A Study of Samuraciclib and Elacestrant in Participants With Metastatic or Locally Advanced HR+/HER2-negative Breast Cancer

Inclusion Criteria: * Histologically confirmed diagnosis of ER-positive, HER2-negative locally advanced or metastatic breast cancer. * Documented objective disease progression while on or within 6 months after the end of the most recent therapy. * Received prior AI in combination with a CDK4/6i as the last therapy * Known TP53 and ESR1 mutation status. * Participants must have measurable disease or bone only disease as defined by Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. * Pre/peri-menopausal participants must have commenced treatment with a luteinizing hormone-releasing hormone (LHRH) agonist at least 4 weeks prior to first dose of study intervention. * Eastern Cooperative Oncology Group (ECOG) performance status ≤1 with no deterioration over the past 2 weeks. * Expected life expectancy of \>12 weeks in the judgement of the treating investigator. Exclusion Criteria: * Inflammatory breast cancer. * Participants with any other active malignancy within 3 years prior to enrollment, except for adequately treated basal cell or squamous cell skin cancer, or carcinoma in situ of the cervix. * More than 1 line of endocrine treatment for locally advanced or metastatic disease treatment. * Inadequate hepatic, renal, and bone marrow function. * Clinically significant cardiovascular disease. * Any current or prior central nervous system metastases, carcinomatous meningitis, or leptomeningeal disease. * Pregnant or breastfeeding women.