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.

Inclusion Criteria:

- Hospitalized (or in emergency room prior to hospitalization) at the time of
randomization and unlikely to be discharged for the first 24 hours after randomization

- Diagnosed with respiratory syncytial virus (RSV) infection based on polymerase chain
reaction (PCR)-based assay with or without co infection with another respiratory
pathogen (eg, influenza, human metapneumovirus, or bacteria)

- With the exception of the RSV disease, medically stable on the basis of medical
history, physical examination, vital signs, and 12-lead electrocardiogram (ECG)
performed at screening. If there are abnormalities, they must be consistent with the
underlying illness in the study population and/or the RSV infection. This
determination must be recorded in the participant's source documents and initialed by
the investigator

- A woman must have a negative urine beta human chorionic gonadotropin at screening

- A woman must agree not to donate eggs (ova, oocytes) during the study and for at least
44 days after receiving the last dose of study drug

- Contraceptive use by women should be consistent with local regulations regarding the
use of contraceptive methods for participants participating in clinical studies A
woman must be of non-childbearing potential defined as either: a) Postmenopausal: a
postmenopausal state is defined as more than (>) 45 years and no menses for 12
consecutive months without an alternative medical cause, OR Permanently sterile:
permanent sterilization methods include hysterectomy, bilateral salpingectomy,
bilateral tubal occlusion/ligation procedures (without reversal operation), and
bilateral oophorectomy. b) Of childbearing potential and, if heterosexually active,
also included: practicing a highly effective method of contraception (failure rate of
less than (<) 1percent (%) per year when used consistently and correctly)

- Participants must have a body weight of at least 50.0 kilogram, at screening

Exclusion Criteria:

- Participants who are not expected to survive for more than 48 hours

- Participants who have had major thoracic or abdominal surgery in the 6 weeks prior to
randomization

- Participants who are considered by the investigator to be immuno-compromised within
the past 12 months, whether due to underlying medical condition (example, malignancy
or genetic disorder) or medical therapy (example, medications other than
corticosteroids for the treatment of chronic obstructive pulmonary disease (COPD) or
asthma exacerbations, chemotherapy, radiation, stem cell or solid organ transplant)

- Participants with a known history of human immunodeficiency virus (HIV) or chronic
viral hepatitis

- Participants undergoing peritoneal dialysis, hemodialysis, or hemofiltration or with
an estimated glomerular filtration rate (GFR, determined by Chronic Kidney Disease
Epidemiology Collaboration [CKD-EPI] equation) of (<) 60 milliliters per minute
(mL/min) per 1.73 meter square (m^2)

- Participants with 1 or more of the following laboratory abnormalities at screening as
defined by the Division of Microbiology and Infectious Diseases (DMID) Adult Toxicity
Table: Hemoglobin <9.5 gram per deciliter (g/dL), Platelet count <75,000 per
millimeter cube (/mm^³), White blood cell count <1,000/mm^³, Absolute neutrophil count
<1,000/mm^³