.01 for both comparisons). Similar results were obtained if PFA results were analyzed using mean closure times (low-dose aspirin, 183 sec; high-dose aspirin, 233 sec; enteric-coated, 173 sec; uncoated, 235 sec; P<0.01 for comparisons). Older patients and women were less likely to have a therapeutic response to aspirin, independent of aspirin dose or formulation.

CONCLUSIONS

A significant proportion of patients taking low-dose aspirin or enteric-coated aspirin have normal platelet function as measured by the PFA-100 test. If these results correlate with clinical events, they have broad implications in determining how aspirin is used and monitored.