Inclusion Criteria:

          -  Histologically confirmed advanced breast cancer not amenable to curative treatment by
             surgery or radiotherapy, that is amenable to biopsy

          -  Radiographically measurable disease by RECIST v1.1

          -  Age ≥ 18 years

          -  Life expectancy of more than 3 months

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0 to 1

          -  Signed informed consent form

          -  Adequate hepatic, bone marrow, and renal function as defined below in the body of the
             protocol

        Exclusion Criteria:

          -  Prior exposure to PARP inhibitor-based therapy

          -  Prior disease progression while receiving anti-PD-1 or anti-PD-L1 therapy within 6
             months of use

          -  Recent severe infection or antibiotic use, or known chronic infection with human
             immunodeficiency virus (HIV) or hepatitis B virus

          -  Diagnosis of immunodeficiency or is receiving systemic steroid or other
             immunosuppressive therapy

          -  Active autoimmune disease that has required systemic treatment in the past 2 years

          -  History of tuberculosis

          -  History of allogenic bone marrow transplant or solid organ transplant

          -  Live vaccine administration within 30 days of planned start of study therapy

          -  Cardiovascular disease problems including unstable angina, therapy for lifethreatening
             ventricular arrhythmia, or myocardial infarction, stroke within the last 6 months, or
             a diagnosis of congestive heart failure

          -  Women who are pregnant or breastfeeding

          -  Patients with known untreated central nervous system (CNS) metastases

          -  Patients with history of another active malignancy within the past 2 years, excluding
             non-melanoma carcinoma of the skin

          -  Patients receiving any other investigational agents