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