Topoisomerase I inhibitors down-regulate HIF-1α leading to tumor growth 
inhibition, but only while maintaining sustained levels of drug exposure. 
EZN-2208, a multi-arm 40 kDa pegylated, releasable SN38-drug conjugate, provides 
higher, longer lasting exposure of tumors to SN38 in contrast to SN38 that is 
released from CPT-11. EZN-2208 also consistently has greater antitumor activity 
than CPT-11 in a variety of solid and hematological tumor models. In this 
report, the ability of PEG-SN38 to down-regulate HIF-1α and its downstream 
targets, in a more potent, sustained manner compared with CPT-11 was examined. 
To do so, U251 glioma xenografts that stably expressed a hypoxia response 
element-dependent luciferase reporter gene were implanted in mice. After 
treatment it was found that EZN-2208 induced potent, sustained HIF-1α 
down-regulation (37% at 48 h and 83% at 120 h) in the tumors, whereas CPT-11 
caused only minor, transient HIF-1α down-regulation. In addition, EZN-2208 
down-regulated mRNA levels of HIF-1α targeted genes (MMP2, VEGF1, Glut1, Glut3 
and TGFβ1). Further, western blot analyses of xenograft tumors demonstrated that 
EZN-2208 had significantly more effect than CPT-11 in down-regulating HIF-1α, 
VEGF, Glut1 and MMP2 protein levels. Significant down-regulation of HIF-1α and 
VEGF proteins translated to EZN-2208's superior anti-angiogenic activity 
compared with CPT-11, confirmed by microvessel density reduction in a 
chorioallantoic membrane assay and in CD-31 immunohistochemistry studies. 
Additional studies done with matrigel implants devoid of tumor cells show that 
EZN-2208 significantly inhibits angiogenesis while CPT-11 has little or no 
effect. It is concluded that the superior antitumor activity of EZN-2208 
compared with CPT-11 is attributed, in part, to an anti-angiogenic effect. 
Ongoing clinical Phase I and Phase II studies will assess safety and efficacy of 
EZN-2208.