Inhibition of MEK with trametinib enhances the efficacy of anti-PD-L1 inhibitor by regulating anti-tumor immunity in head and neck squamous cell carcinoma.

Major histocompatibility complex (MHC) class I downregulation is the primary immune evasion mechanism associated with failure in anti-PD-1/PD-L1 blockade therapies for cancer. Here, we examined the role of MEK signaling pathway inhibition in head and neck squamous cell carcinoma (HNSCC) both in vitro and in vivo . We found that trametinib, a small molecule inhibitor of MEK, significantly enhanced MHC class I and PD-L1 expression in human HNSCC cell lines, and this occurred via STAT3 activation. Trametinib also further upregulated the increase in CXCL9 and CXCL10 expression caused by IFN-γ in HNSCC cells, which is associated with T cell infiltration in tumor tissues. Finally, we evaluated the therapeutic efficacy of trametinib combined with an anti-PD-L1 monoclonal antibody in vivo , using SCCVII mouse syngeneic tumor model for HNSCC. While neither PD-L1 blockade nor trametinib treatment alone affected tumor growth, the combined therapy significantly delayed tumor growth. Our results indicate that in the combined therapy trametinib increases CD8+ T cell infiltration in the tumor site and upregulates antigen presentation, and this may be associated with enhanced PD-L1 blockade efficacy. Furthermore, our results suggest that this combination would therapeutically benefit patients with HNSCC.