• Unresectable Stage III or Stage IV HER2 negative breast cancer (either triple negative or hormone receptor positive)

‣ Triple negative breast cancer for this study is defined as estrogen receptor \<10%, progesterone receptor \<10% by immunohistochemistry, and HER2- negative by Herceptest (0 or 1+) or not amplified by in situ hybridization as per routine clinical testing.

⁃ Hormone receptor positive breast cancer for this study is defined as either estrogen receptor ≥10% or progesterone receptor ≥10% by immunohistochemistry, and HER2- negative by Herceptest (0 or 1+) or not amplified by in situ hybridization as per routine clinical testing.

• Age 18 years or older

• Performance status ECOG 0-2

• Life expectancy ≥ 12 weeks

• Documented progressive disease, based on radiographic, clinical or pathologic assessment, during or subsequent to last anticancer therapy. Patients who need to change systemic therapy for other indications such as toxicity that are otherwise eligible for this study may enroll with approval of the lead principal investigator.

• For triple negative breast cancer patients, subject is in first to fourth line setting of treatment for metastatic or unresectable disease, and have received 0 to 3 prior regimens for metastatic or unresectable disease.

• For hormone receptor positive breast cancer patients, subjects must have received prior cyclin dependent kinase inhibitor in the metastatic setting. They may have received up to 3 prior lines of chemotherapy and/or antibody drug conjugates for metastatic or unresectable disease.

• Among triple negative breast cancer patients enrolled in the first line treatment setting, subjects must be PD-L1 negative by 22C3 assay and not be eligible for FDA approved standard of care chemotherapy and anti-PD-1/PD-L1 combination therapy as alternative to this clinical trial. This does not apply if patients have previously received PD-1 or PD-L1 blockade as part of neoadjuvant or adjuvant therapy regimen.

• Screening laboratory values must meet the following criteria:

‣ Neutrophils ≥ 1500/uL

⁃ Platelets ≥ 100 x10(9)/L

⁃ Hemoglobin ≥ 8 g/dL Patients may receive erythrocyte transfusions to achieve this hemoglobin level at the discretion of the investigator. Initial treatment must not begin earlier than the day after the erythrocyte transfusion.

⁃ Creatinine ≤ 2 mg/dL

⁃ Creatinine clearance \>30 mL/minute

⁃ AST ≤ 2.5 X ULN without, and ≤ 5 x ULN with hepatic metastasis

⁃ ALT ≤ 2.5 X ULN without, and ≤ 5 x ULN with hepatic metastasis

⁃ Total Bilirubin ≤ 1.5 X ULN (except patients with Gilbert's syndrome or liver involvement, who must have a total bilirubin ≤ 2 X ULN)

⁃ Alkaline phosphatase ≤ 2.5 X ULN without, and ≤ 5 x ULN with hepatic metastasis

• All men as well as women of child bearing potential enrolled in this trial must agree to use effective contraception during the course of the trial and for at least 6 months after discontinuing study treatment. Patients and/or partners who are surgically sterile or postmenopausal are exempt from this requirement.

‣ A female of child-bearing potential is any woman (regardless of sexual orientation, marital status, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria: ( 1) has not undergone a hysterectomy or bilateral oophorectomy OR (2) has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months).

• Provision of consent for pre-treatment and on-treatment biopsies. Biopsy sites must be soft tissue tumor lesions or accessible visceral diseases that can be biopsied with acceptable clinical risk (as judged by the investigator); are large enough to allow for the collection of tumor tissue for proposed correlative studies (e.g., anticipated goal of 6-8 cores preferred when feasible using a ≥ 18 gauge needle with an expected core sample length of 5 mm); and have not been irradiated prior to entry. This does not include bone lesions. This may exclude many lung lesions and small lesions.

• Measurable disease allowing for serial assessment of at least one target lesion(s) by RECIST 1.1 criteria \[100\]. Target lesions selected for tumor measurements should be those where additional (e.g., palliative) treatments are not indicated or anticipated.

• All residual toxicity related to prior anticancer therapies (excluding alopecia, grade 2 fatigue, vitiligo, endocrinopathies on stable replacement therapy, grade 2 neuropathy from taxanes or platinum and grade 2 hearing loss from platinum) must resolve to grade 1 severity or less (or returned to baseline) prior to receipt of study treatment.

• Read, understood, and provided written informed consent, and if applicable, Health Insurance Portability and Accountability Act (HIPAA) authorization, after the nature of the study has been fully explained, and must be willing to comply with all study requirements and procedures.