A Study of BTX-A51 in People With Advanced Solid Tumor and Breast Cancer

Inclusion Criteria:

• Demonstration of understanding and voluntarily signing of an informed consent form

• Age ≥ 18 years

• Histologically or cytologically documented, incurable or metastatic solid tumor thatis refractory to or intolerant of all standard therapy or for which no standardtherapy is available

• Phase 1b and 1c only: Histologically confirmed diagnosis of ER+, HER2- mBC notamenable to resection or radiation therapy with curative intent.

• Measurable disease per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST v1.1).

• Adequate organ function

• Females of childbearing age must not be pregnant at time of Screening/beginning oftreatment and agree to either abstain from sexual intercourse or use highlyeffective methods of contraception (for up to 3 months after last dose of studydrug)

• Males sexually active with a woman of childbearing age must agree to use barriermethod of birth control during and after the study (up to 3 months after last doseof study drug)

Exclusion Criteria:

• Life expectancy <3 months, as determined by the Investigator.

• Treatment with any local or systemic antineoplastic therapy (including chemotherapy,hormonal therapy, or radiation) within 3 weeks prior to first dose of BTX-A51

• Chronic use of corticosteroids in excess of 10 mg daily of prednisone or equivalentwithin 4 weeks prior to first dose of BTX-A51

• Major trauma or major surgery within 4 weeks prior to first dose of BTX-A51.

• Adverse events from prior anti-cancer therapy that have not resolved to Grade ≤1except for alopecia or Grade ≤2 immunotherapy-related thyroid toxicity.

• History of, or known, central nervous system (CNS) disease involvement, or priorhistory of NCI CTCAE Grade ≥3 drug-related CNS toxicity.

• Clinically significant cardiac disease

• Active uncontrolled systemic fungal, bacterial, mycobacterial, or viral infection

• Known positive test result for human immunodeficiency virus (HIV) or acquired immunedeficiency syndrome (AIDS)

• Active hepatitis C virus (HCV) or hepatitis B virus (HBV)

• Second primary malignancy that has not been in remission for greater than 3 years

• Any serious underlying medical (e.g., pulmonary, renal, hepatic, gastrointestinal,or neurological) or psychiatric condition (e.g., alcohol or drug abuse, dementia oraltered mental status) or any issue that would limit compliance with studyrequirements

• Pregnant, lactating, or breastfeeding.

• Participation or plans to participate in another interventional clinical study.