Inclusion Criteria:

          -  Histologically or cytologically confirmed recurrent or metastatic non-cutaneous HNSCC
             for which there are no surgical or radiation curative options.

          -  Patients may have received up to 3 prior lines of therapy for metastatic or recurrent
             disease.

          -  ECOG performance status ≤2

        Exclusion Criteria:

          -  Patients with nasopharyngeal HNSCC will be excluded

          -  Patients who have had chemotherapy or radiotherapy within 2 weeks prior to entering
             the study.

          -  Patients who have not recovered from adverse events due to prior anti-cancer therapy

          -  Patients who have previously received PD-1 or PD-L1 inhibitors for
             metastatic/recurrent disease

          -  Patients currently receiving metformin or who have received metformin in the last 6
             months