 The primary purpose of this study is to evaluate the efficacy, safety, and tolerability of GDC-0810 compared with fulvestrant in postmenopausal women with advanced or metastatic estrogen receptor positive ER human epidermal growth factor receptor 2 negative HER2- breast cancer resistant to AI therapy. The development of GDC-0810 has been halted by the Sponsor and the enrollment in this study has been discontinued. Participants currently enrolled in the study who are experiencing clinical benefit may continue receiving GDC-0810 as a single agent or fulvestrant until disease progression PD, unmanageable toxicity, withdrawal of consent, exhaustion of GDC-0810 drug supply, or termination of the study by the Sponsor. 