Study of MK-3475 (Pembrolizumab) in Recurrent or Metastatic Head and Neck Squamous Cell Carcinoma After Treatment With Platinum-based and Cetuximab Therapy (MK-3475-055/KEYNOTE-055)

Inclusion criteria:

* Histologically- or cytologically-confirmed recurrent or metastatic HNSCC of the oral cavity, oropharynx, hypopharynx, and larynx that is considered incurable by local therapies
* Tumor progression or recurrence within 6 months of the last dose of any number of platinum-based and cetuximab therapy lines in the adjuvant, primary, recurrent, or metastatic setting; must be resistant (not responding) to both platinum and cetuximab
* Available tissue for biomarker analysis
* Measurable disease based on RECIST 1.1 as determined by central review
* Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
* Adequate organ function
* Female participants of childbearing potential must have a negative urine or serum pregnancy test and must be willing to use 2 adequate methods of contraception starting with the screening visit through 120 days after the last dose of pembrolizumab
* Male participants with a female partner(s) of childbearing potential must be willing to use 2 adequate methods of contraception from screening through 120 days after the last dose of pembrolizumab

Exclusion criteria:

* Disease that is suitable for local therapy administered with curative intent
* Currently receiving treatment in a study of an investigational agent or using an investigational device \<= 4 weeks prior to the first dose of trial medication
* Diagnosis of immunodeficiency or receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial medication
* Not recovered from AEs due to a previously administered therapy
* Known additional malignancy that is progressing or requires active treatment excluding basal cell carcinoma of the skin, squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cancer
* Known active central nervous system (CNS) metastases and/or carcinomatous meningitis
* Has an active autoimmune disease that has required systemic treatment in past 2 years
* Active, non-infectious pneumonitis
* Active infection requiring systemic therapy
* Pregnant, breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the screening visit through 120 days after the last dose of trial medication
* Human immunodeficiency virus (HIV)
* Hepatitis B or C
* Received live vaccine within 30 days of planned start of study therapy