The primary objective of this study is to compare platelet reactivity between patients receiving a loading dose of clopidogrel and a reloading dose of prasugrel to patients receiving only a loading dose of prasugrel.

This is a prospective, open-label, non-randomized trial of approximately 260 patients with Acute Coronary Syndrome (ACS) undergoing percutaneous coronary intervention (PCI). In order to compare platelet reactivity, as assessed by the VerifyNow P2Y12 assay, we plan to recruit two study groups of interest:

1. Patients who have already received a 600 mg loading dose (LD) of clopidogrel in preparation for PCI will receive a loading dose of 60 mg of prasugrel.
2. Patients who have not received a LD of clopidogrel will receive a 60 mg LD of prasugrel prior to PCI.

A subset of 40 patients from each study group (80 patients total) will undergo additional platelet reactivity testing with the vasodilator stimulated phosphoprotein (VASP) assay.All patients will be followed throughout the duration of the hospital stay.

Inclusion Criteria:

1. Patients age 18 or older, of both genders

2. Presenting with an ACS, defined as at least two of the following:

   Symptoms consistent with myocardial ischemia; ST segment elevation or depression of at least 1 mm in 2 or more contiguous leads on EKG; Cardiac troponin I level above the upper limit of normal.

3. An initial invasive strategy (e.g. early angiography) is planned.

4. No contraindications to prasugrel therapy.

Exclusion Criteria:

1. Known allergies to aspirin, clopidogrel, or prasugrel.
2. Patient known to be pregnant or lactating.
3. Patient with known history of bleeding diathesis, or currently active bleeding.
4. Platelet count <100,000/mm3 at the time of enrollment.
5. Hematocrit <25% at the time of enrollment.
6. On warfarin therapy at the time of PCI, or patient likely to require warfarin therapy post-PCI.
7. Received fibrinolytics within the past 48 hours.
8. Received a glycoprotein IIb/IIIa inhibitor within the past 48 hours, or if a strategy for PCI involving a glycoprotein IIb/IIIa inhibitor is planned.
9. Taking maintenance thienopyridine therapy in the previous 7 days.
10. Known blood transfusion within the preceding 10 days.
11. Patients treated with non-steroidal anti-inflammatory drugs (NSAIDS) within the previous 5 days.
12. Patients with known chronic liver disease.
13. Age greater than 75 years.
14. Body weight less than 60 kg.
15. History of stroke or transient ischemic attack.
16. Surgery planned within 1 month.
17. Patient likely to require coronary artery bypass grafting.
18. Any significant medical condition that, in the investigator's opinion, may interfere with the patient's optimal participation in the study.