The purpose of this study is to test whether SR123781A is a possible treatment for patients with acute coronary syndromes (heart attack) who are scheduled to undergo a percutaneous coronary intervention (PCI) within 48 hours after inclusion in the study. Two doses of SR123781A are currently tested, each in a group of about 180 patients per group to find which is the most adequate, versus heparin and glycoprotein IIbIIIa (GPIIBIIIA) antagonist.

Inclusion Criteria:

• A person with a diagnosis of acute coronary syndrome who is scheduled to undergo a PCI within 48 hours.
• A person who is of the legal age of 21, who is mentally competent, and who has signed a written informed consent

Exclusion Criteria:

• A person with known allergy or any contra-indication to active control.
• A person who has received heparin during more than 48 hours before inclusion in the study.
• A person treated with warfarin (oral anticoagulant).
• A person with current bleeding or recognized increased risk of bleeding or history of intracranial hemorrhage.
• A person who has had a stroke within the last 6 months.
• A person with uncontrolled hypertension despite antihypertensive therapy.
• A person with history of clinically significant reduction in blood platelets or neutrophils (white blood cells).
• A person who has laboratory evidence of significantly reduced renal function or who is dependent on renal dialysis.
• A person who has a coronary bypass performed during the previous month.
• A pregnant or nursing woman or a woman of childbearing potential (before menopause) who does not have a negative pregnancy test and does not use a reliable method of birth control.
• A person who has received any investigational treatment in the preceding month.