This is a prospective, randomized, open-label clinical trial which will enroll 80 acute coronary syndrome (ACS) patients after Percutaneous Transluminal Coronary Intervention (PCI) in China. Patients on maintenance dosing (MD) of aspirin (100 mg/d) and ticagrelor (90 mg twice daily) will be divided into two groups switching from ongoing ticagrelor to clopidogrel 600 mg loading dose (LD)/ 75 mg MD according to their bleeding risk. Then each group will randomly switch at different times(24 hours/ 12 hours after the last MD of ticagrelor). Pharmacodynamic assessments are performed at baseline, and at 4h, 8h, 24h, 48h, 72h hours with platelet aggregation rate by Light Transmittance Aggregometry method (LTA). All patients are followed-up for 30 days.

The primary endpoint of the study was platelet inhibition measured by Light Transmittance Aggregometry method(LTA). Secondary clinical endpoints included a 30-day major adverse cardiovascular endpoint (MACE) defined as a composite of cardiovascular death, recurrent myocardial infarction, target vessel revascularisation or stroke and individual components of the MACE. Safety endpoints of 30-day TIMI major and minor bleed were also evaluated.

Inclusion Criteria:

• ≥ 18 years.
• ACS patients.
• Patients who are treated with ticagrelor and do not tolerate it.
• Volunteer to participate and sign informed consent.
• Approved by national regulatory authorities ethics committees.

Exclusion Criteria:

• Patients who are contraindicated, intolerant or resistant to clopidogrel.
• History of hematological disease or bleeding tendency; platelet count < 100 × 10^9 cells/L, or > 600 × 10^9 cells/L, hemoglobin < 100 g/L.
• Abnormal liver or kidney function (ALT > 3 ULN; estimated CrCl < 30 ml/min calculated by Cockcroft-Gault equation); diagnosed severe pulmonary disease.
• Patients in need of drugs which affect the efficacy of clopidogrel such as miconazole, ketoconazole, andfluconazole.
• Malignancies or other comorbid conditions with life expectancy less than 1 year.
• Pregnant or lactating woman.