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BACKGROUND: The aim of this study was to assess antiplatelet effect of prasugrel in acute coronary syndrome (ACS) patients with high on-treatment platelet reactivity (HTPR) on clopidogrel, undergoing percutaneous coronary intervention (PCI). METHODS: A prospective, platelet reactivity-guided, parallel-group, open-label study including 71 patients pretreated with clopidogrel 600 mg and assigned either to prasugrel (30 mg loading dose, 10 mg maintenance dose; n = 46) or clopidogrel (150 mg maintenance dose for 6 days and thereafter 75 mg maintenance dose; n = 25) regimen, based on vasodilator-stimulated phosphoprotein (VASP)-assessed platelet reactivity index (PRI; > 50% vs. </= 50%) measured next morning post-PCI. RESULTS: Median PRI value after switch to prasugrel sharply declined at 24 h (70.0 [61.3-75.6] vs. 11.9 [6.8-25.7]%; p < 0.000001) and slightly but significantly rose between 24 h and 30 days (27.9 [15.5-46.8]%; p < 0.0006). In contrast, median PRI values in the clopidogrel group were similar at baseline and at 24 h (25.1 [13.7-40.2] vs. 22.0 [18.4-36.8]%; p = NS) and then modestly rose at 30 days (30.3 [20.4-45.7]%; p < 0.03). The prevalence of HTPR decreased in the prasugrel group between baseline and 24 h measurements (100.0 vs. 4.3%; p < 0.0001). Rates of patients with HTPR at 24 h and 30 days were similar in both groups, so were the tendencies in patterns of platelet inhibition evaluated with multiple electrode aggregometry as compared with the VASP assay. CONCLUSIONS: Our study indicates that prasugrel overcomes HTPR on clopidogrel in the acute phase of interventionally treated ACS and maintains its antiplatelet potency in 30-day follow-up. Potential clinical benefits of personalized antiplatelet prasugrel-based therapy warrant further investigation in clinical ACS trials