A Study of Imlunestrant, Investigator's Choice of Endocrine Therapy, and Imlunestrant Plus Abemaciclib in Participants With ER+, HER2- Advanced Breast Cancer

Inclusion Criteria:

* Have a diagnosis of ER+, HER2- locally advanced or metastatic breast cancer
* Have disease that has demonstrated progression on or after an aromatase inhibitor alone or in combination with a cyclin-dependent kinase (CDK)4/6 inhibitor

  \-- Participants are expected to have received prior treatment with a CDK4/6 inhibitor, if this treatment is approved and can be reimbursed
* Must be deemed appropriate for treatment with endocrine therapy
* If female, have a postmenopausal status by natural or surgical means or by ovarian function suppression
* Have RECIST evaluable disease (measurable disease and/or nonmeasurable bone-only disease)
* Have a performance status of 0 or 1 on the Eastern Cooperative Oncology Group scale (Oken et al. 1982)
* Have adequate renal, hematologic, and hepatic organ function
* Must be able to swallow capsules/tablets

Exclusion Criteria:

* Have received prior treatment with chemotherapy (except for neoadjuvant/ adjuvant chemotherapy), fulvestrant, or any investigational-ER-directed therapy (including SERDs and non-SERDs), any PI3K-, mTOR- or AKT- inhibitor
* Have visceral crisis, lymphangitic spread within the lung, or any evidence of leptomeningeal disease.
* Have symptomatic or untreated brain metastasis.
* Have serious preexisting medical conditions that, in the judgment of the investigator, would preclude participation in this study
* Known allergic reaction against any of the components of the study treatment