A Phase 1 Study of S-4321

Inclusion Criteria: 1. Is available for the entire duration of the study and follow up. 2. Is willing and able to comply with all study assessments and adhere to the protocol schedule and restrictions. 3. Voluntarily consents to participation in the trial as documented by signing the study informed consent form (ICF). 4. Has a body mass index (BMI) within 18 to 32 kg/m2, inclusive, and weighs ≥45 kg. 5. Is in good physical and mental health in the opinion of the Investigator or delegate. Major Exclusion Criteria: 1. Has a history of severe allergic or anaphylactic reaction as determined by the Investigator or delegate. 2. Is pregnant, nursing, or is planning to become pregnant or breastfeed during the trial. 3. Has a known immunodeficiency disorder. 4. Has a history of malignancy or confirmed cervical dysplasia. 5. Has a history of human immunodeficiency virus (HIV) or positive serology for HIV at Screening. 6. Has positive laboratory evidence for active hepatitis at screening. 7. Has any other condition or prior therapy that, in the opinion of the Investigator or delegate, may potentially compromise the safety or compliance of the participant, or may preclude the participant from successfully completing the study. Other inclusion/exclusion eligibility criteria apply.