Outcomes in KEYNOTE-355 after chemotherapy discontinuation before pembrolizumab discontinuation and with immune-mediated adverse events

Pembrolizumab plus chemotherapy significantly improved progression-free survival (PFS) and overall survival (OS) versus chemotherapy in the phase 3 randomized KEYNOTE-355 study in participants with previously untreated, advanced triple-negative breast cancer (TNBC) with PD-L1 combined positive score (CPS) ≥ 10. Among participants with clinical benefit from pembrolizumab plus chemotherapy, discontinuation of chemotherapy before pembrolizumab for any reason was associated with similar median PFS and OS to that in the overall population. Additionally, we further characterized immune-mediated adverse events (AEs) in KEYNOTE-355. Most immune-mediated AEs were manageable with treatment interruption and supportive care. In participants treated with pembrolizumab plus chemotherapy with immune-mediated AEs, there appeared to be numerical improvement in efficacy. This analysis provides additional information on the use of pembrolizumab plus chemotherapy as standard-of-care therapy in patients with previously untreated, advanced TNBC with PD-L1 CPS ≥ 10. This trial is registered at ClinicalTrials.gov (NCT02819518; registration date, June 28, 2016).