A Study of S-337395 in Symptomatic Nonhospitalized Adults With Respiratory Syncytial Virus (RSV) Who Are at High Risk of Progression to Severe Disease

• S-337395 (Drug): S-337395 will be administered per schedule specified in the arm description.
• Placebo (Drug): Placebo matched to S-337395 will be administered per schedule specified in the arm description.

Primary outcomes

• Change From Baseline in RSV Ribonucleic Acid (RNA) Load by Quantitative Reverse Transcription Polymerase Chain Reaction (qRT-PCR) (Nasopharyngeal Swab) at Day 2 [Baseline, Day 2]
• Change From Baseline in RSV RNA Load by qRT-PCR (Nasopharyngeal Swab) at Day 4 [Baseline, Day 4]
• Change From Baseline in RSV RNA Load by qRT-PCR (Nasopharyngeal Swab) at Day 6 [Baseline, Day 6]

Secondary outcomes

• Number of Participants With Treatment-emergent Adverse Events (TEAEs) [Day 1 up to Day 28]
• Change From Baseline in Severity of RSV Respiratory Symptom Score Based on the Respiratory Infection Intensity and Impact Questionnaire (RiiQ) Respiratory Symptom Subscale (RiiQ-RSS) [Baseline up to Day 6]
• Change From Baseline in Severity of RSV Systemic Symptoms Score Based on the RiiQ Systemic Symptom Subscale (RiiQ-SSS) [Baseline up to Day 6]
• Change From Baseline in Severity of RSV Total Symptoms Score Based on the Combined RiiQ RSS and RiiQ-SSS [Baseline up to Day 6]
• Time to First Resolution of All Respiratory RSV-related Symptoms Based on the RiiQ-RSS [Baseline up to Day 6]
• Time to Sustained (2 Consecutive Days) Resolution of All Respiratory RSV-related Symptoms Based on the RiiQ-RSS [Baseline up to Day 6]
• Time to First Resolution of All Systemic RSV-related Symptoms Based on the RiiQ-SSS [Baseline up to Day 6]
• Time to Sustained Resolution of All Systemic RSV-related Symptoms Based on the RiiQ-SSS [Baseline up to Day 6]
• Time to First Resolution of All Respiratory and Systemic RSV-related Symptoms Based on the Combined RiiQ RSS and RiiQ-SSS [Baseline up to Day 6]
• Time to Sustained Resolution of All Respiratory and Systemic RSV-related Symptoms Based on the Combined RiiQ RSS and RiiQ-SSS [Baseline up to Day 6]
• Time to First Resolution of Each Separate RSV-related Symptom Based on the RiiQ Symptom Subscale [Baseline up to Day 6]
• Time to Sustained Resolution of Each Separate RSV-related Symptom Based on the RiiQ Symptom Subscale [Baseline up to Day 6]
• Change From Baseline in RSV RNA Load [Baseline up to Day 6]
• Area Under the Curve (AUC) of Change From Baseline in RSV RNA Load [Baseline up to Day 6]
• Time to the First Negative RSV RNA Load by qRT-PCR [Baseline up to Day 6]
• Time to Sustained Negative RSV RNA Load by qRT-PCR [Baseline up to Day 6]
• Percentage of Participants With Negative RSV RNA Load by qRT-PCR [Baseline up to Day 6]
• Time to the First RSV RNA Load <Lower Limit of Quantitation (LLOQ) [Baseline up to Day 6]
• Time to Sustained RSV RNA Load <LLOQ [Baseline up to Day 6]
• Percentage of Participants With RSV RNA Load <LLOQ [Baseline up to Day 6]
• Change From Baseline in Infectious Titer [Baseline up to Day 6]
• AUC of Change From Baseline in Infectious Titer [Baseline up to Day 6]
• Time to the First Negative RSV Infectious Titer [Baseline up to Day 6]
• Time to Sustained Negative RSV Infectious Titer [Baseline up to Day 6]
• Percentage of Participants With Negative RSV Infectious Titer [Baseline up to Day 6]
• Time to the First RSV Infectious Titer <LLOQ [Baseline up to Day 6]
• Time to Sustained RSV Infectious Titer <LLOQ [Baseline up to Day 6]
• Percentage of Participants With RSV Infectious Titer <LLOQ [Baseline up to Day 6]
• Plasma Concentration of S-337395 After Repeated Oral Doses of S-337395 [Baseline up to Day 6]
• Plasma Concentration of S-337395 12 and 24 Hours After the First Dose and the Last Dose (C12 and C24) [12 hours and 24 hours after first dose (Day 2) and last dose (Day 6)]

Sex: All, ages: 18 Years to any

Key Inclusion Criteria:

* Participants who have new onset or worsening (if present chronically) of at least 1 of the following signs and/or symptoms consistent with a viral acute respiratory infection within 72 hours prior to randomization: fever, nasal congestion, nasal discharge, sore throat, cough, sputum production, shortness of breath, or wheezing.
* Participants diagnosed with RSV infection preferably using a rapid polymerase chain reaction (PCR) or other molecular-based diagnostic assay.
* Has at least 1 of the following risk factors for severe RSV disease:

  1. ≥ 75 years of age;
  2. Chronic lung disease that is symptomatic and requiring chronic treatment; and
  3. Chronic cardiovascular disease that is symptomatic and requiring chronic treatment.
* 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.

Key Exclusion Criteria:

* Hospitalized or expected to be hospitalized within 24 hours of screening.
* Is considered by the investigator to be immunocompromised, due to an underlying medical condition or medical therapy, chemotherapy, radiation, stem cell or solid organ transplant.
* Has known allergies, hypersensitivity, or intolerance to S-337395 or to any of the excipients of the S-337395 or placebo formulation.
* Suspicion or known severe renal impairment.
* Any other medical or psychiatric condition making the participant unsuitable for the current study or interfering with the evaluation of response to the study intervention in the opinion of the investigator/subinvestigator.
* Has received a therapy intended to treat RSV infection within 14 days prior to screening.
* Is receiving chemotherapy or immunotherapy for malignancy.
* Has received RSV vaccination within 7 days prior to screening.
* Has had either confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or influenza infection (test was positive) during the 28 days prior to screening.
* Has any other confirmed clinically relevant respiratory infection by any pathogen at, or within 28 days of screening.

Note: Other protocol-specified inclusion and exclusion criteria may apply.