Inclusion Criteria:  Subjects with confirmed diagnosis of Parkinson's disease or Dementia with Lewy Bodies MoCA score 11 to 23, inclusive, at screening Probable Parkinson's Disease Dementia or Lewy Body Dementia BMI between ≥ 16and ≤ 35 kg/m2 for females and between≥ 18 and ≤ 35 kg/m2 for males at Screening Reliable and capable support person/caregiver, who is willing to accept responsibility for supervising the treatment or, if required, administering study drug, and assessing the condition of the subject throughout the study in accordance with all protocol requirements  Exclusion Criteria:  Hoehn-Yahr stage 5 History of significant neurological disease other than PDD or DLB that may affect cognition at onset of dementia Subjects on deep brain stimulation History of brain MRI scan indicative of any other significant abnormality History of unexplained loss of consciousness, and epileptic fits Hearing test result considered unacceptable for auditory ERP P300 assessment Diagnosis of severe major depressive disorder even without psychotic features (GDS score [15-item scale] >7 at Screening) Significant suicide risk based on C-SSRS Significant psychosis (according to Diagnostic and Statistical Manual of Mental Disorders) Moderate or severe substance abuse disorder (according to DSM-5) Myocardial infarction or unstable angina within the last 6 months Clinically significant cardiac arrhythmia (including atrial fibrillation), cardiomyopathy, or cardiac conduction defect (note: pacemaker is acceptable) Clinically significant ECG abnormality at Screening Chronic kidney disease (eGFR < 45 mL/min using Cockcroft and Gault formula) Hepatic impairment with alanine aminotransferase or aspartate aminotransferase > 2 times the upper limit of normal, or Child-Pugh class B and C Malignant tumor within 3 years before Screening Memantine at any dose or combination Donepezil at 23 mg