Targeting chronic myeloid leukemia stem cells with the hypoxia-inducible factor inhibitor acriflavine.

Chronic myeloid leukemia (CML) is a hematopoietic stem cell (HSC)-driven neoplasia characterized by the expression of the constitutively-active tyrosine kinase BCR/Abl. CML therapy based on tyrosine kinase inhibitors (TKi) is highly effective in inducing remission but not in targeting leukemia stem cells (LSC), which sustain minimal residual disease and are responsible for CML relapse following discontinuation of treatment. The identification of molecules capable to target LSC appears therefore of primary importance to aim at CML eradication. LSC home in bone marrow areas at low oxygen tension where HSC are physiologically hosted. This study addresses the effects of the pharmacological inhibition of hypoxia-inducible factor-1 (HIF-1), a critical regulator of LSC survival, on the maintenance of CML stem cell potential. We found that the HIF-1 inhibitor acriflavine (ACF) decreased survival and growth of CML cells. These effects were paralleled by a decreased expression of c-MYC and stemness-related genes. Using different in vitro stem cell assays, we showed that ACF, but not TKi, target stem cell potential of CML cells, including primary cells explanted from twelve CML patients. Moreover, in a murine CML model, ACF decreased leukemia development and reduced LSC maintenance. Importantly, ACF exhibited significantly less severe effects on non-CML hematopoietic cells in vitro and in vivo. Thus, we propose ACF, an FDA-approved drug for non-oncological use in humans, as a novel therapeutic approach to prevent CML relapse and, in combination with TKi, to enhance induction of remission.