The Broad Spectrum Bcl-2 Inhibitor Apogossypol Induces Apoptosis and Differentiation of Blast Crisis Chronic Myeloid Leukemia Stem Cells.

Leukemia stem cells (LSC) are rare, self-renewing cells capable of proliferation and differentiation into the bulk of cells that make up a leukemia. LSC, like normal tissue-specific stem cells, can be highly quiescent and resistant to apoptosis induced by drugs and radiotherapy that target rapidly dividing cells. While traditional chemotherapy may successfully eradicate the bulk of tumor cells, it often fails to kill LSC resulting in their reactivation and eventual relapse of disease.

Apoptosis resistance in cancer often involves deregulation of Bcl-2 family proteins. Bcl-2 family expression also seems to have an important role in normal stem cell function as expression changes of Mcl-1, Bcl-2, and Bcl-X_L in mouse hematopoietic stem and progenitors characterize the differentiation of hematopoietic cells to different lineages.

Several studies have linked expression of Bcl-2 family members to the development of blast crisis chronic myeloid leukemia (BC CML) in vitro, however there has been little data on the role of Bcl-2 expression in apoptosis resistance in human LSC. Also, various data has shown that CML cells become increasingly resistant to BCR-ABL inhibition with progression to blast crisis. In BC CML, LSC are enriched in the progenitor population and can be serially transplanted in immunodeficient mice leading to BC CML in vivo. We hypothesize that human BC CML LSC may resist chemotherapy by overexpression of Bcl-2 family proteins and maintenance of quiescence. We analyzed Bcl-2 family protein expression in BC CML LSC and analyzed whether these cells were sensitive to chemotherapy treatment in vitro. We further analyzed whether BC CML LSC could maintain quiescence in vivo. Finally, we tested the efficacy of the broad spectrum Bcl-2 antagonist apogossypol on BC CML LSC in vitro and in vivo.

Bcl-2 and Mcl-1 protein expression was measured in 1° BC CML LSC by intracellular FACS analysis and compared to expression in normal and chronic phase CML cells. CD34⁺ cells isolated from serially transplanted BC CML LSC were used for all subsequent studies. To assess whether BC CML cells maintained quiescence in vivo, they were stained with DiR, an infrared fluorescent cell membrane dye, transplanted into neonatal mice, and analyzed for DiR fluorescence 18 weeks later. Dividing cells will distribute DiR to all daughter cells leading to dilution of the dye and a decrease in fluorescent signal per cell. LSC drug resistance was tested in vitro by culturing the cells with etoposide, dasatinib, and apogossypol and by measuring apoptosis by FACS using annexin-V/7-AAD staining. Finally, apogossypol efficacy was tested in vivo in LSC transplanted mice. At 8 weeks post-transplantation, mice were treated for 3 weeks with apogossypol or vehicle and then analyzed for human hematopoietic cell engraftment and apoptosis by FACS.

1° BC CML progenitors expressed significantly higher levels of Bcl-2 and Mcl-1 protein compared to normal cord blood and chronic phase CML cells. LSC cultured in vitro were also resistant to etoposide and dasatinib-induced apoptosis. Apogossypol treatment in vitro however led to a dose-dependent increase in cell death and apoptosis and resulted in a significant increase in the frequency of lin⁺ staining cells along with a significant shift in the frequency of the common myeloid progenitor (CMP) and granulocyte-macrophage progenitor (GMP) populations compared to vehicle treated controls.

Mice transplanted with BC CML LSC developed diffuse myeloid sarcomas and had high levels of human engraftment in the liver, spleen, and bone marrow. Human cells in tumors and the liver were uniformly DiR^- while cells engrafted in the spleen and bone marrow retained DiR fluorescence. In vivo treatment with apogossypol led to a significant reduction in human cell engraftment in mouse bone marrow compared to vehicle controls. Consistent with the results in vitro, there was also a significant increase in the frequency of lin⁺ staining cells in engrafted mouse spleens as well as a significant decrease in engrafted GMP in the mouse bone marrow.

Our results demonstrate that BC CML LSC are highly resistant to conventional chemotherapy but are sensitive to apogossypol in vitro and in vivo. Broad-spectrum inhibition of Bcl-2 family proteins may help to eliminate CML LSC by inducing apoptosis as well as by inducing differentiation.

Goff:Coronado Biosciences: Research Funding. Tesi:Coronado Biosciences: President and CEO. Jamieson:Coronado Biosciences: Research Funding. Jamieson:Coronado Biosciences: Research Funding.