title: Inhibition of platelet aggregation by aspirin progressively decreases in long-term treated patients.
abstract: OBJECTIVE
  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.
authors.affiliation: Department of Experimental Medicine and Pathology, University La Sapienza, Viale Regina Elena 324, 00161 Rome, Italy. fabio.pulcinelli@uniroma1.it
authors.forename: Fabio M
authors.initials: FM
authors.lastname: Pulcinelli
authors.forename: Pasquale
authors.initials: P
authors.lastname: Pignatelli
authors.forename: Andrea
authors.initials: A
authors.lastname: Celestini
authors.forename: Silvia
authors.initials: S
authors.lastname: Riondino
authors.forename: Pier Paolo
authors.initials: PP
authors.lastname: Gazzaniga
authors.forename: Francesco
authors.initials: F
authors.lastname: Violi
mesh_terms: D000244:Adenosine Diphosphate; D000368:Aged; D001241:Aspirin / Q000494:pharmacology*; D003094:Collagen; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D010974:Platelet Aggregation / Q000187:drug effects*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology*; D013988:Ticlopidine / Q000494:pharmacology*
chemical_list: D010975:Platelet Aggregation Inhibitors; D000244:Adenosine Diphosphate; D003094:Collagen; D013988:Ticlopidine; D001241:Aspirin