The measurement of platelet function in response to 3 common antiplatelet regimens in patients with peripheral occlusive arterial disease

Background: Anti-platelet therapy reduces vascular complications in patients with peripheral occlusive arterial disease (POAD) with aspirin the most commonly prescribed agent. However, aspirin resistance can be observed in patients who suffer clinical thromboembolic events, or have persistent platelet activity despite regular aspirin treatment. The clinical benefits of clopidogrel or combination aspirin and clopidogrel therapy may be due to superior platelet inhibition or the inhibition of platelets in aspirin resistant patients. Aim: To investigate the effects of aspirin, clopidogrel, and combination therapy on platelet inhibition in patients with POAD, and to test the response of laboratory defined aspirin resistant patients to alternative antiplatelet therapies. Methods: 50 patients were recruited from the vascular outpatient clinic at Royal North Shore hospital, at the consulting rooms of Royal North Shore Hospital vascular surgeons and the associated vascular imaging laboratories. Patients with confirmed symptomatic peripheral vascular disease were randomised to receive either aspirin or clopidogrel for 2 weeks, followed by 2 weeks of combined aspirin and clopidogrel therapy. Patients were then asked to “crossover” to the alternative antiplatelet monotherapy for the final two weeks of the trial. After each 2 week course of antiplatelet therapy, platelet activation was flow cytometrically determined by detecting platelet membrane expression of the activation markers CD62P, PACl, fibrinogen receptor and platelet-leukocyte aggregates. Measurements were performed after the addition of PGE, (for resting activation levels), and after platelet activation by low and high dose (0.5&5uM) ADP. Global platelet function was assessed using the Platelet Function Analyser-100 (PFA-lOO) with the Collagen/Epinephrine and the Collagen/ADP cartridges. Results: By flow cytometry, no significant differences in activation marker expression between aspirin, clopidogrel and combination treated patients were detected in the PGE, treated control groups. There were statistically significant differences in levels of all platelet activation markers expressed between the three antiplatelet therapies (ANOVA, p<0.001), with clopidogrel and combination therapy significantly reducing expression after both low and high dose ADP compared to aspirin (Wilcoxon p<0.05). No significant differences in marker expression were observed between clopidogrel and combination therapies. With the PFA-lOO, 9 of 50 (18%) patients were aspirin resistant by the Coll/Epi cartridge. Six of 9 aspirin resistant patients became sensitive to aspirin after combination antiplatelet therapy. Both PFA-lOO cartridges were insensitive to clopidogrel monotherapy. Statistically significant differences in Coll/ADP closure times were seen between combination therapy and aspirin (Wilcoxon p=0.0102). There was no significant difference in median levels of flow cytometric platelet activation between aspirin resistant and sensitive patients (Mann-Whitney p>0.05). Conclusions: Clopidogrel and combination therapy significantly inhibit platelet activation compared to aspirin in patients with POAD, but no significant difference in platelet inhibition was observed between clopidogrel and combination therapy. A significant proportion of POAD patients were resistant to aspirin by PFA-l 00. The majority of these patients became sensitive to aspirin with combined aspirin and clopidogrel therapy