A Multicenter, Open-Label Study to Evaluate the Safety and Tolerability of Tozadenant as Adjunctive Therapy in Levodopa-Treated Patients With Parkinson's Disease Experiencing End of Dose "Wearing-Off"

Each patient will participate for up to 62 weeks, which includes a Screening Period, followed
by a Baseline Visit and open-label treatment for 1 year with a safety Follow-up 4 weeks after
the last treatment.

- Screening Period: up to 6 weeks.

- Open-Label Treatment Period: 52 weeks (1 year)

- Post-Treatment Safety Follow Up: 4 weeks.

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