Phase 2 Window Study of SAR439859 (Amcenestrant) Versus Letrozole in Post-menopausal Patients With ER+, HER2- Pre-operative Post-menopausal Primary Breast Cancer

Inclusion criteria : * Histological or cytological proven diagnosis of invasive breast adenocarcinoma. * Localized breast cancer eligible for upfront breast conservative surgery or upfront mastectomy: Stage I, Stage II or operable Stage III (excluded T4) as defined in American Joint Committee on Cancer (AJCC) Cancer Staging Manual 8th edition 2017. * Postmenopausal women as defined by one of the following: * Spontaneous cessation of menses greater than (\>) 12 months. * or who had received hormonal replacement therapy but had discontinued the treatment and had follicle stimulating hormone (FSH) level in the postmenopausal range. * or with status post bilateral surgical oophorectomy. * or post bilateral ovarian ablation through pelvic radiotherapy. * Breast tumor size of at least 10 millimeters (mm) in greatest dimension measured by ultrasound. * Primary tumor had to be positive for Estrogen Receptors (ER+) and negative for HER2 (HER2-) receptor by immunohistochemistry. * Ki67 level of at least 15% at diagnosis from immunohistochemistry of the tumor. * Eastern Cooperative Oncology Group (ECOG) performance status 0-1. Exclusion criteria: * Medical history or ongoing gastrointestinal disorders potentially affecting the absorption of SAR439859 or letrozole. * Participants unable to swallow normally and to take capsules or tablets. * Participants with known active hepatitis A, B, C infection; or hepatic cirrhosis. * Participant with any other cancer; adequately treated basal ce…