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

This is a Phase II study of pembrolizumab plus mifepristone in advanced breast cancer patients. The study will include a safety lead in of ten patients. Patients who are deemed eligible and have signed informed consent will be treated with pembrolizumab at a fixed dose of 200 mg intravenously on day 1 of each 21 day cycle for each dose level. Mifepristone 300mg PO be administered daily starting the week prior to pembrolizumab. Once the safety of the combination is confirmed (study will be paused at least 6 weeks after first 10 patients are enrolled for safety evaluation), dose expansion cohorts will be performed in parallel for two cohorts: cohort 1 in triple-negative breast cancer and cohort 2 in hormone receptor positive 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 Patients will be evaluated every 3 cycles until progression of disease unless the patient is otherwise withdrawn from the study.