A US Study That Observes How Parkinson's Disease Changes Over Time in Patients Who Still Have Movement Symptoms Despite Taking Parkinson's Medications

Inclusion Criteria for Patient: * Individual of any sex ≥45 to ≤75 years of age at informed consent (at least 30% ≤60 years of age). * Diagnosis of clinically established Parkinson's disease (PD) as defined by the Movement Disorder Society (MDS) Clinical Diagnostic Criteria for PD ≥4 and \<12 years from time of PD diagnosis at informed consent. * Modified H\&Y stage II-III in the practically defined OFF-medication state (≥12 hours from last dose of antiparkinsonian medications). * Score of ≥30 on MDS-Unified Parkinson's Disease Rating Scale (MDS-UPDRS) part III in the OFF-medication state. * Presence of motor fluctuations with ≥1 hour of absolute time in the OFF state per day as assessed by clinician/patient at screening. * Receiving stable antiparkinsonian medication regimen for ≥4 weeks prior to screening with a levodopa daily dose ≥300 mg or a dosing frequency of ≥3 times per day. * Responsiveness to levodopa as determined by change in the following measures from the practically defined OFF state to ON state after taking typical first-daily dose of PD-medications: i. any degree of improvement (≥0.5 point) in modified H\&Y stage OR. ii. ≥30% improvement in MDS-UPDRS part III score. * Montreal Cognitive Assessment (MoCA) score of ≥24. * Agree to participate and provide signed informed consent. Exclusion Criteria for Patient: * Known history or presence of conditions that may provide an alternative to a PD diagnosis including but not limited to: multiple system atrophy, progressive supranuclear palsy, striatonigral degeneration, corticobasal syndrome/degeneration, vascular Parkinsonism, drug-induced Parkinsonism, essential tremor, diffuse Lewy body disease, Lewy body dementia, Huntington's disease, Wilson's disease, Fahr's disease, Alzheimer's disease, cerebrovascular disease, brain tumor, trauma, and infection. * Known history or presence of significant vascular and/or cardiovascular disease limited to: stroke, transient ischemic attacks, poorly controlled hypertension, poorly controlled diabetes, unstable angina pectoris, or unstable myocardial infarction. * Known history or presence of significant psychosis or impulse control disorder, or untreated or sub optimally treated depression. * Known history or presence of human immunodeficiency virus, hepatitis B virus, hepatitis C virus, syphilis, or tuberculosis. * Current or previously active malignant disease within the past 5 years, except definitively treated cutaneous squamous cell carcinoma, basal cell carcinoma, or in situ uterine cervical carcinoma. * Currently pregnant, nursing, lactating, breastfeeding, or plan to be during study duration. * Known history or current use of percutaneous levodopa/carbidopa intestinal gel, subcutaneous levodopa, or apomorphine pump. * Prior history of brain surgery, including but not limited to: deep brain stimulation (DBS), pallidotomy, focused ultrasound thalamotomy, or other experimental neurosurgical procedure. * Known history or current participation in cell or gene therapy procedures. * Current participation in any interventional clinical trial. Inclusion Criteria for Care Partner: * ≥18 years of age at informed consent. * Identified by the PD patient as their primary care partner. * Agree to participate and the ability to provide signed informed consent independently, without the need for a legal representative. Exclusion Criteria for Care Partner: * Not applicable.