The aim of this study was to evaluate patient profiles, treatment patterns, and outcomes of hormone receptor positive (HR+)/human epidermal growth factor receptor-2 negative (HER2-) metastatic breast cancer (mBC) patients treated with a 1L cyclin dependent kinase 4/6 inhibitor (CDK4/6i) in the real-world setting.

Inclusion criteria:

• A diagnosis of mBC based on at least two diagnoses of breast cancer (International Classification of Diseases, 10th Revision, Clinical Modification [ICD-10-CM] code C50.xxx) along with a diagnosis of secondary malignant neoplasm (ICD-10-CM codes C77.xxx - C80.xxx except C79.8 or C79.81, reporting of a metastatic site by curation), in the period prior to or on the index date.
• Patients with HR+/HER2- status.
• Use of either ribociclib, palbociclib, or abemaciclib in 1L treatment for mBC.
• Continuous care at the contributing practices. Patients with a minimum of two visits up to and including the index date.

Exclusion criteria:

• Use of any prior CDK4/6i before the index date.
• Prior 1L treatment, other than CDK4/6i, in mBC including treatment with endocrine therapy (ET) either with tamoxifen, aromatase inhibitors (AI; anastrozole, exemestane, or letrozole) or fulvestrant. For it to be considered 1L in mBC, the diagnosis of mBC must have preceded treatment initiation, or was indicated so in unstructured data.
• Diagnosis of cancer other than breast cancer and/or mBC on or prior to the index.
• Evidence of participation in a clinical trial at any time during the study observation period.

HR+/HER2- mBC patients who received 1L CDK4/6i treatment and have a medical record in the real-world evidence database.