Synergistic Leukemia Eradication by Combined Treatment with Retinoic Acid and HIF Inhibition by EZN-2208 (PEG-SN38) in Preclinical Models of PML-RARα and PLZF-RARα-Driven Leukemia

Purpose: Retinoic acid-arsenic trioxide (ATRA-ATO) combination therapy is the current standard of care for patients with acute promyelocytic leukemia (APL) carrying the oncogenic fusion protein PML-RAR alpha. Despite the high cure rates obtained with this drug combination, resistance to arsenic is recently emerging. Moreover, patients with APL carrying the PLZF-RAR alpha fusion protein are partially resistant to ATRA treatment. Hypoxia-inducible factor-1 alpha (HIF-1 alpha) activation has been recently reported in APL, and EZN-2208 (PEG-SN38) is a compound with HIF-1 alpha inhibitory function currently tested in clinical trials. This study investigates the effect of EZN-2208 in different preclinical APL models, either alone or in combination with ATRA. Experimental Design: Efficacy of EZN-2208 in APL was measured in vitro by assessing expression of HIF-1 alpha target genes, cell migration, clonogenicity, and differentiation, vis a vis the cytotoxic and cytostatic effects of this compound. In vivo, EZN-2208 was used in mouse models of APL driven by PML-RAR alpha or PLZFRAR alpha, either alone or in combination with ATRA. Results: Treatment of APL cell lines with noncytotoxic doses of EZN-2208 causes dose-dependent downregulation of HIF-1 alpha bona fide target genes and affects cell migration and clonogenicity in methylcellulose. In vivo, EZN-2208 impairs leukemia progression and prolongs mice survival in APL mouse models. More importantly, when used in combination with ATRA, EZN-2208 synergizes in debulking leukemia and eradicating leukemia-initiating cells. Conclusions: Our preclinical data suggest that the combination ATRA-EZN-2208 maybe tested to treat patients with APL who develop resistance to ATO or patients carrying the PLZF-RAR alpha fusion protein. (C)2015 AACR.