Inclusion Criteria:

          -  Patient will have a tumor harboring genomic mutation of ARID1A

          -  Histological or cytological evidence of metastatic or surgically unresectable disease
             harboring ARID1A mutation

          -  All subjects must have measurable disease by computed tomography (CT) or magnetic
             resonance imaging (MRI) per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1
             criteria

          -  Evaluable tumor tissue (archived or new biopsy) must be provided for biomarker
             analysis as formalin-fixed paraffin-embedded (FFPE) tumor block or minimum of 10
             slides. Archived tissue may be from prior biopsy of unresectable or metastatic disease
             or from prior surgical resection

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

          -  Prior palliative radiotherapy must have been completed at least 2 weeks prior to study
             drug administration. Patients must have measurable disease outside the radiation field
             to be eligible. Patients with progression in a previously radiated field will also be
             eligible

          -  White blood cell (WBC) >= 2000/uL (obtained within 7 days prior to first dose)

          -  Neutrophils >= 1500/uL (obtained within 7 days prior to first dose)

          -  Platelets >= 100 x 1 x 10^3/uL (obtained within 7 days prior to first dose)

          -  Hemoglobin >= 9.0 g/dL (obtained within 7 days prior to first dose)

          -  Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) =< 3 x upper limit
             of normal (ULN) (or =< 5 X ULN for participants with liver metastases) (obtained
             within 7 days prior to first dose)

          -  Total bilirubin =< 1.5 x ULN (except subjects with Gilbert Syndrome, who can have
             total bilirubin < 3.0 mg/dL) (obtained within 7 days prior to first dose)

          -  Serum creatinine =< 1.5 x ULN or creatinine clearance (CrCl) >= 30 mL/min (using the
             Cockcroft-Gault formula) (obtained within 7 days prior to first dose)

          -  Re-enrollment: This study permits the re-enrollment of a subject that has discontinued
             the study as a pre-treatment failure (ie, subject has not been treated). If
             re-enrolled, the subject must be re-consented and inclusion/exclusion criteria
             reassessed

          -  Patients are required to participate in PA13-0291 for correlative studies. Patients
             will need to consent for biopsy and peripheral blood collection as documented in the
             study calendar

        Exclusion Criteria:

          -  Prior treatment with an anti-PD-1, anti-PD-L1, anti-CTLA-4 antibody, or any other
             antibody or drug specifically targeting T-cell co-stimulation or immune checkpoint
             pathways

          -  Active brain metastases or leptomeningeal metastases. Subjects with brain metastases
             are eligible if these have been treated and there is no magnetic resonance imaging
             (MRI) evidence of progression for at least 4 weeks after treatment is complete and
             within 28 days prior to first dose of study drug administration. Where MRI is
             contraindicated CT scan is acceptable. Cases must be discussed with the medical
             monitor. Brain lesions are not considered measurable disease. There must also be no
             requirement for immunosuppressive doses of systemic corticosteroids (> 10 mg/day
             prednisone equivalents) for at least 2 weeks prior to study drug administration

          -  Any serious or uncontrolled medical disorder, that in the opinion of the investigator,
             may increase the risk associated with study participation or study drug
             administration, impair the ability of the subject to receive protocol therapy, or
             interfere with the interpretation of study results

          -  Prior malignancy active within the previous 3 years except for locally curable cancers
             that have been apparently cured, such as basal or squamous cell skin cancer, prostate
             cancer without evidence of prostate specific antigen (PSA) progression or carcinoma in
             situ such as the following: gastric, prostate, cervix, colon, melanoma, or breast for
             example

          -  Subjects with active, known or suspected autoimmune disease. Subjects with vitiligo,
             type I diabetes mellitus, residual hypothyroidism, due to autoimmune condition only
             requiring hormone replacement, psoriasis not requiring systemic treatment, or
             conditions not expected to recur in the absence of an external trigger are permitted
             to enroll

          -  Subjects with a condition requiring systemic treatment with either corticosteroids (>
             10 mg daily prednisone equivalents) or other immunosuppressive medications within 14
             days of study drug administration. Inhaled or topical steroids and adrenal replacement
             doses > 10 mg daily prednisone equivalents are permitted in the absence of active
             autoimmune disease

          -  All toxicities attributed to prior anti-cancer therapy other than neuropathy, alopecia
             and fatigue must have resolved to grade 1 (National Cancer Institute [NCI] Common
             Terminology Criteria for Adverse Events [CTCAE] version 5) or baseline before
             administration of study drug. Subjects with toxicities attributed to prior anti-cancer
             therapy which are not expected to resolve and result in long lasting sequelae, such as
             neuropathy after platinum based therapy, are permitted to enroll. Neuropathy must have
             resolved to grade 2 (NCI CTCAE version 5)

          -  Treatment with any chemotherapy, radiation therapy, biologics for cancer, or
             investigational therapy within 28 days of first administration of study treatment.
             Prior palliative radiotherapy must have been completed at least 2 weeks prior to study
             drug administration as indicated in inclusion criteria

          -  Positive test for hepatitis B virus surface antigen (HBV sAg) or hepatitis C virus
             ribonucleic acid (RNA) or hepatitis C antibody (HCV antibody) indicating acute or
             chronic infection

          -  Known history of testing positive for human immunodeficiency virus (HIV) or known
             acquired immunodeficiency syndrome (AIDS)