Pembrolizumab for ER+/HER2− Metastatic Breast Cancer

Checkpoint inhibitors have had a clinical impact in several solid tumors, but to date their potential role in breast cancer has been less clear. In a prior study, the checkpoint inhibitor pembrolizumab demonstrated activity in previously treated patients with programmed death-ligand 1 (PD-L1)–positive, metastatic triple-negative breast cancer (KEYNOTE-012; J Clin Oncol 2016;34:2460).

Now, investigators have conducted an open-label phase 1b trial (KEYNOTE-028) to evaluate the safety and efficacy of pembrolizumab (10 mg/kg every 2 weeks) in 25 patients with PD-L1–positive, ER+/HER2− advanced breast cancer. Patients were heavily pretreated, with a median of nine prior therapies for breast cancer, including endocrine therapy.

With a median follow…