Sapanisertib in Combination With Fulvestrant in Women With Advanced or Metastatic Breast Cancer After Aromatase Inhibitor Therapy

The drug being tested in this study is called sapanisertib. Sapanisertib is being tested to treat postmenopausal women with advanced or metastatic estrogen receptor (ER) positive, human epidermal growth factor receptor-2 (HER2) negative breast cancer that has progressed during or after aromatase inhibitor (AI) therapy. This study will evaluate the efficacy and safety of combination of fulvestrant + daily sapanisertib and fulvestrant + weekly sapanisertib compared with fulvestrant alone. The study will enroll approximately 153 patients. Participants will be randomly assigned (by chance, like flipping a coin) to one of the three treatment groups-which will remain undisclosed to the participant and study doctor during the study (unless there is an urgent medical need): * Fulvestrant 500 mg * Fulvestrant 500 mg + Sapanisertib 4 mg * Fulvestrant 500 mg + Sapanisertib 30 mg All participants will receive either fulvestrant 500 mg intramuscularly (IM), fulvestrant 500 mg + sapanisertib 4 mg daily or fulvestrant 500 mg + sapanisertib 30 mg weekly. This multicenter trial will be conducted Spain and the USA. Participants will make multiple visits to the clinic, and end of treatment (EOT) visit which will occur 30 to 40 days after receiving their last dose of study drug or before the start of any subsequent anticancer therapy. After EOT, participants will be followed for progression free survival (PFS) and overall survival (OS).