Abemaciclib and Endocrine Therapy in Older Patients With Breast Cancer.

Inclusion Criteria: * Documented informed consent of the participant * Age \>= 70 years * Life expectancy \> 6 months * Ability to read and understand English or Spanish * Measurable or non-measurable disease * Histologically or cytologically confirmed diagnosis of: * Estrogen-receptor positive and/or progesterone receptor positive breast cancer determined by immunohistochemistry (IHC) methods according to the local institution standard protocol * HER2-negative breast cancer defined as negative if the IHC status is 0 or 1+, or if IHC is 2+ and in situ hybridization assay is negative per American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines * Radiographically confirmed metastatic breast cancer * Progressed on prior endocrine therapy or palbociclib or ribociclib or chemotherapy * Patients who received chemotherapy recovered from the acute side effects to prior cancer therapy (except alopecia or residual grade 2 peripheral neuropathy) to =\< grade 1 or baseline. A washout period of at least 21 days is required between last chemotherapy dose and randomization (provided the patient did not receive radiotherapy) * Patients who received radiotherapy must have completed and fully recovered from the acute effects of radiotherapy. A washout period of at least 14 days is required between end of radiotherapy and randomization * Absence of central nervous system (CNS) involvement unless they meet ONE of the following criteria: * Untreated …