The Platelet Activity After Clopidogrel Termination (PACT) study.

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. Methods and 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 mu mol/L; thrombin receptor activating peptide (TRAP) 1 and 20 mu mol/L; and collagen/epinephrine 5 mu g/mL/5 mu mol/L, (2) light transmission aggregation with ADP 2.5, 5, and 20 mu mol/L; TRAP 2 and 20 mu mol/L; and collagen 6 mu g/mL, (3) whole blood impedance aggregation with ADP 1.6 and 6.5 mu mol/L; TRAP 4 and 32 mu mol/L; and collagen 3.2 mu g/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.