Healthy adults will be enrolled into this open-label, Phase 1 research study. Participants will spend about 9 nights and 10 days in the clinical research unit (CRU) and the total time in the study will be about 11 weeks.

The goal of this clinical trial is to compare how much of the study drug ibuzatrelvir is in participants' blood after taking one of the two different types of tablets containing the same amount of ibuzatrelvir without food. The study will also measure how much ibuzatrelvir is in participants' blood after taking one type of the tablets dispersed in water and when one type of the tablets is taken with food. This study drug is taken by mouth.

This is a Phase 1, open-label, randomized, 4-period, 6-sequence crossover study in healthy adult participants. The goal of this clinical study is to compare how much of the oral (taken by mouth) study drug ibuzatrelvir is in participants' blood after taking one of the two different types of tablets containing the same amount of ibuzatrelvir without food (fasted). The study will also measure how much ibuzatrelvir is in participants' blood after taking one type of the tablets dispersed in water and when one type of the tablets is taken with food. Safety and tolerability after taking the study medication will also be assessed.

Healthy adult participants will be screened within 28 days prior to the first day of study treatment. Participants will take one dose of study drug on the first day of each of 4 dosing periods that are 2-3 days each. Participants will remain in the clinical research unit (CRU) from Day -1 of Period 1 until completion of procedures on Day 3 of Period 4. This means that participants will stay in the CRU for a total of 9 nights and 10 days. The follow-up phone call will take place approximately 28-35 days after the last dose of study drug. The total time in the study is about 10-11 weeks.

The study treatments are:

Treatment A: ibuzatrelvir (original) fasted Treatment B: ibuzatrelvir (new) fasted Treatment C: ibuzatrelvir (new as dispersion in water) fasted Treatment D: ibuzatrelvir (new) fed

This is a crossover study which means that the treatment groups will receive the same treatments (A, B, C, and D) in a different order during the 4 dosing periods.

Group Period 1 Period 2 Period 3 Period 4

1. A (fasted) B (fasted) C (fasted) D (Fed)
2. A (fasted) C (fasted) B (fasted) D (Fed)
3. B (fasted) A (fasted) C (fasted) D (Fed)
4. B (fasted) C (fasted) A (fasted) D (Fed)
5. C (fasted) A (fasted) B (fasted) D (Fed)
6. C (fasted) B (fasted) A (fasted) D (Fed)

During the screening and time in the CRU participants will have:

• Blood and urine samples collected for safety lab tests
• Pharmacokinetics (PK) blood samples collected to detect concentrations of ibuzatrelvir (up to 40 samples)
• Up to 8 electrocardiograms (ECGs - records the electrical signals controlling your heart)
• To fast (not eat) for at least 10 hours overnight before 3 of the 4 doses

Approximately 18 participants will be enrolled in the study.

Inclusion criteria:

1. 18 years of age 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, and ECGs.
2. BMI of 16-32 kg/m2; and a total body weight >45 kg.
3. Participants who are capable of giving signed informed consent and willing and able to comply with all scheduled visits, treatment plans, laboratory tests, lifestyle considerations, and other study procedures.

Exclusion Criteria

1. 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 condition possibly affecting drug absorption (eg, gastrectomy, cholecystectomy).
   • History of HIV infection, hepatitis B, or hepatitis C; positive testing for HIV, HBsAg, or HCVAb. Hepatitis B vaccination is allowed.

2. Any medical or psychiatric condition including any active suicidal ideation in the past year or suicidal behavior in the past 5 years 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.

3. Use of prescription or nonprescription drugs and dietary and herbal supplements within 14 days or 5 half-lives (whichever is longer) prior to the first dose of study intervention with the exception of moderate or strong CYP3A inducers which are prohibited within 14 days plus 5 half-lives prior to the first dose of study intervention.

4. Previous administration of an investigational product (drug or vaccine) within 30 days or 5 half-lives preceding the first dose of study intervention used in this study (whichever is longer).

5. Participation in studies of other investigational products (drug or vaccine) at any time during participation in this study.

6. A positive urine drug test. A single repeat for positive drug screen may be allowed.

7. Screening supine 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. If systolic BP is ≥140 or 150 mm Hg (based on age) or diastolic ≥90 mm Hg, the BP should be repeated 2 more times and the average of the 3 BP values should be used to determine the participant's eligibility.

8. An eGFR <60 mL/min/1.73 m² as determined by the CKD-EPI equation using Screat.

9. Standard 12 lead ECG that demonstrates clinically relevant abnormalities that may affect participant safety or interpretation of study results (including, but not limited to, QTcF >450 ms, complete LBBB, signs of an acute or indeterminate age myocardial infarction, ST segment and/or T wave changes suggestive of myocardial ischemia, second or third degree AV block, or serious bradyarrhythmias or tachyarrhythmias). If QTcF exceeds 450 ms, or QRS exceeds 120 ms, the ECG should be repeated twice and the average of the 3 QTcF or QRS values used to determine the participant's eligibility. Computer interpreted ECGs with abnormal findings should be overread by an investigator experienced in reading ECGs before excluding a participant.

10. Participants with ANY of the following abnormalities in clinical laboratory tests at screening, as assessed by the study specific laboratory and confirmed by a single repeat test, if deemed necessary:

    • ALT, AST, T Bili >1.5× ULN.

11. History of alcohol abuse or repeated binge drinking and/or any other illicit drug use or dependence within 6 months of screening. Binge drinking is defined as a pattern of 5 (male) and 4 (female) or more alcoholic drinks in about 2 hours. As a general rule, alcohol intake should not exceed 14 units per week (1 unit = 8 ounces (240 mL) beer, 1 ounce (30 mL) of 40% spirit, or 3 ounces (90 mL) of wine).

12. Investigator site staff directly involved in the conduct of the study and their family members, site staff otherwise supervised by the investigator, and sponsor and sponsor delegate employees directly involved in the conduct of the study and their family members.

13. Use of tobacco/nicotine containing products in excess of the equivalent of 5 cigarettes/day or 2 chews of tobacco/day.

14. History of sensitivity reactions to ibuzatrelvir or any of the formulation components of ibuzatrelvir.

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.