The purpose of this study is to evaluate the dose-related safety and tolerability of IPS101A, an adeno-associated virus (AAV) gene therapy, in patients with Parkinson's disease who exhibit severe functional impairment corresponding to Hoehn & Yahr stages 4-5 and whose symptoms are not adequately controlled despite all available monotherapy and combination therapy options. In addition, the study aims to assess the maximum tolerated dose (MTD) of IPS101A, as well as its preliminary efficacy and pharmacokinetic (PK) characteristics.

Inclusion Criteria:

1. Subjects who have a diagnosis of Parkinson's disease that meets the UK Parkinson's Disease Society Brain Bank Clinical Diagnostic Criteria at the time of the Screening Visit.
2. Male or female subjects aged 50 to 80 years (inclusive) at the time of providing written informed consent, with a documented diagnosis of Parkinson's disease.
3. Subjects with a duration of Parkinson's disease of at least 10 years prior to the Screening Visit, based on medical history and/or medical records.
4. Subjects with Parkinson's disease that is inadequately controlled despite all available standard-of-care treatments, including monotherapy or combination therapy, as determined by the Investigator.
5. Subjects with a Hoehn & Yahr stage of 4 or 5 in the off state at the Screening Visit.

Exclusion Criteria:

1. Subjects with Parkinson's disease dementia (PDD) who meet the diagnostic criteria established by the Movement Disorder Society (MDS) Task Force, as determined by the Investigator at Screening.
2. Subjects with a Korean Mini-Mental State Examination (K-MMSE) score ≤ 24 at the Screening assessment.
3. Subjects in whom imaging findings suggestive of Parkinsonism-plus syndrome are observed on PET and MRI performed at the Screening Visit, as assessed by the Investigator and/or a qualified imaging specialist.
4. Subjects who do not meet the diagnostic criteria for Parkinson's disease dementia but present with major visual hallucinations, as judged by the Investigator.
5. Subjects with drug-induced parkinsonism, confirmed by clinical history and/or medical records, and determined by the Investigator.
6. Subjects who are judged by the investigator to be unsuitable for participation in this clinical trial.