Enzalutamide and Metformin Hydrochloride in Treating Patients With Hormone-Resistant Prostate Cancer

PRIMARY OBJECTIVES: I. To determine the maximum tolerated dose (MTD) of enzalutamide when given in combination with metformin (metformin hydrochloride) to patients with castration resistant prostate cancer (CRPC), where fewer than 33% of patients experienced dose limiting toxicity (DLT) attributable to the study regimen and to recommend a phase II dose for the combination. SECONDARY OBJECTIVES: I. To determine prostate-specific antigen (PSA) response in patients with CRPC with given enzalutamide in combination with metformin. II. To determine PSA progression in patients with CRPC with given enzalutamide in combination with metformin. III. To investigate the feasibility and safety of enzalutamide when given in combination with metformin hydrochloride to patients with CRPC. IV. To obtain preliminary evidence of efficacy for this combination. TERTIARY OBJECTIVES: I. To collect computed tomography (CT)-guided biopsies of metastatic soft tissue or bone tumor tissue for analysis of androgen receptor (AR) gene signature as an integrated biomarker (University of California San Francisco \[UCSF\] to conduct analysis). II. To collect serum samples for the measurement of PSA levels and bone re-absorption markers. OUTLINE: This is a dose-escalation study of metformin hydrochloride. Patients receive enzalutamide orally (PO) once daily (QD) and metformin hydrochloride PO twice daily (BID). Courses repeat every 28 days in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up at 4, 8, and 12 weeks.