Inclusion Criteria:

1. Locally advanced or metastatic adenocarcinoma of the breast, not amenable to treatment with curative intent
2. Documented estrogen receptor-positive (ER+) tumor and HER2-negative tumor, assessed locally
3. Patients who have bilateral breast cancers that are both ER+ and HER2-negative are eligible. If patients have bilateral tumors that are of different biomarker status, then proof of the ER and HER2 status of the metastases is required for study entry.
4. Availability of blood sample for circulating-tumor deoxyribonucleic acid (ctDNA) Estrogen Receptor 1 (ESR1) mutation status determination by central testing
5. Prior endocrine therapy (ET) in combination with cyclin-dependent kinase 4/6 inhibitors in either setting as follows:
   • Metastatic setting: Disease progression ≥6 months after initiating ET plus CDK4/6 inhibitor in the locally advanced or metastatic setting. If ET plus CDK4/6 inhibitor is not the most recent therapy, then patient must also have had disease progression after ≥4 months on most recent ET
   • Adjuvant Setting: Relapse either while taking or within 12 months of exposure to combination adjuvant ET and CDK4/6 inhibitor. Patients must have taken at least 12 months of adjuvant ET, 6 months of which was in combination with a CDK4/6 inhibitor.
6. Measurable disease as defined per RECIST v.1.1 or evaluable bone metastases which must have at least one predominantly lytic bone lesion confirmed by computed tomography (CT) or magnetic resonance imaging (MRI) and that can be followed
7. Eastern Cooperative Oncology Group Performance Status 0-1
8. For women who are premenopausal or perimenopausal and for men: treatment with approved luteinizing hormone-releasing hormone (LHRH) agonist therapy for the duration of the study treatment

Exclusion Criteria:

1. Prior treatment with another oral selective estrogen receptor degrader (SERD) in any setting. Prior fulvestrant is allowed if treatment was terminated at least 28 days prior to randomization
2. No more than 2 prior lines of systemic endocrine therapy in the locally advanced or metastatic breast cancer setting
3. Prior chemotherapy for locally advanced or metastatic disease
4. Treatment with the multidrug efflux pump P-glycoprotein (P-gp) and strong Cytochrome P450 3A4 (CYP3A4) inhibitors within 14 days or 5 drug elimination half-lives prior to randomization
5. Treatment with any investigational therapy within 28 days prior to initiation of study treatment
6. Major surgery, chemotherapy, radiotherapy, or other anti-cancer therapy within 28 days prior to randomization
7. History of any other malignancy other than breast cancer within 5 years prior to screening, except for appropriately treated carcinoma in situ of the cervix, nonmelanoma skin carcinoma, papillary thyroid cancer treated with surgery, or Stage I endometrial cancer
8. Advanced, symptomatic, visceral spread that is at risk of life-threatening complications in the short term
9. Known active uncontrolled or symptomatic central nervous system (CNS) metastases, carcinomatous meningitis, or leptomeningeal disease
10. Active cardiac disease or history of cardiac dysfunction
11. Known clinically significant history of liver disease consistent with Child-Pugh Class B or C including active viral or other hepatitis virus, current alcohol abuse, or cirrhosis
12. Active inflammatory bowel disease, chronic diarrhea, short bowel syndrome, or major upper gastrointestinal (GI) surgery including gastric resection
13. Interstitial lung disease or severe dyspnea at rest or requiring oxygen therapy
14. Serious infection requiring oral or intravenous (IV) antibiotics, or other clinically significant infection, within 14 days prior to randomization
15. Any serious medical condition or abnormality in clinical laboratory tests that, in the investigator's judgment, precludes the patient's safe participation in and completion of the study
16. Known allergy or hypersensitivity to any of the study drugs or any of their excipients
17. For premenopausal or perimenopausal women and for men: known hypersensitivity to LHRH agonists
18. Pregnant or breastfeeding

Qualified researchers may request access to individual patient level data through the request platform (www.vivli.org). Further details on Roche's criteria for eligible studies are available here (https://vivli.org/ourmember/roche/).

For further details on Roche's Global Policy on Sharing of Clinical Study Information and how to request access to related clinical study documents, see here (https://www.roche.com/research_and_development/who_we_are_how_we_work/clinical_trials/our_commitment_to_data_sharing.htm).