Pembrolizumab and Enobosarm in Treating Patients With Androgen Receptor Positive Metastatic Triple Negative Breast Cancer

PRIMARY OBJECTIVES: I. To evaluate the safety/tolerability of the combination regimen. II. To determine the response rate (complete response \[CR\] or partial response \[PR\] via Response Evaluation Criteria in Solid Tumors \[RECIST\] 1.1) of the combination of pembrolizumab with enobosarm (GTx-024) in patients with advanced androgen receptor (AR) positive (+) triple negative breast cancer (TNBC). SECONDARY OBJECTIVES: I. To evaluate clinical outcomes by RECIST 1.1 including clinical benefit rate (CBR) at 24 weeks, progression free-survival (PFS), duration of response (DOR), event free survival (EFS), time-to-treatment failure (TTF); and overall survival (OS). II. To evaluate the role of immune-related response criteria (irRECIST). III. To evaluate the association of AR by immunohistochemistry (IHC) and clinical response. EXPLORATORY OBJECTIVES: I. To evaluate the association of an AR gene expression signature and clinical response. II. To evaluate genomic and phenotypic status of breast tumor. III. To evaluate the effect of the combination therapy on peripheral blood circulating tumor cells (CTCs) and circulating tumor deoxyribonucleic acid (DNA) (ctDNA). IV. To evaluate the effect of combination therapy on tumor-derived exosomes (TEX) and TEX associated immune biomarkers. V. Immune correlatives: Va. To evaluate pre-treatment programmed death ligand 1 (PD-L1) and tumor infiltrating lymphocytes (TILs) as a predictor of response to combination therapy. Vb. To evaluate specific TIL subsets (e.g. CD4, CD8, regulatory T cell \[Treg\] distribution) and other immunological correlatives (e.g. T cell receptor \[TCR\] repertoire analysis) as possible predictors of response. Vc. To evaluate change in TILs as a result of the combination therapy. Vd. To evaluate peripheral blood, immune biomarkers. OUTLINE: Patients receive pembrolizumab intravenously (IV) over 30 minutes on day 1 and enobosarm orally (PO) once daily (QD) on days 1-21. Treatment repeats every 21 days for up to 35 cycles in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up at 30 and 90 days, every 3 months, and bi-annually.