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.