- Women with advanced (locoregionally recurrent or metastatic) breast cancer not amenable to curative therapy. - Patient is postmenopausal. Postmenopausal status is defined either by: • Prior bilateral oophorectomy • Age ≥60 • Age <60 and amenorrhea for 12 or more months (in the absence of chemotherapy, tamoxifen, toremifen, or ovarian suppression) and FSH and estradiol in the postmenopausal range per local normal range Note: For women with therapy-induced amenorrhea, serial measurements of FSH and/or estradiol are needed to ensure postmenopausal status. Ovarian radiation or treatment with a luteinizing hormone-releasing hormone agonist (LH-RHa) (goserelin acetate or leuprolide acetate) is not permitted for induction of ovarian suppression in this trial. - No prior systemic anti-cancer therapy for advanced disease. - Patient has a histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive and/or progesterone receptor posit