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The natural peptide N-Acetyl-Seryl-Aspartyl-Lysyl-Proline (Ac-SDKP) decreases inflammation in chronic diseases such as hypertension and heart failure. However, Ac-SDKP effects on acute inflammatory responses during myocardial infarction (MI) are unknown. During the first 72 hours post-MI, neutrophils, M1 macrophages (pro-inflammatory), and M2 macrophages (pro-resolution) and release of myeloperoxidase (MPO) and matrix metalloproteinases (MMP) are involved in cardiac rupture. We hypothesized that in the acute stage of MI, Ac-SDKP decreases the incidence of cardiac rupture and mortality by preventing immune cell infiltration as well as by decreasing MPO and MMP expression. MI was induced by ligating the left descending coronary artery in C57BL/6 mice. Vehicle or Ac-SDKP (1.6 mg/kg/d) was infused via osmotic minipump. Cardiac immune cell infiltration was assessed by flow cytometry, cardiac MPO and MMP levels were measured at 24–48 hrs post-MI. Cardiac rupture and mortality incidence were determined at 7 days post-MI. In infarcted mice, Ac-SDKP significantly decreased cardiac rupture incidence from 51.0% (26 of 51 animals) to 27.3% (12 of 44) and mortality from 56.9% (29 of 51) to 31.8% (14 of 44). Ac-SDKP reduced M1 macrophages in cardiac tissue after MI, without affecting M2 macrophages and neutrophils. Ac-SDKP decreased MMP-9 activation in infarcted hearts with no changes on MPO expression. Ac-SDKP prevents cardiac rupture and decreases mortality post-acute MI. These protective effects of Ac-SDKP are associated with decreased pro-inflammatory M1 macrophage infiltration and MMP-9 activation. [ABSTRACT FROM AUTHOR]