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

PRIMARY OBJECTIVE: I. To describe the effect of cemiplimab and yttrium Y90 resin microspheres (90Y TARE) on immune activation in the treated tumor based on CD8+ T cells. SECONDARY OBJECTIVES: I. To evaluate other measures of changes (both in treated and untreated lesions after the first TARE treatment) in the tumor microenvironment (TME) of 10 metastatic breast cancer (MBC) patients with liver-dominant disease. II. To identify systemic immune activation in the peripheral blood. III. To determine the safety and tolerability of cemiplimab and hepatic radioembolization in patients with liver dominant metastatic breast cancer using Common Terminology Criteria for Adverse Events (CTCAE) version 5.0 criteria. IV. To evaluate objective tumor response of the target lesion treated with Y90 by Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 and modified RECIST (mRECIST). OUTLINE: Patients receive cemiplimab intravenously (IV) over 30 minutes on day 1 of each cycle. Cycles repeat every 21 days for up to 1 year in the absence of disease progression or unacceptable toxicity. Patients receive Y90 SIR-Spheres intratumorally via TARE over 60-90 minutes once during days 7-14 and on day 67. Patients also undergo tumor biopsy at baseline and at day 42 after immunotherapy begins. Additionally patients also undergo blood sample collection and computed tomography (CT) or magnetic resonance imaging (MRI) throughout the study. After completion of study treatment, patients are followed up at 30 days, every 3 months within 1 year of start of treatment, and then every 6 months for up to 2 years.