Human Epidermal Growth Factor Receptor Type 2 (HER2)-Positive Metastatic Breast Cancer With HER2-Negative Conversion: A Case Report and Treatment With Cyclin-Dependent Kinase (CDK) 4/6 Inhibitor for the Luminal Type

A 37-year-old woman was diagnosed with stage IIIA, estrogen receptor (ER)-positive/human epidermal growth factor receptor type 2 (HER2)-positive breast cancer. The patient received neoadjuvant chemotherapy with epirubicin and cyclophosphamide, followed by docetaxel, trastuzumab, and pertuzumab. The surgical specimen remained ER-positive/HER2-positive. Liver metastasis was detected after the completion of postoperative adjuvant trastuzumab and pertuzumab. A liver biopsy following treatment with trastuzumab emtansine (T-DM1) and trastuzumab deruxtecan (T-Dxd) revealed HER2-negative status. Cyclin-dependent kinase (CDK) 4/6 inhibitor combination endocrine therapy has been continued for 16 months to date while maintaining tumor shrinkage. It is essential to perform a rebiopsy during treatment to optimize therapy based on the subtype.