Voruciclib, an Oral, Selective CDK9 Inhibitor, Enhances Cell Death Induced By the Bcl-2 Selective Inhibitor Venetoclax in Acute Myeloid Leukemia

Introduction: Patients with acute myeloid leukemia (AML) face overall 5-year survival rates of 65% and 27% for children and adults, respectively, leaving significant room for improvement. Relapse remains a major contributor to such low overall survival rates, and leukemic stem cells (LSCs) that survive treatment are believed to be responsible for AML relapse. The anti-apoptotic protein Bcl-2 is overexpressed in bulk AML cells and LSCs and is associated with poor clinical outcomes. Thus, Bcl-2 represents a promising therapeutic target for the treatment of AML.

Venetoclax (ABT-199) is a selective Bcl-2 inhibitor that has shown great potential for treating a number of malignancies, including AML. Venetoclax inhibits Bcl-2, preventing it from sequestering pro-apoptotic Bcl-2 family protein Bim, leading to Bim activated Bax/Bak, resulting in apoptosis. However, Mcl-1 can also sequester Bim and prevent apoptosis. We previously showed that directly targeting Mcl-1 can enhance the antileukemic activity of venetoclax (Luedtke DA, et al. Signal Transduct Target Ther. Apr 2017). Alternatively, we proposed that indirect targeting of Mcl-1 may preserve or enhance the antileukemic activity of venetoclax, and prevent resistance resulting from Mcl-1.

It has been reported that inhibition of CDK9 can downregulate cell survival genes regulated by superenhancers, including Mcl-1, MYC, and Cyclin D1. One CDK9 inhibitor in clinical development, flavopiridol (alvocidib), has progressed to phase II clinical trials in AML. However, off target effects and dose-limiting toxicities remain a concern. Voruciclib is an oral, selective CDK inhibitor differentiated by its potent inhibition of CDK9 as compared to other CDK inhibitors. This selectivity may potentially circumvent toxicities resulting from inhibition of non-CDK targets like MAK and ICK that are inhibited by flavopiridol. Voruciclib has been shown in vitro to promote apoptosis and decrease Mcl-1 expression levels in chronic lymphocytic leukemia (CLL) cells (Paiva C, et al. PLOS One. Nov 2015) and inhibit tumor growth in mouse xenograft models of diffuse large B-cell lymphoma (DLBCL) in combination with venetoclax (Dey J. et al Scientific Reports. Dec 2017). Based on these data, voruciclib may downregulate Mcl-1 in AML cells and therefore synergistically enhance the antileukemic activity of venetoclax.

Methods/Results: Culturing AML cell lines (THP-1, U937, MOLM-13, MV4-11, and OCI-AML3) and primary patient samples with various concentrations of voruciclib resulted in a concentration-dependent increase in Annexin V+ cells (2 μM voruciclib induced 13.8-55.8% Annexin V+ cells) along with increased levels of cleaved caspase 3 and PARP, demonstrating that voruciclib induces apoptosis in AML cells. Next, we tested the combination of voruciclib and venetoclax in AML cell lines and primary AML patient samples at clinically achievable concentrations. Annexin V/PI staining, flow cytometry analysis, and combination index calculation (using CalcuSyn software) revealed synergistic induction of apoptosis by voruciclib and venetoclax combination (combination index values for MV4-11, U937, THP-1, and MOLM-13 cells were <0.73; treatment with 2 µM voruciclib and venetoclax for 24 h resulted in >80% apoptosis). Importantly, synergy was observed in both venetoclax sensitive and resistant cell lines. This was accompanied by increased cleavage of caspase 3 and PARP. Lentiviral shRNA knockdown of Bak and Bax partially rescued AML cells from voruciclib-induced apoptosis, showing that voruciclib induces apoptosis at least partially through the intrinsic apoptosis pathway. However, Bak and Bax knockdown had little to no effect on induction of apoptosis by the combination treatment, indicating that there might be other molecular mechanisms underlying the synergistic interaction between the two agents. Treatment with the pan-caspase inhibitor Z-VAD-FMK partially rescued cells from combination treatment induced-apoptosis.

Discussion: Collectively, these results demonstrate that voruciclib and venetoclax synergistically induce apoptosis in AML cells in vitro and reverse venetoclax resistance. Further studies to determine the mechanism of action and in vivo efficacy of this promising combination in AML xenografts and PDX models are underway.