Cemiplimab and Transarterial Radioembolization With Y-90 SIR-S Spheres for the Treatment of Liver Directed Metastatic Breast Cancer

Inclusion Criteria: * Documented informed consent of the participant and/or legally authorized representative. * Assent, when appropriate, will be obtained per institutional guidelines * Agreement to allow the use of archival tissue from diagnostic tumor biopsies * If unavailable, exceptions may be granted with study principal investigator (PI) approval * Age: ≥ 18 years * Eastern Cooperative Oncology Group (ECOG) 0-1 * Must have a life expectancy of at least 12 weeks * Evidence of post-menopausal status or negative urinary or serum pregnancy test for female pre-menopausal patients. Women will be considered post-menopausal if they have been amenorrheic for 12 months without an alternative medical cause. The following age-specific requirements apply: * Women \< 50 years of age would be considered post-menopausal if they have been amenorrheic for 12 months or more following cessation of exogenous hormonal treatments and if they have luteinizing hormone and follicle-stimulating hormone levels in the post-menopausal range for the institution or underwent surgical sterilization (bilateral oophorectomy or hysterectomy) * Women ≥ 50 years of age would be considered post-menopausal if they have been amenorrheic for 12 months or more following cessation of all exogenous hormonal treatments, had radiation-induced menopause with last menses \> 1 year ago, had chemotherapy-induced menopause with last menses \> 1 year ago, or underwent surgical sterilization (bilateral oophorectomy, bilateral salpingectomy, or hysterectomy) * Male patients must be surgically sterile, or if sexually active and having a pre-menopausal female partner then must be using an acceptable form of contraception * Patients with metastatic breast cancer, previously treated with ≥ 1 line of chemotherapy in metastatic setting * Candidate for TARE based on evaluation by board certified interventional radiologist; must have measurable metastatic liver disease, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded) as outlined in RECIST version 1.1; not all liver tumors will be treated with TARE in a single session * Liver tumor burden \< 50% * No radiographic or clinical evidence of cirrhosis * Other treated stable metastases are allowed, including treated stable brain metastases * Fully recovered from the acute toxic effects (except alopecia) to ≤ grade 1 to prior anti-cancer therapy * Body weight \> 30 kg * No untreated, active hepatitis * Hemoglobin ≥ 9.0 g/dL (within 14 days prior to day 1 of protocol therapy unless otherwise stated) * Absolute neutrophil count (ANC) ≥ 1500/uL (within 14 days prior to day 1 of protocol therapy unless otherwise stated) * Platelet count ≥ 75000/uL (within 14 days prior to day 1 of protocol therapy unless otherwise stated) * Total bilirubin \< 2.0mg/dL (within 14 days prior to day 1 of protocol therapy unless otherwise stated) * Aspartate aminotransferase (AST/serum glutamic oxaloacetic transaminase \[SGOT\]) and alanine aminotransferase (ALT/serum glutamic pyruvic transaminase \[SGPT\]) ≤ 5 x upper limit of normal (ULN) (within 14 days prior to day 1 of protocol therapy unless otherwise stated) * Albumin ≥ 2.8 g/dL (within 14 days prior to day 1 of protocol therapy unless otherwise stated) * International normalized ration (INR) ≤ 1.6 (within 14 days prior to day 1 of protocol therapy unless otherwise stated) * Measured creatinine clearance (CL) \> 40 mL/min or calculated creatinine CL \> 40 mL/min by the Cockcroft-Gault formula (Cockcroft and Gault 1976) or by 24-hour urine collection for determination of creatinine clearance (within 14 days prior to day 1 of protocol therapy unless otherwise stated) Exclusion Criteria: * Prior immunotherapy in the past 12 months * Progressing or untreated extra-hepatic metastatic disease * Autoimmune disease requiring therapy; immunodeficiency, or any disease process requiring immunosuppressive therapy * Any other condition that would, in the investigator's judgment, contraindicate the patient's participation in the clinical study due to safety concerns with clinical study procedures * Portal vein invasion * Evidence of diffuse metastatic disease (tumor burden occupying \> 50% of liver) * Major surgical procedure (as defined by the investigator) within 28 days prior to radioembolization. Note: Local surgery of isolated lesions for palliative intent is acceptable * Participation in another clinical study with an investigational product during the last 4 weeks * Concurrent enrollment in another clinical study, unless it is an observational (non-interventional) clinical study or during the follow-up period of an interventional study * Receipt of the last dose of anticancer therapy (chemotherapy, immunotherapy, endocrine therapy, targeted therapy, biologic therapy, tumor embolization, monoclonal antibodies) ≤ 28 days prior to the first dose of study drug. If sufficient wash-out time has not occurred due to the schedule or pharmacokinetics (PK) properties of an agent, a longer wash-out period will be required, as agreed by Regeneron and the investigator * Prior exposure to anti-PD-1/PD-L1 inhibitor or anti-CTLA4 inhibitor, including cemiplimab in the past 12 months * Any unresolved toxicity National Cancer Institute (NCI) CTCAE grade ≥ 2 from previous anticancer therapy with the exception of alopecia, vitiligo, and the laboratory values defined in the inclusion criteria * Patients with grade ≥ 2 neuropathy will be evaluated on a case-by-case basis after consultation with the study physician * Patients with irreversible toxicity not reasonably expected to be exacerbated by treatment with cemiplimab may be included only after consultation with the study physician * Active or prior documented autoimmune or inflammatory disorders (including inflammatory bowel disease \[e.g., colitis or Crohn's disease\], diverticulitis \[with the exception of diverticulosis\], systemic lupus erythematosus,