This study will evaluate the use of a prasugrel 60 mg loading dose (LD) administered during percutaneous coronary intervention (PCI) with and without a prior LD of clopidogrel on platelet inhibition in patients presenting with acute coronary syndrome (ACS). Platelet inhibition following a prasugrel LD in clopidogrel pretreated patients' will be determined in a time-dependent manner for two different prasugrel loading doses (30 mg and 60 mg). Understanding the effects of this combination on platelet inhibition will provide guidance to physicians on the use of prasugrel in patients who have already been pretreated with clopidogrel.