Objective.:In patients with head and neck squamous cell carcinoma (HNSCC),cetuximab [a monoclonal antibody targeting epidermal growth factor receptor (EGFR)] has been shown to improve overall survival when combined with radiotherapy in the locally advanced setting or with chemotherapy in first-line recurrent and/or metastatic (R/M) setting,respectively.While biomarkers of resistance to cetuximab have been identified in metastatic colorectal cancer,no biomarkers of efficacy have been identified in HNSCC.Here,we aimed to identify biomarkers of cetuximab sensitivity/resistance in HNSCC.Methods:HNSCC patients treated with cetuximab at the Curie Institute,for whom complete clinicopathological data and formalin-fixed paraffin-embedded (FFPE) tumor tissue collected before cetuximab treatment were available,were included.Immunohistochemistry analyses of PTEN and EGFR were performed to assess protein expression levels.PIK3CA and H/N/KRAS mutations were analyzed using high-resolution melting (HRM) and Sanger sequencing.We evaluated the predictive value of these alterations in terms of progression-free survival (PFS).Results:Hot spot activating PIK3CA and KRAS/HRAS mutations were associated with poor PFS among HNSCC patients treated with cetuximab in the first-line R/M setting,but not among HNSCC patients treated with cetuximab in combination with radiotherapy.Loss of PTEN protein expression had a negative predictive value among HNSCC patients treated with cetuximab and radiotherapy.High EGFR expression did not predict cetuximab sensitivity in our patient population.Conclusions:Hot spot activating PIK3CA and RAS mutations predicted cetuximab resistance among HNSCC patients in the first-line R/M setting,whereas loss of PTEN protein expression predicted resistance to cetuximab when combined to radiotherapy.