Currently approved drug therapy for AIS is limited by the need to treat within 3 hours of symptom onset. Alfimeprase acts to degrade fibrin directly and is inactivated locally by circulating alpha-2 macroglobulin. This study will determine whether treatment with alfimeprase facilitates rapid restoration of arterial blood flow with avoidance of symptomatic hemorrhagic conversion in subjects with AIS within 3 to 9 hours of symptom onset.