Real-life data on adjuvant trastuzumab emtansine treatment in early-stage HER2-positive breast cancer: a Turkish Oncology Group study.

In this Turkish Oncology Group study, we aimed to evaluate the effectiveness of trastuzumab emtansine (TDM1) in the adjuvant treatment of early-stage human epidermal growth factor receptor-2 (HER2)-positive breast cancer with residual disease using real-life data. A total of 13 Turkish centers participated in the study between September 2019 and October 2024. Patients with early-stage HER2-positive breast cancer who underwent surgery after completing neoadjuvant chemotherapy with HER2-targeted therapies had residual invasive disease in the breast or axillary lymph nodes and received adjuvant TDM1 therapy. The patients' files were retrospectively scanned from the hospitals' archives. The study included 79 female patients. The 36-month median disease-free survival rate was 92%, and the 36-month median overall survival rate was 85%. Neoadjuvant anthracyclines were administered to 93.6% of the patients. All patients received neoadjuvant trastuzumab and 86.1% of patients received neoadjuvant pertuzumab in addition to trastuzumab. Twelve (15.2%) patients developed progression during or after adjuvant TDM1 therapy. The most common adverse events were grade 1 fatigue (34.2%), grade 1 anemia (27.8%), and grade 1 AST increase (25.3%). Toxicity of grade 3 or above developed in five (5%) patients. TDM1 was stopped for one patient due to thrombocytopenia and for two patients due to cardiotoxicity. This study describes the sociodemographic and clinicopathological characteristics of patients with early-stage HER2-positive breast cancer with residual disease after neoadjuvant therapy and provides real-life data on treatment with adjuvant TDM1. The findings support the manageable safety profile of the adjuvant TDM1 regimen with a low discontinuation rate.