This is a single-center, randomized, single-blind, investigator-initiated, pharmacological study with a parallel design. Patients with ST elevation myocardial infarction undergoing primary percutaneous coronary intervention and presenting high platelet reactivity as assessed with the Verify Now P2Y12 assay-Accumetrics(Platelet Reactivity Units -PRU≥235) at 2 hours post-clopidogrel 600mg LD (Day 0), as assessed with the Verify Now P2Y12 assay, will be randomized after informed consent, in a 1:1 ratio to the following treatment groups: