This study aims to assess the treatment patterns and real-world outcomes of HR+/HER- metastatic breast cancer patients who have progressed on 1L ET + CDK4/6i and started a second line (2L) treatment within a real-world cohort in the United States Flatiron Health Database.

This analysis will assess real-world outcomes in HR+/HER2- mBC patients who initiated a subsequent line of therapy (LOT) (i.e., 2L) after progressing on 1L treatment with ET+CDK4/6i. This study will also characterize the treatment patterns, clinical characteristics, and outcomes in this patient population. No study drug was administered in this non-interventional study.

Inclusion criteria

• Patients with evidence of Stage IV or recurrent mBC with a metastatic diagnosis date on or after January 1, 2017
• Aged ≥18 years at mBC diagnosis date
• Received 1L ET + CDK 4/6i, with no other systemic agents, in the metastatic setting
• Had evidence of a rwP event during 1L ET + CDK 4/6i
• Evidence of a subsequent LOT following 1L ET + CDK 4/6i (start date of subsequent LOT = index treatment date)
• A record of HR+ status in the closest estrogen receptor (ER) or progesterone receptor (PR) test result occurring prior to or within 30 days after the index treatment date
• A record of HER2- status in the closest HER2 test result recorded prior to or on the index treatment date
• Index treatment date occurring at least 90 days prior to end of study period (30Sep2024)

Exclusion Criteria

• A record of estrogen receptor 1 (ESR1), phosphatidylinositol4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), V-akt murine thymoma viral oncogene homolog (AKT1), phosphatase and tensin homolog (PTEN), or germline breast cancer gene (gBRCA) alteration/mutation prior to or within 30 days after index treatment date

The study cohort consists of adults in the Flatiron database who are diagnosed with HR+/HER2- mBC and initiated a subsequent LOT (i.e., 2L) after progressing on 1L treatment with ET+CDK4/6i.