The primary objective of this study is to evaluate the effects of presatovir on respiratory syncytial virus (RSV) viral load in RSV-positive adults who have been hospitalized with acute respiratory infectious symptoms.

Participants will receive 1 dose of presatovir on Day 1 and followed for 27 days postdose. Nasal swabs will be collected at each study visit (excluding Day 28) and assayed for change in viral load as the primary endpoint.

Key Inclusion Criteria:

• Current inpatient

• New onset of acute respiratory infectious symptoms, or acute worsening of chronic symptoms related to ongoing respiratory disease for ≤ 5 days prior to screening:

  • Upper respiratory tract symptoms: nasal congestion, runny nose, sore throat, or earache
  • Lower respiratory tract symptoms: cough, sputum production, wheezing, dyspnea, or chest tightness

• Documented to be RSV-positive at the current admission within 72 hours of screening, or as evaluated at screening

Key Exclusion Criteria:

• Related to concomitant or previous medication use:

  • Use of oral prednisone or other corticosteroid equivalent to:

    • > 20 mg/day for > 14 days prior to screening is not permitted.
    • > 20 mg/day for ≤ 14 days, including corticosteroids received during current hospitalization (ie, bolus doses), is permitted.
    • ≤ 20 mg/day, regardless of duration, is permitted.

  • Individuals taking a moderate or strong cytochrome P450 enzyme (CYP) inducer including but not limited to rifampin, St John's Wort, carbamazepine, phenytoin, efavirenz, bosentan, etracirine, modafinil, and nafcillin within 2 weeks prior to the first dose of study drug

• Related to medical history:

  • Pregnant, breastfeeding, or lactating females
  • Individuals requiring > 50% supplemental oxygen (while the individual is awake) at screening
  • Individuals with a Clinical Frailty Scale (CFS) > 7 at Baseline
  • Known significant abnormality altering the anatomy of the nose or nasopharynx that in, the opinion of the investigator, will preclude obtaining adequate nasal swab sampling in either nasal passage
  • Waiting for or recently (within the past 12 months) received a bone marrow, stem cell, or solid organ transplant, or who have received radiation or chemotherapy within 12 months prior to Screening
  • Individuals with HIV/AIDS and a known CD4 count < 200 cells/uL
  • History of severe dementia or Alzheimer's disease
  • History of drug and/or alcohol abuse that, in the opinion of the investigator, may prevent adherence to study activities

• Related to medical condition at screening:

  • Influenza-positive as determined by local diagnostic test
  • Known Middle East Respiratory Syndrome coronavirus (MERS-CoV) infection or known coinfection with other coronavirus
  • Use of mechanical ventilation during the current admission, not including noninvasive ventilation
  • Clinically significant bacteremia or fungemia that has not been adequately treated prior to Screening, as determined by the investigator
  • Inadequate treatment of confirmed bacterial, fungal, or non-RSV pneumonia, as determined by the investigator
  • Excessive nausea/vomiting at admission, as determined by the investigator, that precludes administration of an orally administered study drug

• Related to allergies:

  • Known allergy to components of the study drug (microcrystalline cellulose, mannitol, croscarmellose sodium, magnesium stearate, polyvinyl alcohol, titanium dioxide, polyethylene glycol and talc)
  • Documented history of acute (anaphylaxis) or delayed (Stevens-Johnson syndrome or epidermal necrolysis) allergy to sulfa drugs

Note: Other protocol defined Inclusion/Exclusion criteria may apply.