Study of PDS0101 and Pembrolizumab Combination I/O in Subjects With HPV16 + Recurrent and/or Metastatic HNSCC

Inclusion Criteria: * The participant (or legally acceptable representative if applicable) provides written informed consent for the study. * Be ≥18 years of age on the day of signing the informed consent. * Checkpoint-naïve subjects: Have a history of histologically- confirmed diagnosis of squamous cell cancer of the head and neck (HNSCC) that is recurrent, metastatic, or persistent with: * Confirmed HPV16 infection * Confirmed tumor PDL1 expression defined as a combined positive score (CPS) ≥1 using the FDA-approved Dako PD-L1 immunohistochemistry (IHC) 22C3 PharmDx Assay. * No prior receipt of any immunological therapy for metastatic disease. * Checkpoint experienced subjects have a history of histologically-confirmed diagnosis of HNSCC that is recurrent, metastatic, or persistent with: * Confirmed HPV16 infection * Characterization of tumor PDL1 expression using the FDA-approved PD-L1 IHC 22C3 PharmDx Assay. * Receipt of prior treatment with checkpoint inhibitors as a single agent or in combination, and have received at least 2 doses of the agent or a minimum of 6 weeks on treatment * Have documented clinical progression or recurrence that has been radiologically confirmed * Have recurrent and/or metastatic measurable disease based on RECIST 1.1 as assessed by the local PI/radiology. There must be confirmation that the subject's imaging shows at least 1 lesion that is appropriate for selection as a target lesion per RECIST 1.1. Lesions situated in a previ…