Inclusion Criteria:

          -  Female or male patients aged:

               -  Cohort 1: ≥ 18 years old

               -  Cohort 2: ≥ 65 years old

          -  Histologically or cytologically confirmed breast cancer

          -  Most recent biopsy must be HER2 negative

          -  Cohort 1 only: Most recent biopsy must be hormone receptor (HR) (estrogen receptor and
             progesterone receptor) negative

          -  Measurable disease per RECIST 1.1.

               -  Patients with bone-only metastatic cancer must have a measurable lytic or mixed
                  lytic-blastic lesion

               -  Known metastases to the CNS are permitted but not required

          -  Documented (including de novo): (a) locally advanced breast cancer that is not
             considered curable by surgery and/or radiation; or (b) metastatic breast cancer

          -  Disease-free interval of at least 12 months after the completion of systemic
             neoadjuvant or adjuvant chemotherapy for patients previously treated with systemic
             chemotherapy for a tumor surgically resected with curative intent

          -  Cohort 2 only: Prior endocrine therapy with or without a cyclin-dependent kinase (CDK)
             4/6 inhibitor unless endocrine therapy is not indicated. Any prior targeted therapies
             are permitted. There is no limit to the number of prior endocrine therapies.

          -  Cohort 1 only: For central determination of PD-L1 expression, adequate, newly obtained
             or archival core of excisional biopsy

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0, 1, or 2

          -  Adequate bone marrow, hepatic, and renal function

        Exclusion Criteria:

          -  Prior chemotherapy for locally advanced or metastatic disease

          -  Cohort 1 only: prior treatment with pembrolizumab, nivolumab, atezolizumab, any other
             PD-(L)1/PD-L2 inhibitor, or a cytotoxic T-lymphocyte-associated protein 4 (CTLA-4)
             inhibitor

          -  Current evidence or history of leptomeningeal disease

          -  Known human immunodeficiency virus infection, unless well controlled

          -  Known active hepatitis B or known active hepatitis C infection

          -  Other severe acute or chronic medical or psychiatric condition or laboratory
             abnormality that may increase the risk associated with Study participation or
             investigational product administration or may interfere with the interpretation of
             Study results

          -  Presence of neuropathy Grade > 1

          -  History of hypersensitivity to any of the Study drugs or any of their ingredients, as
             applicable

          -  Cohort 1 only:

               -  Chronic autoimmune disease

               -  Evidence of active, non-infectious pneumonia (eg, pneumonia due to autoimmune or
                  connective tissue disease)

               -  Treatment with a live vaccine within 30 days prior to the first dose of
                  nivolumab, pembrolizumab, or atezolizumab

               -  History of active tuberculosis

               -  Prior organ transplantation including allogeneic stem cell transplantation

               -  Active infection requiring systemic therapy

               -  Current or prior use of immunosuppressive medication within 7 days prior to Cycle
                  1, Day 1

               -  Active autoimmune disease that might deteriorate when receiving an
                  immunostimulatory agent

          -  Anticancer treatment, including endocrine therapy, radiotherapy (except stereotactic
             brain radiosurgery), chemotherapy or biologic therapy, ≤ 14 days prior to Enrollment

          -  Major surgery ≤ 28 days prior to Enrollment

          -  Less than 2 weeks or 5 plasma half-lives (whichever is greater) since last use of a
             medication or ingestion of an agent, beverage, or food that is a known clinically
             relevant strong inhibitor or known clinically relevant inducer of the cytochrome P450
             (CYP) 3A pathway