Triple-Negative Metaplastic Breast Cancer Ameliorated Following KEYNOTE-522: A Case Report.

Metaplastic breast cancer (MpBC) is a rare and aggressive subtype of breast cancer, with the triple-negative variant (TN-MpBC) being particularly resistant to standard systemic therapies and associated with poor outcomes. We present the case of a 48-year-old African American female diagnosed with TN-MpBC, incidentally identified during cardiac evaluation for sarcoidosis. Imaging and biopsy revealed a 5.8 cm high-grade tumor with a Ki-67 index above 30%. The patient was treated with the KEYNOTE-522 regimen, which includes neoadjuvant chemotherapy (NAC) (paclitaxel, carboplatin, doxorubicin, and cyclophosphamide) and pembrolizumab immunotherapy. Post-treatment imaging demonstrated substantial tumor regression, and subsequent bilateral mastectomy confirmed a complete pathological response with no residual malignancy. She is currently undergoing adjuvant pembrolizumab and proton radiation therapy. Given the historically poor response of TN-MpBC to chemotherapy alone, this case illustrates the promising role of immunotherapy, particularly in programmed death-ligand 1 (PD-L1)-expressing tumors. It supports emerging evidence that chemoimmunotherapy combinations such as KEYNOTE-522 may improve prognosis in TN-MpBC, emphasizing the need for continued investigation into targeted therapeutic strategies for this challenging breast cancer subtype.