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.

Inclusion Criteria:

• Must carry a diagnosis of idiopathic PD according to UK Parkinson's Disease Society Brain Bank Clinical Diagnostic Criteria, and be Hoehn & Yahr Stage II to IV (inclusive) when OFF.
• Must be on a stable dose of L-3,4-dihydroxyphenylalanine (L-DOPA)/carbidopa or L-DOPA/benserazide for at least 4 weeks prior to enrollment.
• Except for L-DOPA and certain allowed dopamine agonists, must not be receiving any other PD medications. (Current treatment with certain dopamine agonists is allowed but subjects must have been on a stable dose for at least 4 weeks prior to enrollment).
• Some subjects must demonstrate a definite end of L-DOPA dose wearing off (at least 2 hours OFF time per waking day) and must be able to keep accurate patient diaries of PD activity.

Major Exclusion Criteria:

• A Mini Mental State Examination (MMSE) score <26.
• History or clinical features consistent with an atypical parkinsonian syndrome.
• Any significant non-Parkinson's central nervous system disorder.
• Any significant AXIS I psychiatric disease from the Diagnostic and Statistical Manual of Mental Disorders (DSM).
• Any previous surgical intervention for Parkinson's Disease.
• History of certain malignancies.
• History of severe allergic anaphylactic reactions to any drug.
• Clinically significant baseline electrocardiogram (ECG).
• Orthostatic hypotension.
• HbA1c >7.0%