Response to trastuzumab-deruxtecan in metastatic triple-negative breast cancer with both HER2 mutation and low expression.

Breast cancer is one of the most prevalent and heterogeneous cancers worldwide; however, it remains a complex and enigmatic disease. Advances in targeted therapies combined with increasing knowledge about tumor subtypes and molecular profiling have placed precision medicine on the frontier of oncological treatments. The recent classification of certain HER2 tumors as HER2-low, along with emerging evidence from studies using antibody-drug conjugates to target specific tumor profiles, highlights the relevance of this case study. This report describes the treatment of a 72-year-old woman diagnosed with metastatic triple-negative breast cancer, classified as HER2-low with specific HER2 mutations. The patient showed disease progression despite standard treatments, leading to the use of trastuzumab deruxtecan (T-DXd), an antibody-drug conjugate targeting HER2, as a third-line therapy. Her positive response, which surpassed the median progression-free survival rates seen in clinical trials, highlights the potential of precision medicine, where treatment is customized based on genetic and molecular tumor profiles. These new therapeutic options for difficult-to-treat cancer subtypes expand treatment possibilities, ushering in a new era of personalized and precision-driven oncology.