Despite the demonstrated safety and efficacy of pyrotinib and capecitabine in treating human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer, their efficacy in recurrent breast cancer in which the HER2 status has changed to negative remains unexplored. Here, we report a case of a 38-year-old female diagnosed with invasive ductal adenocarcinoma of the left breast, staged as mcT2N0M0. Fluorescence in situ hybridization (FISH) confirmed that the tumor was hormone receptor (HR) positive with low HER2 expression (2+) and a HER2/CEP17 ratio of 3.56. Following neoadjuvant targeted therapy and chemotherapy, she underwent a modified radical mastectomy. Post-surgical histopathological examination revealed a non-pathological complete response, classified as ypT1cypN1M0. The tumor remained HR positive with low HER2 expression (2+), but the FISH result was negative (HER2/CEP17 ratio of 1.65). For 1 year, she was administered dual-targeted therapy with goserelin and exemestane. Sequential therapy with neratinib was initiated; however, it was discontinued due to grade IV diarrhea. Despite ongoing endocrine therapy, she experienced tumor recurrence on the left chest wall. A biopsy of the recurrent lesion revealed it to be HR positive with low HER2 expression (2+) and a negative FISH result (HER2/CEP17 ratio of 1.33). The recurrent lesion responded to combination therapy consisting of pyrotinib and capecitabine, with tolerable adverse events. This case highlights the potential advantages of combining pyrotinib and capecitabine when the HER2 status changes to negative following systemic therapy.

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