Inclusion Criteria:
* Participant must be 45 to 75 years of age inclusive, at the time of signing the informed consent.
* \>10 years since diagnosis of PD (at time of consenting / Screening Visit 1)
* Presence of bradykinesia plus any of the following:
* Rigidity
* Resting tremor
* Postural instability
* Modified Hoehn and Yahr stage III-IV in the practically defined OFF state
* Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) Part III score \>40 in the practically defined OFF state
* Stable anti-PD medication regimen for at least 4 weeks prior to Screening Visit 1 and through Baseline Visit
* ≥30% reduction in MDS-UPDRS Part III following a levodopa challenge
* Must agree to use barrier method protection when engaging in intercourse/sexual activity with another person for at least 3 months post-dosing.
* Male participants must refrain from donating sperm for at least 3 months post-dosing.
* Female participants cannot be pregnant or breastfeeding at the time of screening. A woman of childbearing potential (WOCBP) must have a negative highly sensitive pregnancy test (urine or serum as required by local regulations) at the required assessments
* Provision of signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol
* Provision of signed consent to also participate in LTFU study ASK-PD0-CS002
Exclusion Criteria:
* Evidence of secondary or atypical parkinsonism (as determined by the neurologist)
* Presence or history of psychosis or impulse control disorder (as determined by the neurologist)
* Presence or history (within 2 years prior to screening) of substance use disorder (including alcohol) as defined by the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) criteria (or in the judgment of the neurologist)
* Presence of untreated or sub optimally treated depression (Beck Depression Inventory \[BDI\]-II score ≥20)
* Current suicidal ideation as indicated by positive response to items 4 or 5 on the Columbia-Suicide Severity Rating Scale (C SSRS), or any history of a suicide attempt
* Clinically significant cognitive impairment (Montreal Cognitive Assessment \[MoCA\] score \<25)
* Presence or history of malignancy other than treated cutaneous squamous or basal cell carcinomas
* Presence of clinically active infection, including acute or chronic scalp infection
* Known contraindications to MRI
* Presence or history of significant cerebrovascular or cardiovascular disease, including:
* Stroke, transient ischemic attack, or other suspected cerebrovascular accident within 1 year prior to screening
* Unstable angina pectoris or myocardial infarction within 1 year prior to screening
* Revascularization procedure(s) within 1 year prior to screening
* Poorly controlled hypertension, poorly controlled diabetes mellitus or prediabetes mellitus with known significant microvascular injury, or other significant cardiovascular history or risk factor
* Known history of complications of anesthesia including difficult airway management and/or difficult endotracheal intubation, malignant hyperthermia, or other related issues that would compromise participant safety during general anesthesia (as determined by the anesthesiologist)
* Inability to identify a safe trajectory to each putamen via an occipitoparietal entry point, or known contraindications to brain surgery in prone position (as determined by the neurosurgeon)
* Known allergy or sensitivity to ingredients in the intervention formulation and/or to gadolinium-based contrast agents
* Concurrent use of percutaneous levodopa/carbidopa intestinal gel, subcutaneous levodopa, or apomorphine pump
* History of brain surgery (including deep brain stimulation \[DBS\] or focused ultrasound)
* Chronic immunosuppressive therapy
* History of prior cell or gene therapy
* Participation in other interventional clinical trials within 12 weeks prior to screening or unwilling to refrain from starting new investigational agents throughout the course of the study
* Laboratory values at screening:
* Platelets ≤100,000/mm3
* PT \>15 s, aPTT \>40 s, and/or INR \>1.3
* Absolute neutrophil count (ANC) ≤1500/mm3
* Hemoglobin ≤10.0 g/dL
* Aspartate aminotransferase or alanine aminotransferase ≥2.5 times the upper limit of normal
* Total bilirubin ≥2.5 mg/dL
* eGFR \<45 mL/min/1.73 m2
* HbA1C ≥8%
* Unable to comply with the protocol procedures, including frequent and prolonged follow-up assessments (during LTFU study ASK-PD0-CS002)
* Unwilling to defer any vaccination from screening through 1 month after surgery
* Any significant issue raised by the neurologist, neurosurgeon, or anesthesiologist that may make a participant unsuitable for the study
