Baseline Platelet Reactivity as Determined by TRAP-6 Induced Aggregation in Whole Blood Is Related to the Rate of Non-Responsiveness to Clopidogrel

Background: Platelet inhibition by clopidogrel is often determined using ADP induced aggregometry. TRAP-6 stimulates platelets via the PAR receptors and is influenced to a much lesser extend by aspirin or clopidogrel. The value of the determination of TRAP-6 induced aggregometry in patients treated with clopidogrel is unknown. We evaluated the relation of clopidogrel non-responsiveness as determined by multiple electrode aggregometry (MEA) (1) vs. TRAP-6 induced aggregation. MEA provides a measurement of platelet aggregation in whole blood by monitoring changes in electrical impedance via disposable test cells each containing two pairs of electrodes (1).

Methods: Following IRB approval hirudin-anticoagulated blood (Dynabyte, Munich, Germany) was sampled from 993 patients. Aggregation induced by arachidonic acid (0.5 mM), ADP (6.4 μM) or TRAP-6 (32μM) was determined in whole blood using a new generation impedance aggregometer. The device is called Multiplate analyzer (Dynabyte, Munich, Germany), indicating the multiplicity of channels and sensors per channel of the device. After dilution (1:2 with 0.9% NaCl solution) of hirudin-anticoagulated whole blood and stirring for three minutes in the test cuvettes at 37°C, the activator was added and aggregation was continuously recorded for six minutes. The increase of impedance due to the attachment of platelets to the electrodes is detected for each sensor unit separately and transformed to arbitrary aggregation units. The aggregation is quantified as area under the curve (AUC).

Results: 618 patients were on chronic therapy with 100 mg aspirin/d, 295 patients on clopidogrel 75 mg/d (237 on dual therapy). Aspirin therapy had no influence on TRAP-6- induced aggregation (112+−260 vs. 110+−25; mean +− sd; aspirin vs. no aspirin, p> 0.05 Mann-Whitney U-test). In contrast clopidogrel induced a minor but significant reduction of TRAP6-induced aggregation (107+− 277 vs. 113+−246; p<0.05). Patients on clopidogrel had a significantly lower ADP induced aggregation (41+−30 vs. 86 +−33) compared to controls (p<0.05), and patients on aspirin had a significantly lower arachidonic acid induced aggregation compared to controls (23+−28 vs. 94 +− 35, p<0.05). 89 of 295 patients (30%) were stratified as clopidogrel non-responders based on the cut-off presented in (2). Patients were divided into 4 quartiles (q1-4) depending on their TRAP-6 induced aggregation. The rate of non-responsiveness in Q1 was 8.1%, in Q2 20%, in Q3 38% and 54% in Q4. ADPinduced aggregation was significantly higher in Q4 than in any other quartile (p<0.05).

Conclusion: A high TRAP-6 induced aggregation is associated with an increased rate of clopidogrel non-responsiveness as determined by multiple electrode aggregometry. TRAP-6 induced aggregometry seems to indicate the global platelet potential and might allow a prediction of clopidogrel responsiveness before the induction of therapy.