Ribociclib And Endocrine Treatment of Physician's Choice for Locoregional Recurrent, Resected Hormone Receptor Positive HER2 Negative Breast Cancer

This is an open label, multicenter, single arm phase II study to evaluate the efficacy and safety of ribociclib and ET in patients with locoregional recurrence of HR-positive, HER2-negative breast cancer.

Eligibility Criteria to Collect Optional Correlative Blood and Tissue at Local Recurrence * Written informed consent (stage I) and HIPAA authorization for release of personal health information obtained prior to performing any study-specific procedures. NOTE: HIPAA authorization may be included in the informed consent or obtained separately. * Male or female age ≥ 18 years at the time of consent. * Patient has a histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive and/or progesterone receptor positive breast cancer based on the most recently analyzed tissue sample and all tested by local laboratory. * Patient has HER2-negative breast cancer defined as a negative in situ hybridization test or an IHC status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization (FISH, CISH, or SISH) test is required by local laboratory testing and based on the most recently analyzed tissue sample. If there is insufficient tissue from the most recently collected sample, earlier tissue may be used on a case-by-case basis if permission is granted by the sponsor investigator. * Patient has locoregional recurrence of breast cancer: locoregional recurrence is defined as invasive recurrence in the ipsilateral breast, axilla, regional nodes, or chest wall. Inclusion Criteria for Treatment Phase: Subject must meet all of the following applicable inclusion criteria to participate in this study: * Written informed consent (stage II/ main consent) and HIPAA authorization for release of personal health information obtained prior to performing any study-specific screening procedures. NOTE: HIPAA authorization may be included in the informed consent or obtained separately. * Male or female age ≥ 18 years at the time of consent. NOTE: Both pre- and post-menopausal women are eligible. Post-menopausal status is defined as: * Prior bilateral oophorectomy * Age ≥60 * Age \160 or \