The primary goal of this study is to evaluate the effectiveness of elacestrant versus standard endocrine therapy in participants with node-positive, Estrogen Receptor-positive (ER+), Human Epidermal Growth Factor-2 negative (HER2-) early breast cancer with high risk of recurrence.

Key Inclusion Criteria:

• Histopathologically or cytologically confirmed ER-positive (≥ 10% by immunohistochemistry [IHC]), HER2-negative [IHC = 0, 1+, 2 and in situ hybridization [ISH]-negative)] on tumor biopsy or final surgical pathology specimen early stage resected invasive breast cancer without evidence of recurrence or distant metastases, per local laboratory, according to the American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines
• Participants considered at high risk of recurrence at initial staging
• Participants who have received at least 24 months but not more than 60 months of endocrine therapy (AIs or tamoxifen) with or without a CDK 4 and CKD 6 inhibitor (CDK4/6i)
• Participants who received prior CDK4/6i or a poly adenosine diphosphate-ribose polymerase (PARP) inhibitor must have already completed or discontinued these treatments.

Key Exclusion Criteria:

• Participants with inflammatory breast cancer
• History of any prior (ipsilateral and/or contralateral) invasive breast cancer
• Participant with history of malignancy within 3 years of the date of randomization, except for adequately treated basal or squamous cell skin cancer, superficial bladder cancer or carcinoma in situ of the cervix
• Participants who have had more than a 6-month continuous interruption of prior SoC adjuvant endocrine therapy or who are off current adjuvant endocrine therapy more than 6 months prior to randomization

Note: Other protocol defined Inclusion/Exclusion criteria may apply.