Background: Head and neck squamous cell carcinoma (HNSCC) is characterized by epidermal growth factor receptor (EGFR) overexpression but treatments targeting EGFR have met with limited clinical success. We hypothesized that everolimus (EV), an mTOR inhibitor, potentiates the activity of erlotinib (ER), an EGFR tyrosine kinase inhibitor, in platinum-resistant recurrent and/or metastatic HNSCC. Methods: This single-arm two-stage phase II clinical trial evaluated EV 5 mg and ER 150 mg orally daily in patients (pts) with platinum-resistant recurrent and/or metastatic HNSCC. Primary endpoints were response rate and 12-week progression-free survival (PFS). Plasma biomarkers by multianalyte profiling (Luminex Corp) and ELISA at baseline (20 pts), 4 weeks (18 pts) and 12 weeks (16 pts) as well as p16 expression in baseline tumor tissue (29 pts) were assessed in correlation with clinical results. Results: Of the 36 evaluable pts (median age 61 years, 29 males, 19 with oropharynx primaries, 15 with ≥2 lines of chemotherapy, 17 with prior cetuximab), 3 (8%) achieved partial response at 4 weeks, one of which was confirmed at 12 weeks, while 27 (75%) pts achieved disease stabilization at 4 weeks, 11 of which were confirmed at 12 weeks. The 12-week PFS was 49%, median PFS was 11.9 weeks and median overall survival (OS) was 10.25 months. Grade 3 toxicities included mucositis (17%), fatigue (14%), skin (8%), diarrhea (8%), and 5 pts withdrew from trial secondary to toxicity. High neutrophil gelatinase-associated lipocalin(NGAL) levels at baseline were associated with worse OS (HR 4.59; 95%CI 1.36, 15.46; p=0.014) and at 4 weeks with worse PFS (HR=12.29; 95% CI 1.26, 119.87; p=0.03) and OS (HR=6.21; 95% CI 0.95, 40.62; p=0.057). High carbonic anhydrase-9 (CA-9) levels at 4 weeks were associated with worse OS (HR=1.18; 95% CI 1.001, 1.41; p=0.049), while decreased CA-9 levels at 4 weeks were associated with better PFS (p=0.77). Conclusions: Efficacy for the combination ofEV and ER is reasonable however plasma markers of tumor invasion and hypoxia are associated with worse outcomes, and further biomarker analysis may define subsets of pts with significant therapeutic benefit. Clinical trial information: NCT00942734.