Ph II Study of Pembrolizumab & Eribulin in Patients With HR+/HER2- MBC Previously Treated With Anthracyclines & Taxanes

Inclusion Criteria

• Written informed consent prior to beginning specific protocol procedures.
• Female patients ≥18 years of age.
• Eastern Cooperative Oncology Group (ECOG) performance status must be 0 or 1 which the Investigator believes is stable at the time of screening.
• Life expectancy ≥ 12 weeks.
• Patients have a histologically and/or cytologically confirmed diagnosis of breast cancer.
• Patients have radiologic evidence of inoperable locally recurrent or MBC.
• Patients have HER2-negative breast cancer (based on most recently analyzed biopsy) defined as a negative in situ hybridization (ISH) test or an immunohistochemistry (IHC) status of 0, 1+, or 2+ (if IHC 2+, a negative ISH test is required) by local laboratory testing.
• Patients have HR-positive breast cancer defined as estrogen receptor (ER) and/or progesterone receptor (PR) with >1% of tumor cells positive for ER and/or PR by IHC irrespective of staining intensity.
• Available tumor tissue for PD-L1 biomarker analysis from a newly obtained core or excisional biopsy since last progression of a metastatic tumor lesion not previously irradiated.

Note: Subjects for whom tumor biopsies cannot be obtained (e.g., inaccessible tumor or subject safety concern) may submit an archived metastatic tumor specimen only upon agreement from the Sponsor.

• Patients have measurable disease based on Response Evaluation Criteria in Solid Tumors (RECIST) criteria version 1.1 as assessed by site Investigator and local radiology review.
• Patients have received at least one, but not more than two, prior chemotherapeutic regimens for locally recurrent and/or metastatic disease. Prior therapy must have included an anthracycline and a taxane in any combination or order and either in the early or metastatic disease setting unless contraindicated for a given patient. Prior anti-hormonal therapy is mandatory.
• Patients have adequate bone marrow and organ function as defined by the following laboratory values:
• Absolute neutrophil count (ANC) ≥ 1.5 x 10^9/L.
• Platelets ≥ 100 x 10^9/L.
• Hemoglobin ≥ 9 g/dL.
• Potassium, calcium (corrected for serum albumin), and magnesium within normal limits for the institution.
• Serum creatinine ≤ 1.5 x upper limit of normal (ULN).
• Alanine aminotransferase (AST) and aspartate aminotransferase (ALT) ≤ 2.5 x ULN (or ≤ 5.0 x ULN if liver metastases are present).
• Total serum bilirubin within normal range (or ≤ 1.5 x ULN if liver metastases are present). Patients with known Gilbert disease who have serum bilirubin ≤ 3 x ULN may be enrolled.
• Patients must be accessible for treatment and follow-up.

Exclusion Criteria

• Patients have received previous treatment with eribulin and an/or anti-PD1 or anti-PD-L1 agents.
• Patients have a known hypersensitivity to any of the excipients of MK3475 (pembrolizumab) or eribulin.
• Patients who have received chemotherapy, targeted small molecule therapy, or radiotherapy within two weeks of first dose of study treatment.
• Patients who have received monoclonal antibodies for direct antineoplastic treatment or an investigational agent/device within four weeks of first dose of study treatment.
• Patients have known active central nervous system (CNS) metastases and/or carcinomatous meningitis.

Note: Known brain metastases are considered active, if any of the following criteria is applicable:

• Brain imaging during screening demonstrates progression of existing metastases and/or appearance of new lesions compared to brain imaging performed at least four weeks earlier.
• Neurological symptoms attributed to brain metastases have not returned to baseline.
• Steroids were used for brain metastases within 28 days of first dose of study treatment.
• Patients have peripheral neuropathy grade 2 or more.
• Patients have a concurrent malignancy or malignancy within five years of study enrollment (with the exception of adequately treated, basal or squamous cell skin carcinoma or curatively res