Real-World Evaluation of Trastuzumab Emtansine Biosimilar in Early-Stage HER2-Positive Breast Cancer: Results from a Single-Center Retrospective Study in India

Background: Trastuzumab emtansine (T-DM1), an antibody-drug conjugate targeting HER2, is an established adjuvant therapy for residual invasive HER2-positive breast cancer after neoadjuvant treatment. Real-world evidence on trastuzumab emtansine biosimilars in early-stage disease is limited. This study evaluated invasive disease-free survival (iDFS) and safety outcomes with a trastuzumab emtansine biosimilar in routine practice. Methods: This retrospective, single-center observational study include 24 women with stage II and III HER2-positive breast cancer who received adjuvant trastuzumab emtansine biosimilar (3.6 mg/kg every 21 days) after neoadjuvant chemotherapy and surgery. The primary endpoint was iDFS; secondary endpoints were relapse patterns and treatment-emergent adverse events. Results: Median age was 60.3 years; 83.3% were postmenopausal. At diagnosis, 20.8% had stage II and 79.2% had stage III disease. With a median follow-up of 14 months, 5 iDFS events occurred and no deaths were reported. The 12-month iDFS rate was 79.2%. Relapse occurred in 20.8% (5/24 patients); sites of recurrence included bone (n=2, 40%), liver (n=2, 40%), and brain (n=1, 20%), reflecting a predominantly skeletal and visceral, distant metastatic pattern. Common adverse events were thrombocytopenia (95.8%), neutropenia (79.2%), and anemia (41.7%), predominantly grade 1–2. Left ventricular ejection fraction declined to <50% in 20.8%. Conclusion: Adjuvant trastuzumab emtansine biosimilar use was feasible with a manageable safety profile in this single-center real-world cohort. Efficacy findings are exploratory and require confirmation in larger prospective studies.