Potential of Pembrolizumab in Metastatic or Recurrent Head and Neck Cancer: Evidence to Date.

Relapsed and/or metastatic head and neck squamous cell carcinoma (R/M HNSCC) is a heterogeneous disease previously associated with poor prognosis and limited treatment options until the advent and implementation of immune checkpoint inhibitors (ICIs). The fully humanized monoclonal antibody pembrolizumab alone, or in combination with chemotherapy, was shown to have significantly improved overall survival (OS) when compared to the standard of care (SOC) EXTREME regimen consisting of the monoclonal antibody cetuximab combined with a platinum and 5-fluorouracil. Pembrolizumab with or without chemotherapy will soon supplant the EXTREME regimen that has been in use for over a decade. Given the fast-approaching significant change in the treatment algorithm for R/M HNSCC and the novelty of ICIs in general, it is important to review the literature to date to understand how this rapidly growing treatment class has come about and explore potential areas of research for the plethora of questions that remain unanswered in selecting patients appropriate for treatment with ICIs in the R/M setting. In this review, we explore the landmark trials leading to the use of ICIs for R/M HNSCC with a particular focus on pembrolizumab, the most well-studied ICI in this setting. We also provide an overview of the rationale behind the use of ICIs in relation to the immune system and challenges surrounding tumor heterogeneity and PD-L1 expression status, human papilloma virus (HPV) and the efficacy of ICI, potential of radiation therapy for enhancement of ICI response, and complications of immune-related adverse events (irAEs).