Inclusion Criteria:

          1. Signed informed consent.

          2. Be 18 years of age or older.

          3. Have histologically or cytologically documented locally advanced unresectable or
             metastatic/recurrent SCCHN and meet the criteria of either Cohort A or B.

             Cohort A: Patients with Ongoing CPI Therapy

               1. Patients currently receiving a checkpoint inhibitor (CPI: anti-PD-1 and
                  anti-PD-L1 agents).

               2. Patients currently receiving a CPI must be considered by Investigator to have the
                  potential to derive clinical benefit from continued treatment with pembrolizumab.

               3. Based on RECIST 1.1/iRECIST criteria on current CPI treatment (prior to
                  initiation of this study), patients must have a best response of stable disease
                  (SD) or first evidence of progressive disease (PD) after a minimum of 12 weeks of
                  a CPI.

               4. Patients on other CPI therapy than pembrolizumab must be willing to switch over
                  to pembrolizumab therapy.

             Cohort B: Patients without Previous CPI Therapy

               1. Patients must be checkpoint inhibitor naïve (anti-PD-1 and anti-PD-L1 agents)

               2. Patients should receive study treatment as first line (PD-L1 positive) or as
                  second line (PD-L1 negative) systemic therapy in the advanced/metastatic setting.

          4. Have measurable disease by RECIST 1.1.

          5. Eastern Cooperative Oncology Group (ECOG) Performance Scale 0-1.

          6. Have a life expectancy of ≥ 3 months.

          7. Be willing to provide a pre-treatment tissue sample (archived or fresh).

          8. Demonstrate adequate organ function: hematological, renal, hepatic, coagulation
             parameters.

          9. For women of childbearing potential: agreement to remain abstinent (refrain from
             heterosexual intercourse) or use two highly effective contraceptive methods during the
             treatment period and for at least 180 days after the last dose of study treatment. For
             male patients: Agree that during the period specified above, men will not father a
             child. Male patients must remain abstinent, must be surgically sterile during the
             treatment period and for at least 180 days after the last dose of study treatment.

        Exclusion Criteria:

          1. Any approved anti-cancer therapy including chemotherapy, targeted small molecule
             therapy or radiation therapy within 2 weeks prior to trial Day 0.

          2. Participated on a clinical trial of an investigational agent and/or investigational
             device within 28 days prior to Day 0.

          3. Uncontrolled tumor-related pain.

          4. Malignancies other than indications open for enrollment within 3 years prior to Day 0.

          5. History of severe allergic, anaphylactic, or other hypersensitivity reactions to
             chimeric or humanized antibodies or fusion proteins.

          6. Known hypersensitivity allergy or contraindication to biopharmaceuticals produced in
             Chinese hamster ovary cells or any component of the PD-1/PD-L1 inhibitor formulation.

          7. Active autoimmune disease that has required systemic treatment in the past 2 years

          8. History or any evidence of interstitial lung disease.

          9. History of HIV. HIV antibody testing recommended per investigator's clinical
             suspicion.

         10. Active hepatitis B (hepatitis B surface antigen reactive) or active hepatitis C (HCV
             qualitative RNA detected); testing recommended per investigator's clinical suspicion.

         11. Severe infections within 4 weeks prior to enrollment.

         12. Received therapeutic oral or IV antibiotics within 2 weeks prior to Day 0.

         13. History or current evidence of any condition, therapy or laboratory abnormality that
             in the opinion of the treating investigator might confound the results of the trial.

         14. Prior allogeneic stem cell or solid organ transplant.

         15. Known previous or ongoing, active psychiatric or substance abuse disorders that would
             interfere with the requirements of the trial.

         16. Treatment with systemic immunomodulating agents (including but not limited to IFNs,
             IL-2, ipilimumab) within 6 weeks or five half-lives of the drug, whichever is shorter,
             prior to first dose.

         17. Treatment with systemic immunosuppressive medication within 2 weeks prior to
             initiation of study treatment, or anticipation of need for systemic immunosuppressive
             medication during the course of the study.