Trial of H3B-6545, in Women With Locally Advanced or Metastatic Estrogen Receptor-positive, HER2 Negative Breast Cancer

Inclusion Criteria

• •1. Pre- or post-menopausal women.
• •2. ER-positive, HER2-negative breast cancer that is advanced or metastatic.
• •3. Progressed on prior therapy. Multiple prior lines of therapy allowed in Phase 1 and 2. Participants under amendment 6 (or subsequent amendments) must have received prior cyclin-dependent kinase (CDK4/6) inhibitor therapy. Up to one prior chemotherapy in the metastatic setting is allowed.
• •4. A recent archival tumor tissue obtained within 6 months prior to enrollment or a fresh tumor biopsy must be provided. A second biopsy after initiating trial therapy is not required.
• •5. Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1.
• •6. Adequate bone marrow and organ function.
• •7. Participants under amendment 6 (or subsequent amendments) must have measurable disease at baseline as per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria.
• •8. Participants under amendment 6 (or subsequent amendments) must have ESR1 Y537S mutation in absence of ESR1 D538G mutation as per the results of a central laboratory from a Nucleic Acids Whole Blood sample.