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

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 ( 110 kg/242 lbs

- Body mass index (BMI) of 18 to 34 kilograms per meter-squared (kg/m^2)

- A diagnosis of PD for 2 years or less at screening

- Hoehn and Yahr Stage I or II

- A screening brain DaT-SPECT consistent with PD (central reading)

- Clinical status does not require dopaminergic PD medication and is not expected to
require dopaminergic treatment within 52 weeks from baseline

- If presently being treated for PD, a stable dose of MAO-B inhibitor (rasagiline or
selegiline) for at least 90 days prior to baseline and not expected to change within
52 weeks

- For women of childbearing potential: use of highly effective contraceptive methods
(that result in a failure rate of <1 percent [%] per year) during the treatment period
and for at least 30 days (or longer if required by local regulations) after the last
dose of study drug

- For men with female partners of childbearing potential or pregnant female partners,
must use a condom during the treatment period and for at least 30 days (or longer if
required by local regulations) after the last dose of study drug to avoid exposing the
embryo. Men must refrain from donating sperm during this same period. The female
partners should use a contraception method with a failure rate of <1% per year during
the treatment period and for at least 30 days (or longer if required by local
regulations) after the last dose of study drug

Exclusion Criteria:

- Medical history indicating a Parkinson syndrome other than idiopathic PD, including
but not limited to, progressive supranuclear gaze palsy, multiple system atrophy,
drug-induced parkinsonism, essential tremor, primary dystonia

- Known carriers of certain familial PD genes (as specified in study protocol)

- History of PD related freezing episodes or falls

- A diagnosis of a significant CNS disease other than Parkinson's disease; history of
repeated head injury; history of epilepsy or seizure disorder other than febrile
seizures as a child

- Mini Mental State Examination (MMSE)
- Reside in a nursing home or assisted care facility

- History of or screening brain magnetic resonance imaging (MRI) scan indicative of
clinically significant abnormality

- Concomitant disease or condition that could interfere with, or treatment of which
might interfere with, the conduct of the study, or that would, in the opinion of the
Investigator, pose an unacceptable risk to the participant in this study or interfere
with the participant's ability to comply with study procedures or abide by study
restrictions, or with the ability to interpret safety data

- Any significant cardiovascular condition

- Any significant laboratory abnormality

- Lactating women

- Prior treatment with dopaminergic medication (for example, levodopa or a dopaminergic
agonist) with no clinical treatment response or a clinical treatment response
inconsistent with PD (for example, absence of observable response to a sufficiently
high-dose of levodopa [i.e., ≥ 600 mg/day])

- Use of any of the following: catechol-O-methyl transferase (COMT) inhibitors
(entacapone, tolcapone), amantadine or anticholinergics, or dopaminergic medication
(levodopa and both ergot and non-ergot [pramipexole, ropinirole, rotigotine] dopamine
agonists) for more than a total of 60 days or within 60 days of baseline

- Anti-epileptic medication for non-seizure-related treatment which has not remained
stable for at least 60 days prior to baseline

- Anti-depressant or anxiolytic use that has not remained stable for at least 90 days
prior to baseline. The use of fluoxetine and fluvoxamine is not permitted. For
patients treated with a MAO-B inhibitor and an antidepressant (except fluoxetine and
fluvoxamine), a 6-month period of stable and tolerated dosing before baseline is
required.

- Use of any of the following within 90 days prior to baseline: antipsychotics
(including clozapine and olanzapine), metoclopramide, alpha methyldopa, clozapine,
olanzapine, flunarizine, amoxapine, amphetamine derivatives, reserpine, bupropion,
buspirone, cocaine, mazindol, methamphetamine, methylphenidate, norephedrine,
phentermine, phenylpropanolamine, and modafinil

- Participated in an investigational drug, device, surgical , or stem cell study in PD

- Any prior treatment with an investigational PD-related vaccine (including active
immunization or passive immunotherapy with monoclonal antibodies).

- Prior participation in any RO7046015 or PRX002 study

- Receipt of any non-PD investigational product or device, or participation in a non-PD
drug research study within a period of 30 days (or 5 half-lives of the drug, whichever
is longer) before baseline

- Receipt of any monoclonal antibody or an investigational immunomodulator within 180
days (or 5 half-lives, whichever is longer) before baseline

- Immunomodulating drugs within 30 days prior to baseline

- Allergy to any of the components of RO7046015 such as citrate, trehalose and
polysorbate (Tween) 20 or a known hypersensitivity or an Infusion-related reaction
(IRR) to the administration of any other monoclonal antibody

- Any contraindications to obtaining a brain MRI. Patients with a hypersensitivity to
iodine may receive an alternative thyroid blocking agent.

- For participants consenting to provide optional cerebrospinal fluid (CSF) samples by
lumbar puncture (LP): LP will only be performed if the participant does not have any
contraindication to undergoing an LP

- Donation of blood over 500 milliliters (mL) within three months prior to screening