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.