Study of Pembrolizumab and Mifepristone in Patients With Advanced HER2-negative Breast Cancer

Eligible patients will be treated with pembrolizumab on day 1 of every 21 day cycle, given at a dose of 200 mg. Mifepristone 300mg PO will be administered daily starting the week prior to pembrolizumab. Enrollment will be paused after the first 10 patients are enrolled in the study for a safety evaluation. Once safety is confirmed, dose expansion cohorts will be performed in parallel for each indication * Hormone receptor positive, hormone refractory advanced breast cancer (n = 25-34) * Triple-negative advanced breast cancer (n = 27-40) Mandatory pre- and on-treatment tumor biopsies will be obtained in order to evaluate immunological changes in these tissues. Cycle length is 21 days. Patients will be evaluated every 3 cycles until progression of disease unless the patient is otherwise withdrawn from the study.