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:

* 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