Inclusion Criteria:

          -  Women who are postmenopausal or premenopausal/perimenopausal

          -  For women who are premenopausal or perimenopausal and for men: willing to undergo and
             maintain treatment with approved LHRH agonist therapy for the duration of study
             treatment

          -  Locally advanced or metastatic adenocarcinoma of the breast, not amenable to treatment
             with curative intent

          -  Documented ER-positive tumor and HER2-negative tumor, assessed locally

          -  Disease progression after treatment with one or two lines of systemic therapy (but not
             more than one prior targeted therapy) in the locally advanced or metastatic setting

          -  Measurable disease as defined per RECIST v.1.1 or bone only disease which must have at
             least one predominantly lytic bone lesion confirmed by CT or MRI which can be followed

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status 0-1

          -  Adequate organ function

        Exclusion Criteria:

          -  Prior treatment with a selective estrogen receptor degrader (SERD), with the exception
             of fulvestrant, if fulvestrant treatment was terminated at least 28 days prior to
             randomization

          -  Treatment with any investigational therapy within 28 days prior to randomization

          -  Advanced, symptomatic, visceral spread that is at risk of life-threatening
             complications

          -  Known active uncontrolled or symptomatic CNS metastases, carcinomatous meningitis, or
             leptomeningeal disease

          -  Active cardiac disease or history of cardiac dysfunction

          -  Pregnant or breastfeeding