Preoperative Fulvestrant With or Without Enzalutamide in ER+/Her2- Breast Cancer

This is a randomized two arm phase II study to further evaluate the efficacy of fulvestrant plus enza compared to single agent fulvestrant in postmenopausal women with locally advanced AR+/ER+/Her2- BC who will have local surgery after \~4 months on treatment.

Inclusion Criteria: * ER+ Her2- breast cancer * Stage at least T2 or greater * Planned to get local surgery * Postmenopausal, or if pre- or peri- menopausal, then will need to have concurrent ovarian suppression. * At least 18 years of age * Not on anticoagulants * PS 0-2 * Able to swallow study drug and comply with study requirements * ANC \>1000/uL, platelets \>75,000/uL at screening visit * Total bilirubin \< 1.5 times upper limit of normal (ULN) at the screening visit unless an alternate nonmalignant etiology exists (eg, Gilbert's disease) * Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \< 3 times ULN or \< 5 times ULN if patient has documented liver metastases * Creatinine \< 1.5 times ULN * INR \< 1.5 times ULN, or if on warfarin, can safely transition off for biopsy * Willing to donate blood for research at 4 time points * Willing to undergo core biopsies for research at study entry and at \~4 weeks. * Willing to donate tissue to research from the surgical specimen * Written informed consent obtained prior to biopsies and blood samples Exclusion Criteria: * Current or previously treated brain or leptomeningeal metastases * History of seizures * Prior treatment with an anti-androgen (abiraterone, ARN-509, bicalutamide, enzalutamide, ODM-201, TAK-448, TAK-683, TAK-700, VT-464). * Systemic estrogens or androgens within 14 days before initiating therapy. Vaginal estrogens are allowed if necessary for patient comfort.