The primary aim of the study is to assess the safety and tolerability of S-309309 after oral administration in healthy adult or obese but otherwise healthy adult participants. The secondary aim of the study is to assess the pharmacokinetics of S-309309 and the effects on ECG parameters after oral administration in healthy or obese but otherwise healthy participants.

This study will consist of 2 Parts. Part 1 will include healthy participants who will be assigned to a dose group. Participants will then be randomized to receive a single oral administration of S-309309 or placebo in a fasted state. Administration will be initiated in the lowest dose group, and administration in the next dose group will not occur until a review of safety and tolerability has been completed for the preceding group.

Part 2 will include 2 groups. Healthy participants and obese but otherwise healthy participants will receive multiple oral administrations of S-309309 or placebo in a fed state.

Inclusion Criteria:

• Healthy as determined by medical evaluation including medical history, physical examination, clinical laboratory tests, vital sign measurements, and 12-lead ECG at the Screening Visit and upon admission to the clinical research unit (CRU).
• Body weight ≥50 kilograms (kg), and body mass index (BMI) within the range ≥18.5 to <30.0 kilogram/meter square (kg/m^2) for all groups except Group G-2. For Group G-2, BMI ≥ 30 to < 40 kg/m2 at the Screening Visit.

Exclusion Criteria:

• History or presence of/significant history of or current cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrinological, hematological, or neurological disorders capable of significantly altering the absorption, metabolism, or elimination of drugs; constituting a risk when taking the study intervention; or interfering with the interpretation of data.
• Lymphoma, leukemia, or any malignancy within the past 5 years, except for basal cell or squamous epithelial carcinomas of the skin that have been resected with no evidence of metastatic disease for 3 years.
• Breast cancer within the past 10 years.
• Unable to swallow capsules.
• Chronic history of or current liver disease or known hepatic or biliary abnormalities (with the exception of Gilbert's syndrome or asymptomatic gallstones).