A Study of Imlunestrant Versus Standard Endocrine Therapy in Participants With Early Breast Cancer

Inclusion Criteria: * Have a diagnosis of ER+, HER2- early-stage, resected, invasive breast cancer without evidence of distant metastasis. * Participants must have received at least 24 months but not more than 60 months of any adjuvant ET, from time of adjuvant ET initiation. * Participants may have received (neo) adjuvant chemotherapy and/or targeted therapy with a CDK4/6- or PARP- inhibitor. * Must have an increased risk of disease recurrence based on clinical-pathological risk features. * Have a Performance Status of 0 or 1 on the Eastern Cooperative Oncology Group scale. * Have adequate organ function. Exclusion Criteria: * Have any evidence of metastatic disease (including contralateral ALN) or inflammatory breast cancer at primary breast cancer diagnosis. * Participants with more than a 6-month consecutive gap in therapy during the course of prior adjuvant ET. * Participants who have completed or discontinued prior adjuvant ET \>6 months prior to screening. * Participants with a history of previous breast cancer are excluded, with the exception of ipsilateral DCIS treated by locoregional therapy alone ≥5 years ago. * Pregnant, breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the screening visit through 180 days after the last dose of study intervention. * Participant has previously received ET of any duration for breast cancer prevention (tamoxifen or AIs) or raloxifene. * Participants with a history of any other cancer. * Have serious preexisting medical conditions that, in the judgment of the investigator, would preclude participation in this study.