Study Overview: This research is for people with early-stage Parkinson's disease (PD). It tests a drug called Prasinezumab, which may help with PD symptoms. People in the study must already be taking a medicine called levodopa.

Phase 3 Study: This is an advanced stage of testing to learn more about Prasinezumab's safety and effectiveness. Participants will either get the actual drug or a "placebo," which is a fake version to compare results. After finishing, you might join an "open-label" study where everyone gets the real drug for about two more years.

• Duration: The study lasts until the open-label part, which is approximately 2 years.
• Eligibility: You can join if you've had Parkinson's for 3 months to 3 years and are on a stable dose of levodopa.
• Exclusions: You cannot join if you're pregnant, plan to become pregnant, or have uncontrolled high blood pressure.