Resistance to aspirin in vitro is associated with increased platelet sensitivity to adenosine diphosphate

Background: Platelet activation plays an important role in arterial thrombosis and the widespread use of aspirin has reduced major events by 25% in the secondary prevention of cardiovascular diseases. However, it appears that aspirin antiplatelet effect is not uniform and 8-45%. of the population are, in vitro, aspirin resistant, and it is well recognized that platelets can be activated by pathways that are not blocked by aspirin, such as adenosine diphosphate (ADP). Objectives: To investigate whether aspirin-resistant patients have a modified sensitivity to ADP-induced platelet activation Material and Methods: Seventy-two patients were enrolled. Platelet function was measured by the PFA-100(R) analyser; platelet GP IIb-IIIa activation by ADP 10 muM was assessed by flow cytometry using PAC-1 MoAb. Results: Using a collagen/epinephrine coated cartridge on the PFA-100, the prevalence of aspirin resistance was 29.2% (n=21). For aspirin-resistant patients, the collagen/ADP coated cartridge showed a closure time significantly shorter (p=0.004) compared to the sensitive and control groups. Platelets from aspirin-resistant patients bound PAC-1 significantly more (p=0.03) than the aspirin-sensitive patients and controls when activated with 10 muM ADP. Conclusions: Platelets from aspirin-resistant patients appear to be more sensitive and activable by ADP. This hypersensitivity could provide a possible explanation for the so-called aspirin resistance, and this could justify therapeutic improvement with alternative antiplatelet agents. (C) 2002 Elsevier Science Ltd. All rights reserved.