Background: Trastuzumab emtansine (T-DM1) treatment for human epidermal growth factor receptor-2 (HER2)-positive metastatic breast cancer after taxane with trastuzumab and pertuzumab is standard therapy. However, treatment strategies beyond T-DM1 are still in development with insufficient evidence of their effectiveness. Here, we aimed to evaluate real-world treatment choice and efficacy of treatments after T-DM1 for HER2-positive metastatic breast cancer. Methods: In this multi-centre retrospective cohort study involving 17 hospitals, 325 female HER2-positive metastatic breast cancer patients whose post-T-DM1 treatment began between April 15, 2014 and December 31, 2018 were enrolled. The primary end point was the objective response rate (ORR) of post-T-DM1 treatments. Secondary end points included disease control rate (DCR), progression-free survival (PFS), time to treatment failure (TTF), and overall survival (OS). Results: The median number of prior treatments of post-T-DM1 treatment was four. The types of post-T-DM1 treatments included (1) chemotherapy in combination with trastuzumab and pertuzumab (n = 102; 31.4%), (2) chemotherapy concomitant with trastuzumab (n = 78; 24.0%), (3), lapatinib with capecitabine (n = 63; 19.4%), and (4) others (n = 82; 25.2%). ORR was 22.8% [95% confidence interval (CI): 18.1–28.0], DCR = 66.6% (95% CI 60.8–72.0), median PFS = 6.1 months (95% CI 5.3–6.7), median TTF = 5.1 months (95% CI 4.4–5.6), and median OS = 23.7 months (95% CI 20.7–27.4). Conclusion: The benefits of treatments after T-DM1 are limited. Further investigation of new treatment strategies beyond T-DM1 is awaited for HER2-positive metastatic breast cancer patients.
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