Study to Evaluate the Efficacy and Safety of Eribulin Mesylate in Combination With Pembrolizumab in Participants With Metastatic Triple-Negative Breast Cancer (mTNBC)

Inclusion Criteria

• Females or males, aged >=18 years at the time of signing the informed consent form (ICF).
• mTNBC (confirmed from most recent tissue sample) meeting the following criteria:
• Estrogen receptor (ER) and progesterone receptor negative (a tumor is ER and/or progesterone receptor positive if at least 1 percent (%) of the cells examined have estrogen and/or progesterone receptors) and human epidermal growth factor receptor 2 (HER2) negative (defined as immunohistochemistry [IHC] less than ( =10 millimeter (mm) in long axis diameter for nonlymph nodes or >=15 mm in short axis diameter for lymph nodes that is serially measurable according to Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST 1.1) using computerized tomography (CT) or magnetic resonance imaging (MRI) or panoramic and close-up color photography.
• Lesions that have had radiotherapy must show subsequent radiographic evidence of increased size to be deemed a target lesion.
• Life expectancy of >=3 months.
• Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1.
• Adequate renal function as evidenced by serum creatinine less than or equal to ( =50 millimeter per minute (mL/min) according to the Cockcroft and Gault formula.
• Adequate bone marrow function, defined as:
• Absolute neutrophil count (ANC) >=1.5*10^9/L.
• Hemoglobin (Hb) >=10.0 gram per deciliter (g/dL) (can be corrected by growth factor or transfusion).
• Platelet count >=100*10^9/L.
• Adequate liver function, defined as:
• Total bilirubin = 2 (except for alopecia and Grade 2 sensory neuropathy) according to Common Terminology Criteria for Adverse Events (CTCAE v4.03).
• Any other malignancy that required treatment or has shown evidence of recurrence (except for nonmelanoma skin cancer, or histologically confirmed complete excision of carcinoma in situ) during the 5 years prior to enrollment in this study.
• History of significant cardiovascular disease, defined as:
• congestive heart failure greater than New York Heart Association (NYHA) Class II according to the NYHA Functional Classification.
• unstable angina or myocardial infarction within 6 months of enrollment.
• serious cardiac arrhythmia.
• Clinically significant electrocardiogram (ECG) abnormality, including a marked Baseline prolonged QT interval/corrected QT interval ([QT/QTc], example, a repeated demonstration of a QTc interval >500 millisecond [ms]).
• History of concomitant medical conditions or infectious diseases that, in the opinion of the investigator, would compromise the participant's ability to safely complete the study.
• Hypersensitivity to the active substance or any other excipients of the eribulin mesylate drug product, or severe hypersensitivity (>=Grade 3) to pembrolizumab and/or any of its excipients.
• Scheduled for major surgery during the study.
• Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment. The use of physiologic doses of corticosteroids may be approved after consultation with the sponsor.
• Has a history of (non-infectious) pneumonitis that required steroids or current pneumonitis.
• Has a history of interstitial lung disease.
• Has an active infection requiring systemic therapy.
• Has received a live-virus vaccination within 30 days of planned start of study therapy. Seasonal flu vaccines that do not contain live virus are permitted.
• The investigator's belief that the participant is medically unfit to receive eribulin mesylate and pembrolizumab or unsuitable for any other reason.