Testing an add-on medicine when movement symptoms vary

Key Inclusion Criteria: Participants must meet all of the following criteria to take part in the study: 1. Be a man or woman between 30 and 80 years of age at the time of screening. 2. Have a diagnosis of Parkinson's disease, confirmed using standard medical criteria, including slowness of movement and symptoms that affect one side of the body more than the other. 3. Have mild to moderate Parkinson's disease, defined as stage 3 or lower on the Hoehn and Yahr scale when medications are working ("ON" state). 4. Experience an average of at least 3 hours of OFF time per day, based on home symptom diaries, with at least 2.5 hours of OFF time each day during the baseline period. 5. Have a Montreal Cognitive Assessment (MoCA) score of 24 or higher at screening. 6. Be able to walk independently, with or without the use of a walking aid. 7. Be able to swallow oral medication. 8. Have been on a stable Parkinson's medication regimen for at least 4 weeks before screening. Medications known as MAO-B inhibitors must have been stable for at least 12 weeks. 9. Be taking levodopa at least four times daily (immediate- or controlled-release formulations) or three times daily (extended-release formulations such as Rytary or Crexont). Key Exclusion Criteria: Participants cannot take part in the study if any of the following apply: 1. Have a form of parkinsonism that is not typical Parkinson's disease, such as secondary or atypical parkinsonism. 2. Have previously received, or plan to receive during the study, advanced Parkinson's therapies such as continuous levodopa or dopamine delivery systems, or Parkinson's disease-related brain surgery. 3. Have dyskinesias (involuntary movements) that are severe enough, in the study doctor's opinion, to interfere with participation. 4. Have a history of only certain types of dyskinesias (such as OFF-state, diphasic, myoclonic, or dystonic dyskinesias) without typical peak-dose dyskinesias. 5. Are currently taking medications that block dopamine, except for low-dose quetiapine (up to 50 mg per day) used for insomnia. 6. Routinely use on-demand "rescue" Parkinson's medications more than three times per week.