Inclusion Criteria:

• Men and women aged ≥ 55 years. 
• Subject or subject's legally authorized representative is willing and able to provide written informed consent. 
• Probable DLB and identified cognitive deficits, according to current consensus criteria (McKeith et al, 2017), specifically one core clinical feature and a positive DaTscan. If a negative DaTscan, but the subject has historical PSG-verified RBD, the subject would also qualify. 
• MMSE score of 15-28, inclusive, during Screening. 
• Currently receiving cholinesterase inhibitor therapy, having received such therapy for greater than 3 months and on a stable dose for at least 6 weeks at the time of randomization. Except for reducing the dose for tolerability reasons, the dose of cholinesterase inhibitor may not be modified during the study. 
• Normal or corrected eye sight and auditory abilities, sufficient to perform all aspects of the cognitive and functional assessments. 
• No history of learning difficulties that may interfere with their ability to complete the cognitive tests. 
• Must have reliable informant or caregiver.

Exclusion Criteria: 

• Diagnosis of any other ongoing central nervous system (CNS) condition other than DLB, including, but not limited to, post-stroke dementia, vascular dementia, Alzheimer's disease (AD), or Parkinson's disease (PD).
• Suicidality, defined as active suicidal thoughts within 6 months before Screening or at Baseline, defined as answering yes to items 4 or 5 on the C-SSRS, or history of suicide attempt in previous 2 years, or, in the Investigator's opinion, at serious risk of suicide. 
• Ongoing major and active psychiatric disorder and/or other concurrent medical condition that, in the opinion of the Investigator, might compromise safety and/or compliance with study requirements. 
• Diagnosis of alcohol or drug abuse within the previous 2 years. 
• Poorly controlled clinically significant medical illness, such as hypertension (blood pressure > 180 mmHg systolic or 100 mmHg diastolic); myocardial infarction within 6 months; uncompensated congestive heart failure or other significant cardiovascular, pulmonary, renal, liver, infectious disease, immune disorder, or metabolic/endocrine disorders or other disease that would interfere with assessment of drug safety. 
• Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) > 2 × the upper limit of normal (ULN), total bilirubin > 1.5 × ULN, and/or International Normalized Ratio (INR) > 1.5. 
• Known human immunodeficiency virus, hepatitis B, or active hepatitis C virus infection. 
• Participated in a study of an investigational drug less than 3 months or 5 half-lives of an investigational drug, whichever is longer, before enrollment in this study.
• History of previous neurosurgery to the brain. 
• If male with female partner(s) of child-bearing potential, unwilling or unable to adhere to contraception requirements specified in the protocol. 
• If female who has not has not reached menopause > 1 year previously or has not had a hysterectomy or bilateral oophorectomy/salpingo-oophorectomy, has a positive pregnancy test result during Screening and/or is unwilling or unable to adhere to the contraception requirements specified in the protocol.