This study will investigate the effects of a potential neuroprotectant compound, YM872, in the treatment of acute ischemic stroke. The study will determine if a 24-hour infusion of YM872, given within 6 hours of stroke onset, reduces the ischemic lesion volume as measured by MRI at 28 days after the infusion is given. The clinical effects of YM872 will also be determined by neurological function and disability scores at follow up visits through Day 90 of the study.