KEYNOTE-B49 — Pembrolizumab plus chemotherapy vs placebo plus chemotherapy for HR-positive/HER2-negative metastatic breast cancer

Phase 3 · Breast Cancer

Population

HR-positive/HER2-negative locally recurrent inoperable or metastatic breast cancer after progression on endocrine therapy, receiving first-line chemotherapy.

Primary Endpoint

Progression-free survival and overall survival (by PD-L1 CPS)

Rationale

Immunotherapy is established in triple-negative breast cancer but its role in HR-positive/HER2-negative disease is unproven. KEYNOTE-B49 tests whether adding pembrolizumab to first-line chemotherapy improves outcomes once endocrine options are exhausted, and whether PD-L1 expression identifies who benefits.

What It Could Change

A positive result would extend checkpoint immunotherapy into the largest breast cancer subtype for patients who have progressed beyond endocrine therapy.