 36 patients with high on treatment platelet reactivity to clopidogrel will be randomized to receive either an additional loading dose of 600mg clopidogrel n24 or to continue normal treatment as a control group n12. Pharmacokinetics and pharmacodynamics will be reassessed and those patients, who are tested again to have high on treatment platelet reactivity in spite of the additional loading dose, will now be randomized to receive either ticagrelor or prasugrel. The investigators expect about six patients per group. The twelve patients in the control group will continue normal treatment 75mgday until the end of the study. Pharmacokinetics and pharmacodynamics of ticagrelor and prasugrel will be assessed. Any patient, who is tested again with high on treatment platelet reactivity in spite of receiving prasugrel or ticagrelor, will be finally switched to the opposite drug and a final high on treatment platelet reactivity testing will be conducted. 