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 brain metastases (e.g., lesions \< 1 cm) not needing immediate local therapy
  * Previously treated brain metastases not needing immediate local therapy

    * At least 4 weeks from the last date of prior therapy completion (including radiation and/or surgery) to starting the study treatment
    * Clinically stable CNS tumor at the time of screening and not receiving steroids and/or enzyme-inducing anti-epileptic medications for brain metastases
* Absence of interstitial lung disease/pneumonitis
* Absolute neutrophil count (ANC) \>= 1.5 X 10\^9/L
* Platelets \>= 100 x 10\^9/L
* Hemoglobin \>= 8 g/dL

  * (Patients may receive erythrocyte transfusions to achieve this hemoglobin level at the discretion of the investigator. Initial treatment must not begin earlier than the day after the erythrocyte transfusion)
* In the absence of liver metastases, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) =\< 3.0 x upper limit of normal (ULN)

  * If the patient has liver metastases, ALT and AST \< 5 x ULN
* In patients without Gilbert's syndrome, total bilirubin =\< 1.5 x ULN; In patients with Gilbert's syndrome, total bilirubin =\< 2.0 x ULN or direct bilirubin within normal limits (WLN)
* Creatinine clearance of \>= 30 mL/min per 24 hour urine test or the Cockcroft-Gault formula

Exclusion Criteria:

* Major surgery within 14 days prior to receiving study drug or has not recovered from major side effect
* Patient is currently receiving any of the prohibited medications detailed below and cannot be discontinued 7 days prior to starting study drug

  * Other investigational therapy should be given to participants
  * Anticancer agents other than the study medications administered as part of this study protocol should be given to participants. If such agents are required for a participant then the participant must first be withdrawn from the study
  * Co-medication that may interfere with study results; e.g. immune-suppressive agents other than corticosteroids, such as systemic cyclosporine and tacrolimus are prohibited during the treatment phase of the study, unless discussed with principal investigator felt to be of low clinical risk to the participant
  * Use of herbal medications may have unknown interactions with the metabolism of the study agents, and therefore are prohibited from use during the treatment phase of the trial
* Known hypersensitivity to any of the excipients of abemaciclib
* Active systemic bacterial infection (requiring intravenous \[IV\] antibiotics at time of initiating study treatment), fungal infection, or detectable viral infection (such as known human immunodeficiency virus positivity or with known active hepatitis B or C (for example, hepatitis B surface antigen positive). Screening is not required for enrollment
* Impairment of gastrointestinal (GI) function or GI disease that in the investigator's opinion may significantly alter the absorption of the study drugs (e.g., ulcerative diseases, uncontrolled nausea, vomiting, diarrhea, malabsorption syndrome, or small bowel resection)
* History of any of the following conditions: syncope of cardiovascular etiology, ventricular arrhythmia of pathological origin (including, but not limited to, ventricular tachycardia and ventricular fibrillation), or sudden cardiac arrest
* Patient has any other concurrent severe or uncontrolled medical condition that would, in the investigator's judgment, cause unacceptable safety risks, contraindicate patient participation in the clinical study or compromise compliance with the protocol (e.g. chronic pancreatitis, chronic active hepatitis)
* Inability to swallow oral medications
* Serious or uncontrolled preexisting medical condition(s) that, in the judgment of the investigator, would preclude participation in this study (for example, interstitial lung disease, severe dyspnea at rest or requiring oxygen therapy, severe renal impairment \[e.g. estimated creatinine clearance \< 30 ml/min\], history of major surgical resection involving the stomach or small bowel, or preexisting Crohn's disease or ulcerative colitis or a preexisting chronic condition resulting in baseline grade 2 or higher diarrhea)
* History of non-compliance to medical regimen
* Patients with a prior malignancy diagnosed within 2 years and with evidence of disease (except adequately treated, basal or squamous cell carcinoma, non-melanomatous skin cancer or curatively resected cervical cancer