A Study of LY3303560 in Participants With Mild Cognitive Impairment or Alzheimer's Disease

Inclusion Criteria:

- Mild Cognitive Impairment (MCI) due to Alzheimer's Disease (AD) or mild-to-moderate AD
based on National Institute of Aging and Alzheimer's Association diagnostic criteria

- Female participants: women not of child-bearing potential may participate, and include
those who are:

- Infertile due to surgical sterilisation (hysterectomy, bilateral oophorectomy, or
for countries outside of Japan, tubal ligation), congenital anomaly such as
mullerian agenesis; or

- Postmenopausal defined as women at least 50 years of age with an intact uterus
who have not taken hormones or oral contraceptives within 1 year, who have had
either cessation of menses for at least 1 year, or 6 to 12 months of spontaneous
amenorrhea with follicle-stimulating greater than (>) 40 milli-international
units per millilitre (mIU/mL)

- Have a body weight of at least 50 kilogram (kg) (except for Japanese sites) and have a
body mass index (BMI) of 18.0 to 35.0 kilogram per meter square (kg/m²) (for all
sites), inclusive, at screening

Exclusion Criteria:

- Are currently enrolled in a clinical trial involving an investigational product (IP)
or any other type of medical research judged not to be scientifically or medically
compatible with this study

- Have participated, within the last 30 days (3 months and 4 months for sites in the
European Union [EU] and Japan, respectively) in a clinical trial involving an IP. If
the previous IP has a long half-life, 3 months (4 months for sites in Japan) or 5
half-lives (whichever is longer) should have passed

- Have known allergies to LY3303560, related compounds or any components of the
formulation, or history of significant atopy

- Have significant allergies to humanised monoclonal antibodies, diphenhydramine,
adrenaline, or methylprednisolone; or have a history of clinically significant
multiple or severe drug allergies, or intolerance to topical corticosteroids, or
severe post treatment hypersensitivity reactions (including, but not limited to,
erythema multiforme major, linear immunoglobulin A dermatosis, toxic epidermal
necrolysis, or exfoliative dermatitis)

- Have an increased risk of seizures as evidenced by a history of head trauma with loss
of consciousness within the last 5 years or any seizure; prior electroencephalogram
with epileptiform activity; surgery to the cerebral cortex; or history within the last
5 years of a serious infectious disease affecting the brain

- Have any contraindications for Magnetic Resonance Imaging (MRI) studies, including
claustrophobia, or the presence of metal (ferromagnetic) implants or a cardiac
pacemaker

- Have a history of intracranial haemorrhage, cerebrovascular aneurysm, or arteriovenous
malformation, carotid artery occlusion, or epilepsy

- Have received acetylcholinesterase inhibitors (AChEIs), memantine, and/or other AD
therapy for less than 4 weeks, or have less than 4 weeks of stable therapy on these
treatments by time of randomisation (including less than 4 weeks since stopping AChEIs
and/or memantine); or have received medications that affect the central nervous
system, except treatments for AD, for less than 4 weeks at a stable dose

- Have used stable medical therapy for less than 2 months by time of randomization for
any concurrent medical condition that is not exclusionary

- Are currently using or intend to use drugs known to significantly prolong the QT
interval, or who have a known risk factor for Torsades de Pointes. A participant will
not be excluded if they have been using stable medication that is known to potentially
cause significant prolongation of the QT interval, but does not present with any
clinically significant prolongation of the QT interval at screening, in the opinion of
the investigator

- History of cancer within the last 5 years, with the exception of non-metastatic basal
and/or squamous cell carcinoma of the skin, in situ cervical cancer, non-progressive
prostate cancer, or other cancers with low risk of recurrence or spread.