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 inhibitoralone or in combination with a cyclin-dependent kinase (CDK)4/6 inhibitor

-- Participants are expected to have received prior treatment with a CDK4/6inhibitor, 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 ovarianfunction suppression

• Have RECIST evaluable disease (measurable disease and/or nonmeasurable bone-onlydisease)

• 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/ adjuvantchemotherapy), fulvestrant, or any investigational-ER-directed therapy (includingSERDs and non-SERDs), any PI3K-, mTOR- or AKT- inhibitor

• Have visceral crisis, lymphangitic spread within the lung, or any evidence ofleptomeningeal disease.

• Have symptomatic or untreated brain metastasis.

• Have serious preexisting medical conditions that, in the judgment of theinvestigator, would preclude participation in this study

• Known allergic reaction against any of the components of the study treatment