A Study to Evaluate the Efficacy and Safety of Intravenous (IV) Prasinezumab in Participants With Early-Stage Parkinson's Disease

The purpose of this study is to evaluate the efficacy, safety, and pharmacokinetics (PK) of prasinezumab compared with placebo in participants with early-stage Parkinson's disease (PD) on stable symptomatic monotherapy with levodopa.

Inclusion Criteria: * Body weight within 40-110 kilograms (kg) (88-242 pounds \[lbs\]) and a body mass index within the range 18-34 kg/m2 * Diagnosis of idiopathic PD based on Movement Disorder Society (MDS) criteria * Has received monotherapy treatment * An MDS-UPDRS Part IV score of 0 at screening and prior to randomization * Hoehn and Yahr (H\&Y) Stage 1 or 2 off medication at screening and prior to randomization * Agreement to adhere to the contraception requirements Exclusion Criteria: * Pregnant or breastfeeding, or intention of becoming pregnant during the study or within the time frame in which contraception is required * Medical history indicating a parkinsonian syndrome other than idiopathic PD * Diagnosis of a significant neurologic disease other than PD * Chronic uncontrolled hypertension