Your diagnosis of Parkinson’s disease will be confirmed based on specific criteria established by the Movement Disorder Society. This means your condition must include bradykinesia (slowness of movement) plus at least one other main sign such as resting tremor (shaking when muscles are relaxed) or rigidity (stiffness).

Your current stage of Parkinson’s disease will be assessed using the Hoehn and Yahr scale, which measures disease progression. You must be at Stage 1 or 2 when not taking medication, indicating early-stage disease.

Your score on the MDS-UPDRS Part IV assessment, which evaluates motor complications, must be 0. This means you should not have motor complications from your current treatment.

You must have been taking Parkinson’s disease medication for at least 3 months before starting the study.

You will be randomly assigned to receive either prasinezumab or placebo. The placebo is an inactive substance that looks identical to the actual medication but contains no active ingredient.

Neither you nor your medical team will know which treatment you are receiving. This is called a double-blind study design and helps ensure accurate results.

You will receive your assigned treatment through an intravenous infusion, which means the medication will be delivered directly into your vein through a needle.

The infusion will be given at regular intervals throughout the study period.

Your motor function will be regularly assessed using the MDS-UPDRS Part III scale. This evaluation measures movement-related symptoms when you are not taking your regular Parkinson’s medication.

The medical team will monitor for any motor progression events, which means a confirmed worsening of your movement symptoms.

Your vital signs, including blood pressure, heart rate, and temperature, will be measured at each visit.

Blood samples will be taken at specific times to check various laboratory parameters, including hematology (blood cell counts), clinical chemistry (chemical components in blood), coagulation (blood clotting ability), and urinalysis (urine tests).

An electrocardiogram (ECG) will be performed to monitor your heart’s electrical activity.

You will be assessed for any suicidal thoughts or behaviors using the Columbia Suicide Severity Rating Scale.

The medical team will monitor for any side effects or adverse reactions, including infusion-related reactions that may occur during or after the treatment administration.

At Week 104 (approximately 2 years after starting), a comprehensive assessment of your motor function will be conducted using the MDS-UPDRS Part III score while you are off your regular medication.

Changes in your motor function from the beginning of the study will be evaluated.

Your self-reported motor function will be assessed to determine if there is any worsening.

The Clinical Global Impression of Change (CGI-C) Overall Disease Subscale will be used to evaluate meaningful changes in your overall condition.

Any increases in your LEDD (levodopa equivalent daily dose), which measures the total amount of Parkinson’s medication you take, will be monitored.

Blood samples will be collected at specified times to measure the concentration of prasinezumab in your blood and to check for anti-drug antibodies, which are proteins your immune system might produce in response to the medication.

You must follow specific contraception requirements throughout the study period as agreed upon at the start.

The study is expected to continue until July 2031, though your individual participation duration may vary.

Throughout the entire study period, any adverse events will be classified according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) version 5.0, which provides a standardized way to describe the severity of side effects.