A Randomized, Multicenter, Double-blind Phase 3 Study of Amcenestrant (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

Primary Objective: To determine whether Amcenestrant (SAR439859) in combination with palbociclib improves progression free survival (PFS) when compared with letrozole in combination with palbociclib in participants with estrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer who have not received any prior systemic anticancer therapies for advanced disease. Secondary Objective: * To compare the overall survival in both treatment arms. * To evaluate the objective response rate in both treatment arms. * To evaluate the duration of response in both treatment arms. * To evaluate the clinical benefit rate in both treatment arms. * To evaluate progression-free survival on next line of therapy. * To evaluate the pharmacokinetics of amcenestrant, and palbociclib. * To evaluate health-related quality of life in both treatment arms. * To evaluate the time to first chemotherapy in both treatment arms. * To evaluate safety in both treatment arms.