The purpose of this clinical trial is to learn about the dose proportionality on the PK of the study medicine (called atirmociclib) when administered in the various doses range under the fed condition in healthy participants.

This study is seeking participants who are:

1. male and female aged 18 to 65 years are healthy as determined by medical evaluation including medical history, physical examination, and laboratory tests
2. with BMI of 17.5-30.5 kg/m2; and a total body weight >50 kgs (110 lbs.).

All participants (72 total) in this study will receive atirmociclib at Dose (A), Dose (B), Dose (C), and Dose (D) oral dose in 1 of the 12 treatment sequences among 6 cohorts under fed conditions.

Atirmociclib will be given by mouth at the study research unit once single dose about 30 minutes after a moderate fat standard calorie meal.

Dose proportionality will be evaluated on the pharmacokinetics (PK), safety and tolerability of atirmociclib at Doses (A), (B), (C), and (D) oral dose under the fed condition.

Including the 28 days of screening window and the 35 days safety follow-up period, the total study duration for each participant can be up to 71 days, containing 2 periods (6 days for each period), minimum 7-day interval between two periods, and follow-up period 28 to 35 days from administration of the final dose of study intervention. During this time, they will undergo safety laboratory and serial blood PK samplings up to 120 hours after administration of atirmociclib to determine plasma concentrations of atirmociclib. Participants will be discharged from the research unit on Period 2 Day 6 following completion of all assessments.

Inclusion:

• Participants who are overtly healthy as determined by medical evaluation including medical history, physical examination, and laboratory tests.
• Body mass Index (BMI) of 17.5-30.5 kg/m2; and a total body weight >50 kg (110 lb.).

Exclusion:

• 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 (e.g., gastrectomy, cholecystectomy).
• History of human immunodeficiency virus (HIV) infection, hepatitis B, or hepatitis C; positive testing for HIV, hepatitis B surface antigen (HBsAg), or hepatitis C antibody (HCVAb). Hepatitis B vaccination is allowed.
• Concomitant use of any medications or substances that are strong inducers or inhibitors of CYP3A4 or UGT2B7 are prohibited within 5 half-lives plus 14 days (up to 28 days) prior to first dose of atirmociclib.
• Previous exposure to atirmociclib or participation in studies requiring atirmociclib 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.