12-Week, Double-Blind, Placebo-Controlled Study of 20 and 60 mg/Day Istradefylline in Parkinson's Disease Patients on Levodopa/Carbodopa

Inclusion Criteria: 1. United Kingdom Parkinson's Disease Society brain bank diagnostic criteria (Steps 1 and 2). 2. Modified Hoehn and Yahr in the OFF state of II-IV. 3. Treated with levodopa/carbidopa for at least one year with a stable regimen for 4 weeks prior to randomization. 4. Taking at least 4 doses of levodopa/carbidopa per day (3 doses if at least 2 doses contained slow-release formulation) with predictable end of dose wearing off. 5. Successfully competed Parkinson's disease patient diary training with at least 120 minutes of OFF time per day. 6. Stable regimen of other antiparkinson's medications for 4 weeks prior to randomization. 7. At least 30 years of age and able to give written informed consent. Exclusion Criteria: 1. Treatment with liquid levodopa/carbidopa within 4 weeks of randomization. 2. Treatment with MAO inhibitors except selegiline. 3. Treatment within 3 months with centrally acting dopamine antagonists (6 months for depot formulations), e.g., antipsychotic neuroleptics, metoclopramide, buspirone, amoxapine. 4. Neurosurgical operation for Parkinson's disease. 5. Atypical parkinsonism or secondary parkinsonism variants. 6. Diagnosis of cancer or evidence of continued disease within 5 years. 7. Clinically significant illness of any organ system (e.g., ALT or AST \> 1.5 times the upper limit of normal). 8. Mini-Mental Status Examination score of 25 or less. 9. History of drug or alcohol abuse or dependence within 2 years. 10. History of psychotic illness or seizures. 11. Clinically relevant depression disorder. 12. History of neuroleptic malignant syndrome. 13. Pregnancy or lactation. Women of child bearing potential must use a reliable method of contraception.