A retrospective study of de-identified (to preserve patient privacy) patient information from the Flatiron Health Database to describe 3L and 4L real-world (rw) outcomes (e.g. time to next treatment, time to discontinuation, and overal survival) of the tucatinib-trastuzumab-capecitabine triplet therapy immediately following T-DXd therapy in patients diagnosed with HER2+ metastatic breast cancer (mBC) in the United States.

Inclusion Criteria:

Patients must meet all of the following inclusion criteria to be eligible for inclusion in the study:

1. mBC diagnosis between January 1, 2017 and 6 months prior to database cutoff
2. Evidence of human epidermal growth factor receptor 2 (HER2) receptor positivity prior to or up to 90 days following the mBC diagnosis date
3. At least 18 years old on the mBC diagnosis date
4. Treated with systemic anticancer treatment in the metastatic setting, i.e., post mBC diagnosis.
5. Treated with T-DXd in the 1L or 2L or 3L setting, followed immediately by the tucatinib triplet, and at least 1 additional prior HER2-targeted regimen

Exclusion Criteria:

Patients meeting any of the following criteria will not be included in the study:

1. Patients with evidence of other cancers six months prior to the mBC diagnosis date will be identified with the following International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) and International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) codes in all databases: 140.xx - 195.xx, 200.xx - 208.xx, C00.xx - C76.xx, C81.xx - C96.xx. (exclude - ICD-9: 174.X, 175.X; ICD-10: C50.XX)
2. Patients treated with tucatinib prior to T-DXd
3. Patients participating in clinical trials
4. Patients receiving concomitant endocrine therapy on index line

The study will include participants in the US Flatiron Dataset diagnosed with HER2+ mBC treated with the tucatinib triplet in the post-T-DXd setting.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.