The purpose of this study is to determine if YM872 in combination with t-PA can reduce disability and brain damage from stroke. YM872 or placebo will be given as a continuous intravenous (iv) infusion for 24 hours. It is important that the study medication, YM872 or placebo, is administered prior to the completion of the t-PA administration. The clinical effects of YM872 in addition to t-PA will be determined by assessing neurological function and disability scores at follow up visits through Day 90 of the study.

Inclusion Criteria:

• Patients with acute ischemic stroke who are treated with alteplase within 3 hours of stroke onset (Onset is defined as the time that which the patient was last seen in a normal state, or bedtime for unwitnessed strokes occurring during the night).
• Patients who are able to provide written informed consent or have consent provided by a legally authorized representative.
• Patients who are at least 18 years of age.
• Patients who have a National Institutes of Health Stroke Scale (NIHSS) score of at least 7 but not more than 23 and who are conscious.
• Other criteria as specified in the study protocol

Exclusion Criteria:

• Patients who are not eligible to receive treatment with alteplase (t-PA) due to brain hemorrhage, risk for hemorrhage, or other criteria.
• Patients who have stroke of the brainstem or cerebellum.
• Patients who have renal (kidney) disease or insufficiency.
• Patients who have active epilepsy or convulsions during the current stroke episode.
• Patients who are IV drug users or are inebriated.
• Patients who have a history of drug-related anaphylaxis.
• Patients who have taken sedatives, anticonvulsants, or any medication with sedating effects in the 10 days prior study enrollment.
• Patients who have taken more than 1.3 g of aspirin per day in the 2 days prior to enrollment.
• Patients who have a known vitamin hypersensitivity.
• Other exclusion criteria as specified by the study protocol