EILEEN was a non-interventional/observational, retrospective, multi-center, real life cohort study conducted in 14 private and academic oncology clinics in Turkey. Group I cohort of the study was based on secondary use of data of postmenopausal hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (MBC) patients treated with ribociclib in combination with letrozole (LET) or fulvestrant (FUL) after June 2020. Group II cohort of the study was a parallel, comprehensive chart review for detecting all postmenopausal HR+, HER2- MBC patients who were eligible for cyclin-dependent kinase inhibitors (CDKis) but received chemotherapy. The study used secondary data which was retrieved from electronic or paper medical records or clinical databases available at the sites. Regular follow up with close monitorization was used for the effective management of patients with breast cancer. Data sources included information about diagnosis, treatment and monitorization of patients at an individual level. The study used medical patient records at hospitals e.g. hospital discharge files, primary clinical records and electronic medical records.

Inclusion Criteria:

Group I: Postmenopausal female patients who had histologically or cytologically confirmed HR-positive (10% or more ER+), HER2-negative, metastatic breast cancer and initiated ribociclib combinations per local label.

• Treated with ribociclib plus letrozole relapsed after 12 months completing adjuvant treatment or without any prior endocrine treatment for metastatic disease, 1 line or less chemotherapy was allowed.
• Treated with ribociclib plus fulvestrant, clinical and/or radiological disease progression after at least 6 months and at least 1 aromatase inhibitor treatment for metastatic disease or relapsed after 12 months adjuvant aromatase inhibitor treatment and/or relapsed within 12 months completing adjuvant aromatase inhibitor, 2 lines or less prior treatments and 1 line or less prior chemotherapy at metastatic setting were allowed.
• Had first treatment response rate assessment within 6 months after ribociclib initiation.
• Initiated ribociclib in combination with letrozole/fulvestrant after MBC diagnosis after local approval of ribociclib in June 2020.

Group II: Postmenopausal female patients who had histologically or cytologically confirmed HR-positive (10% or more ER+), HER2-negative, metastatic breast cancer eligible for cyclin-dependent kinase (CDK) inhibitors according to local label but started chemotherapy.

Exclusion Criteria:

• Pre- or perimenopausal women.
• Enrollment in an interventional clinical trial for MBC during the study observation period.
• Evidence of prior treatment with any CDK4/6 inhibitor in the adjuvant setting.
• Evidence of another primary cancer within 3 years prior to the initial line containing ribociclib.

Postmenopausal female patients who had histologically or cytologically confirmed HR-positive (10% or more estrogen receptor-positive [ER+]), HER2-negative, metastatic breast cancer

This was a retrospective, noninterventional cohort study.