TEEL Study- Phase 1 Tamoxifen and Ribociclib (LEE011) in Advanced ER+ (HER2 Negative) Breast Cancer

Phase I Dose Escalation: The phase I portion of the study is a dose escalation to confirm the safety of the combination and to determine the Maximum Tolerated Dose (MTD) and the Recommended Phase II Dose (RP2D) for ribociclib with Tamoxifen. Phase I will be conducted in all participants with Hormone Receptor Positive (HR+)/Human Epidermal growth factor Receptor 2 Negative (HER2-) locally advanced or metastatic breast cancer with any prior endocrine therapy and up to three prior cytotoxic chemotherapy regimens administered in the metastatic or locally advanced setting. Phase Ib Dose Expansion: Phase I trials are increasingly including dose-expansion cohorts (Ib) after the maximum-tolerated dose has been reached to better characterize the toxicity profile and identify early signs of efficacy within this specific disease population. The investigators' goal is to assess the anti-tumor activity Ribociclib + Tamoxifen and to further evaluate their safety in adult patients with HR+/HER2- locally advanced or metastatic breast cancer. Patients in the phase 1b expansion will have the same exclusion and inclusion criteria except that they will only be allowed to have up to two lines of cytotoxic chemotherapy in the metastatic setting.