Treatment with homoharringtonine reduces levels of BCR-ABL, Mcl-1, and Hsp90 in leukemia cell lines and the survival of leukemic stem cells

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Homoharringtonine (HHT) [omacetaxine mepesuccinate-USAN/INN designation] is active clinically for patients with chronic myeloid leukemia (CML) who are resistant to or failing treatment with the tyrosine kinase inhibitor, imatinib mesylate (IM). HHT is a natural product alkaloid that affects protein synthesis by binding to the 80S subunit of the ribosome. Currently, laboratory studies are underway to determine the effects of HHT on leukemic cells focusing on proteins that may be critical for CML cell survival. Human K562 CML (BCR-ABL+) cells, BCR-ABL+ murine B-lymphoid leukemia (+/- T315I mutation) and murine leukemia stem cells (Lin^-c-Kit⁺Sca-1⁺) were used in these studies. HHT dose response studies explored concentrations ranging from 20-2000nM for periods up to 6 days. For K562 cells, the IC50 for HHT at 48hr was approximately 10nM. Similar results were also observed for the murine B lymphoid line independent of T315I mutation status. In our in vitro stem cell assay, greater than 90% of leukemic stem cells were killed after exposure to 50nM HHT for 6 days. In contrast, less than 8 % of the leukemic stem cells were killed with exposure to IM (2 μM) for the same time period. Additionally, levels of BCR-ABL (oncoprotein for CML phenotype), Mcl-1 (anti-apoptotic protein) and Hsp-90 (chaperone protein which stabilizes oncoproteins such as BCR-ABL) were examined by Western blot. For K562 cells exposed to HHT (50-150nM) for 48hr, levels of these proteins were reduced. These findings support the hypothesis that HHT can alter the levels of proteins critical to maintain the oncogenic phenotype of CML.