Enzalutamide is an androgen-receptor inhibitor, which means that it blocks the activity of the hormone testosterone. In ovarian, fallopian tube, and primary peritoneal cancers that express the androgen receptor, blocking the androgen-receptor may possibly slow or stop tumor growth. Enzalutamide has been studied in women with breast cancer, but this is the first study using enzalutamide for the treatment of patients with ovarian, primary peritoneal, or fallopian tube cancer.