The purpose of this study is to determine whether KAI-9803 is safe and effective in reducing infarct size in subjects with ST elevation myocardial infarction (heart attack) undergoing a percutaneous coronary intervention (PCI). A select number of sites will also participate in a substudy where eligible patients will undergo an additional procedure;cardiac magnetic resonance imaging.

Inclusion Criteria:

• Acute STEMI and has a planned emergent primary PCI procedure
• Continuous symptoms of cardiac ischemia and present to the primary PCI facility within 6 hours of symptom onset

Exclusion Criteria:

• Persistent systolic blood pressure < 90 mm Hg