The aim of this study is to evaluate the safety, tolerability and PK of LY3876602 after administering it as single ascending doses and, following a data review, proceeding to multiple ascending doses in healthy participants. Blood tests will also be performed to check how much LY3876602 gets into the bloodstream and cerebrospinal fluid (CSF) and how long it takes the body to eliminate it.

Following the screening visit for Part A, the study will last up to approximately 20 weeks., and a subgroup of participants will consent to CSF collections The study will last up to approximately 28 weeks for Part B. All participants will consent to CSF collections.

Inclusion Criteria:

• Male and female participants who are overtly healthy as determined by medical evaluation
• Have a body mass index with the range of 18 and less than or equal to (≤) 32 kilograms per square meter (kg/m²)
• Male or female participants of nonchildbearing potential

Exclusion Criteria:

• Have a family history, defined as a parent or first-degree relative, of genetic neurodegenerative disorders
• Have allergies to diphenhydramine, epinephrine, or methylprednisolone
• Have serious or unstable medical conditions,
• History of skin wounding within 14 days of screening or current skin infection
• Are unwilling to stop alcohol consumption 48 hours prior to each dosing
• Have active or latent tuberculosis
• Participants in the cohorts undergoing lumbar puncture have an allergy to local anesthetics