A Critical Appraisal of the Phenomenon of Aspirin Resistance

Global Trends in Aspirin Resistance-Related Research from 1990 to 2015: A Bibliometric Analysis

Aspirin resistance can be defined as the inability of the usual dose of aspirin medication to produce its antithrombotic effect. Patients with diabetes or cardiovascular disease are at higher risk of stroke, myocardial infarction or cardiovascular death due to aspirin resistance. The aim of this bibliometric study was to identify and analyse the status and trends of aspirin resistance research production at global level through publications indexed in the Scopus database; this will shed new light on future research trends and help researchers predict dynamic direction of research. Literature search using the Scopus database was conducted to assess publications related to aspirin resistance. The selected publications included the terms related to aspirin resistance in the title, abstract or keywords. The searching was accomplished on 20 March 2016 and can be considered to include all publications up to 31 December 2015. Global cumulative publication output on aspirin resistance consists of 986 papers during 1990-2015. Among the 986 documents, 19 (1.9%) were published before 2000, 567 (57.5%) were published from 2000 to 2009 and 400 (40.6%) were published from 2010 to 2015, with peak of publications on this topic in 2008. The leading country in the field of aspirin resistance was the United States, which had the greatest counts of independent articles (165) and international collaboration articles (44). Turkey was in the second rank with 78 articles, followed by Italy (68), the UK (62) and Poland (60). The total number of citations for all documents was 26,342, and the average citations per document were 26.7. The h-index for all aspirin resistance publications was 82. This study presents the results of the first bibliometric study (including quantitative and qualitative analysis) of scientific publications in the field of aspirin renitence at global level. Aspirin resistance-related researches have notably increased in the last years, especially from 2000 to 2015. The United States is the most prolific country, not only in research quantity but also in quality. Furthermore, Turkey and European countries provided more research related to aspirin resistance than other regions such as the developing countries.

Endothelial dysfunction in young healthy men is associated with aspirin resistance

The aim of this study was to investigate the relation between endothelial dysfunction and aspirin response in a young healthy population (102 men aged 18-40). Initial concentrations of the NO pathway metabolites (ADMA, l-arginine, SDMA), cardiovascular risk markers, oxidative stress markers (MDA, thiol index), sICAM1, sVCAM1, PAI-1, sE-selectin, sP-selectin, VEGF, thromboxane B2, 6-keto-PGF1α and arachidonate-induced platelet aggregation (to separate aspirin resistant from sensitive group) were measured. Flow-mediated-vasodilation (FMD) was measured before and after intravenous infusion of 16.0 g of l-arginine. Measurements were repeated following aspirin administration (75 mg/24 h) for 4 days. Both groups were homogenous regarding demographic and biochemical characteristics reflecting cardiovascular risk. Aspirin resistant subjects were characterized by lower baseline FMD and higher FMD following aspirin and l-arginine treatment, as compared to aspirin sensitive control. MDA and nitrotyrosine were greater, whereas thiol index was lower in aspirin resistant men. The sICAM1, sVCAM1, PAI-1, sE-selectin, sP-selectin and VEGF levels were similar in the analyzed groups. Thromboxane in aspirin resistant subjects was greater both at baseline and following aspirin therapy. However, a significant decrease following aspirin treatment was present in both groups. Aspirin resistance in young men is associated with endothelial dysfunction, which could be due to oxidative stress resulting from lipid peroxidation.

Serum thromboxane B2 compared to five other platelet function tests for the evaluation of aspirin effect in stable cardiovascular disease

BACKGROUND

To assess the role of serum thromboxane B(2) (TXB(2)) measurements and the correlation between platelet function studies, in patients with stable cardiovascular disease on aspirin or clopidogrel.

METHODS

76 patients (47 on aspirin, 16 clopidogrel, 13 both) underwent assessment of TXB(2), whole blood aggregometry (WBA) after stimulation with (i) arachidonic acid (0.5mM), (ii) ADP (5 microM), (iii) collagen (1 and 5 microg/ml), PFA-100, and Cone and Plate Analyzer. Clopidogrel patients were additionally assessed by the VerifyNow System.

RESULTS

TXB(2) values ranged between 0.2 and 56.2 ng/ml, with significant separation between those taking aspirin, clopidogrel and controls (0.45 ng/ml vs 6.85 ng/ml vs 12.97 ng/ml, p<0.001). There was moderate correlation between WBA-AA and TXB(2) (r=0.487, p<0.001), PFA-100((R)) (r=0.599, p<0.001), WBA-Col1 (r=0.424, p<0.001), WBA-Col1:5 (r=0.417, p<0.001), and between TXB(2) and PFA-100((R)) (r=0.509, p<0.001). The prevalence of aspirin non-responders for WBA-AA, TXB(2), PFA-100((R)), CPA and Coll1:5 was 13.1%, 8.2%, 14.8%, 9.7% and 16.4% respectively. Individual patients were not consistently classified as aspirin non-responders in all tests. Those with inadequate aspirin response on > or =3 tests had higher TXB(2) levels (mean 1.57+/-1.66, range 0.553-4.45 vs mean 0.45+/-0.18, range 0.23-1.50) (p=0.001). Clopidogrel suppressed TXB(2) (p=0.02), WBA-AA (p<0.001), WBA-Col1 (p=0.012) and WBA-ADP (p<0.001) compared to controls. TXB(2) in patients ingesting fish oil tablets was lower compared to those without (0.4 ng/ml vs 0.52 ng/ml, p=0.004). Obesity was associated with higher TXB(2) values (0.61 vs 0.41, p=0.01).

CONCLUSION

Serum TXB(2) measurements are a direct measure of the pharmacological effect of aspirin, are easily performed and correlate with other measures of platelet function. Serum TXB(2) measurements could be a useful sole measure of aspirin non-response, and may be even more predictive when performed in tandem with a global measure of platelet function. Aspirin and clopidogrel both suppressed several platelet pathways.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Prevalence of Platelet Dysfunction and Abnormal Coagulation: Results of a Population-Based Study

The prevalence of impairments in the hemostatic process is unknown in acutely ill people. Data on hemostasis (PFA 100®) and the coagulation cascade of 1015 people are presented here, establishing a cohort of unselected emergency patients in a population-based approach. A high prevalence of reduced platelet function (38%) was found, which was more frequent than expected. In contrast, there was a lower prevalence (20%) of abnormal plasmatic coagulation, which was almost always explained by medication, whereas medication could not predict abnormal platelet function. Moreover, a history of disproportionate bleeding did not correlate well with abnormal platelet or coagulation factor function and could not substitute for a screening in this setting. The effect of acetylsalicylic acid (ASA) on PFA-closure time was frequently missing (34%), indicating a considerable prevalence of ASA nonresponse among the study population. These data should be applicable in similar settings. The high prevalence of unexpectedly abnormal platelet function in acute illness as well as the high prevalence of possible ASA nonresponders suggests a functional platelet assay to be effective in screening certain subpopulations of emergency patients.

Laboratory Evaluation of Platelet Function

Platelets play a central role in hemostasis. Consequently, they lie at the heart of many inherited and acquired bleeding disorders and thrombotic events. The diagnosis of these disorders and monitoring of antiplatelet therapy require a thorough understanding of tests that measure platelet quantity and function. This article outlines basic concepts of platelet physiology and describes the tests that are commonly used in the clinical assessment of platelet function.

The frequency of aspirin resistance and its risk factors in patients with metabolic syndrome

BACKGROUND

Although different populations were examined for the incidence of aspirin resistance, the frequency and related risk factors for aspirin resistance in patients with metabolic syndrome have not been reported yet. This study aimed to determine the frequency of aspirin resistance and its risk factors in patients with metabolic syndrome.

METHODS

We performed a cross-sectional study in 110 patients with metabolic syndrome. After one week of 100 mg/day aspirin, blood samples were obtained. Platelet function analyzer (PFA-100) was used to determine the frequency of aspirin resistance. Endothelial functions, carotid intima media thickness, and the presence of plaques in the carotid arteries were evaluated for subclinical atherosclerosis and the levels of inflammatory markers were assessed as risk factors for aspirin resistance. The presence of subclinical atherosclerosis was defined as a maximum carotid intima media thickness of > or = 0.9 mm and/or the presence of carotid atheroma.

RESULTS

Aspirin resistance was detected in 21.9% of the patients. In the multivariate analysis, hs-CRP levels (odds ratio [95% CI]=2.8 [1.3-5.9], p=0.009), diastolic blood pressure, (0.9 [0.8-1.0], p=0.007), and the presence of subclinical atherosclerosis (4.1 [1.4-12.2], p=0.012) were statistically significant risk factors for aspirin resistance.

CONCLUSIONS

We concluded that the frequency of aspirin resistance confirmed in this cohort of patients with metabolic syndrome was higher in patients with a lower diastolic blood pressure, higher hs-CRP levels and atherosclerotic changes in their carotid arteries.

Variation and importance of aspirin resistance in patients with known cardiovascular disease

AIM

To investigate whether aspirin resistance is a persistent condition, and to evaluate if aspirin resistance affects one-year clinical outcome.

METHODS AND RESULTS

Previously we studied 298 patients admitted to hospital with symptoms suggestive of an acute myocardial infarction (MI) despite treatment with aspirin, and 70 patients (23.5%) were aspirin resistant. In the present study, platelet function was reassessed by use of a Platelet Function Analyzer-100 one year later. A total of 187 patients were re-examined, and 17 (9.1%) demonstrated aspirin resistance. Of these 17 patients, 12 also exhibited aspirin resistance at baseline resulting in a 6% (12/187) prevalence of persistent aspirin resistance. A total of 34 patients had changed from aspirin resistant at baseline to aspirin sensitive at follow-up. We found a significant decrease in the prevalence of aspirin resistance from baseline (43%) to follow-up (11%) in patients with MI at baseline (p=0.0018). Furthermore, a significant decrease was found for patients without MI at baseline (20% to 9%, p=0.0009). During follow-up, 17% (12/70) of the patients with aspirin resistance at baseline suffered death, MI or stroke compared to 16% (37/227) of aspirin sensitive patients (p=0.868).

CONCLUSION

The prevalence of aspirin resistance varies with the clinical status of the patients, and indeed an acute MI is associated with temporary aspirin resistance. We also found that 6% of patients demonstrate persistent aspirin resistance. The presence of aspirin resistance did not affect one-year clinical outcome.

Using the Platelet Function Analyzer-100 for monitoring aspirin therapy

INTRODUCTION

The aim of the study was to evaluate the test characteristics of the Platelet Function Analyzer-100 (PFA-100) in patients treated with aspirin.

METHODS AND RESULTS

The study consisted of two sub-studies. In study 1, 10 patients with ischemic heart disease (IHD) and 10 controls had platelet function assessed by optical platelet aggregation and the PFA-100 method in two 5-week periods. Patients with IHD were treated with aspirin 150 mg/day (first 5-week period), and 300 mg/day (second 5-week period), whereas the controls only received aspirin (150 mg/day) during the second 5-week period. From the results of study 1, we found that a cut-off value for the PFA-100 collagen/epinephrine cartridge <165 s identified patients not taking aspirin (sensitivity 0.91, specificity 1.00). A good agreement between the PFA-100 method and optical platelet aggregation was found. Within-subject variation for the PFA-100 collagen/epinephrine cartridge was +/-28%, as compared to +/-17% for the optical platelet aggregation. Study 2 included 298 aspirin treated patients who were admitted with symptoms suggestive of an acute myocardial infarction. Platelet function was assessed in duplicate by the PFA-100 collagen/epinephrine cartridge, and a 95% Limit of Agreement interval at [-65%, 65%] indicated a limited precision.

CONCLUSION

We defined a cut-off value below which patients not taking aspirin can be identified. However, due to imprecision of the PFA-100 method repeated duplicate assessment of the collagen/epinephrine Closure Time is recommended.

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.

Prevalence of aspirin resistance in patients with an evolving acute myocardial infarction

OBJECTIVE

To study the prevalence and importance of aspirin resistance in patients with an evolving acute myocardial infarction (AMI) by use of the Platelet Function Analyzer-100.

INTRODUCTION

Previous studies have demonstrated the existence of aspirin resistance, but the clinical relevance of the phenomenon remains to be clarified. If aspirin resistant patients comprise a high-risk subgroup, it might be expected that the prevalence of aspirin resistance in patients with AMI would be higher than in patients without AMI. We hypothesized that the prevalence of aspirin resistance in patients with AMI was twice the prevalence in patients without AMI.

METHODS

We included 298 consecutive patients with known cardiovascular disease who were admitted to hospital with symptoms suggestive of an AMI. All had been taking aspirin 150 mg/day for at least 7 days prior to hospital admission. Platelet function was measured immediately at admission, and aspirin resistance was defined as a collagen/epinephrine Closure Time (CT(CEPI))<165 s.

RESULTS

We found that 70 (23.5%) patients were aspirin resistant, and 70 (23.5%) patients ended up with the diagnosis of an AMI. The prevalence of aspirin resistance was significantly higher in patients with AMI as compared to patients without (36% versus 20%, OR 2.26, CI 95% 1.19-4.22, p=0.0058). The CT(CEPI) measured at admission was an independent factor associated with an AMI.

CONCLUSIONS

Aspirin resistance is present in almost one fourth of patients admitted to hospital with symptoms suggestive of an AMI, and aspirin resistance is significantly associated with the diagnosis of a definite AMI.