BEMPEG With Pembrolizumab vs Pembrolizumab Alone in Patients With Metastatic or Recurrent HNSCC (PROPEL-36)

Inclusion Criteria: * Provide written, informed consent to participate in the study and follow the study procedures. * Male or female patients, age 18 years or older at the time of signing the informed consent form (ICF). * Have histologically or cytologically-confirmed recurrent or metastatic HNSCC that is considered incurable by local therapies. * No prior systemic therapy for recurrent or metastatic disease. * The eligible primary tumor locations are oropharynx, oral cavity, hypopharynx, and larynx. * Patients may not have a primary tumor site of nasopharynx (any histology) and/or unknown primary. * Have measurable disease based on RECIST 1.1 as determined by the local site Investigator. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. Measure Description: GRADE - ECOG PERFORMANCE STATUS 0 - Fully active, able to carry on all pre-disease performance without restriction 1 - Restricted in physically strenuous activity but ambulatory and able to carry out work of a light or sedentary nature, e.g., light housework, office work 2- Ambulatory and capable of all selfcare but unable to carry out any work activities; up and about more than 50% of waking hours • The tumor must have positive PD-L1 expression (i.e., CPS ≥1) Exclusion Criteria: * Has disease that is suitable for local therapy administered with curative intent. * Has progressive disease within 6 months of completion of curatively intended systemic treatment for locoregionally advanced HNSCC. * Has had radiation therapy (or other non-systemic therapy) within 2 weeks prior to initiation of study drug * Has a life expectancy of less than 3 months and/or has rapidly progressing disease (e.g., tumor bleeding, uncontrolled tumor pain) as determined by the Investigator. * Has a known additional malignancy that is progressing or has required active treatment within 5 years prior to the first dose of study drug * Has an active autoimmune disease that has required systemic treatment in the past 2 years * Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy * Use of an investigational agent or an investigational device within 28 days before the first dose of study drug. * Prior treatment with an anti PD-1, anti PD-L1, anti-PD-L2, or anti CTLA-4 antibody, agents that target IL-2 pathway, or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways. NOTE: Other protocol defined inclusion/exclusion criteria may apply