BTX-9341 and Fulvestrant for Advanced Breast Cancer

Inclusion Criteria: * Metastatic and/or locally advanced HR+/HER2- breast cancer (dose escalation: measurable disease and/or at least 1 lytic or mixed \[lytic + sclerotic\] bone lesion that can be assessed by CT or MRI or non-measurable disease \[including bone lesions\]; dose expansion: measurable disease) * Dose escalation: (a) received not more than 1 chemotherapy in the metastatic/advanced setting; (b) no limit to the lines of endocrine therapy (monotherapy or combination therapy) in the metastatic setting; (c) received CDK4/6 inhibitor therapy * Dose expansion: (a) received not more than 1 chemotherapy in metastatic/advanced setting; (b) received not more than 2 lines of endocrine therapy (monotherapy or combination therapy) and must have been on prior endocrine therapy for at least 6 months before progression; (c) received at most 2 lines of CDK4/6 inhibitor therapy (1 in the adjuvant setting and 1 in the metastatic setting) and must have been on prior CDK4/6 inhibitor therapy for at least 6 months * Acceptable hematologic function 1. ANC ≥ 1500 per mL. Note: Use of growth-factors to maintain the ANC criterion is prohibited. 2. Platelet count ≥ 100,000 per mL. Note: Use of transfusions or thrombopoietic agents to achieve the baseline platelet count criterion is prohibited. 3. Hemoglobin ≥ 9.0 g/dL. Note: Packed red blood cell transfusion is allowed up to 14 days prior to trial entry. * Acceptable liver function 1. Bilirubin ≤ 2.0 × institutional upper limit of normal (ULN) (or \< 3.0 × institutional ULN if Gilbert's disease is present) 2. Alanine transaminase (ALT)/aspartate aminotransferase (AST) ≤ 3.0 × institutional ULN (≤ 5.0 × institutional ULN if liver metastases present) 3. Alkaline phosphatase ≤ 2.5 × institutional ULN (≤ 5.0 × institutional ULN if bone or liver metastases present) * Able and willing to sign informed consent * Meets all study requirements in the opinion of the Investigator Exclusion Criteria: * RB1 (retinoblastoma) gene mutation * Symptomatic visceral disease * Clinical evidence or history of central nervous system metastasis * Abnormalities in coagulation, such as bleeding diathesis, or treatment with anticoagulants precluding injections of fulvestrant or luteinizing hormone-releasing hormone (LHRH) agonist