Inhibition of platelet aggregation by aspirin progressively decreases in long-term treated patients

Objectives We sought to investigate, during a two-year follow-up period, the effects of aspirin on platelet aggregation. Background The platelets of patients given aspirin may be less sensitive to antiplatelet treatment, although the extent of such phenomenon over long-term follow-up is unclear. Methods Adenosine diphosphate (ADP) and collagen-induced platelet aggregation was periodically monitored before and after 2, 6, 12, and 24 months of treatment with aspirin (n = 150) or ticlopidine (n = 80) in patients matched for gender, age, and risk factors for atherothrombosis. Results Compared with baseline values, two months of aspirin treatment significantly inhibited platelet aggregation; thereafter, this inhibitory effect progressively decreased. At 24-month follow-up, collagen-induced platelet aggregation was significantly higher than that observed at two months (p <0.05); a more pronounced difference was observed when collagen-induced lag phase was considered (p <0.01). Restoration of platelet aggregation was less evident when ADP was used as an agonist. Conversely, the inhibition induced by ticlopidine was constant throughout follow-up with both agonists. Conclusions The study demonstrates that a long-term treatment with aspirin is associated with a progressive reduction in platelet sensitivity to this drug.