Dipyridamole Decreases Protease-Activated Receptor and Annexin-V Binding on Platelets of Poststroke Patients with Aspirin Nonresponsiveness

Abstract

Background: Although controversial, the phenomenon of aspirin resistance (AR) has been correlated in some small studies with poor clinical outcomes in patients with coronary artery disease. Even less is known regarding the role of AR in the poststroke population. The reason for and the underlying mechanism of AR is unknown. We hypothesized that excessive formation of thrombin on the platelet surface may contribute to this phenomenon and assessed how dipyridamole affects multiple platelet and thrombin generation biomarkers in AR patients after ischemic stroke. Methods: Whole blood samples from 20 poststroke AR patients were pretreated with dipyridamole, simulating the therapeutic range, and then incubated for 45 min at 37°C. Platelet characteristics were assessed by aggregometry, cartridge-based analyzer, and receptor expression by flow cytometry. Markers of thrombin generation were measured in the autologous plasma by ELISA. Results: Pretreatment of blood with dipyridamole resulted in 22–26% diminished expression of intact PAR-1 receptor (p = 0.021 and p = 0.024) and 28–31% decrease of annexin V binding (p = 0.031 and p = 0.02) after incubation with 2 µg/ml and 4 µg/ml of dipyridamole, respectively. Platelet aggregation and thrombin generation markers were not affected in vitro by dipyridamole. Conclusions: Dipyridamole may be capable of overcoming increased prothrombinase complex formation and be in part able to compensate for AR in patients with moderate carotid stenosis. This phenomenon may explain the clinical advantages of Aggrenox^®, known to reduce ischemic events in poststroke patients as proven in clinical trials, though an additional antithrombotic benefit beyond the platelet inhibition by aspirin alone.