Fulvestrant Plus Enzalutamide in ER+/Her2- Advanced Breast Cancer

Inclusion Criteria: 1. ER+ Her2- breast cancer 2. Metastatic 3. Female, at least 18 years of age 4. Candidate for fulvestrant therapy - patients who have started fulvestrant may enter this trial if within 3 months of starting fulvestrant 5. Measurable or evaluable by RECIST 1.1 6. ECOG PS 0-2 7. Able to swallow study drug and comply with study requirements 8. Tumor available for fresh biopsy (two biopsies - pretreatment as regards enzalutamide, and during treatment at 4 weeks). The patient will be also be asked if they would be willing to provide a third biopsy at time of progression. 9. If patient is pre- or peri- menopausal, then will need to have concurrent ovarian suppression. Patients may have already gotten the loading dose of ovarian suppression. Pre- or peri- menopausal subjects must have a negative urine pregnancy test confirmed at screening. 10. ANC \>1000/uL and platelets \>75,000/uL at screening visit 11. Total bilirubin \< 1.5 times upper limit of normal (ULN) at the screening visit unless an alternate nonmalignant etiology exists (eg, Gilbert's disease) 12. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \< 3 times ULN or \< 5 times ULN if patient has documented liver metastases 13. Creatinine \< 1.5 times ULN 14. INR \< 1.5 times ULN, or if on warfarin, can safely transition off for biopsy 15. Willing to donate blood for research at 4 time points 16. Written informed consent obtained prior to biopsies and blood samples 17. Agreement to exercise appropriate use of contraception. Subjects should use 2 acceptable methods of birth control (one of which must include a condom as a barrier method of contraception) starting at the time of screening for an enzalutamide study and continuing throughout the course of treatment and for at least three months after enzalutamide is discontinued. Exclusion Criteria: 1. Current or previously treated brain or leptomeningeal metastases 2. History of seizures 3. Prior treatment with an anti-androgen (abiraterone, ARN-509, bicalutamide, enzalutamide, ODM-201, TAK-448, TAK-683, TAK-700, VT-464) 4. Systemic estrogens or androgens within 14 days before initiating therapy. Vaginal estrogens are allowed if necessary for patient comfort.