A Study of LY3154207 in Participants With Dementia Due to Lewy Body Dementia (LBD) Associated With Idiopathic Parkinson's Disease (PD) or Dementia With Lewy Bodies (DLB)

Inclusion Criteria

• Have dementia as defined by a decline in cognitive function, which in the opinion of the investigator has resulted in functional impairment.
• Meet diagnostic criteria for PD per MDS criteria or DLB per 4th Consensus Report of the DLB Consortium.
• Have a score on the MoCA of 10 - 23.
• Are Modified Hoehn and Yahr Stages 0 - 4.
• Have a blood pressure (BP) or pulse rate at screening and randomization, as determined by three sequential BP/pulse rate measurements in a seated position:
• Participants <60 years old:
• A mean systolic BP less than or equal to 140 millimeters of mercury (mmHg), a mean diastolic BP less than or equal to 90 mmHg and a mean pulse rate less than or equal 90 beats/minute in a seated position.
• Each of the 3 systolic BP measurement must be less than 180 mmHg
• Participants ≥60 years old:
• A mean systolic BP less than or equal to 150 mmHg, a mean diastolic BP less than or equal to 90 mmHg and a mean pulse rate less than or equal to 90 beats/min in a seated position.
• Each of the 3 systolic BP measurement must be less than 180 mmHg
• If on anti-parkinsonian agents, participants must be on stable dosage for at least 3 weeks prior to screening, and should remain on stable doses during the course of the study.
• If on medications affecting cognition (rivastigmine, galantamine, donepezil, memantine), participants must be on stable dosage for at least 3 weeks prior to screening and should remain at a stable dosage during the course of the study.
• If on antidepressant medications, participants must be on stable dosage for at least 3 weeks prior to screening and should remain at a stable dosage during the course of the study.
• If on clozapine, quetiapine, and pimavanserin to address drug induced or disease related psychosis, participants must be on stable dosage for 3 weeks prior to screening and should remain at a stable dosage during the course of the study.
• If on antihypertensive medications, participants must be on stable dosage for at least 3 weeks prior to screening.
• Men should use appropriate contraception.
• All participants must have a reliable caregiver who is in frequent contact with the participant (defined as at least 10 hours per week) and will accompany the participant to screening, baseline, day 7, day 42, day 84 and follow-up.

Exclusion Criteria

• Are women of childbearing potential.
• Have significant central nervous system or psychiatric disease, other than PD or DLB, that in the investigator's opinion may affect cognition or the ability to complete the study.
• Have a history in the last 6 months of transient ischemic attacks or ischemic stroke.
• Have a history of intra cerebral hemorrhage due to hypertension.
• Have a history of hypertensive encephalopathy.
• Have atypical or secondary parkinsonism due to drugs (e.g., antipsychotics) or disease (such as progressive supranuclear palsy, essential tremor, multiple system atrophy (e.g. striatonigral degeneration, olivopontocerebellar atrophy), or postencephalitic parkinsonism).
• Have a current implantable intracranial stimulator or history of intracranial ablation surgery (e.g., subthalamic, globus pallidus-internal segment [GPi]).
• Have a history of substance abuse within the past 1 year (drug categories defined by the Diagnostic and Statistical Manual of Mental Disorder, 5th Edition [DSM-5], and/or substance dependence within the past 1 year, not including caffeine and nicotine.
• Have a serious or unstable medical illness, other than idiopathic LBD (PDD or DLB), including cardiovascular, hepatic, respiratory, hematologic, endocrinologic, neurologic, or renal disease, or clinically significant laboratory or electrocardiogram (ECG) abnormality as determined by the investigator.
• Have a history in the last 6 months of exertional angina, unstable angina, myocardial infarction, and acute coronary syndrome.
• Have a history of heart failure of either New York Heart Association Class III or IV.
• A histor