Small Molecule Therapeutics Monensin Inhibits Epidermal Growth Factor Receptor Traf fi cking and Activation : Synergistic Cytotoxicity in Combination with EGFR Inhibitors

Targeting theEGFR,with inhibitors such as erlotinib, represents a promising therapeutic option in advanced head and neck squamous cell carcinomas (HNSCC). However, they lack significant efficacy as single agents. Recently,we identified the ability of statins to induce synergistic cytotoxicity inHNSCCcells through targeting the activation and trafficking of the EGFR. However, in a phase I trial of rosuvastatin and erlotinib, statininduced muscle pathology limited the usefulness of this approach. To overcome these toxicity limitations, we sought to uncover other potential combinations using a 1,200 compound screen of FDA-approved drugs. We identified monensin, a coccidial antibiotic, as synergistically enhancing the cytotoxicity of erlotinib in two cell linemodels ofHNSCC, SCC9 and SCC25.Monensin treatmentmimicked the inhibitory effects of statins on EGFR activation anddownstream signaling. RNA-seq analysis ofmonensin-treated SCC25 cells demonstrated a wide array of cholesterol and lipid synthesis genes upregulated by this treatment similar to statin treatment. However, this pattern was not recapitulated in SCC9 cells as monensin specifically induced the expression of activation of transcription factor (ATF) 3, a key regulator of statin-induced apoptosis. This differential response was also demonstrated in monensin-treated ex vivo surgical tissues in which HMG-CoA reductase expression and ATF3 were either not induced, induced singly, or both induced together in a cohort of 10 patient samples, including fourHNSCC.These results suggest thepotential clinical utility of combiningmonensinwith erlotinib in patients with HNSCC. Mol Cancer Ther; 13(11); 2559–71. 2014 AACR.