Acute Myocardial Infarction of Left Ventricle

The goal of this phase IIc clinical trial is to learn if recombinant human thymosin beta 4 injection (NL005) works to treat heart damage in people who have had a serious type of heart attack called ST-segment elevation myocardial infarction (STEMI) and have been treated with emergency percutaneous coronary intervention (PCI, a procedure to open the blocked artery). It will also learn about the safety of NL005. The main questions it aims to answer are: * Does NL005 lower the size of permanent heart muscle damage measured by cardiac magnetic resonance (CMR) scan 90 days after treatment? * What medical problems do participants have when taking NL005? Researchers will compare two different doses of NL005 to a placebo (a look-alike substance that contains no drug) to see if NL005 works better to reduce heart damage caused by the heart attack. Participants will: * Receive NL005 or placebo through a vein within 4 hours after the PCI procedure, then once a day for 7 days * Stay in the hospital for the first week for monitoring, blood draws, and electrocardiograms (heart tracings) * Have a CMR scan on Day 6 and Day 90 to measure the size of the heart injury * Return to the hospital for checkups on Day 30 and Day 90 * Be contacted by the study team (by phone or online) 3 times during the first year and come back to the hospital on Day 360 to check long-term recovery