Phase 3 Trial of Elacestrant Versus Standard of Care for the Treatment of ER+/HER2- Advanced Breast Cancer

Critical Inclusion Criteria:

1. Participants with proven diagnosis of adenocarcinoma of the breast with evidence ofeither locally advanced disease not amenable to resection or radiation therapy withcurative intent or metastatic disease not amenable to curative therapy.

2. Participants must be appropriate candidates for endocrine monotherapy

3. Participants must have measurable disease or bone only disease with evaluablelesions

4. Female or male participants age ≥ 18 years; female participants must bepostmenopausal women, and male participants must not allow pregnancy with theirsperm (abstain, do not donate sperm, et cetera).

5. Participants must have ER+ and HER2- tumor status

6. Participants must have previously received at least one and no more than two linesof endocrine therapy for advanced/metastatic breast cancer and meet additionalprevious treatment criteria.

7. Participants must have received prior treatment with a CDK4/6 inhibitor incombination with either fulvestrant or an aromatase inhibitor (AI) foradvanced/metastatic breast cancer (mBC).

8. Participants may have received no more than one line of chemotherapy in theadvanced/metastatic setting.

Critical Exclusion Criteria:

1. Prior treatment with elacestrant or other investigational selective estrogenreceptor degrader (SERD) or ER antagonist (D-0502, GDC-0810, GDC-0927, GDC-9545,G1T-48, LSZ102, AZD9496, SAR439859, ZN-c5, H3B-6545, bazedoxifene, lasofoxifene).

2. Prior anticancer or investigational drug treatment within the following windows:

3. Fulvestrant treatment < 42 days before first dose of study drug

4. Any endocrine therapy < 14 days before first dose of study drug

5. Chemotherapy < 21 days before first dose of study drug

6. Any investigational anti-cancer drug therapy < 28 days or five half-lives (whichever is shorter) before the first dose of study drug. Enrollment ofparticipants whose most recent therapy was an investigational agent should bediscussed with the Sponsor

7. Bisphosphonates or RANKL inhibitors initiated or dose changed < 3 months priorto first dose of study drug

8. Presence of symptomatic visceral disease as defined in protocol.