Tamoxifen Citrate or Z-Endoxifen Hydrochloride in Treating Patients With Locally Advanced or Metastatic, Estrogen Receptor-Positive, HER2-Negative Breast Cancer

Inclusion Criteria: * PRE-REGISTRATION ELIGIBILITY CRITERIA * Women who agree to undergo a standard of care core biopsy of recurrent or metastatic breast cancer to confirm the ER+ (\>= 10% nuclear staining) and HER2 negative status * Patient must have been previously treated with an aromatase inhibitor (either letrozole, anastrozole or exemestane) either in the adjuvant or metastatic setting, and have one of the following types of primary or secondary endocrine resistant disease * Primary clinical resistance is defined as one of the following: * Recurrence within the first 2 years of adjuvant endocrine therapy while on aromatase inhibitor therapy * Progression within first 6 months of initiating first-line endocrine therapy (either aromatase inhibitor or fulvestrant containing regimen) for the treatment of metastatic breast cancer * Secondary clinical resistance is defined as one of the following: * Recurrence after year 2 while receiving adjuvant aromatase inhibitor therapy, or within 12 months of completing adjuvant aromatase inhibitor therapy * Progression occurring 6 or more months after initiating the first endocrine therapy for metastatic disease (either fulvestrant or aromatase inhibitor containing regimen) * Patients with a history of measurable disease as defined by RECIST criteria or bone only disease are eligible; Note: those patients with non-measurable disease and bone metastases are eligible * No history of tumors involving spinal cord or…