Pembrolizumab and Capecitabine in Treating Patients With Locally Advanced or Metastatic Triple Negative or Hormone-Refractory Breast Cancer That Cannot Be Removed by Surgery

Inclusion Criteria

• •Patients must have histologically-confirmed unresectable, locally advanced or metastatic breast cancer that meets one of the following:
• •* Triple negative, defined as estrogen receptor (ER) negative, progesterone receptor (PR) negative, human epidermal growth factor receptor 2 (HER2) negative; HER2 negative defined as immunohistochemistry (IHC) 0 or 1+ or fluorescence in situ hybridization (FISH) negative
• •* Hormone-refractory breast cancer which denotes progression to endocrine therapy (e.g., tamoxifen, aromatase inhibitors, fulvestrant) unless contraindicated
• •Patients must have an Eastern Cooperative Oncology Group (ECOG) performance status =\< 2
• •Patients must have a life expectancy of \>= 90 days
• •Patients must have baseline laboratory tests within the following parameters at least 4 weeks (28 days) prior to registration:
• •Absolute neutrophil count (ANC) \>= 1,500/mcL
• •Platelets \>= 100,000/mcL
• •Hemoglobin \>= 9 g/dL or \>= 5.6 mmol/L without transfusion or erythropoietin (EPO) dependency
• •Serum creatinine =\< 1.5 x upper limit of normal (ULN) OR measured or calculated creatinine clearance (GFR can also be used in place of creatinine or CrCl) \>= 60 mL/min for subject with creatinine levels \> 1.5 x institutional ULN
• •Serum total bilirubin =\< 1.5 x ULN OR direct bilirubin =\< ULN for subjects with total bilirubin levels \> 1.5 ULN
• •Aspartate aminotransferase (AST) (serum glutamic-oxaloacetic transaminase \[SGOT\]) and alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase \[SGPT\]) =\< 2.5 x ULN OR =\< 5 x ULN for subjects with liver metastases
• •Albumin \>= 2.5 mg/dL
• •International normalized ratio (INR) or prothrombin time (PT) =\< 1.5 x ULN unless subject is receiving anticoagulant therapy as long as PT or partial thromboplastin time (PTT) is within therapeutic range of intended use of anticoagulants
• •Activated partial thromboplastin time (aPTT) =\< 1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use of anticoagulants
• •Females of child-bearing potential (FOCBP) must have a negative serum or urine pregnancy test within 7 days prior to registration and must be at least within 3 days prior to first dose of study drug. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required
• •* (Note: a FOCBP is any woman \[regardless of sexual orientation, having undergone a tubal ligation, or remaining celibate by choice\] who meets the following criteria:
• •* Has not undergone a hysterectomy or bilateral oophorectomy
• •* Has had menses at any time in the preceding 12 consecutive months (and therefore has not been naturally postmenopausal for \> 12 months)
• •Female subjects of childbearing potential (FOCBP) must be willing to use an adequate method of contraception; contraception must be used for the course of the study through 120 days after the last dose of study medication
• •* Note: abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject
• •Male subjects of childbearing potential must agree to use an adequate method of contraception; contraception must be used starting with the first dose of study therapy through 120 days after the last dose of study therapy
• •* Note: abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject
• •Patients must have the ability to understand and the willingness to sign a written informed consent prior to registration on study
• •Patients must be willing and able to comply with scheduled visits, treatment plan and laboratory tests
• •Patient must be able to swallow and retain oral medication