Inclusion Criteria: - Age ≥18 years; - Written informed consent obtained from the
        patient/legal representative; - Histologically or cytologically confirmed recurrent or
        metastatic SCCHN; - Tumor progression or recurrence during or after only one palliative
        systemic treatment regimen for recurrent or metastatic disease that must have contained a
        platinum agent OR progression within 6 months of the last dose of platinum given as part of
        multimodality therapy with curative intent; - Confirmed PD-L1-positive or -negative SCCHN
        by the Ventana PD-L1 SP263 IHC assay; - WHO/Eastern Cooperative Oncology Group (ECOG)
        performance status of 0 or 1; At least 1 measurable lesion, - Not previously irradiated; -
        No prior exposure to immune-mediated therapy; - Adequate organ and marrow function;
        Evidence of post-menopausal status or negative urinary or serum pregnancy test for female
        pre-menopausal patients. Exclusion Criteria: - Histologically or cytologically confirmed
        squamous cell carcinoma of any other primary anatomic location in the head and neck; -
        Received more than 1 palliative systemic regimen for recurrent or metastatic disease; -Any
        concurrent chemotherapy, Investigational Product, biologic, or hormonal therapy for cancer
        treatment; - Receipt of any investigational anticancer therapy within 28 days or 5
        half-lives; - Receipt of last dose of an approved (marketed) anticancer therapy
        (chemotherapy, targeted therapy, biologic therapy, mAbs, etc) within 21 days prior to the
        first dose of study treatment; - Major surgical procedure within 28 days prior to the first
        dose of Investigational Product; - Any unresolved toxicity NCI CTCAE Grade ≥2 from previous
        anticancer therapy with the exception of alopecia, vitiligo, and the laboratory values
        defined in the inclusion criterion; - Current or prior use of immunosuppressive medication
        within 14 days before the first dose of their assigned Investigational Product; - History
        of allogeneic organ transplantation; - Active or prior documented autoimmune or
        inflammatory disorders; - Uncontrolled intercurrent illness; - Patients with a history of
        brain metastases, spinal cord compression, or leptomeningeal carcinomatosis; - Mean QT
        interval corrected for heart rate (QTc) ≥470 ms calculated from 3 electrocardiograms (ECGs)
        using Fridericia's Correction; - History of active primary immunodeficiency; - Active
        tuberculosis; - Active infection including hepatitis B, hepatitis C or human
        immunodeficiency virus (HIV); - Receipt of live, attenuated vaccine within 30 days prior to
        the first dose of Investigational Product; - Pregnant or breast-feeding female patients; -
        Known allergy or hypersensitivity to Investigational Product