Tumor regression and curability of preclinical neuroblastoma models by PEGylated SN38 (EZN-2208), a novel topoisomerase I inhibitor

Purpose: Treatment of neuroblastoma is successful in less than half of patients with high-risk disease. The antitumor activity of a water soluble pegylated SN38 drug conjugate, EZN-2208, was compared with CPT-11 (a prodrug for SN38) in preclinical models of human neuroblastoma. Experimental Design: The in vitro cytotoxicity of EZN-2208 was tested by counting trypan blue dye-and Annexin V-positive cells, whereas its therapeutic efficacy was evaluated, in terms of survival, and antitumor and antiangiogenic activities, in s.c. luciferase-transfected, pseudometastatic, and orthotopic neuroblastoma animal models. Results: EZN-2208 was about 100-fold more potent than CPT-11 in vitro, by inducing apoptosis/ necrosis and p53 expression and by reducing hypoxia-inducible factor (HIF)-1α/HIF-2α expression. EZN-2208 gave superior antitumor effects compared with CPT-11 in neuroblastoma xenografts. EZN-2208 treatment always resulted in lack of tumor detection at the end of trials whereas only small therapeutic effects were observed with CPT-11, as assessed by luciferase assay or tumor size, or even by staining histologic sections of tumors with antibodies recognizing neuroblastoma cells and cell proliferation. In a neuroblastoma model resistant to doxorubicin, cisplatin, vincristine, fenretinide, and topotecan, EZN-2208 induced 100% curability. It also blocked tumor relapse after topotecan-vincristine-doxorubicin combined treatment. Mechanistic experiments showed statistically significantly enhanced terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling and Histone H2ax staining as well as decreased vascular endothelial growth factor, CD31, matrix metalloproteinase (MMP)-2, and MMP-9 expression in tumors removed from EZN-2208-treated mice and radiating vessels invading the tumor implanted onto the chorioallantoic membranes. Conclusions: EZN-2208 should be considered a most promising novel antineuroblastoma agent. An ongoing phase I study in pediatric patients should identify the optimal dose for a phase II study.