This research study is being done to see the effectiveness and safety of a new drug called prasinezumab as a possible treatment for Parkinson's Disease. It has not been approved by the Food and Drug Administration. Eligible participants will be randomly assigned to received either prasinezumab or placebo (an inactive substance) The drug or placebo is given as an intravenous infusion every 4 weeks. Participation in the study is expected to last approximately 3 years. There is a Screening period that will last up to six weeks and a study treatment period that can last 104 weeks. There are two options at the end of the study, 1. Safety follow-up 70 days after last dose or Option 2: Two years of Open label prasinezumab and safety follow-up 70 days after last dose. The Screening, Randomization and Treatment period, Year 1 is 19 visits and treatment period Year 2 is 17 visits. Each visit will last to 4 to 6 hours. The OLE Year 1 is 16 visits and OLE Year 2 is 15 visits, and each visit will last 4 to 6 hours.

Researchers at the Medical University of South Carolina are studying speech and language in people with Parkinson's disease (PD), progressive supranuclear palsy (PSP), and multiple system atrophy (MSA), and in healthy adult volunteers. These conditions can change how a person speaks and uses language, and the changes may differ from one condition to another. The study uses a tablet-based set of short speech and language tasks, called SLANG, together with computer software that measures features of the recorded speech, such as pitch, timing, and word choice. Participants also complete standard speech, language, and thinking tests and a brief exam by a neurologist, which researchers compare against the tablet measurements. The purpose of this study is to gather early research data: to build a database of these measurements across the groups, to check whether the tablet captures them reliably, and to explore whether they differ between conditions. SLANG is not used to diagnose participants or to guide their medical care in this study. The long-term goal of this research is to develop a tool that could someday help clinicians recognize and tell these conditions apart earlier, but testing such a tool for diagnosis is beyond the scope of the current study.