Safety and Tolerability of Tozadenant as Adjunctive Therapy in Levodopa-Treated Patients With Parkinson's Disease.

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