150 patients admitted to 4 tertiary care hospitals (Heart Hospital London, London Chest, Essex Cardiothoracic Center and Leeds General Infirmary) for heart attack will be randomly assigned to receive MRA treatment or placebo. The first dose of the MRA will be applied intravenously immediately in the catheter suite, even before re-opening of the occluded vessel. From the second day on, patients will be prescribed oral MRA treatment, as a pill, for a total of three months. Before hospital discharge and after three months, a magnetic resonance image (MRI) of the heart will accurately investigate the evolution of infarct (scar) size and the contractile heart function and compare the group of patients who received the MRA drug versus the placebo control group. Of note, patients with an ejection fraction <40% AND signs of heart failure OR diabetes will go on open label eplerenone according to current guidelines, instead of the study drug.