Fulvestrant With or Without Ganetespib in HR+ Breast Cancer

Inclusion Criteria: * Histologically confirmed invasive breast cancer that is metastatic or unresectable locally advanced * Estrogen and/or progesterone receptor positive breast cancer * HER2 negative * Measurable disease is required (effective 4/30/14: all non-measurable \[evaluable\] disease slots have been filled) * Endocrine resistant breast cancer * May have received up to one prior line of chemotherapy for metastatic or unresectable locally advanced breast cancer * May have initiated bisphosphonate therapy prior to start of protocol therapy * Must be at least 2 weeks from prior chemotherapy or radiotherapy * ECOG performance status of 0 or 1 * Availability of tissue block from initial breast cancer diagnosis and/or metastatic recurrence * For subjects with biopsy-accessible disease, must be willing to undergo a required on-treatment research biopsy * Adequate IV access Exclusion Criteria: * Pregnant or breastfeeding * Prior treatment with HSP90 inhibitor * Prior treatment with fulvestrant * Concurrent treatment with commercial agents or other agents with the intent to treat the participant's malignancy * Untreated or progressive brain metastases * Pending visceral crisis, in the opinion of the treating investigator * History of allergic reactions attributed to compounds of similar chemical or biologic composition to fulvestrant or ganetespib * Uncontrolled intercurrent illness * Other malignancies within 3 years