High platelet reactivity unit (PRU) after loading dose clopidogrel in patients undergoing percutaneous coronary intervention (PCI) is related to high risk of short and long term recurrent ischemic events including stent thrombosis.

The investigators hypothesize that additional loading of prasugrel in patients with high PRU after clopidogrel loading would be superior to additional loading of clopidogrel in reducing platelet reactivity and thereby result in lower risk of short term recurrent ischemic events.

Dual antiplatelet therapy with acetylsalicyclic acid (ASA) and additional clopidogrel is now standard regimen for the prevention of recurrent ischemic events in patients who undergo PCI.

But decreased effect of clopidogrel in a group of patients was reported and they are known to be associated with high risk of recurrent ischemic event. Decreased effect of clopidogrel is mainly resulted from decreased function to metabolite prodrug, clopidogrel to active form of drug.

Prasugrel, newer thienopyridine has been recently developed and showed advantages to clopidogrel. Prasugrel is known to have shorter onset time to achieve steady state level than clopidogrel and constant pharmacologic effect regardless of patient diversity.

High PRU after loading dose clopidogrel in patients undergoing PCI is known to be related to increased risk of short and long term recurrent ischemic events including stent thrombosis. Prasugrel has been reported to be effective in reducing platelet reactivity in patients showing resistance to clopidogrel and high PRU.

The investigators hypothesize that additional loading of prasugrel in patients with high PRU after clopidogrel loading would be superior to additional loading of clopidogrel in reducing platelet reactivity and thereby result in reduced risk of short term recurrent ischemic events.

The investigators plan to include 70 acute coronary syndrome patients who are planned to undergo PCI and show high platelet reactivity. Most patients with ACS administer loading dose of ASA and clopidogrel as soon as they are assumed to be ACS.

The investigators plan to perform platelet reactivity test by VeryfyNow (VN) before PCI and enroll patients with high PRU defined by 235 or more. They are to randomly administered additional 300mg of clopidogrel or 20mg of prasugrel.

Inclusion Criteria:

• Acute coronary syndrome
• Patients planned to undergo percutaneous transluminal coronary angioplasty
• Patients who agreed to the experimental plan which was permitted by IRB

Exclusion Criteria:

• Low body weight (<50kg)
• Urgent PCI for ACS
• Use of glycoprotein IIb/IIIa inhibitor in recent 24hrs or planned to
• History of transient ischemic attack
• History of upper gastrointestinal bleeding in recent 6 months
• Renal dysfunction defined as serum creatinine > 2.5 mg/dl
• Severe hepatic dysfunction defined as serum transaminase > 3 times normal limit
• Bleeding tendency
• Anticoagulation treatment including warfarin
• Thrombocytopenia defined by platelet < 100,000/ml
• Anemia defined by hemoglobin < 10 g/dl
• Contraindication for study drugs