Overcoming cetuximab resistance in HNSCC by Hsp90 inhibition to enhance EGFR degradation.

Cetuximab, a monoclonal antibody targeting the epidermal growth factor receptor (EGFR), exhibits limited clinical benefit in head and neck squamous cell carcinoma (HNSCC) despite high receptor expression. We screened a panel of HNSCC cell lines and confirmed that neither total nor phospho-EGFR expression levels correlate with cetuximab sensitivity, identifying KRAS mutations as a primary determinant of de novo resistance. While cetuximab successfully inhibits EGFR phosphorylation and dimerization, it fails to induce significant receptor degradation in cetuximab-resistant lines. Here, we identify a novel "monomer trap" mechanism wherein Hsp90 stabilizes dephosphorylated, cetuximab-bound monomers to prevent their turnover and facilitate recycling to the plasma membrane. Using chemical crosslinking and biochemical assays, we show that disrupting the EGFR:Hsp90 interaction with a sub-cytotoxic minimal essential dose (MED) of an Hsp90 inhibitor selectively redirects monomeric EGFR toward proteasomal and lysosomal degradation. This strategy resulted in 70 to 90% EGFR depletion and synergistic cell killing independent of cell cycle effects, effectively blocking compensatory signaling that drives cetuximab resistance, including in KRAS-mutant models. In vivo, combination therapy significantly suppressed growth in aggressive KRAS-mutant HNSCC xenografts without overt systemic toxicity. Histopathological analysis confirmed that therapeutic efficacy was driven by a significant reduction in EGFR protein levels. Our findings establish Hsp90-mediated stabilization of monomeric EGFR as a novel resistance mechanism and provide a translational rationale for employing low-dose Hsp90 inhibition in combination with cetuximab to improve clinical outcomes for HNSCC patients currently lacking effective targeted options.