Background: Immunotherapy has shown therapeutic promise in several cancers, including breast cancer. Monotherapy with anti-PD-1/anti-PD-L1 antibodies has demonstrated durable responses in patients with metastatic triple-negative breast cancer (mTNBC) (Nanda et al, JCO 2016) and metastatic estrogen receptor-positive (mER+)/HER2-negative breast cancer (Rugo et al, SABCS 2015). Furthermore, response rates have been increased with combination approaches with chemotherapy (Adams et al, ASCO 2016). Based on these results, we seek to study the anti-tumor efficacy and safety of pembrolizumab (anti-PD-1 antibody) and nab-paclitaxel, the impact of therapy on the tumor microenvironment, and predictive markers of response. Methods: This is an ongoing single-arm open-label multi-cohort phase II study of pembrolizumab and nab-paclitaxel in patients treated with ≤2 prior lines of therapy for metastatic HER2-negative breast cancer (n = 50) (ClinicalTrials.gov: NCT02752685). Thirty patients with mTNBC and 20 patients with mER+/HER2-negative breast cancer will be enrolled. Enrollment of patients with metaplastic breast cancer is encouraged. Patients will receive pembrolizumab 200 mg IV on day 1 plus nab-paclitaxel 100 mg/m² IV on day 1 and 8 (21-day cycle). Prior taxane therapy given > 3 months before cycle 1 is allowed. Primary objective is treatment efficacy, as determined by overall response rate (RECIST 1.1). Secondary objectives include safety, progression-free survival, overall survival, and duration of response. Serial tumor biopsies will be performed to assess changes in the tumor microenvironment from baseline with chemotherapy alone (cycle 1) and then with chemoimmunotherapy (cycle 2 and subsequent cycles). Mutational and neoantigen load, TILs by histopathological assessment, TCR by immunosequencing, and immune gene profiles in tumors will be evaluated. PD-L1 expression in tumor tissue is not required for enrollment but will be assessed as a predictive marker. The potential role of the gut microbiome in modulating the immune response will also be evaluated by 16S rRNA. An initial safety run-in with 12 subjects has been completed with no unexpected toxicity. Clinical trial information: NCT02752685