Hemin Confers Resistance to Imatinib (Glivec)-Induced Cell Death but Enhances Nilotinib (AMN107) Cytotoxicity in Human K-562 CML Leukemia Cells.

Imatinib (Glivec) and Nilotinib (AMN107) are selective inhibitors of the Bcr-Abl, Kit and PDGFR tyrosine kinases, developed by Novartis Pharma AG to treat CML and other malignancies by promoting apoptosis (Manley P.W., et al. Biochim Biophys Acta 1754(1–2):3–13, 2005). Heme (iron protoporphyrin-IX) and its oxidized form Hemin, are key regulators of key cellular functions in mammalian cells and Hemin protects normal and leukemia cells from cytotoxic agents (Tsiftsoglou A.S. et al., Pharmacology & Therapeutics 111(2):327–345, 2006). We have recently observed that Hemin, like Erythropoietin (EPO), protects human Ph⁺ K-562 CML leukemia cells from Imatinib-induced cell death. We extended this observation and investigated whether Hemin also counteracts Nilotinib-induced cytotoxicity in cells. K-562 CML leukemia cells were treated (from 0h to 96h) separately with each of the inhibitors added at different concentrations (0.01 μM to 1 μM) in the presence and absence of Hemin (90 μM) and assessed for their ability to proliferate, differentiate into erythroid lineage or undergo cell death. K-562 cells pre-treated with Hemin prior to be being exposed to Imatinib and/or Nilotinib were used to demonstrate whether induction of differentiation by Hemin alters their developmental stage and susceptibility to either inhibitor. Our results thus indicate that:

1. Hemin confers resistance to Imatinib in a substantial proportion of cells (20–30%). Priming cells with Hemin gradually decreased susceptibility to Imatinib-induced cell death. Considering that the intracellular Heme in CML cells may overcome the Imatinib effect, this resistance could affect the clinical outcome of CML patients treated with Imatinib.

2. Conversely, Hemin enhanced cell death induced by Nilotinib in the concentration range 0.01 μM to 1 μM.

These findings indicate that Imatinib and Nilotinib induce cell death in K-562 CML leukemia cells, via pathways affected differently by Hemin. Experiments are now in progress to delineate the mechanisms via which Hemin exerts such disparate action (antagonistic and synergistic) on these inhibitors in human leukemias.