Combination of Venetoclax and CUDC-907 Shows Superior Antileukemic Activity Against Acute Myeloid Leukemia Ex Vivo

Overall survival rates for adults and children with acute myeloid leukemia (AML) remain unacceptably low. Resistance to chemotherapy is the major factor contributing to such dismal overall survival rates. Chemoresistance in leukemia cell line models has been associated with overexpression of the anti-apoptotic Bcl-2 family members. As such, small molecule Bcl-2 inhibitors represent a promising strategy for treating AML.

Venetoclax (ABT-199) is a Bcl-2-selective inhibitor that has demonstrated promising antileukemic activity against AML. However, initial resistance to ABT-199 remains a concern. In our most recent study, we identified a novel Mcl-1-mediated intrinsic mechanism of resistance to ABT-199 in AML cells: ABT-199 treatment results in increased sequestration of Bim by Mcl-1, preventing Bim from inducing apoptosis (Niu X, et al. Clinical Cancer Research. 2016; epub ahead of print). We also found that ABT-199 in combination with DNA damaging agents results in enhanced DNA damage and synergistic antileukemic activity against AML cells. Based on these findings, we hypothesized that simultaneously downregulating Mcl-1, upregulating Bim, and enhancing DNA replication stress and/or DNA damage would maximally enhance ABT-199-induced cell death, leading to potent synergistic antileukemic activity against AML.

CUDC-907, a dual histone deacetylase inhibitor (HDACI) and PI3K inhibitor, is currently being tested in Phase I and Phase II clinical trials for the treatment of lymphoma, multiple myeloma, and advanced/relapsed solid tumors (www.clinicaltrials.gov). It inactivates both PI3K/AKT and MEK/ERK signaling pathways in different cancer cell types. Inhibition of these pathways has been shown to cause downregulation of Mcl-1 and upregulation of Bim and HDACIs have been demonstrated to downregulate CHK1 and Wee1, as well as upregulate Bim, leading to DNA damage and cell death. Furthermore, CHK1 and Wee1 inhibitors have been shown to cause DNA replication stress through downregulation of ribonucleotide reductase (RNR). Therefore, CUDC-907 would be an ideal compound to combine with ABT-199 to enhance its antileukemic activity against AML.

In this study, we investigated the antileukemic activity of CUDC-907 alone and in combination with ABT-199 in both AML cell lines and primary patient samples. CUDC-907 treatment resulted in increased Bim expression and decreased Mcl-1, CHK-1, Wee1, and RRM1 (the regulatory subunit of RNR) expression in both AML cell lines and primary patient samples. Ectopic overexpression of Mcl-1 and shRNA knockdown of Bim demonstrated that both were at least partially involved in CUDC-907-induced apoptosis. Treatment with a CHK1-selective inhibitor LY2603618, the Wee1-selective inhibitor MK-1775, or hydroxyurea (RNR inhibitor) enhanced CUDC-907-induced apoptosis in a synergistic fashion, demonstrating that downregulation of Wee1, CHK1, and RRM1 was also an important contributor to CUDC-907-induced apoptosis. Consistent with our hypothesis, the combination of CUDC-907 and ABT-199 resulted in significantly increased apoptosis compared to single drug treatment and excellent synergy in both AML cell lines (n=6) and primary patient samples (n=18), regardless of their sensitivities to ABT-199. Synergy was also detected when AML cells were treated with CUDC-907 first for 16 h and then followed by ABT-199 treatment for another 8 h. Western blots revealed that combined treatment caused further decrease of Mcl-1, CHK1, and Wee1, while comet assays revealed that the combination caused significantly increased DNA strand breaks in both AML cell lines and primary patient samples. Our results demonstrate that CUDC-907 synergizes with ABT-199 in AML cells, and support the clinical development of the combination of CUDC-907 and ABT-199 in the treatment of AML.