A Randomized, Multicenter, Double-blind Phase 3 Study of SAR439859 Plus Palbociclib Versus Letrozole Plus Palbociclib for the Treatment of Patients With ER (+), HER2 (-) Breast Cancer Who Have Not Received Prior Systemic Anti-cancer Treatment for Advanced Disease


To determine whether SAR439859 in combination with palbociclib improves progression-free survival (PFS) when compared with letrozole in combination with palbociclib in participants with ER+, HER2- advanced breast cancer who have not received any prior systemic anticancer therapies for advanced disease.

Study Arms

  • Experimental: SAR439859 with Letrozole-matching placebo Arm
  • Active Comparator: Letrozole with SAR439859-matching placebo Arm


  • Adult participants with loco-regional recurrent or metastatic disease not amenable to curative treatment
  • Confirmed diagnosis of ER+/HER2- breast cancer
  • No prior systemic treatment for loco-regional recurrent or metastatic disease
  • Measurable or non-measurable disease evaluable per Response Evaluation Criterion in Solid Tumors (RECIST) v.1.1.
  • Eastern Cooperative Oncology Group (ECOG) performance status 0-1.
  • Participants should be willing to provide tumor tissue
  • Capable of giving informed consent