Abemaciclib and Endocrine Therapy in Older Patients With Breast Cancer.

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 hospitalizations. III. To estimate median (and 95% confidence interval \[CI\]) failure-free survival, progression-free survival and overall survival. IV. To describe the results of Was It Worth It (WIWI) and Overall Treatment Utility (OTU) questionnaires. V. To describe the rate of adherence to abemaciclib. VI. To determine average plasma steady-state abemaciclib Ctrough concentrations. VII. To evaluate the association of adherence rate with abemaciclib plasma t-rough concentrations. VIII. To describe associations between cancer-specific, comprehensive Geriatric Assessment (cGA) scores and the incidence of toxicities and their grade. EXPLORATORY OBJECTIVE: I. To determine the association between biomarkers of aging and grades 3 or higher toxicity. OUTLINE: Patients receive abemaciclib orally (PO) twice daily (BID) on days 1-28. Cycles repeat every 28 days in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up at 30 days, then every 6 months for 2 years.