This study will explore treatment patterns and clinical outcomes using the US-based Flatiron Health database to describe patients with HER2+ mBC who were previously treated with T-DXd to better characterize this population and inform internal decision making in this rapidly changing therapeutic landscape.

This study will utilize a longitudinal, demographically and geographically diverse database derived from Electronic Health Record data. No study medication will be supplied or administered for this protocol. The primary research objective is to describe the real-world progression-free survival (rwPFS) in HER2+ mBC patients who initiated a subsequent line of therapy (LOT) after a T-DXd-containing LOT in the metastatic setting.

Secondary research objectives include describing the real-world overall survival (rwOS), patient demographics and clinical characteristics, real-world time to next treatment (rwTTNT), and real-world time to treatment discontinuation (rwTTD) in HER2+ mBC patients who initiated a subsequent LOT after a T-DXd-containing LOT in the metastatic setting. Treatment patterns and sequencing in HER2+ mBC patients for subsequent LOTs following initial treatment with T-DXd will also be assessed.

Inclusion Criteria

Patients must meet all the inclusion criteria below to be included in the study population:

1. Patients with evidence of stage IV or recurrent mBC with a metastatic diagnosis date on or after January 1, 2011.
2. Patients aged ≥18 years old at mBC diagnosis date.
3. Patients with ≥1 T-DXd-containing LOT in the metastatic setting.
4. Patients with evidence of a subsequent LOT following the first T-DXd-containing LOT in the metastatic setting. The LOT subsequent to the first T-DXd-containing LOT in the metastatic setting will be the index LOT. Index date will be the index LOT start date.
5. Patients whose closest HER2 test result (defined as immunohistochemistry [IHC] 3+ or in situ hybridization [ISH]+) recorded prior to or on the index date was a HER2+ test result.
6. Patients with index date occurring at least 90 days prior to end of study period (March 31st, 2024).

Exclusion Criteria No exclusion criteria will be imposed.

The study population will consist of adults in the Flatiron mBC EDM who were diagnosed with HER2+ mBC during the study period and initiated a subsequent LOT after the initial T-DXd-containing LOT in the metastatic setting.