The study evaluated the safety and antitumor activity of pembrolizumab, a programmed cell death protein 1 (PD-1) inhibitor, in patients with advanced triple-negative breast cancer (TNBC). The KEYNOTE-012 trial was a multicenter, nonrandomized phase Ib study that included patients with PD-L1-positive TNBC, gastric cancer, urothelial cancer, and head and neck cancer. The TNBC cohort, which included 32 patients, received pembrolizumab intravenously at 10 mg/kg every 2 weeks. The median number of doses administered was five, and common toxicities were mild, similar to those observed in other tumor cohorts. Among the 27 patients evaluable for antitumor activity, the overall response rate was 18.5%, with a median time to response of 17.9 weeks and a median duration of response not yet reached. The study demonstrated an acceptable safety profile and preliminary evidence of clinical activity for pembrolizumab in heavily pretreated advanced TNBC. A phase II study is ongoing to further evaluate the efficacy of pembrolizumab in this patient population.The study evaluated the safety and antitumor activity of pembrolizumab, a programmed cell death protein 1 (PD-1) inhibitor, in patients with advanced triple-negative breast cancer (TNBC). The KEYNOTE-012 trial was a multicenter, nonrandomized phase Ib study that included patients with PD-L1-positive TNBC, gastric cancer, urothelial cancer, and head and neck cancer. The TNBC cohort, which included 32 patients, received pembrolizumab intravenously at 10 mg/kg every 2 weeks. The median number of doses administered was five, and common toxicities were mild, similar to those observed in other tumor cohorts. Among the 27 patients evaluable for antitumor activity, the overall response rate was 18.5%, with a median time to response of 17.9 weeks and a median duration of response not yet reached. The study demonstrated an acceptable safety profile and preliminary evidence of clinical activity for pembrolizumab in heavily pretreated advanced TNBC. A phase II study is ongoing to further evaluate the efficacy of pembrolizumab in this patient population.