Study of Alfimeprase to Rapidly Dissolve Blood Clots in the Leg and Help Prevent the Need for Surgery on Leg Arteries

Inclusion Criteria: Must give written informed consent Ages 18 or older Acute PAO of a lower extremity with onset of symptoms within 14 days prior to randomization Acute index limb ischemia classified as SVS/ISCVS Class I or IIA caused by occlusion of a native artery and/or bypass graft (vein or prosthetic) Need for open vascular surgical intervention in the event of unsuccessful thrombolysis Available for follow-up assessments Exclusion Criteria: Contraindication to systemic anticoagulation History of endovascular procedure or open vascular surgery on the index limb within the last 30 days History of significant acute or chronic kidney disease that would preclude contrast angiography Known allergy to contrast agents History of heparin-induced thrombocytopenia (HIT) Participation in any study of an investigational device, medication, biologic, or other agent within 30 days prior to randomization Any thrombolytic therapy within 30 days prior to randomization Past participation in any alfimeprase clinical trial History of hypersensitivity to aspirin Pregnant, lactating, or actively menstruating women and women of child-bearing potential who are not using adequate contraceptive precautions (e.g. intrauterine device, oral contraceptives, barrier methods, or other contraception deemed adequate by the investigator) Uncontrolled hypertension: systolic blood pressure (BP) > 180 mmHg, or diastolic BP > 110 mmHg at the time of baseline assessment Hematocrit < 30%; subjects with a low hematocrit who are not actively bleeding can be entered into this study if after transfusion their hematocrit is >= 30% Platelet count <100 X 10(9)/L on baseline labs Investigator inability to advance guidewire through index occlusion Medically unable to withstand an open vascular surgical procedure Any other feature that, in the opinion of the investigator, should preclude study participation