Inclusion Criteria:
1. Diagnosis of idiopathic Parkinson's disease according to the United Kingdom Parkinson's Disease Society (UKPDS) Brain Bank Criteria.
2. Subjects have 4 or 5 in modified Hohn and Yahr (H \& Y) classification of disease severity at "Off" state determined by the UPDRS Part V at Screening Visit 1.
3. The subject's advanced Parkinson's disease must be levodopa-responsive as judged by the Investigator.
4. Subjects have had optimal treatment with available Parkinson's disease medication as defined by local standards of care and, based upon the judgment of the Investigator, and their symptoms are judged inadequately controlled on this optimized treatment. Optimized treatment is defined as the maximum therapeutic effect obtained with available anti-parkinsonian pharmacological therapy when no further improvement is expected regardless of any additional manipulations of levodopa and/or other anti parkinsonian medication; this will be based on the Investigator's best clinical judgment.
5. Presence of a recognizable "Off" and "On" state (motor fluctuations) as confirmed by UPDRS Part III (in both "On" and "Off" states), and by the Parkinson's Disease Diary© which must be observed and confirmed at Screening Visit 1.
6. Subjects must be experiencing a minimum of 3 hours per day of "Off" time, as estimated by the Investigator and supported by the UPDRS at Screening Visit 1 and the Parkinson's Disease Diaries at baseline. The "Off" time must occur during a continuous 16-hour interval, including the portion of the day during which the subject is awake the majority of the time (e.g., 5 AM to 9 PM, 7 AM to 11 PM).
Exclusion Criteria:
1. Parkinson's disease diagnosis is unclear or a suspicion of other parkinsonian syndromes exists, such as secondary Parkinsonism (caused by drugs, toxins, infectious agents, vascular disease, trauma, brain neoplasm), Parkinson's-plus syndromes (e.g., multiple system atrophy, progressive supranuclear palsy) or the other neurodegenerative diseases that might mimic the symptoms of Parkinson's disease .
2. Subjects who have undergone neurosurgery for the treatment of Parkinson's disease .
3. Current primary psychiatric diagnosis of acute psychotic disorder or other uncontrolled primary psychiatric diagnoses, (e.g., bipolar disorder or major depressive disorder per Diagnostic and Statistical Manual of Mental Disorders 4th edition, Text Revision (DSM-IV-TR) criteria.
4. Alzheimer's disease; or other significant cognitive impairment or dementia (defined as Mini-Mental State Examination (MMSE) total score \< 24).
5. Subject has significant current suicidal ideation within the previous year as evidenced by answering "yes" to questions 4 or 5 on the suicidal ideation portion of the Columbia-Suicide Severity Rating Scale (C-SSRS) completed at Screening or any history of suicide attempts.
6. A low B12 level or low-normal B12 level (less than 300 pg/mL) with elevated methylmalonic acid (MMA). Note: Abnormal Vitamin B12 of questionable clinical significance (i.e., indeterminate or low normal results) prior to or at Screening Visit 2 require appropriate interpretation in conjunction with MMA and homocysteine laboratory values prior to proceeding further into the study.
