A Study of Samuraciclib in Combination With Fulvestrant in Metastatic or Locally Advanced Breast Cancer in Adult Participants

Inclusion Criteria:

• Histologically confirmed diagnosis of ER-positive, HER2-negative locally advanced ormetastatic breast cancer.

• Documented objective disease progression while on or within 6 months after the endof 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 byResponse Evaluation Criteria in Solid Tumors (RECIST) version 1.1.

• Pre/peri-menopausal participants must have commenced treatment with a luteinizinghormone-releasing hormone (LHRH) agonist at least 4 weeks prior to first dose ofstudy intervention.

• Eastern Cooperative Oncology Group (ECOG) performance status ≤1 with nodeterioration 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 carcinomain situ of the cervix.

• More than 1 line of endocrine treatment for locally advanced or metastatic diseasetreatment.

• Inadequate hepatic, renal, and bone marrow function.

• Clinically significant cardiovascular disease.

• Any current or prior central nervous system metastases, carcinomatous meningitis, orleptomeningeal disease.

• Pregnant or breastfeeding women.