Aspirin Resistance: A Review Of Diagnostic Methodology, Mechanisms, and Clinical Utility

Comparative Study of Ex Vivo Antiplatelet Activity of Aspirin and Cilostazol in Patients with Diabetes and High Risk of Cardiovascular Disease

BACKGROUND

The role of aspirin in primary cardiovascular disease prevention in patients with diabetes remains controversial. However, some studies have suggested beneficial effects of cilostazol on cardiovascular disease in patients with diabetes. We prospectively investigated the antiplatelet effects of cilostazol compared with aspirin in patients with diabetes and cardiovascular risk factors.

METHODS

We randomly assigned 116 patients with type 2 diabetes and cardiovascular risk factors but no evident cardiovascular disease to receive aspirin at a dose of 100 mg or cilostazol at a dose of 200 mg daily for 14 days. The primary efficacy outcome was antiplatelet effects of aspirin and cilostazol assessed with the VerifyNow system (aspirin response units [ARU]) and PFA-100 (closure time [CT]). Secondary outcomes were changes of clinical laboratory data (ClinicalTrials.gov Identifier: NCT02933788).

RESULTS

After 14 days, there was greater decrease in ARU in aspirin (-28.9%±9.9%) compared cilostazol (-0.4%±7.1%, P<0.001) and was greater increase in CT in aspirin (99.6%±63.5%) compared cilostazol (25.7%±54.1%, P<0.001). The prevalence of aspirin resistance was 7.5% according to VerifyNow (defined by ARU ≥550) and 18.9% according to PFA-100 (CT <192 seconds). Compared with aspirin, cilostazol treatment was associated with increased high density lipoprotein cholesterol (7.1%±12.7% vs. 4.2%±18.0%, P=0.006) and decreased triglycerides (-9.4%±33.7% vs. 4.4%±17.57%, P=0.016). However, there were no significant changes in total and low density lipoprotein cholesterol, C-reactive protein level, and cluster of differentiation 40 ligand between cilostazol and aspirin groups.

CONCLUSION

Aspirin showed better antiplatelet effects assessed with VerifyNow and PFA-100 compared with cilostazol. However, there were favorable changes in atherogenic dyslipidemia only in the cilostazol.

Ischemic stroke as a presenting manifestation of polycythemia vera: a narrative review

Polycythemia vera (PV) is a myeloproliferative disorder associated with an increased risk of cerebrovascular diseases. In this narrative review, we aimed to analyze the relationships between acute ischemic stroke and PV. We conducted a PubMed/Medline and Web of Sciences Database search using MeSH major terms. We found 75 articles and finally considered 12 case reports and 11 cohort studies. The ischemic stroke resulted as the first manifestation of PV in up to 16.2% of cases; the cumulative rate of cerebrovascular events was up to 5.5 per 100 persons per year and stroke accounted for 8.8% of all PV-related deaths; age, mutations, and a previous history of thrombosis were the main risk factors. The best approach to reduce stroke recurrence risk is unclear, even if some evidence suggests a potential role of lowering hematocrit below 45%. Ischemic stroke represents one of the most common PV manifestations but, despite their relationship, patients with both diseases have a very heterogeneous clinical course and management. PV-related strokes often remain underdiagnosed, especially for the low prevalence of PV. An early diagnosis could lead to prompt treatment with phlebotomy, cytoreduction, and low-dose aspirin to decrease the risk of recurrences. Clinicians should be aware of PV as a risk factor for stroke when approaching the differential diagnosis of cryptogenic strokes. An early diagnosis could positively influence patients' management and clinical outcomes. Further studies are required to evaluate the role of PV treatments in the prevention of cerebrovascular disease.

Aspirin resistance: a clinical review focused on the most common cause, noncompliance

Aspirin is an inexpensive, readily available medication that reduces the risk of subsequent vascular disease by about 25% in patients with known occlusive vascular disease. Aspirin's beneficial effect is mediated via inhibition of arachidonic acid (AA) activation of platelets and is detected by demonstrating a decrease in platelet function and/or a decrease in prostaglandin metabolites. Patients who are assumed to be taking their aspirin, but who do not demonstrate an aspirin effect are labeled as, "aspirin resistant." This is an unfortunate designation as the vast majority of patients labeled as "aspirin resistant" are noncompliant. Noncompliance is demonstrated in multiple studies that use repeat testing for platelet inhibition in patients with an initial inadequate response to aspirin. When the test is repeated under condition where ingestion of the test aspirin is assured, the patients' platelets are inhibited. Instead of using the term "aspirin resistance," this review will use "inadequate response to aspirin." Patients with an inadequate aspirin response have an increased likelihood for subsequent vascular events. Detection and treatment of an inadequate aspirin response would be facilitated by the development of a bedside assay that uses whole blood, is technically simple, inexpensive, sensitive, specific, reproducible, and provides an answer in a few minutes. Future research in patients with an inadequate response to aspirin should focus on mechanisms to improve compliance, which should decrease their risk of future vascular events.

Clinical importance of aspirin and clopidogrel resistance

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

Impaired anti-platelet effect of aspirin, inflammation and platelet turnover in cardiac surgery

A reduced platelet inhibitory response to acetyl salicylic acid (ASA) has been associated with an increased risk of graft thrombotic occlusion after coronary artery bypass grafting (CABG). We performed a prospective, observational study of 18 patients on 100 mg/day ASA before and after CABG. We assessed antiplatelet response to ASA and its relationship with platelet turnover, inflammatory markers, and soluble thrombomodulin (sTM) levels. All patients showed optimal response to ASA preoperatively but had higher values during follow-up. Platelet aggregation and platelet count in the perioperative period were significantly associated (P=0.05). Platelet turnover was defined as the average daily turnover (ADTO). The lowest inhibitory value (28% of patients > or =6 Omega) was recorded at the same time of the highest platelet turnover (>10% daily in 77.77% of patients), one week after CABG. ADTO >10% was associated with an increased risk of platelet aggregation > or =6 Omega. Levels of sTM were significantly higher one week after CABG (median 13 vs. 3 ng/ml preoperatively, P=0.0011). There is a transient impairment in ASA antiplatelet effect after CABG related to an increased platelet turnover caused by the inflammatory process. This could be responsible for the high risk of occlusive thrombosis.

Non-compliance is the predominant cause of aspirin resistance in chronic coronary arterial disease patients

Background

Our previous publication showed that 9% of patients with a history of myocardial infarction MI. could be labeled as aspirin resistant; all of these patients were aspirin resistant because of non-compliance. This report compares the relative frequency of aspirin resistance between known compliant and non-compliance subjects to demonstrate that non-compliance is the predominant cause of aspirin resistance.

Methods

The difference in the slopes of the platelet prostaglandin agonist (PPA) light aggregation curves off aspirin and 2 hours after observed aspirin ingestion was defined as net aspirin inhibition.

Results

After supposedly refraining from aspirin for 7 days, 46 subjects were judged non-compliant with the protocol. Of the remaining 184 compliant subjects 39 were normals and 145 had a past history of MI. In known compliant subjects there was no difference in net aspirin inhibition between normal and MI subjects. Net aspirin inhibition in known compliant patients was statistically normally distributed. Only 3% of compliant subjects (2 normals and 5 MI) had a net aspirin inhibitory response of less than one standard deviation which could qualify as a conservative designation of aspirin resistance. A maximum of 35% of the 191 post MI subjects could be classified as aspirin resistant and/or non-compliant: 9% aspirin resistant because of non-compliance, 23% non-compliant with the protocol and possibly 3% because of a decreased net aspirin inhibitory response in known compliant patients.

Conclusion

Our data supports the thesis that the predominant cause of aspirin resistance is noncompliance.

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 in coronary artery bypass surgery: new aspects of and alternatives for an old antithrombotic agent

The success of coronary artery bypass graft surgery (CABG) depends mainly on the patency of the graft vessels. Aortocoronary vein graft disease is comprised of three distinct but interrelated pathological processes: thrombosis, intimal hyperplasia and atherosclerosis. Early thrombosis is a major cause of vein graft attrition during the first month after CABG, while during the remainder of the first year, intimal hyperplasia forms a template for subsequent atherogenesis, which thereafter predominates. Platelets play a crucial role in the pathophysiology of graft thrombosis and aspirin is the primary antiplatelet drug that has been shown to improve vein graft patency within the first year after CABG. Nevertheless, a significant number of grafts still occlude in the early postoperative period despite 'appropriate' aspirin treatment. Moreover, laboratory investigations showed that the expected inhibition of platelet function is not always achieved. This has been called 'aspirin nonresponse' or 'aspirin resistance', although a uniform definition is lacking. The finding that a considerable number of patients show an impaired antiplatelet effect of aspirin after CABG brought new insight into the discussion concerning poor patency rates of bypass grafts: the early period after CABG shows a coincidence of an increased risk for bypass thrombosis (amongst others, due to platelet activation and endothelial cell disruption of the graft) and an increased prevalence of aspirin resistance. Hitherto, the underlying mechanisms of aspirin resistance are uncertain and largely hypothetical; amongst others, increased platelet turnover, enhanced platelet reactivity, systemic inflammation, and drug-drug interaction are discussed. Up to now available data concerning the clinical outcome of aspirin resistant CABG patients are limited, and there is evidence that platelets of patients with graft thrombosis are more likely to be resistant to aspirin compared with patients without thrombotic events. Many publications concerning aspirin resistance are available today, but reports addressing this topic in CABG patients are sparse. This review summarises recent insights into the antiplatelet treatment after CABG and describes the clinical benefit, but also the therapeutic failure of the well-established drug aspirin. Moreover, possible pharmacological approaches to improve antithrombotic therapy in aspirin nonresponders among CABG patients are discussed.