Inhibitory Effects of Homoharringtonine on Leukemic Stem Cells and BCR-ABL Induced Chronic Myeloid Leukemia and Acute Lymphoblastic Leukemia in Mice.

The Abl tyrosine kinase inhibitors (TKIs) imatinib mesylate (IM) and dasatinib, targeting BCR-ABL for the treatment of Philadelphia-positive (Ph+) leukemia including chronic myeloid leukemia (CML) and B-cell acute lymphoblastic leukemia (B-ALL), have produced impressive results in terms of therapeutic outcome and safety for patients. However, clinical resistance to these TKIs likely at the level of leukemic stem cell negates curative results in Ph+ leukemia. At present, an anti-stem cell strategy has not been developed for treating these leukemia patients. Homoharringtonine (HHT) (omacetaxine mepesuccinate - USAN/INN designation) has shown significant clinical activity in CML in combination with IM or alone for patients failing IM. However, little is known about whether HHT has an inhibitory effect on leukemic stem cells. The purpose of this study is to determine whether HHT inhibits BCR-ABL-expressing leukemic stem cells (Lin-c-Kit+Sca-1+) that we identified previously (Hu et al. Proc Natl Acad Sci USA 103(45):16870–16875, 2007) and to evaluate therapeutic effects of HHT on CML and B-ALL in mice. We find that in our in vitro stem cell assay, greater than 90% of leukemic stem cells were killed after being treating with HHT (12.5, 25, and 50 nM) for 6 days, and in contrast, greater than 75% or 92% of leukemic stem cells survived the treatment with dasatinib (100 nM) or imatinib (2 mM). We next treated CML mice with HHT (0.5 mg/kg, i.p., once a day). 4 days after the treatment, FACS analysis detected only 2% GFP+Gr–1+ myeloid leukemia cells in peripheral blood of HHT -treated CML mice and in contrast, 41% GFP+Gr–1+ myeloid leukemia cells in placebo-treated mice. We also treated mice with BCR-ABL induced B-ALL with HHT, and found that only 0.78% GFP+B220+ lymphoid leukemia cells were detected in peripheral blood compared to 34% GFP+B220+ lymphoid leukemia cells in placebo-treated mice. Furthermore, HHT significantly inhibited in vitro proliferation of K562 and B-lymphoid leukemic cells isolated from mice with B-ALL induced by BCR-ABL wild type and BCR-ABL-T315I resistant to both imatinib and dasatinib. In sum, HHT has an inhibitory activity against CML stem cells, and is highly effective in treating CML and B-ALL induced by BCR-ABL in mice.