Effect of Ticagrelor Versus Clopidogrel on Endothelial Dysfunction and Vascular Inflammation

Inclusion Criteria: * Provision of informed consent prior to any study specific procedures * Men and women ≥20 years of age * Documented history of non-ST-segment acute coronary syndrome occurring 30 \~ 365 days prior to randomization and successfully treated with percutaneous coronary intervention using drug-eluting stent * Patient currently prescribed and tolerating aspirin 100mg and clopidogrel 75mg. * Patient who have demonstrated endothelial dysfunction defined as percent flow-mediated dilation values lower than 7% at baselines test Exclusion Criteria: * Patients with angina related symptoms * Patients who did not undergo or failed invasive treatment * Patients with a history of hypersensitivity to ticagrelor or clopidogrel * Patients who took an anti-coagulant, anti-thrombotic regularly before the study, or plan to have continuous treatment during the study * Patients who took vasoactive agents or caffeine ingestion for \<48 * Patients with decompensated congestive heart failure of cardiogenic shock (Killip classification III or IV) * Patients with intractable arrhythmia * Patients with intractable arrhythmia * Patients with second or third degree atrioventricular block * Patients with uncontrolled hypertension * Patients with high risk of hemorrhage like blood coagulation disorders, gastrointestinal bleeding, gross hematuria, intraocular bleeding, hemorrhagic stroke, intracranial hemorrhage * Patients with more than moderate chronic obstructive pulmonary disease diagnosed by symptoms or documented by pulmonary function test * Patients who required renal replacement therapy * Patients with moderate to severe hepatic impairment * Patients with platelet \<100,000/μL * Patients with hematocrit \<30% * Concomitant oral or parenteral therapy with strong cytochrome P450 3A4 inhibitors, cytochrome P450 3A substrates with narrow therapeutic indices, or strong cytochrome P450 3A4 inducers i) Strong inhibitors: ketoconazole, itraconazole, voriconazole, telithromycin, clarithromycin (but not erythromycin or azithromycin), nefazadone, ritonavir, saquinavir, nelfinavir, indinavir, atanazavir, over 1 litre daily of grapefruit juice ii) Substrates with narrow therapeutic index: cyclosporine, quinidine, simvastatin at doses \>40 mg daily or lovastatin at doses \>40 mg daily iii) Strong inducers: rifampin/rifampicin, rifabutin, phenytoin, carbamazepine, phenobarbital * Patient who need to take drugs other than study medications and allowed concomitant medications during study period. * Patients who have planned elective surgery or invasive procedure requiring temporary discontinued study medication during study period. * Patients who are pregnant, breast feeding and not using medically acceptable birth control. * Patients considered as unsuitable based on medical judgment by investigators.