Fulvestrant With or Without Ganetespib in HR+ Breast Cancer

Ganetespib is a drug that may stop cancer cells from growing. This drug has been used in other research studies and laboratory experiments. It has also been studied in phase I trials, where the appropriate dosing has been determined. Ganetespib is considered an "HSP90 inhibitor". By blocking HSP90, ganetespib is thought to reduce the ability of cancer cells to become resistant to treatment. Fulvestrant is a hormonal therapy that works by attaching to estrogen receptors. In doing so, it can block the effect of estrogen on cancer cells. In addition, fulvestrant causes a decrease in the number of estrogen receptors. Fulvestrant is a drug that is approved by the FDA for treatment of metastatic, hormone receptor positive breast cancer, based upon the results of phase III clinical trials. In the laboratory, adding ganetespib to fulvestrant appears to improve its effectiveness. It is not known whether this is true in humans. In this research study, we are evaluating the effect of the addition of ganetespib to fulvestrant in participants with hormone receptor-positive, metastatic breast cancer.