The purpose of this study is to evaluate the safety and efficacy of samuraciclib in combination with fulvestrant versus fulvestrant alone in adult participants with metastatic or locally advanced Hormone Receptor (HR) positive and Human Epidermal Growth Factor Receptor (HER)2-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 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.