<br>        Inclusion Criteria:<br><br>          -  Patient understands study requirements and has given his/her written informed consent<br>             on an Institutional Review Board (IRB) or Independent Ethics Committee (IEC) approved<br>             consent form.<br><br>          -  Parkinson's disease diagnosis consistent with UK Parkinson's Disease Society Brain<br>             Bank Diagnostic criteria<br><br>          -  Minimum of 3 years since diagnosis.<br><br>          -  Meet Hoehn and Yahr PD stage<br><br>          -  Good response to levodopa<br><br>          -  Stable regimen of anti-PD medications<br><br>          -  Patients must have been taking a levodopa-containing anti-PD medication continuously<br>             for at least the previous 12 months<br><br>          -  Patient has documented a minimum amount of Off time.<br><br>          -  If of childbearing potential (male and female) must use an acceptable method of<br>             contraception<br><br>        Exclusion Criteria:<br><br>          -  Previous tozadenant study participation<br><br>          -  Current or recent participation in another study.<br><br>          -  Secondary or atypical parkinsonism<br><br>          -  Neurosurgical intervention for PD<br><br>          -  Patient is taking apomorphine, budipine, istradefylline, tolcapone, or DUOPA™/Duodopa®<br><br>          -  Treatment with excluded medications<br><br>          -  Untreated or uncontrolled hyperthyroidism or hypothyroidism<br><br>          -  Clinically significant out-of-range laboratory<br><br>          -  MMSE out of range<br><br>          -  Current episode of major depression (stable treatment for depression is permitted).<br><br>          -  Recent suicide attempt or suicidal ideation type 4 or type 5 of the Columbia-Suicide<br>             Severity Rating Scale (C-SSRS)<br><br>          -  Women lactating or pregnant<br><br>          -  Hypersensitivity to any components of tozadenant or excipients<br><br>          -  Abnormal findings on the physical or neurological examination, or medical history that<br>             would make the patient unsuitable for the study<br><br>          -  History of hepatitis or cholangitis<br>