Exceptional and Durable Complete Response to Ado-Trastuzumab Emtansine in HER2-Positive Metastatic Breast Cancer: A Case Report.

Ado-trastuzumab emtansine (T-DM1) is an established treatment for patients with human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer who have previously received trastuzumab-based therapy. However, complete and durable responses to T-DM1 are uncommon, and long-term complete remission has rarely been reported. We report the case of a 72-year-old woman with HER2-positive metastatic breast cancer who achieved a sustained complete response following T-DM1 therapy. The patient was initially diagnosed with left breast invasive ductal carcinoma with axillary and abdominal lymph node metastases. She received first-line chemotherapy with trastuzumab, pertuzumab, and docetaxel, followed by maintenance anti-HER2 therapy and surgery for local disease control. During maintenance therapy, chest wall recurrence with rib and mediastinal lymph node involvement was detected. Re-biopsy confirmed persistent HER2 overexpression. After progression on endocrine therapy, T-DM1 was initiated. Marked tumor regression was observed on CT imaging three months after initiation of T-DM1, and complete radiological remission has been maintained for more than five years with continuous T-DM1 treatment. Adverse events were limited to mild liver dysfunction, and no clinically significant thrombocytopenia occurred. This case may represent an exceptional response to T-DM1 and highlights the potential for durable complete remission in selected patients with HER2-positive metastatic breast cancer. Further investigation is warranted to identify predictive factors for exceptional responses and to determine optimal treatment duration.