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 cancerdetermined by immunohistochemistry (IHC) methods according to the localinstitution 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 AmericanSociety 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 priorcancer 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 lastchemotherapy dose and randomization (provided the patient did not receiveradiotherapy)

• Patients who received radiotherapy must have completed and fully recovered from theacute effects of radiotherapy. A washout period of at least 14 days is requiredbetween end of radiotherapy and randomization

• Absence of central nervous system (CNS) involvement unless they meet ONE of thefollowing criteria:

• Untreated brain metastases (e.g., lesions < 1 cm) not needing immediate localtherapy

• Previously treated brain metastases not needing immediate local therapy

• At least 4 weeks from the last date of prior therapy completion (includingradiation and/or surgery) to starting the study treatment

• Clinically stable CNS tumor at the time of screening and not receivingsteroids and/or enzyme-inducing anti-epileptic medications for brainmetastases

• 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 levelat the discretion of the investigator. Initial treatment must not begin earlierthan the day after the erythrocyte transfusion)

• In the absence of liver metastases, alanine aminotransferase (ALT) and aspartateaminotransferase (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 patientswith Gilbert's syndrome, total bilirubin =< 2.0 x ULN or direct bilirubin withinnormal limits (WLN)

• Creatinine clearance of >= 30 mL/min per 24 hour urine test or the Cockcroft-Gaultformula

Exclusion Criteria:

• Major surgery within 14 days prior to receiving study drug or has not recovered frommajor side effect

• Patient is currently receiving any of the prohibited medications detailed below andcannot 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 thisstudy protocol should be given to participants. If such agents are required fora participant then the participant must first be withdrawn from the study

• Co-medication that may interfere with study results; e.g. immune-suppressiveagents other than corticosteroids, such as systemic cyclosporine and tacrolimusare prohibited during the treatment phase of the study, unless discussed withprincipal investigator felt to be of low clinical risk to the participant

• Use of herbal medications may have unknown interactions with the metabolism ofthe study agents, and therefore are prohibited from use during the treatmentphase of the trial

• Known hypersensitivity to any of the excipients of abemaciclib

• Active systemic bacterial infection (requiring intravenous [IV] antibiotics at timeof initiating study treatment), fungal infection, or detectable viral infection (such as known human immunodeficiency virus positivity or with known activehepatitis B or C (for example, hepatitis B surface antigen positive). Screening isnot required for enrollment

• Impairment of gastrointestinal (GI) function or GI disease that in theinvestigator's opinion may significantly alter the absorption of the study drugs (e.g., ulcerative diseases, uncontrolled nausea, vomiting, diarrhea, malabsorptionsyndrome, 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 thatwould, in the investigator's judgment, cause unacceptable safety risks,contraindicate patient participation in the clinical study or compromise compliancewith 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 ofthe 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], historyof major surgical resection involving the stomach or small bowel, or preexistingCrohn's disease or ulcerative colitis or a preexisting chronic condition resultingin 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 ofdisease (except adequately treated, basal or squamous cell carcinoma,non-melanomatous skin cancer or curatively resected cervical cancer