Study to Evaluate the Antiviral Activity, Clinical Outcomes, Safety, Tolerability, and Pharmacokinetics of Orally Administered Lumicitabine Regimens in Adult Participants Hospitalized With Respiratory Syncytial Virus

The study will be conducted in 3 phases: a screening phase, a treatment phase from Day 1 to Day 5/6 (depending on the timing of the loading dose), and a follow-up phase for a total of 28 days post randomization. Participants will have assessments completed at Day 7, Day 10, Day 14, and Day 28. Depending on discharge date, assessments will be completed either while hospitalized or during outpatient visits. The duration of the participant's participation will be approximately 28 days. The study will be performed in 2 parts. Participants will be randomly assigned to one of 2 treatment groups in part 1, and to one of 3 treatment groups in part 2. Treatment groups will be evaluated for PK and safety after a target of approximately 24 participants have been enrolled in part 1 and before initiating part 2 (approximately 90 participants in part 2). An Independent Data Monitoring Committee (IDMC) will be established to monitor the safety of participants and will review data in an unblinded manner on a regular basis to ensure the continuing safety of the participants enrolled in this study and to evaluate whether efficacy objectives are met. The committee will meet periodically to review interim data. Based on the recommendations of the IDMC following interim analyses/reviews, an increase in duration may be implemented.