A PHASE 1, RANDOMIZED, SPONSOR OPEN, TWO-PART CROSSOVER STUDY TO ASSESS SAFETY, TOLERABILITY, PHARMACOKINETICS AND FOOD EFFECT OF MULTIPLE DOSES IN PART 1 AND PALATABILITY OF A SINGLE DOSE OF SISUNATOVIR IN PART 2, IN HEALTHY ADULT PARTICIPANTS

Brief Summary This study is seeking healthy participants who are: 1. Aged 18 to 65 years of age. All fertile participants must agree to use a highly effective method of contraception. 2. Male and female participants who are overtly healthy as determined by medical evaluation. This includes medical history, physical examination, blood pressure, pulse rate, standard 12-lead ECG (electrocardiogram), and laboratory tests. 3. BMI (body mass index) of 17.5 to 35 kg/m2; and a total body weight \>50 kg (110 lb). This study will consist of up to 2 cohorts (groups of participants).: Cohort 1 is a randomized, 2-part, crossover cohort. Part 1 has 3 periods. Periods 1 and 2 are to evaluate the safety and effects of sisunatovir. Participants will take sisunatovir tablets or placebo by mouth once every 12 hours. A placebo looks like the study medicine but does not contain any active medicine in it. Period 3 is an open label period to evaluate the food effect of the planned higher dose of sisunatovir. Participants will take the planned higher dose sisunatovir tablets every 12 hours. In Part 2, participants will take sisunatovir prepared in 4 different vehicles (water, infant formula, apple juice, and saline) to assess how palatable each form is. Participants will complete a questionnaire after tasting each form of sisunatovir. The palatability questionnaire will be completed for each vehicle, the questionnaire asks participants to assess each vehicle at 4 different time increments after tasting. At least 60 minutes will pass between tasting each vehicle. Period 4 may start after the last PK draw of Period 3. A minimum 7-day washout period will occur between the last dose of Periods 1 and 2 and the first dose of Periods 2 and 3. We will assess the safety of participants following each study period and doses and adjust the doses for subsequent study periods as needed. Cohort 2 is an optional cohort; the design of Cohort 2 is the same as Part 1 of Cohort 1. Dose levels studied in Cohort 2 will be determined after the completion of Cohort 1. Participants will take part in this study for approximately 2 months, excluding the screening period. During this time there are 3 separate 7-day in-patient stays at the study clinic and a follow-up phone call that takes place 28-35 days after the last dose of study medicine.