The KEYNOTE-012 phase Ib trial evaluated pembrolizumab, a PD-1 inhibitor, in patients with advanced triple-negative breast cancer (TNBC). The study enrolled 32 patients with PD-L1-positive TNBC, who received pembrolizumab at 10 mg/kg every 2 weeks. The overall response rate (ORR) was 18.5%, with one complete response and four partial responses. The median time to response was 17.9 weeks, and the median duration of response was not reached. Common adverse events were mild, with five patients experiencing grade 3 or higher toxicity and one treatment-related death. Pembrolizumab showed an acceptable safety profile and clinical activity in heavily pretreated, advanced TNBC. A phase II trial (KEYNOTE-086) is ongoing to further evaluate pembrolizumab in mTNBC. The study highlights the potential of immune checkpoint inhibitors in TNBC, particularly in PD-L1-positive patients. The results suggest that pembrolizumab may offer a new treatment option for patients with advanced TNBC who have limited therapeutic options. The study also underscores the importance of PD-L1 expression in predicting response to immunotherapy in TNBC.The KEYNOTE-012 phase Ib trial evaluated pembrolizumab, a PD-1 inhibitor, in patients with advanced triple-negative breast cancer (TNBC). The study enrolled 32 patients with PD-L1-positive TNBC, who received pembrolizumab at 10 mg/kg every 2 weeks. The overall response rate (ORR) was 18.5%, with one complete response and four partial responses. The median time to response was 17.9 weeks, and the median duration of response was not reached. Common adverse events were mild, with five patients experiencing grade 3 or higher toxicity and one treatment-related death. Pembrolizumab showed an acceptable safety profile and clinical activity in heavily pretreated, advanced TNBC. A phase II trial (KEYNOTE-086) is ongoing to further evaluate pembrolizumab in mTNBC. The study highlights the potential of immune checkpoint inhibitors in TNBC, particularly in PD-L1-positive patients. The results suggest that pembrolizumab may offer a new treatment option for patients with advanced TNBC who have limited therapeutic options. The study also underscores the importance of PD-L1 expression in predicting response to immunotherapy in TNBC.