©2015 American Association for Cancer Research. Tumor growth in the context of EGFR inhibitor resistance may remain EGFRdependent and is mediated by mechanisms including compensatory ligand upregulation and de novo gene alterations. Sym004 is a two-antibody mixture targeting nonoverlapping EGFR epitopes. In preclinical models, Sym004 causes signifi cant EGFR internalization and degradation, which translates into superior growth inhibition in the presence of ligands. In this phase I trial, we observed grade 3 skin toxicity and hypomagnesemia as mechanism-based dose-limiting events during dose escalation. In dose-expansion cohorts of 9 and 12 mg/kg of Sym004 weekly, patients with metastatic colorectal cancer and acquired EGFR inhibitor resistance were enrolled; 17 of 39 patients (44%) hadtumor shrinkage, with 5 patients (13%) achieving partial response. Pharmacodynamic studies confi rmed marked Sym004-induced EGFR downmodulation. MET gene amplifi cation emerged in 1 patient during Sym004 treatment, and a partial response was seen in a patient with EGFR S492R mutation that is predictive of cetuximab resistance. SIGNIFICANCE: Potent EGFR downmodulation with Sym004 in patients with metastatic colorectal cancer and acquired resistance to cetuximab/panitumumab translates into signifi cant antitumor activity and validates the preclinical hypothesis that a proportion of tumors remains dependent on EGFR signaling. Further clinical development and expanded correlative analyses of response patterns with secondary RAS/EGFR mutations are warranted.