Treatment strategy after the discontinuation of immunotherapy for head and neck cancer: a review

First-line systemic therapy with immune checkpoint inhibitors (ICIs) is currently the mainstream treatment for recurrent or metastatic squamous cell carcinoma of the head and neck (R/M SCCHN) that is not amenable to local therapy. However, the optimal treatment after discontinuation of ICIs is still unknown, and no large-scale analysis has been conducted in R/M SCCHN. In this narrative review, we summarize the treatment strategies available to continue ICI therapy using the currently available treatment modalities. After the administration of ICIs, unlike cytotoxic agents, the following responses are observed: durable response, pseudo-progressive disease, hyper-progressive disease, mixed response, immune-related adverse events (irAEs), and improved sensitivity to subsequent chemotherapy. Some patients show a durable response to ICIs and a good prognosis; however, many patients will require salvage therapy. We need to select the subsequent treatment according to the various responses unique to ICIs to obtain a durable response. For instance, there are reports on the effectiveness of reapplication of local therapy for a mixed response, retreatment with ICIs, and off-treatment in responsive cases complicated by irAEs. If a durable response can be maintained with ICI therapy, a long-term prognosis that cannot be obtained with conventional chemotherapy can be achieved.