A 12-week Randomized Study to Evaluate Oral Istradefylline in Subjects With Moderate to Severe Parkinson's Disease

This is a study to assess the efficacy and safety of oral Istradefylline (KW-6002) in patients with moderate to severe Parkinson\'s Disease. While on this study, participants will continue to take their usual, prescribed, stable regimen of Levodopa/Carbidopa or Levodopa/Benserazide therapy plus adjunct Parkinson\'s medications. Patients will be randomized 1:1:1 to receive either Istradefylline 20 mg per day, or Istradefylline 40 mg per day or an equivalent placebo. Patients will be treated for a 12 week period to demonstrate the effectiveness of Istradefylline in improving Parkinson\'s disease symptoms (referred to as improvement in patient OFF time) and that Istradefylline has an acceptable safety profile in this group.