The purpose of this study is to evaluate how well LY3866288 is tolerated and what side effects may occur in healthy participants. The study drug will be administered orally. Blood tests will be performed to check how much LY3866288 gets into the bloodstream and how long it takes the body to eliminate it.

The study will last up to approximately 9 weeks including screening.

Inclusion Criteria:

• Females must be of nonchildbearing potential or of childbearing potential and agree to use contraception
• Males must either be infertile via bilateral orchiectomy or vasectomized at least 90 days prior to screening with proper documentation
• Body mass index (BMI) between 18.5 and 32.0 kilogram per square meter (kg/m²)
• In good general health, determined by no clinically significant findings from medical history, physical examination, 12 lead electrocardiogram (ECGs), vital signs measurements, or clinical laboratory evaluations at screening and/or check-in as assessed by the investigator (or designee)

Exclusion Criteria:

• Females only: Are lactating or pregnant

• Any history or presence of disease(s), deemed clinically significant:

  • Dermatological disease
  • Liver disease
  • Metabolic disease, including congenital nonhemolytic hyperbilirubinemia (e.g., Gilbert syndrome)
  • Gastrointestinal disease, including peptic ulcer disease, active malabsorption syndrome or other condition likely to affect gastrointestinal absorption of the study drug, or gastric reduction surgery. Note: participants with a history of appendectomy and/or inguinal hernia repairs will be acceptable
  • Biliary disease, including cholecystectomy
  • Cancer within the past 5 years (except localized basal cell, squamous, or in situ cancer of the skin)
  • History or current evidence of ophthalmic disorder, such as central serous retinopathy or retinal vein occlusion, active wet age-related macular degeneration, diabetic retinopathy with macular edema, uncontrolled glaucoma, corneal pathology such as keratitis, keratoconjunctivitis, keratopathy, corneal abrasion, inflammation or ulceration
  • Clinically significant, active cardiovascular disease or history of myocardial infarction within 6 months prior to first dose, or 12-lead ECG abnormalities that are clinically significant at screening

• History of alcohol and/or drug abuse within 2 years prior to screening

• Have been on a diet incompatible with the on-study diet, in the opinion of the investigator (or designee), and as confirmed by the sponsor, within the 30 days prior to Day 1 and throughout

• Use of tobacco, smoking cessation products, products containing nicotine, or e-cigarettes (nicotine and non-nicotine) within 90 days prior to screening and throughout