Neoadjuvant Endocrine Therapy, Palbociclib, Avelumab in Estrogen Receptor Positive Breast Cancer

Patients will be treated with endocrine therapy +/- palbociclib (125mg, taken orally, daily for 21 days, 7 days off ) for 1 cycle (1 cycle =28 days) and undergo a repeat biopsy, MRI, and blood draw at Cycle 2, Day 1. Afterwards avelumab will be added to both arms. Avelumab will be administered 10mg/kg IV every 14 days (1 cycle = 2 doses = 28 days). Patients will be treated for 3 cycles of avelumab with endocrine therapy +/- palbociclib (thus 4 cycles total, including run-in without avelumab). The kind of endocrine therapy given will depend on menopausal status and will be a standard treatment. For premenopausal women, the endocrine therapy will be daily tamoxifen (20mg taken orally daily for 28 days), plus either Leuprolide (3.75mg taken intramuscularly) or Goserelin (3.6mg taken subcutaneously) on Day 1 of each cycle. For postmenopausal women, it will be daily letrozole (2.5mg taken orally). Patients will be treated so long as there is no clinical evidence of progression and therapy is tolerated. Patients who experience progressive disease (25% increase) of their tumor will cease study treatment and undergo end-of-study assessment (including repeat MRI) and surgery. Otherwise, patients completing all 4 cycles of therapy will undergo MRI and surgery.