East Asian patients will be required optimal dose of newer P2Y12 inhibitors (prasugrel or ticagrelor) to determine the safer treatment and better outcome. Whether lower dose of these regimens are more adequate for clinical practice in Korea is unclear. Therefore, the investigators aim to evaluate efficacy and safety of half dose of new oral P2Y12 inhibitors in Korean patients with acute coronary syndrome (ACS) undergoing percutaneous coronary intervention (PCI).

In recent years, newer oral P2Y12 receptor blockers (prasugrel or ticagrelor) have been strong recommendations for management of patients with ACS undergoing (PCI). These drugs provided more profound inhibitory effects than clopidogrel, which could lead to marked reduction in ischemic events, with relatively increase in bleeding complication, specific to low body weight, especially in women and East Asian patients.

Inclusion Criteria:

• Patients present with acute coronary syndrome undergoing primary PCI.
• Patients receive DAPT (conventional dose of P2Y12 inhibitors+aspirin) at least 1 month.
• Patients provide written informed consent prior to enrollment.

Exclusion Criteria:

• Low body weight (<60kg).
• History of transient ischemic attack or stroke.
• 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
• On treatment with oral anticoagulant (Vitamin K antagonists, dabigatran, rivaroxaban).
• Bleeding tendency.
• Thrombocytopenia defined by platelet < 100,000/ml.
• Anemia defined by hemoglobin < 10 g/dl.
• Current treatment with drugs interfering with CYP3A4 metabolism (to avoid interaction with Ticagrelor): Ketoconazole, itraconazole, voriconazole, clarithromycin, nefazodone, ritonavir, saquinavir, nelfinavir, indinavir, atazanavir, and telithromycin.
• Known severe chronic obstructive pulmonary disease or bradycardia (sick sinus syndrome (SSS) or high degree AV block without pacemaker protection).
• Contraindication for study drugs.

Open-labeled