Inclusion Criteria:

• Clinical diagnosis of AIS defined as the sudden onset of an acute focal neurological deficit presumed to be due to cerebral ischemia
• Arterial occlusion of the carotid T or a M1, M2, or M1-M2 branch of the middle cerebral artery (MCA) as documented by CT angiography or magnetic resonance angiography
• Arteriographically confirmed occlusion of the carotid T or a M1, M2, or M1-M2 branch of the MCA
• The subject (or legally acceptable representative) must give written informed consent
• Age 18 years to 85 years
• Onset of symptoms of AIS (i.e., last known well time) within 3-9 hours
• Baseline NIHSS of 4 to 25
• Available for follow-up assessments at 30 and 90 days

Exclusion Criteria:

• Contraindication to systemic anticoagulation including any history of prior intracranial hemorrhage
• Uncontrolled hypertension at study entry as defined by systolic blood pressure greater than 180 mmHg or diastolic blood pressure greater than or equal to 100 mmHg on repeated measures prior to study entry despite the use of IV antihypertensive agents
• Expectation based on timing of presentation that alfimeprase administration will not be able to be completed by 9 hours after stroke onset
• Inability to initiate alfimeprase within 120 minutes of the qualifying imaging scan
• Coma
• Rapidly improving neurological symptoms at the time of screening
• Brain CT or MRI evidence of intracranial bleeding of any age
• High clinical suspicion for subarachnoid hemorrhage despite a negative baseline CT or MRI
• CT evidence of an acute and/or evolving hypodensity greater than 1/3 of the MCA territory in the vascular territory to be treated or Alberta Stroke Program Early CT Score (ASPECTS) of less than or equal to 5
• MRI diffusion weighted imaging lesion greater than 1/3 of the MCA territory in the vascular distribution to be treated
• Carotid artery and/or intracranial artery stenosis that precludes safe passage of a microcatheter to treat the primary AOL
• Life expectancy of less than 6 months
• History of significant acute or chronic kidney disease, including known nephrotic syndrome, that would preclude safe contrast angiography
• Known allergy to contrast agents
• History of immune deficiency
• History of heparin-induced thrombocytopenia
• Participation in any study of an investigational device, medication, biologic, or other agent within 30 days prior to enrollment (Stage I)/randomization (Stage II)
• Any stroke, myocardial infarction, or use of thrombolytic therapy (including investigational thrombolytic therapy) within 3 months prior to enrollment (Stage I)/randomization (Stage II)
• Past participation in any alfimeprase clinical trial
• Pregnant, lactating, or actively menstruating women and women of child-bearing potential who are not using adequate contraceptive precautions
• Current use of oral anticoagulants or an international normalized ratio (INR) greater than 1.4
• Any non-atherosclerotic arteriopathy
• Any prior neurologic event that would obscure the radiographic or clinical evaluation of the new index neurological deficits
• Subjects with known renal insufficiency defined as a serum creatinine >2 mg/dL (>180 mmoL/L)
• Subjects with known clinically significant hepatic disease defined as transaminase values > 3x upper limit of normal
• Subjects with any malignant neoplasm diagnosed within five years prior to screening, with the exception of basal cell carcinoma of the skin and fully resected squamous cell carcinoma of the skin
• Subjects with a platelet count less than 100,000/mm3
• Subjects with a baseline serum glucose level less than 50 mg/dL or greater than 300 mg/dL
• Subjects receiving any dose of a heparinoid or a non-prophylactic intensity dose of a low molecular weight heparin within the 24-hour period prior to study drug administration
• Any other subject feature that in the opinion of the investigator should preclude study participation