This study investigates whether a platelet function testing guided approach with a short-term (1 week) prasugrel treatment and a switch over to clopidogrel treatment in adequate responders to clopidogrel is non-inferior regarding the combined incidence of bleeding and thrombotic complications to a 12 month standard treatment with prasugrel in acute coronary syndrome (ACS) patients treated with percutaneous coronary intervention (PCI).

Patients suffering of heart attack have highly activated blood platelets. During and after invasive treatment of blocked coronary vessels (percutaneous coronary intervention = PCI) a potent platelet inhibition is needed to reduce the risk of thrombotic complications which is particularly high within the first week after PCI. On the other hand, the use of potent platelet inhibitors such as prasugrel is associated with higher bleeding risk particularly when used at long-term. A combination of a potent antiplatelet drug (prasugrel) within the first week with a less potent antiplatelet drug (clopidogrel) thereafter might lead to a higher net clinical benefit - means less bleeding and thrombotic complications. This hypothesis is being investigated in the current trial.

Inclusion Criteria:

• Patients with Troponin positive ACS
• Successful PCI (defined as a post PCI diameter stenosis <20% and TIMI flow ≥2)
• A planned treatment of Prasugrel for 12 months after the procedure
• written informed consent

Exclusion Criteria:

• Age <18 years and >80 years
• Subjects with known contraindications to Clopidogrel treatment, which are hypersensitivity to the drug substance or any component of the product and active pathological bleeding such as peptic ulcer or intracranial hemorrhage
• Subjects with known contraindications to Prasugrel treatment, which are hypersensitivity to the drug substance or any component of the product, active pathological bleeding such as peptic ulcer or intracranial hemorrhage and a history of prior transient ischemic attack (TIA) or stroke
• Cardiogenic shock
• Subjects requiring concomitant treatment with an anticoagulant agent (Vitamin-K antagonists or novel oral anticoagulants such as Rivaroxaban, Dabigatran or Apixaban)
• Indication for major surgery (per decision of the treating physician) for the planned duration of the study
• Simultaneous participation in another clinical trial or participation in any clinical trial involving administration of an investigational medicinal product within 30 days prior to clinical trial beginning
• Known or persistent abuse of medication, drugs or alcohol
• Current or planned pregnancy or nursing women, women 90 days after childbirth. Females of childbearing potential, who do not use and are not willing to use medically reliable methods of contraception for the entire study duration (such as oral, injectable, or implantable contraceptives, or intrauterine contraceptive devices) unless they are surgically sterilized / hysterectomized or there are any other criteria considered sufficiently reliable by the investigator in individual cases
• Evidence of significant active neuropsychiatric disease, in the investigator's opinion