This is a retrospective cohort study to assess the real-world analysis of concomitant medication use among metastatic breast cancer patients treated with CDK4/6 inhibitors utilizing the US Optum research administrative claims database.

This is a retrospective cohort study utilizing the US Optum research administrative claims database. Adult female patients who had HR+/HER2- mBC were identified and included in the study. These patients were stratified into three cohorts based on the first CDK4/6i received (ribociclib, palbociclib, or abemaciclib).

• Study period: Period during which all relevant medical and pharmacy information in the database was retrieved
• Index event identification period: time frame during which index treatment was identified
• First CDK4/6i: the date on which patients initiated their first treatment after being diagnosed with HR+/HER2- mBC; this date was considered as the index date
• Baseline period: Time frame during which patient characteristics, prior mBC treatments, and concomitant treatments received by CDK4/6i-naïve patients were evaluated
• Follow-up period: time frame on and after the index date during which patients' treatments and dosing patterns were observed

Establishment of baseline period Health plan enrollment in the three months prior to the index treatment was included in the baseline period. Patient demographics, clinical characteristics, National Cancer Institute (NCI) comorbidity index, concomitant medication use that can potentially lead to a DDI, and other BC treatments prior to the index treatment were evaluated during this period.

Establishment of follow-up period Patients were observed for ≥3 months starting from the index date until the end of the study period or continuous enrollment or until the time when patients switched to another therapy (including another CDK4/6i), whichever came first. Dosing patterns for CDK4/6i were evaluated in the follow-up period. Analyses were conducted at key time points of 3, 6, and 12 months

Inclusion Criteria:

• Treated with CDK4/6i from 2015-02-01 to 2019-06-30
• ≥2 claims with a BC diagnosis recorded ≥30 days apart from the index date
• ≥2 claims with an mBC diagnosis, with a time interval between the first BC diagnosis date and the first mBC diagnosis date no longer than 30 days
• Female patients aged ≥18 years
• Continuous health plan enrollment for ≥3 months prior to the index date and ≥3 months post-index date

Exclusion Criteria:

• None

Adult women with HR+/HER2- mBC

Study analyses were conducted among adult women with HR+/HER2- mBC receiving ribociclib, palbociclib, or abemaciclib, regardless of the line of therapy. Patients were identified from the US Optum Claims Data.