The purpose of this study to understand how the study medicine atirmociclib behaves in the body and how safe it is for people with different levels of liver health.

The study is seeking participants who are:

• Aged 18 years or older
• Have a BMI of 17.5 to 40 kg/m2, and body weight more than 50 kg.
• Have normal liver function or have stable mild, moderate, or severe liver disease

Participants will receive study medicine by mouth. Blood samples will be collected to see how much of the medicine is in the body over time. This will help understand whether liver disease changes how the medicine is handled.

Participants will be in the study for about 9 weeks. Those with normal liver function will stay in the clinic for about 7 days. People with liver disease will stay for about 9 days. After leaving the clinic, people may be contacted for follow-up. This may be done by phone or through a clinic visit.

Inclusion criteria including but not limited to:

• BMI of 17.5-40 kg/m2; and a total body weight >50 kg (110 lb).
• Willing and able to comply with all scheduled visits, treatment plan, laboratory tests, lifestyle considerations, and other study procedures.
• Capable of giving signed informed consent.

Normal hepatic function group only:

- Overtly healthy as determined by medical evaluations including medical history, physical examination, laboratory tests, vital signs and standard 12-lead ECGs.

Hepatic impairment groups only:

- Stable hepatic impairment meeting the criteria for Child-Pugh Class A, B, or C . Stable hepatic impairment is defined as no clinically significant change in disease status within the last 28 days prior to the screening visit, as documented by the participant's recent medical history.

Exclusion criteria including but not limited to:

• Any condition or history of surgery possibly affecting drug absorption.
• Use of prohibited prior or concomitant medications.

Normal hepatic function group only:

- Evidence or history of clinically significant hematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, psychiatric, neurological, or allergic disease (including drug allergies, but excluding untreated, asymptomatic, seasonal allergies at the time of dosing).

Hepatic impairment groups only:

- A diagnosis of hepatic dysfunction secondary to any acute ongoing hepatocellular process that is documented by medical history, PE, liver biopsy, hepatic ultrasound, CT scan, or MRI.

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.