A Phase IIA Trial Assessing the Tolerability of Abemaciclib in Combination With Endocrine Therapy in Patients Age 70 and Older With Hormone Receptor Positive Metastatic Breast Cancer Who Have Progressed on or After Prior CDK 4/6 Inhibition

PRIMARY OBJECTIVE: I. To estimate the incidence of grade 3 or higher toxicities attributed to abemaciclib monotherapy in adults aged 70 or older with hormone receptor positive metastatic breast cancer. SECONDARY OBJECTIVES: I. To describe the full toxicity profile including all grade 2 and higher adverse events, and patient-reported adverse events (AEs) using Patients Reported Outcomes (PRO)-Common Terminology Criteria for Adverse Events (CTCAE) measures. II. To describe rates of dose reductions, dose holds, treatment discontinuations due to factors other than progression, and hospitalizat…