This is a Phase 2, multicenter, multiple-arm, open-label study to evaluate the efficacy, safety, and tolerability of EZN-2208. EZN-2208 will be administered as a single agent in patients with K-RAS mutations in the tumors. Patients with wild type K-RAS in tumors will be randomized to EZN-2208 + cetuximab or to standard of care (Camptosar® + cetuximab), patients must have failed regimens containing irinotecan (Camptosar®, CPT-11), oxaliplatin (Eloxatin®), and fluoropyrimidine. After discontinuation of study treatment, patients will receive care as considered appropriate by the investigator. Patients will continue to be followed for disease progression, subsequent anticancer therapy, and survival.

EZN-2208 will be administered by i.v. infusion weekly for 3 weeks in 4-week cycles. The cetuximab infusion will be administered before the EZN-2208 (Arm B) or irinotecan (Arm C) infusion. Study treatment will be continued until evidence of disease progression, unacceptable toxicity, or withdrawal of the patient's consent for participation in the study. Approximately 220 patients will be enrolled in this study: approximately 100 patients in the K-RAS mutated arm and approximately 120 patients in the wild-type K-RAS arm.