Standard of Care Chemotherapy Plus Pembrolizumab for Breast Cancer

In the pilot phase, patients will be enrolled to one of two experimental arms, which will be selected by the treating investigator (arm A: pembrolizumab + weekly paclitaxel; arm B: pembrolizumab + capecitabine). Subjects will receive pembrolizumab via intravenous (IV) infusion at 200mg every three weeks (Q3W), and continue treatment Q3W until progression of disease, initiation of alternative cancer therapy, unacceptable toxicity, or other reasons to discontinue treatment occur, up to 24 months. Paclitaxel will be administered intervenously on a weekly schedule at a dose of 80mg/m2. Oral capecitabine will be administered at total daily dose of 4,000 mg (2,000 mg two times each day (abbreviated BID)). Capecitabine will be administered as intermittent therapy given on days 1-7 in 14-day cycles.