Platelet function testing for aspirin resistance is reasonable to do: yes!

Comparison of Different Laboratory Tests to Identify “Aspirin Resistance” and Risk of Vascular Events among Ischaemic Stroke Patients: A Double-Blind Study

“Aspirin resistance” (AR) is associated with increased risk of vascular events. We aimed to compare different platelet function tests used in identifying AR and assess their implications on clinical outcome. We performed platelet aggregation studies on non-cardioembolic ischaemic stroke patients taking aspirin 100 mg/day and 30 non-stroke controls. Data were collected on demographics, vascular risk factors, and concomitant medications. Cut-offs for AR were (1) light transmission aggregometry (LTA) of ≥20% using arachidonic acid (AA), ≥70% using ADP, or ≥60% using collagen; and (2) VerifyNow® assay ≥ 550 ARU. Telephone follow-ups were conducted by study staff blinded to AR status to ascertain the occurrence of vascular outcomes (stroke, myocardial infarction, amputation, death). A total of 113 patients were recruited, mean age 65 ± 8 years, 47% women, 45 ± 15 days from index stroke. 50 (44.3%, 95% CI 34.9–53.9) had AR on at least 1 test. Frequency of AR varied from 0% to 39% depending on method used and first vs. recurrent stroke. There were strong correlations between LTA AA, VerifyNow® and Multiplate® ASPItest (r = 0.7457–0.8893), but fair to poor correlation between LTA collagen and Multiplate® COLtest (r = 0.5887) and between LTA ADP and Multiplate® ADPtest (r = 0.0899). Of 103 patients with a mean follow up of 801 ± 249 days, 10 (9.7%) had vascular outcomes, of which six had AR by LTA-ADP. AR by LTA-ADP is associated with increased risk of vascular outcome (p = 0.034). Identification of AR is not consistent across different platelet function tests. LTA of ≥70% using 10 µM ADP in post-stroke patients taking aspirin is associated with increased risk of vascular outcome.

Evolving Approaches to Antithrombotics in Stroke Prevention and Treatment

The optimization of antithrombotic therapy for acute stroke treatment and secondary prevention is an evolving process based on an increasing array of studies that provide an evidence-based approach. Options have increased dramatically with the release of the non-vitamin K oral anticoagulants and with the results of recent randomized clinical trials designed to assess potential benefits versus risks for patients in an individualized fashion. Recent studies have provided important information to guide choice and dosing of antiplatelet agents as well as the length of treatment. Anticoagulant use is particularly pertinent for stroke prevention in patients at higher risk of atrial fibrillation and may have a place in certain other stroke mechanisms. One important focus of study is the potential benefit of combined antiplatelet and anticoagulant therapy. Options for our patients, when the initial choice of therapy does not demonstrate benefit or is not well tolerated, clearly, are valuable. For example, short-term dual antiplatelet therapy for minor stroke and transient ischemic attack is being adopted, but with the recognition that longer-term combined therapy is not worth the increased risk of bleeding. Alternative antiplatelet choices, such as cilostazol and possibly ticagrelor, may be of benefit for refractory patients and this could affect the decision-making process. This review represents an effort to incorporate the information from more recent stroke prevention and treatment studies with information gleaned from prior studies.

Platelet function-guided modification in antiplatelet therapy after acute ischemic stroke is associated with clinical outcomes in patients with aspirin nonresponse

PURPOSE

To investigate the association of clinical outcomes with platelet function-guided modification in antiplatelet therapy in patients with ischemic stroke.

RESULTS

Among 812 patients, 223 patients had aspirin nonresponse, 204 patients was modified in antiplatelet therapy after platelet function testing. Mean follow-up period was 4.8 ± 1.7 years (ranged from 1 to 6.4 years). The incidence rates of ischemic events, death, or bleeding events were not significantly different between the patients with and without antiplatelet therapy modification. However, in patients with aspirin nonresponse, antiplatelet therapy modification was associated with decreased ischemic events (hazard ratio, 0.67; 95% confidence interval [CI], 0.62-0.97; P = 0.01) and ischemic stroke (hazard ratio, 0.70; 95% CI, 0.63-0.98; P = 0.03) compared with no modification in antiplatelet therapy.

CONCLUSIONS

In patients with aspirin nonresponse, platelet function-guided modification in antiplatelet therapy after an ischemic stroke was associated with significantly lower rate of ischemic events. The platelet function testing may be useful to guide antiplatelet therapy modification.

METHODS

This is a retrospective, multicentre study. From August 2010 to December 2014, 812 patients with ischemic stroke underwent platelet function testing using platelet aggregation. Antiplatelet therapy modification was defined as any change in antiplatelet therapy after testing, including increasing aspirin dosage, adding an additional antiplatelet agent to aspirin, or switching to a more potent antiplatelet agent. The primary outcome was ischemic events. Secondary outcomes included death and bleeding events. Clinical outcomes were compared between patients with and without antiplatelet therapy modification using univariate and propensity score-adjusted analyses.

Dose-dependent effect of early antiplatelet therapy in acute ischaemic stroke

Antiplatelet agents are essential in treating patients with acute ischaemic stroke (AIS) to prevent recurrent ischaemic events. The aim of this study was to evaluate the effectiveness of early antiplatelet therapy with different aspirin (ASA) dosages in patients with AIS. This observational study included 454 patients with AIS in whom antiplatelet treatment was initiated. The antiplatelet effect was determined by whole blood aggregometry within 48 hours after antplatelet therapy was initiated. An impedance change exceeding 0 Ω after stimulation with arachidonic acid was defined as ASA low response (ALR) and ≥5 Ω in ADP-stimulated specimen as clopidogrel LR. Of the study group 53.5% patients were treated with 200 mg ASA orally, 27.5% with 500 mg ASA intravenously, 8.6% with 100 mg ASA orally, and 7.7% with 75 mg clopidogrel. A dose-dependent antiplatelet effect of ASA treatment was found: 18.4% of patients with 500 mg ASA intravenously were ALR, in contrast to 32.5% on 200 mg and 35.9% on 100 mg ASA orally. Clopidogrel treatment without a loading dose resulted in a high proportion of LR (45.7%). Using the propensity score method revealed a three times higher risk for ALR for patients treated with ASA 200 mg [odds ratio 2.99 (1.55–5.79)] compared to treatment with ASA 500 mg. In conclusion, initiating antiplatelet therapy in patients with AIS resulted in a dose-dependent insufficient platelet inhibitory effect. Our findings suggest using a loading dose of 500 mg ASA intravenously as this seems to be favourable when a sufficient early platelet inhibitory effect is wanted.

ClinicalTrials.gov Identifier: NCT01273935

Platelet Function Testing in Patients with Acute Ischemic Stroke: An Observational Study

BACKGROUND

The measurement of platelet reactivity in patients with stroke undergoing antiplatelet therapies is not commonly performed in clinical practice. We assessed the prevalence of therapy responsiveness in patients with stroke and further investigated differences between patients on prevention therapy at stroke onset and patients naive to antiplatelet medications. We also sought differences in responsiveness between etiological subtypes and correlations between Clopidogrel responsiveness and genetic polymorphisms.

METHODS

A total of 624 stroke patients on antiplatelet therapy were included. Two different groups were identified: "non-naive patients", and "naive patients". Platelet function was measured with multiple electrode aggregometry, and genotyping assays were used to determine CYP2C19 polymorphisms.

RESULTS

Aspirin (ASA) responsiveness was significantly more frequent in naive patients compared with non-naive patients (94.9% versus 82.6%, P < .0010). A better responsiveness to ASA compared with Clopidogrel or combination therapy was found in the entire population (P < .0010), in non-naive patients (P < .0253), and in naive patients (P < .0010). Multivariate analysis revealed a strong effect of Clopidogrel as a possible "risk factor" for unresponsiveness (odds ratio 3.652, P < .0001). No difference between etiological subgroups and no correlations between responsiveness and CYP2C19 polymorphisms were found.

CONCLUSION

In our opinion, platelet function testing could be potentially useful in monitoring the biological effect of antiplatelet agents. A substantial proportion of patients with stroke on ASA were "resistant", and the treatment with Clopidogrel was accompanied by even higher rates of unresponsiveness. Longitudinal studies are needed to assess whether aggregometry might supply individualized prognostic information and whether it can be considered a valid tool for future prevention strategies.

Clinical Implications of Changes in Individual Platelet Reactivity to Aspirin Over Time in Acute Ischemic Stroke

BACKGROUND AND PURPOSE

Time-dependent changes in individual platelet reactivity have been detected in patients with coronary artery disease. Therefore, we sought to evaluate the time-dependent changes in platelet reactivity to aspirin during the acute stage after ischemic stroke and the clinical implications of variable patient responses to aspirin in acute ischemic stroke.

METHODS

We conducted a single-center, prospective, observational study. The acute aspirin reaction unit (ARU) was measured after 3 hours of aspirin loading, with higher values indicating increased platelet reactivity despite aspirin therapy. The follow-up ARU was measured on the fifth day of consecutive aspirin intake. The numeric difference between the follow-up ARU and the acute ARU was defined as ΔARU and was stratified into quartiles. Early neurological deterioration was regarded as an early clinical outcome.

RESULTS

Both the acute ARU (476±69 IU) and the follow-up ARU (451±68 IU) were measured in 349 patients in this study. Early neurological deterioration was observed in 72 patients (20.6%). Changes in aspirin platelet reactivity over time showed an approximately Gaussian distribution. The highest ΔARU quartile was independently associated with early neurological deterioration (odds ratio, 3.19; 95% confidence interval, 1.43-7.10; P=0.005) by multivariate logistic regression analysis.

CONCLUSIONS

The results of our study showed that the increase in platelet reactivity to aspirin over time is independently associated with early neurological deterioration in patients with acute ischemic stroke. In addition, during the acute stage of ischemic stroke, serial platelet reactivity assays may be more useful than a single assay for identifying the clinical implications of aspirin platelet reactivity after ischemic stroke.

No Additional Prognostic Value of Genetic Information in the Prediction of Vascular Events after Cerebral Ischemia of Arterial Origin: The PROMISe Study

Background

Patients who have suffered from cerebral ischemia have a high risk of recurrent vascular events. Predictive models based on classical risk factors typically have limited prognostic value. Given that cerebral ischemia has a heritable component, genetic information might improve performance of these risk models. Our aim was to develop and compare two models: one containing traditional vascular risk factors, the other also including genetic information.

Methods and Results

We studied 1020 patients with cerebral ischemia and genotyped them with the Illumina Immunochip. Median follow-up time was 6.5 years; the annual incidence of new ischemic events (primary outcome, n=198) was 3.0%. The prognostic model based on classical vascular risk factors had an area under the receiver operating characteristics curve (AUC-ROC) of 0.65 (95% confidence interval 0.61-0.69). When we added a genetic risk score based on prioritized SNPs from a genome-wide association study of ischemic stroke (using summary statistics from the METASTROKE study which included 12389 cases and 62004 controls), the AUC-ROC remained the same. Similar results were found for the secondary outcome ischemic stroke.

Conclusions

We found no additional value of genetic information in a prognostic model for the risk of ischemic events in patients with cerebral ischemia of arterial origin. This is consistent with a complex, polygenic architecture, where many genes of weak effect likely act in concert to influence the heritable risk of an individual to develop (recurrent) vascular events. At present, genetic information cannot help clinicians to distinguish patients at high risk for recurrent vascular events.

Chinese herbal medicine for aspirin resistance: a systematic review of randomized controlled trials

Aspirin resistance (AR) is a prevalent phenomenon and leads to significant clinical consequences, but the current evidence for effective interventional strategy is insufficient. The objective of this systematic review is thus to assess the efficacy and safety of Chinese herbal medicine (CHM) for AR. A systematical literature search was conducted in 6 databases until December 2012 to identify randomized controlled trials (RCTs) of CHM for AR. As a result, sixteen RCTs with a total of 1011 subjects were identified, suggesting that the interests of the medical profession and the public in the use of CHM for AR have grown considerably in the recent years. Tongxinluo capsule and Danshen-based prescriptions were the most frequently used herbal prescriptions, while danshen root, milkvetch root, Leech, and Rosewood were the most frequently used single herbs. Despite the apparent reported positive findings, it is premature to determine the efficacy and safety of CHM for the treatment of AR due to poor methodological quality and insufficient safety data. However, CHMs appeared to be well tolerated in all included studies. Thus, CHM as a promising candidate is worthy of improvement and development for further clinical AR trials. Large sample-size and well-designed rigorous RCTs are needed.

Aspirin resistant patients with recent ischemic stroke

Some patients with a recent ischemic stroke who are being treated with aspirin as an antiaggregant suffer a new ischemic stroke. These patients (15-25%) have been called unresponsive to aspirin or aspirin resistant. The aspirin-resistant patients have a four-time greater risk of suffering a stroke. Furthermore, these strokes are generally more severe, with increased infarct volume and greater risk of recurrence. There is currently no ideal laboratory test to detect the resistance to the antiaggregant effect of aspirin. The study of resistance to aspirin would only be indicated in selected cases. In these patients, one should first rule out any "pseudo-resistance" to aspirin (lack of compliance, concomitant treatments that interfere with the action of the aspirin).

Advances in our understanding of "resistance" to antiplatelet agents for prevention of ischemic stroke

We review the role of aspirin and clopidogrel for prevention of ischemic stroke and explore the concept of antiplatelet therapy resistance both from a laboratory and clinical perspective and genetic polymorphisms that might influence platelet reactivity with clopidogrel administration. Debates have raged over the years about the application of platelet function tests in clinical practice. We conclude that platelet function testing is not indicated in routine clinical practice. This recommendation is supported by clinical guideline statements, a lack of a global platelet function measure, and limitations of current platelet function test methods as applied in practice. We discuss a recently hypothesized hierarchy of patient characteristics in relation to which patients are most likely to benefit from platelet function studies based on acuity (i.e., risk) of cardiovascular disease. A focus of antiplatelet therapy administration should include emphasis on compliance/adherence and in the example of aspirin, use of well-absorbed forms of aspirin and avoidance of drugs that may interact with aspirin to inhibit its mechanism of action (e.g., certain nonsteroidal anti-inflammatory drugs).

Antiplatelet therapy in cerebrovascular disorders

Antiplatelet treatment is a mainstay in acute and long-term secondary stroke prevention. Aspirin is still most widely used worldwide, however, there is increasing evidence from small randomised trials that dual antiplatelet therapy combining aspirin with dipyridamole or clopidogrel might be more effective in the acute and early chronic post-ischemic phase (i.e. first 90 days). Both clopidogrel and the combination of aspirin and extended-release dipyridamole are recommended by current guidelines in long-term secondary stroke prevention in patients who are at high risk for a recurrent ischemic stroke, since they are more effective compared with aspirin monotherapy. Antiplatelet agents are the therapy of choice in patients with ischemic stroke due to intracranial stenosis and patent foramen ovale. In contrast, oral anticoagulation is clearly superior to single or double antiplatelet therapy in patients with cardioembolic stroke, mainly caused by atrial fibrillation.Concerning newer antiplatelet agents, only cilostazol appears to be a promising therapeutic option in patients with ischemic stroke in the near future, but so far, only studies in Asian stroke patients have been performed.

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.

Antiplatelet resistance in stroke

Although the exact prevalence of antiplatelet resistance in ischemic stroke is not known, estimates about the two most widely used antiplatelet agents - aspirin and clopidogrel - suggest that the resistance rate is high, irrespective of the definition used and parameters measured. Inadequate antiplatelet responsiveness correlates with an increased risk of recurrent ischemic vascular events in patients with stroke and acute coronary syndrome. It is not currently known whether tailoring antiplatelet therapy based on platelet function test results translates into a more effective strategy to prevent secondary vascular events after stroke. Large-scale clinical trials using a universally accepted definition and standardized measurement techniques for antiplatelet resistance are needed to demonstrate whether a 'platelet-function test-guided antiplatelet treatment' strategy translates into improved stroke care. This article gives an overview of the clinical importance of laboratory antiplatelet resistance, describes the challenges for platelet-function test-guided antiplatelet treatment and discusses practical issues about the management of patients with aspirin and/or clopidogrel resistance.