Before you can start receiving the study drug, you will have "screening tests." These tests will help the doctor decide if you are eligible to take part in this study. Your complete medical history will be recorded. You will be asked about any drugs you have taken and are taking. You will have a physical exam, including measurement of vital signs (blood pressure, heart rate, temperature, and breathing rate). You will have a neurological exam. Blood (about 2 tablespoons) will be drawn for routine tests. This routine blood draw will include a pregnancy test for women who are able to have children. To be eligible to take part in this study, the pregnancy test must be negative. Urine will be collected over a 24-hour period to test kidney function and for routine tests. You will have a magnetic resonance imaging (MRI) scan of the brain and spine.