The purpose of this clinical trial is to learn about the safety, effects, and tolerability of the study medicine (sisunatovir).

This study is looking for healthy adult participants who meet the following criteria:

1. Males age 18 to 55 years
2. All fertile participants must agree to the use of highly effective contraception
3. Body mass index (BMI) of 18 to 32.0 kg/m2; body weight of 55.0 to 100.0 kg.
4. 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.

This study will enroll up to 10 participants. All study participants will receive 1 dose of sisunatovir by mouth. The study duration is expected to be about 9 weeks. This includes a 28-day screening period, 16-day inpatient stay, and 2 overnight follow-up visits, if needed.

Inclusion Criteria (key):

• Males
• Age 18- 55 years
• Body mass index (BMI) 18.0 kg/m2 to 32.0 kg/m2
• Body weight 55.0 kg to 100.0 kg
• Considered to be in good health
• Nonsmoker
• Content of 14C in urine, whole blood, and/or plasma samples does not exceed the general environmental background level of 14C

Exclusion Criteria (key):

• Clinically significant abnormal medical history or any abnormal finding on physical examination, vital signs, ECG, laboratory tests
• History of cancer that has not been in full remission for >5 years
• Acute illness within 14 days prior to Day 1
• History of significant drug allergies
• History or presence of alcohol or drug abuse
• Recent history of incomplete bladder emptying with voiding or awakening more than once at night to void
• Usual habit of <1 or >3 bowel movements per day
• Exposure to radiation for therapeutic, diagnostic, or occupational reasons within the 12 months prior to Day 1.
• Participation in another clinical study in which a [14C]-labeled drug was administered within 1 year prior to Check-in
• Administration of another investigational medication within 60 days (or 5 half-lives, whichever is longer) prior to Day 1
• Participation in an investigational-device study within 60 days prior to Day 1
• Any ECG abnormality considered to be clinically significant by the Investigator
• QTcF interval >450 msec and QRS interval >120 msec
• Family history of long QT syndrome or of unexplained sudden death in a first-degree relative under 50 years of age
• Documented congenital or acquired long QT syndrome
• Presence of clinically significant hypertension
• Presence of clinically significant hypotension
• Loss or donation of more than 500 mL blood within 60 days prior to the Screening
• Excessive consumption of alcohol
• Use of any live vaccinations within 30 days
• Use/consumption of food or drugs known to be moderate or strong cytochrome P450 3A4 (CYP3A4) inducers or potent CYP3A4 inhibitors within 14 days or 5 half-lives, whichever is longer, prior to RV521 administration

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.