Study of Alfimeprase's Ability to Dissolve Blood Clots in the Leg and Help Prevent the Need for Surgery

Inclusion Criteria: * Age 18 or older * Arteriographically confirmed acute PAO of the 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). Only Class I subjects with abrupt onset of ischemic rest pain or abrupt onset/progression of lifestyle-limiting claudication are eligible * Acute PAO with a need for urgent surgical intervention to restore arterial blood flow in the event of unsuccessful thrombolytic therapy * 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 past 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 * Participation in any study of an investigational device, medication, biologic, or other agent within 30 days prior to randomization * Any thrombolytic therapy within 5 days prior to randomization * Past participation in any alfimeprase trial * Pregnant, lactating, or actively menstruating women or women of childbearing potential who are not using adequate contraceptive precautions * Investigator inability to advance guidewire through index occlusion * Any other subject feature that in the opinion of the investigator should preclude study participation