Dose-Finding Safety Study of BIIB014 in Combination With Levodopa in Moderate to Late Stage Parkinson's Disease

The main purpose of this study is to determine the safety of BIIB014 and how well BIIB014 is tolerated when given at different doses to patients with moderate to late-stage Parkinson's Disease who are also taking the Parkinson's medication, levodopa (L-DOPA). This study will also explore: 1. the pharmacokinetics of BIIB014 in Parkinson's patients who are also taking L-DOPA (this will be done by measuring the levels of BIIB014 in the blood at several different times during the study), and 2. the activity of BIIB014 when given to Parkinson's patients who are also taking L-DOPA (this will be done by performing different Parkinson's Disease assessments during the study to examine change in waking OFF time, change in time with troublesome dyskinesia, change in Unified PD Rating Scale (UPDRS) scores, and Clinical Global Improvement). Patients who enter this study will be randomly assigned to receive either BIIB014 or a placebo but because the study is blinded, neither they nor their study doctor will know which study treatment they are taking. The study will be divided into 2 parts: * Part A: a, rapid, sequential cohort, dose escalation to establish MTD, followed by * Part B: a parallel-group exploration of the two highest tolerated doses versus placebo. Note: As Part A of the study is now concluded, some of the study design information presented below (e.g., number of study arms) pertains only to Part B.