Letrozole Plus Ribociclib or Placebo as Neo-adjuvant Therapy in ER-positive, HER2-negative Early Breast Cancer

Key Inclusion Criteria: * Pathologically confirmed invasive breast cancer by core needle biopsy * Female subjects, age ≥ 18 years * Only postmenopausal women will be eligible. * Performance Status: Eastern Cooperative Oncology Group (ECOG) score 0-2 * Invasive breast cancer must be ER+ in ≥66 % of the cells or ER Allred score 6-8 * Invasive breast cancer must be HER2 negative. * Clinical Stage II or III (by clinical measurement and/or breast imaging) Key Exclusion Criteria: * Subjects meeting any of the exclusion criteria at baseline will be excluded from study participation. * Current use of other investigational agents * Inflammatory breast cancer defined as clinically significant erythema of the breast and/or documented dermal lymphatic invasion (not direct skin invasion by tumor or peau d'orange without erythema) * An excisional biopsy of this breast cancer * Surgical axillary staging procedure prior to study entry * Hormone replacement therapy of any type, megestrol acetate, or raloxifene within four weeks prior to first study treatment * Clinical or radiographic evidence of metastatic disease * Clinically significant, uncontrolled heart disease * Herbal preparations/medications as listed in Appendix B of the protocol