Effects of Ribociclib and Palbociclib on Tumor and Blood Characteristics in Patients With Metastatic Breast Cancer

PRIMARY OBJECTIVE: I. To identify predictive immune biomarkers and mechanisms of response to ribociclib or palbociclib in advanced, hormone receptor positive breast cancer patients. SECONDARY OBJECTIVES: I. Identify changes in antigen presentation machinery and costimulatory molecules on circulating myeloid cells as a result of ribociclib or palbociclib treatment. II. Characterize the dynamic remodeling of circulating myeloid cell composition that occur as a result of ribociclib or palbociclib treatment. III. Characterize acquired immune tumor microenvironment features of resistance in patients that progress while undergoing ribociclib or palbociclib treatment. EXPLORATORY OBJECTIVE: I. To study the association between immune biomarkers and clinical response. OUTLINE: Patients are assigned to 1 of 2 cohorts. PROSPECTIVE COHORT: Patients receive standard of care (SOC) treatment consisting of ribociclib or palbociclib plus aromatase inhibitor (AI). Patients undergo biopsy of tumor tissue at baseline and post-treatment. Patients also undergo collection of blood samples at baseline, on day 1 of SOC treatment cycles 2, 4, and 6, every 6 cycles thereafter, and at post-treatment. RETROSPECTIVE COHORT: Patients' tumor tissue collected during previous SOC treatment (ribociclib or palbociclib plus AI) is used for analysis.