Antiviral Activity, Clinical Outcomes, Safety, Tolerability, and Pharmacokinetics of Oral Lumicitabine Regimens in Hospitalized Adult Participants Infected With Human Metapneumovirus

The purpose of this study is to determine in hospitalized adult participants infected with human metapneumovirus (hMPV - a virus closely related to respiratory syncytial virus (RSV) and has been identified as an important cause of acute respiratory infections, affecting all age groups) the dose-response relationship of multiple regimens of lumicitabine on antiviral activity based on nasal hMPV shedding using quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) assay.

The study consists of 3 phases: screening phase, treatment phase (Day 1 to Day 5/6 \[depending on timing of loading dose\]), and follow-up phase of 28 days post randomization. Participants will have assessments at Days 7, 10, 14, and 28 to evaluate safety, efficacy, and pharmacokinetics (PK). The primary hypothesis of study is a positive dose-response relationship of active treatment on average hMPV viral load area under concentration versus time curve (AUC) over 7 days, meaning that either average AUC on pooled active treatments is lower than on placebo, or average AUC on high active dose is lower than average AUC on placebo using multiple contrast testing. Based on review of PK, efficacy and safety data, Independent Data Monitoring Committee (IDMC) may recommend modifications to study design that is changes in dose and treatment duration.

Eligibility Criteria:
Inclusion Criteria:

* Participants 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
* Participants diagnosed with human metapneumovirus (hMPV) infection using a rapid polymerase chain reaction (PCR)-based molecular diagnostic assay, with or without coinfection with another respiratory pathogen (respiratory virus or bacteria)
* Participants with an acute respiratory illness with signs and symptoms consistent with a viral infection (for example, fever, cough, nasal congestion, runny nose, sore throat, myalgia, lethargy, shortness of breath, or wheezing) with onset less than or equal to (\<=)5 days from the anticipated time of randomization
* With the exception of the symptoms related to hMPV infection, participants must be medically stable on the basis of physical examination, medical history, 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 hMPV infection. This determination must be recorded in the participant's source documents and initialed by the investigator
* A woman must have a negative urine pregnancy test (beta-human chorionic gonadotropin \[b-hCG\]) 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 immunocompromised within the past 12 months, whether due to underlying medical condition (for example, malignancy or genetic disorder) or medical therapy (for 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 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 a known history of human immunodeficiency virus (HIV) or chronic viral hepatitis