Utilization and safety of trastuzumab emtansine (T-DM1): a nationwide population-based study using the French National Health Data System.

Background: Trastuzumab emtansine (T-DM1), the first antibody-drug conjugate targeting human epidermal growth factor receptor 2 (HER2), has provided new alternatives for metastatic breast cancer (mBC) treatment, and for early-stage breast cancer (esBC) since 2020. We characterized T-DM1 users and assessed the risk of hospitalisation for certain adverse events. Materials and methods: Using data from the French National Health Data System (SNDS), we identified patients receiving T-DM1 from 2014 to 2022. Sociodemographic data, medical history, and overall survival were described by indication. The risk of adverse event-related hospitalisation was quantified. Results: We included 12 822 patients initiating T-DM1 treatment in the study: 9232 (72%) with mBC (median age: 59 years, 54.6% with at least one comorbid condition) and 3590 (28%) with esBC (median age: 54 years, 33.9% with at least one comorbid condition). In mBC, median overall survival was 32.6 months, showing a gradual improvement over the years. At 1 year, 75.8% of patients had discontinued treatment and 22.0% had died. Patients in the study faced higher hospitalisation risks than those with incident breast cancer, with increases of 5.7 and 10.5 times for thrombocytopenia, 3.0 and 4.5 times for interstitial lung disease, 3.3 and 5.9 times for acute liver injury, and 2.3 and 7.3 times for sepsis, for esBC and mBC, respectively. Conclusions: Real-world T-DM1 users frequently present with comorbid conditions and prior treatments, with a higher risk of hospitalisation for severe toxicity events than the general population of incident breast cancers. This study highlights the importance of monitoring treated patients to manage the risk of toxicity and prevent treatment discontinuation.