The purpose of this study is to learn about how a class of medicines called CDK4/6 inhibitors, are used for the treatment of breast cancer in patients in Australia. The study looks at how the CDK4/6 inhibitor class of drugs are used for treating breast cancer that is advanced or metastatic (has spread to other parts of the body).

This study does not include patients. Instead, this study looks at already available data to describe characteristics (like age and sex) of patients who have already taken these medicines.

This study will focus on one of these medicines called palbociclib and will look at the data to learn what amounts of the study medicine (palbociclib) patients receive, and how long patients take it for in the real-world in Australia. This study will also learn what other anti-cancer medicines patients receive, including chemotherapy. Chemotherapy is the treatment that uses medicines to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing.

Inclusion Criteria:

• 18 years or older when first received CDK4/6i breast cancer treatment
• Patient who has received CDK4/6i during study period

Exclusion Criteria:

• There are no exclusion criteria for this study

Patients receiving CDK4/6i for the first time for HR+ HER2- metastatic breast cancer

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.