Inclusion Criteria:

          -  Ability to comprehend and willingness to sign a written informed consent form.

          -  Confirmed diagnosis of locally advanced or metastatic SCAC.

          -  Must have received (or been intolerant to or ineligible for) at least 1 prior line of
             platinum-based chemotherapy and received no more than 2 prior systemic treatments.

          -  Must have measurable disease by RECIST v1.1.

          -  Eastern Cooperative Oncology Group performance status of 0 to 1.

          -  If HIV-positive, then all of the following criteria must also be met: CD4+ count ≥
             300/μL, undetectable viral load, and receiving highly active antiretroviral therapy.

        Exclusion Criteria:

          -  Receipt of anticancer therapy or participation in another interventional clinical
             study within 21 days before the first administration of study drug; 6 weeks for
             mitomycin C.

          -  Radiotherapy within 14 days of first dose of study treatment with the following
             caveats: 28 days for pelvic radiotherapy, 6 months for thoracic region radiotherapy
             that is > 30 Gy.

          -  Prior treatment with programmed cell death protein 1 (PD-1) or programmed cell death
             ligand protein 1 (PD-L1)-directed therapy.

          -  Active autoimmune disease requiring systemic immunosuppression.

          -  Known central nervous system (CNS) metastases and/or carcinomatous meningitis.

          -  Known active hepatitis infection.

          -  Active infections requiring systemic therapy.

          -  Is pregnant or breastfeeding or is expecting to conceive or father children within the
             projected duration of the study, from screening through 6 months after the last dose
             of study drug.