A Study of LY3154207 in Participants With Parkinson's Disease Dementia

Inclusion Criteria:

- Have Idiopathic Parkinsons's Disease (PD) per MDS criteria with at least 2 years of PD
symptoms.

- Have dementia as defined by a decline in cognitive function, which in the opinion of
the investigator has resulted in functional impairment.

- Have a score on the MoCA of 10 - 23.

- Are Modified Hoehn and Yahr Stages 1 - 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:

1. 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.

2. Each of the 3 systolic BP measurement must be less than 180 mmHg

- Participants ≥60 years old:

1. 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.

2. 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 4
weeks prior to baseline.

- If on medications affecting cognition (rivastigmine, galantamine, donepezil,
memantine), participants must be on stable dosage for at least 8 weeks prior to
baseline.

- If on antidepressant medications, participants must be on stable dosage for at least 8
weeks prior to baseline.

- If on Clozapine, quetiapine and pimavanserin to address drug-induced or
disease-related psychosis, participants must be on stable dosage for 4 weeks prior to
baseline.

- If on anti-hypertensive medications, participants must be on stable dosage for at
least 3 months 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 disease, other than PD, that may affect
cognition or the ability to complete the study, including but not limited to, other
dementias (e.g., Alzheimer's disease).

- 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 current or any previous diagnosis of bipolar disorder, schizophrenia, or other
primary psychotic disorder.

- Have poorly controlled psychosis (hallucinations or delusions) that in the opinion of
the investigator would interfere with the subject's ability to be compliant with the
study protocol.

- Have any other psychiatric disorder that, in the judgment of the investigator, would
interfere with compliance with the study protocol.

- 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 Geriatric Depression Scale - Short Form (GDS-S) score of >6 at screening.

- Have a serious or unstable medical illness, other than idiopathic PD, 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 history of additional risk factors for Torsades de Pointes (TdP; [e.g., chronic
hypokalemia, family history of Long QT Syndrome]).

- Participants with acute liver disease (e.g. acute viral hepatitis, alcoholic
hepatitis); participants with a known chronic liver disease (e.g. hepatitis B, C,
alcoholic liver disease, cirrhosis); alanine aminotransferase (ALT) or aspartate
aminotransferase (AST) equal to or higher than 2X upper limit of normal (ULN); total
bilirubin (TBL) equal to or higher than 1.5X ULN; (except for participants with
Gilbert's syndrome); or alkaline phosphatase (ALP) equal to or higher than 2X ULN.

- Participants have answered 'yes' to either Question 4 (Active Suicidal Ideation with
Some Intent to Act, Without Specific Plan) or Question 5 (Active Suicidal Ideation
with Specific Plan and Intent) on the "Suicidal Ideation" portion of the Columbia
Suicide Severity Rating Scale (C-SSRS)- Children's version, or answer "yes" to any of
the suicide-related behaviors (actual attempt, interrupted attempt, aborted attempt).

- Have used antipsychotic medications, with the exception of clozapine, quetiapine,
pimavanserin in the 6 months prior to screening.

- Have used trihexyphenidyl and benztropine in the 4 weeks prior to screening.

- Have motor conditions for which the antiparkinsonian treatment is expected to change
during the course of the study, as well as unpredictable motor fluctuations that in
the investigator's opinion would interfere with administering assessments.

- Are currently taking any medications or food, herbal or dietary supplements that are
inhibitors (e.g., ketoconazole, grapefruit juice) or strong/moderate inducers of
CYP3A4 (e.g. rifampicin).