Inclusion Criteria:

• Age ≥18 years;
• Written informed consent obtained from the patient/legal representative;
• Histologically confirmed recurrent or metastatic SCCHN; tumor progression or recurrence during or after treatment with only 1 systemic palliative regimen for recurrent or metastatic disease that must have contained a platinum agent; Patients who have only received chemo-radiation with curative intent for treatment of their locally advanced disease or recurrent disease are not eligible. Patients who received concurrent chemo-radiation as part of treatment of their recurrent disease are also not eligible.
• Written consent to provide newly acquired tumor tissue (preferred) or archival tissue for the purpose of establishing PD-L1 status.
• Confirmed PD-L1-negative SCCHN by Ventana SP263;
• WHO/ECOG performance status of 0 or 1;
• At least 1 measurable lesion at baseline;
• No prior exposure to immune-mediated therapy;
• Adequate organ and marrow function; Evidence of post-menopausal status or negative urinary or serum pregnancy test.

Exclusion Criteria:

• Histologically confirmed squamous cell carcinoma of any other primary anatomic location in the head and neck;
• Received more than 1 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;
• another primary malignancy
• Patients with history of brain metastases, spinal cord compression, or a history of leptomeningeal carcinomatosis;
• History of active primary immunodeficiency;
• Known history of previous 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;
• Mean QT interval corrected for heart rate (QTc) ≥470 ms calculated from 3 electrocardiograms (ECGs) using Fridericia's Correction
• Known allergy or hypersensitivity to Investigational Product.
• Any condition that, in the opinion of the Investigator, would interfere with evaluation of the IP or interpretation of patient safety or study results