The purpose of this study is to see how rabeprazole affects the level of the study medicine sisunatovir in the blood of healthy adult participants. Rabeprazole is a medicine that reduces the amount of acid the stomach makes.

The information from this study may inform how sisunatovir will be used in the future with medicines that reduce stomach acid.

This study is seeking healthy participants who:

• are aged 18 years of age or older.
• are confirmed to be healthy by some medical tests. This study can include both men and women. Women who can produce a baby must agree to use a highly effective method of birth control.
• have body mass index (BMI) of 16 to 32 kilograms per meter squared.
• a total body weight of more than 45 kilograms. Participants will receive sisunatovir tablets by mouth on Day 1. After at least 48 hours, participants will take rabeprazole tablets by mouth once a day for 7 days. On the last (7th) day of rabeprazole dosing, another dose of sisunatovir tablets will be taken by mouth.

The study will look at the blood levels of sisunatovir with and without taking rabeprazole. This will help see how sisunatovir should be given in the future.

The total planned time of participation is about 10 to 11 weeks. The study consists of:

• screening period of up to 28 days before taking sisunatovir.
• 13 days of staying in the study clinic.
• a follow-up contact that will occur 28 to 35 days after taking sisunatovir the last time.

Inclusion Criteria:

• Male and female participants aged 18 years or older (or the minimum age of consent in accordance with local regulations) at screening who are overtly healthy as determined by medical evaluation including medical history, physical examination, laboratory tests, blood pressure, pulse rate and standard 12-lead electrocardiogram (ECG).
• Body mass index (BMI) of 16-32 kg/m2; and a total body weight >45 kg

Exclusion Criteria:

• Evidence or history of clinically significant hematological, renal, endocrine,pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, neurological, or allergic disease (including drug allergies, but excluding untreated, asymptomatic, seasonal allergies at the time of dosing).
• Any medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality or other conditions that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study.
• Previous administration with an investigational product (drug or vaccine) within 30 days (or as determined by the local requirement) or 5 half-lives preceding the first dose of study intervention used in this study (whichever is longer). Participation in studies of other investigational products (drug or vaccine) at any time during their participation in this study.
• A positive urine drug test at screening or admission.
• A positive serum pregnancy test at screening and/or positive urine/serum pregnancy test in woman/women of childbearing potential (WOCBP) at Day -1
• Current use of any prohibited concomitant medication(s) for sisunatovir and/or medication(s) that are contraindicated in the rabeprazole prescribing information.
• Use of tobacco/nicotine containing products
• Screening supine blood pressure (BP) ≥140 mm Hg (systolic) or ≥90 mm Hg (diastolic) for participants <60 years; and ≥150/90 mm Hg for participants ≥60 years old, following at least 5 minutes of supine rest.
• Renal impairment as defined by an estimated glomerular filtration rate (eGFR) (units of mL/min/1.73m²) <60 mL/min(/1.73m²) based on 2021 chronic kidney disease epidemiology (CKD-EPI).
• Standard 12-lead ECG that demonstrates clinically relevant abnormalities that may affect participant safety or interpretation of study results
• Alanine aminotransferase (ALT), aspartate aminotransferase (AST), bilirubin (Bili) ≥ 1.05 × upper limit of normal (ULN). Participants with an elevated total bilirubin consistent with Gilbert's Disease should have a direct bilirubin measured and would be eligible for this study provided the direct bilirubin level is ≤ ULN.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.