title: The Platelet Activity After Clopidogrel Termination (PACT) study.
abstract: BACKGROUND
  "Rebound" platelet hyperreactivity after discontinuation of clopidogrel has been proposed to lead to increased thrombotic risk, including late stent thrombosis. However the hypothesis that discontinuation of clopidogrel results in platelet hyperreactivity has never been rigorously tested. We therefore performed a randomized, double-blind, placebo-controlled, crossover study: the Platelet Activity after Clopidogrel Termination (PACT) study.
  
  
  RESULTS
  Platelet reactivity in 15 healthy subjects was measured at baseline, during clopidogrel 75 mg or placebo daily for 14 days, and on days 1, 4, 8, 11, 15, and 45 after discontinuation of clopidogrel or placebo. Platelet reactivity was assessed by (1) platelet surface activated GPIIb-IIIa and surface P-selectin (by whole blood flow cytometry) in response to ADP 0.5, 1, and 20 μmol/L; thrombin receptor activating peptide (TRAP) 1 and 20 μmol/L; and collagen/epinephrine 5 μg/mL/5 μmol/L, (2) light transmission aggregation with ADP 2.5, 5, and 20 μmol/L; TRAP 2 and 20 μmol/L; and collagen 6 μg/mL, (3) whole blood impedance aggregation with ADP 1.6 and 6.5 μmol/L; TRAP 4 and 32 μmol/L; and collagen 3.2 μg/mL, and (4) plasma soluble CD40 ligand (by ELISA). Immature platelet fraction was measured in the Sysmex XE-2100. At no time point after discontinuation of clopidogrel was platelet reactivity, as determined by any assay end point, or the immature platelet fraction significantly greater than after discontinuation of placebo.
  
  
  CONCLUSIONS
  This randomized, double-blind, placebo-controlled, crossover study demonstrates that discontinuation of clopidogrel does not result in "rebound" platelet hyperreactivity, as determined by multiple time points, assays, agonists, and agonist concentrations.
  
  
  BACKGROUND
  URL: http://www.clinicaltrials.gov. Unique identifier: NCT00619073.
authors.affiliation: Center for Platelet Research Studies, Division of Hematology/Oncology, Children's Hospital Boston, Harvard Medical School, Boston, MA 02115-5737, USA. andrew.frelinger@childrens.harvard.edu
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Frelinger
authors.forename: Marc R
authors.initials: MR
authors.lastname: Barnard
authors.forename: Marsha L
authors.initials: ML
authors.lastname: Fox
authors.forename: Alan D
authors.initials: AD
authors.lastname: Michelson
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000328:Adult; D000368:Aged; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism / Q000473:pathology; D023201:CD40 Ligand / Q000097:blood; D000077144:Clopidogrel; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D019007:P-Selectin / Q000378:metabolism; D015539:Platelet Activation / Q000187:drug effects; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism; D013988:Ticlopidine / Q000008:administration & dosage / Q000009:adverse effects / Q000031:analogs & derivatives*; D028761:Withholding Treatment
chemical_list: D019007:P-Selectin; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D023201:CD40 Ligand; D000244:Adenosine Diphosphate; D000077144:Clopidogrel; D013988:Ticlopidine