The investigators hypothesize that reduced loading dose of prasugrel followed by reduced maintenance dose of prasugrel in acute coronary syndrome patients with CYP2C19 polymorphism undergoing percutaneous coronary intervention might exhibit lower platelet reactivity 24 hours and 30 days later which is associated with major adverse cardiovascular events.

Antiplatelet treatment is recommended worldwide for the secondary prevention and clopidogrel is an essential drug. But clopidogrel has limited value because of its pharmacodynamic interpatient variability and delayed onset time.

It is well known that patients who carry a common reduced-function CYP2C19 allele have a lower level of active metabolite of clopidogrel, diminished platelet inhibition, and furthermore, higher rate of major adverse cardiovascular events than noncarriers.

To achieve maximum plateau more rapidly and reduce the rate of high on-treatment platelet reactivity, higher loading dose of clopidogrel, up to 600 mg, is recommended. Although, however, the higher loading dose of clopidogrel, many patients still remain as non-responder.

Incidence of patients with clopidogrel resistance, especially CYP2C19*2 and *3, which encounter loss function, is higher in Eastern Asian peoples than Western peoples. Some studies report incidence rate of clopidogrel resistance in Eastern Asian peoples up to 99%.

However, the metabolism is not influenced by the presence of CYP2C19 genetic variation and prasugrel shows potent platelet inhibition. Although prasugrel exhibit potent platelet inhibition, recent reports describe the possible over inhibition of platelet especially in the East Asian people.

The investigators are going to compare the efficacy and safety of loading dose of prasugrel 30 mg which is lower than conventional loading dose followed by 5 mg/day which is also lower than conventional maintenance dose for 30 days and loading dose of clopidogrel 600 mg followed by 75 mg/day for 30 days.

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)
• History of stroke or transient ischemic attack
• History of upper gastrointestinal bleeding in recent 6 months
• Renal dysfunction defined by serum creatinine > 2.5 mg/dl
• Severe hepatic dysfunction defined by Child-Pugh criteria B or C
• Bleeding tendency
• Anticoagulation treatment including warfarin
• Thrombocytopenia defined by platelet < 100,000/ml
• Anemia defined by hemoglobin < 10 g/dl
• Contraindication for antiplatelet treatment or anticoagulation treatment
• History of administer glycoprotein IIb/IIIa inhibitor in recent 24hrs or planned to