Real World-clinical Outcomes of Tucatinib, Trastuzumab, and Capecitabine Following Trastuzumab Deruxtecan (T-DXd) for the Treatment of HER2-positive Metastatic Breast Cancer.

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