A Study to Evaluate the Efficacy of Prasinezumab (RO7046015/PRX002) in Participants With Early Parkinson's Disease

This multicenter, randomized, double-blind, placebo-controlled, Phase 2 study will evaluate the efficacy of intravenous prasinezumab (RO7046015/PRX002) versus placebo over 52 weeks in participants with early Parkinson's Disease (PD) who are untreated or treated with monoamine oxidase B (MAO-B) inhibitors since baseline. The study will consist of three parts: a 52-week, double-blind, placebo-controlled treatment period (Part 1) after which eligible participants will continue into an all-participants-on-treatment blinded dose extension for an additional 52 weeks (Part 2). Participants who complete Part 2 (including the 12-week treatment-free follow up visit assessing long term safety and efficacy of RO7046015) will be offered participation in Part 3 open-label extension (all-participants-on-RO7046015-treatment) for an additional 520 weeks.

Inclusion Criteria: * Idiopathic PD with bradykinesia plus one of the other cardinal signs of PD (resting tremor, rigidity) being present, without any other known or suspected cause of PD untreated or treated with MAO-B inhibitor * Body weight range between: \>/=45 kg/ 99 pounds (lbs) and less than or equal to (\