Cooperative Targeting of Bcl-2 Family Proteins By ABT-199 (GDC-0199) and Tyrosine Kinase Inhibitors to Eradicate Blast Crisis CML and CML Stem/Progenitor Cells

Bcr-Abl tyrosine kinase supports CML cell survival in part by regulating antiapoptotic Bcl-2 proteins such as Bcl-xL and Mcl-1. Tyrosine kinase inhibition, the front-line therapy for patients with chronic phase CML, is less effective in blast crisis (BC) patients and inactive against quiescent CML stem/progenitor cells. We reported that ABT-737, a dual Bcl-2/Bcl-xL inhibitor, induces apoptosis in BC CML cells including CD34⁺quiescent CML cells. ABT-199, a potent Bcl-2 specific inhibitor, has entered clinical trials for various hematological malignancies. We hypothesized that cooperative targeting of antiapoptotic Bcl-2 proteins using a combination of ABT-199 and tyrosine kinase inhibitors (TKIs) would exert enhanced activity against BC CML and CML stem/progenitor cells.

Cells from patients (n=4) with TKI-resistant BC CML were treated with ABT-199, TKIs, and combinations. Although exerting low activity by itself, ABT-199 in combination with TKIs synergistically induced apoptosis (CI<0.1) in bulk and CD34⁺38^- cells from these patients regardless of their previous clinical responses to TKIs. The combinations had minimal activity against normal CD34⁺cells (n=3). Mechanistic studies demonstrated that nilotinib inhibited the expression of Bcl-xL and Mcl-1 mRNA and protein, even in cells from TKI (including nilotinib) resistant patients. Individual inhibition of Bcl-xL or Mcl-1, and even more so inhibition of both, by siRNAs increased the sensitivity of cells to ABT-199, suggesting that cooperative inhibition of Bcl-2 by ABT-199 and Bcl-xL/Mcl-1 by TKIs contributes to the synergy.

To evaluate the effect of these combinations on TKI-insensitive quiescent stem/progenitor CML cells, BC CML patient cells were stained with the cell division-tracking dye carboxyfluorescein succinimidyl ester (CFSE) and then co-cultured with human bone marrow (BM)-derived mesenchymal stromal cells (MSCs). Once proliferating and quiescent cells were distinguishable by flow cytometry, cells were treated with ABT-199, TKIs, and their combinations for 48 hours with or without MSC co-culture. Apoptosis was measured in proliferating and quiescent progenitor cells, defined as the percentage of annexin V positivity in CD34⁺CFSE^dim and CD34⁺CFSE^bright cells, respectively. ABT-199 as a single agent decreased viability of CML cells cultured alone or co-cultured with MSCs in both proliferating (IC₅₀=191±103nM and 194±64nM, respectively) and quiescent (IC₅₀=221±75nM and 205±123nM, respectively) CD34⁺ CML cells. Combinations of ABT-199 with TKIs, including imatinib, nilotinib, dasatinib, or ponatinib, synergistically induced death (CI<0.2) and decreased the number of viable cells in proliferating as well as quiescent CD34⁺progenitor cell populations (n=6). All 6 patients were resistant to TKIs, and 4 had mutations in the BCR-ABL gene, including three with the T315I mutation.

To further test the ability of ABT-199 and TKI combinations to eradicate CML stem cells, we used an inducible transgenic CML mouse model in which the BCR-ABL gene is expressed under control of a tet-regulated enhancer of the murine stem cell leukemia (Scl) gene, allowing targeted BCR-ABL expression in stem/progenitor cells. Once BM cells from transgenic Scl-tTa-BCR-ABL/GFP mice were engrafted in wild type recipient mice, the mice were treated with ABT-199, nilotinib, or both. At the end of a 3-week treatment period, each single agent alone, and even more so with the combinations, significantly decreased blood total GFP⁺ WBC (12.9±1.4, 5.2±0.3, 6.1±0.4, and 1.6±0.3 x10⁶/ml in controls, ABT-199, nilotinib, and combination, respectively) and neutrophils (1.43±0.03, 0.49±0.06, 0.32±0.03, and 0.25±0.05 x10⁶/ml in the respective groups). ABT-199 (P=0.02), and more so with the combination (P<0.01) but not nilotinib alone (P=0.29), significantly decreased BM GFP⁺ LSK cells (12.0±1.2, 6.8±0.6, 9.5±1.6, and 2.2±0.2 x10³ cells in the respective groups). The in vivo experiments are ongoing.

Conclusions: ABT-199 and TKIs cooperatively target antiapoptotic Bcl-2 family proteins. This combination is highly effective in killing bulk and CD34⁺38^- CML cells and quiescent CD34⁺ CML stem/progenitor cells from BC CML patients in vitro and in suppressing leukemia and leukemia stem cells in vivo. This strategy has the potential to eradicate BC CML cells and CML stem/progenitor cells, neither of which are effectively targeted by TKIs alone.