A Study Evaluating the Efficacy and Safety of Giredestrant Compared With Physician's Choice of Endocrine Monotherapy in Participants With Previously Treated Estrogen Receptor-Positive, HER2-Negative Locally Advanced or Metastatic Breast Cancer (acelERA Breast Cancer)

Inclusion Criteria:

1. Women who are postmenopausal or premenopausal/perimenopausal
2. 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
3. Locally advanced or metastatic adenocarcinoma of the breast, not amenable to treatment with curative intent
4. Documented ER-positive tumor and HER2-negative tumor, assessed locally
5. 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
6. 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
7. Eastern Cooperative Oncology Group (ECOG) Performance Status 0-1
8. Adequate organ function

Exclusion Criteria:

1. 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
2. Treatment with any investigational therapy within 28 days prior to randomization
3. Advanced, symptomatic, visceral spread that is at risk of life-threatening complications
4. Known active uncontrolled or symptomatic CNS metastases, carcinomatous meningitis, or leptomeningeal disease
5. Active cardiac disease or history of cardiac dysfunction
6. Pregnant or breastfeeding