EZN-2208 (PEG-SN38) is a polymeric prodrug of anticancer agent SN38 (7-ethyl-10-hydroxyl-camptothecin) which is prepared by coupling a 40 kDa, 4-arm polyethylene glycol (PEG) with SN38, through a glycine spacer. The coupling strategy was developed to selectively link the 20-OH group of SN38, thus stabilizing the E-ring of SN38 in the active lactone form. EZN-2208 markedly increased the solubility of SN38 by about 1,000 fold, thus allowing systemic administration of SN38. In preclinical studies, EZN-2208 had excellent anticancer efficacy in several human cancer models derived from solid tumors and hematological malignancies. It outperformed the prodrug irinotecan (CPT-11) in these studies, including tumor models that failed to respond to CPT-11. Associated with enhanced activity, it was found that EZN-2208 prolonged the circulation half-life of SN38 and increased tumor-site accumulation of both PEG conjugate and released free SN38 compared to CPT-11. These observations may explain the enhanced anti-angiogenic activity that has been found with EZN-2208 compared to CPT-11. In two completed phase I studies, EZN-2208 was well tolerated in patients with advanced malignancies. The dose-limiting toxicity (DLT) was febrile neutropenia with or without fever, unlike CPT-11, for which unpredictable diarrhea was the DLT. The maximum tolerated dose and recommended phase II dose of EZN-2208 have been identified. Stable disease, sometimes prolonged and associated with tumor shrinkage, was observed as the best response in Phase I. EZN-2208 is being evaluated in phase II clinical studies in patients with metastatic colorectal and breast cancers and a phase I/II study in pediatric patients with cancer.