Long-Term Response to Pembrolizumab in Metastatic Metaplastic Breast Carcinoma: A Case Report

BACKGROUND Metaplastic breast carcinoma (MpBC), a rare subtype of breast malignancy characterized by low response rates to conventional chemotherapy, often presents as triple-negative breast cancer (TNBC). Recently, systemic therapy including immune checkpoint inhibitors (ICIs) has emerged as a promising treatment option for TNBCs in perioperative or metastatic settings. However, evidence regarding the efficacy of ICIs for MpBC remains limited. CASE REPORT A 52-year-old Japanese woman underwent radical surgery for primary MpBC of the breast, which confirmed the triple-negative subtype. Despite adjuvant chemotherapy with epirubicin and cyclophosphamide, metastases to the sternum and lungs developed during treatment. Because the initial surgical specimen was PD-L1-positive, the treatment was changed to chemoimmunotherapy consisting of pembrolizumab, gemcitabine, and carboplatin based on the KEYNOTE-355 trial. Although the precordial mass and multiple lung metastases temporarily increased in size soon after treatment initiation, the metastatic lesions subsequently showed marked shrinkage. The patient developed treatment-related adrenal insufficiency, rash, pruritus, and anemia, all categorized as Grade 2. Hydrocortisone replacement therapy was initiated for adrenal insufficiency, while pembrolizumab was continued without interruption. Ten months after treatment initiation, carboplatin plus gemcitabine was discontinued because of adverse events, including myelosuppression, and the patient was transitioned to pembrolizumab monotherapy. At 30 months after the initiation of chemoimmunotherapy, PET/CT showed only mild ¹⁸F-fluorodeoxyglucose uptake in the sternal metastasis, with no evidence of disease progression. CONCLUSIONS This report describes the successful treatment of metastatic MpBC with pembrolizumab-containing chemoimmunotherapy. Pembrolizumab combination therapy may be a promising treatment option for metastatic MpBC.