Effect of platelet turnover on whole blood platelet aggregation in patients with coronary artery disease

Aspirin resistance and platelet turnover: a 25-year old issue

The evidence of an incomplete inhibition of platelet function by aspirin, despite therapeutic doses of the drug proved to be clinically effective are employed, was first reported in the '80s, in the frame of studies devoted to platelet turnover. Because inhibition of platelet aggregation by aspirin is irreversible, the return after an interval of time of the ability to form thromboxane by platelets in circulating blood should reflect the entry into the circulation of platelets whose cyclooxygenase activity has not been affected by aspirin. Based on this concept, the possibility of monitoring the entry of newly formed platelets into the circulation after aspirin ingestion was documented by measuring the return of thromboxane biosynthesis by platelets challenged in vitro by pairs of aggregating agents. The data obtained showed that platelets with intact cyclooxygenase activity could be detected into the circulation of control individuals as early as 4-6 h after aspirin ingestion, and at shorter time intervals in diabetic angiopathy. In the latter setting,the data allowed to conclude that "schedules of aspirin which may suffice in normals are not effective in patients with diabetic angiopathy, presumably because these patients have a high rate of entry of new platelets into the circulation".

Aspirin failure in patients presenting with acute cerebrovascular ischaemia

Aspirin is the most commonly used antiplatelet drug for prevention of ischaemic stroke. In order to determine the prevalence and nature of aspirin failure, we studied 51 adults admitted with suspected ischaemic stroke and already prescribed daily aspirin. Within 48 hours (h) of onset, blood and urine samples were collected to assess platelet aggregation, activation and aspirin response by a range of methods. All tests were then repeated on a second sample taken 24 h after witnessed administration of 75 mg or 150 mg aspirin. At entry to the study, incomplete response to aspirin, measured by arachidonic acid (AA)-stimulated platelet aggregation, was found in 43% of patients. Following in-hospital aspirin administration, there was a significant decrease in AA-aggregation (p=0.001) suggesting poor adherence to therapy prior to admission. However, residual aggregation (10-15%) persisted in 11 subjects - suggesting alternative causes. In incomplete responders on admission, platelet aggregation with adenosine diphosphate (ADP) was significantly higher compared with responders (p<0.05) but there were no significant differences in collagen aggregation, platelet fibrinogen binding or P-selectin expression, plasma von Willebrand factor, fibrinogen, high-sensitivity C-reactive protein, or the urinary metabolite, 11-dehydro-TxB2. Incomplete platelet inhibition is common around the time of acute cerebrovascular ischaemic events in patients prescribed aspirin. Up to 50% of these observations appear due to incomplete adherence to aspirin therapy. Intervention studies are required to determine the clinical relevance of measured platelet response to aspirin in terms of outcome, and the effectiveness of improved pharmacotherapy for stroke prevention.

The influence of CYP 2C19*2 polymorphism on platelet function testing during single antiplatelet treatment with clopidogrel

Background

Different platelet function tests can be used to evaluate the degree of achieved platelet inhibition in patients treated with clopidogrel. The presence of CYP 2C19*2 polymorphism can reduce the formation of the active metabolite of clopidogrel, resulting in less platelet inhibition.

Patients and Methods

Patients with symptomatic coronary artery disease, all on chronic single aspirin treatment were randomized to continue on aspirin or change to clopidogrel. In 219 randomly selected clopidogrel treated patients, platelet reactivity was evaluated by VASP-PRI determination and by use of VerifyNow P2Y12-PRU. The CYP 2C19*2 G/A polymorphism was further determined.

Results

The total frequency of clopidogrel resistance was 29.0% by VASP-PRI and 31.6% by VerifyNow-PRU. The number of patients being hetero- and homozygous combined for the CYP 2C19*2 polymorphism (GA/AA) was 64 (29%). Platelet reactivity was significantly higher in patients with the polymorphism compared to wild-type patients (GG). VASP-PRI was 50.9% (SD19) in patients having the polymorphism compared to 38.3% (SD21) in patients with the GG genotype (p = 0.001). Correspondingly, the mean PRU was 165 (SD67) compared to 124 (SD69) (p < 0.001). The frequency of clopidogrel resistance in patients with the polymorphism was 32% compared to 16% in wild-type patients when defined by VASP-PRI (p = 0.006). When defined by PRU (VerifyNow), the corresponding frequencies were 53% and 22% (p < 0.001).

Conclusions

Clopidogrel treated patients with the CYP 2C19*2 polymorphism have significantly increased platelet reactivity compared to patients with the wild-type, evaluated with the VASP determination, and even more pronounced with the VerifyNow P2Y12 method.

Trial Registration

ClinicalTrials.gov: NCT00222261