Preclinical Efficacy and Biological Effects of Venetoclax and Ixazomib in Combination in Lymphoma Cells

Introduction. Bcl-2 family proteins comprise anti-apoptotic and pro-apoptotic proteins. Interaction between these proteins, as well as severe regulation of their expression, mediates cell survival and can quickly induce cell death. Venetoclax is Bcl-2-targeting that has shown preclinical and clinical activity in hematologic malignancies. Due to the development of resistance and the loss of dependence on the target protein, the monotherapy may be insufficient for maximal effectiveness. To circumvent the resistance mechanisms, many preclinical studies have shown that combination of venetoclax with other agents may represent a more effective therapeutic strategy. Ubiquitin-proteasome signaling pathway is a potential target that plays an important role in the proteolysis of key regulatory proteins. Proteasome inhibitors include ixazomib that inhibits cell growth and induces apoptosis in hematological malignancies cells resistant to conventional therapies and bortezomib.

Objective: To analyze the preclinical efficacy and associated biological effects of venetoclax combined with ixazomib in a panel of lymphoma cell lines with diverse expression levels of Bcl-2 and other Bcl-2 family proteins.

Methods: 12 lymphoma cell lines including FL (RL, WSU-NHL, Karpas422), MCL (Jeko1, Granta519), DLBCL (OCI-LY3, OCI-LY18), CTCL (Hut-78), ALCL (Karpas299), HL (L1236, L540), CLL (Mec1) and two MCL primary patient samples were exposed to venetoclax (0.01 - 8 µM) and ixazomib (10 - 2000 nM) alone for 24 - 72 hours to calculate IC₅₀. Subsequently, lymphoma cells were exposed to venetoclax (0.015 - 25 nM) in combination with ixazomib (0.015 - 0.5 nM) for 24 hours. Cell viability was determined by MTT. Coefficient of synergy (combination index - CI) was calculated using CalcuSyn. Cell cycle and induction of apoptosis were evaluated by flow cytometry and changes in Bcl-2 family members, caspase activation and AKT phosphorylation were determined by western blotting.

Results. In vitro, venetoclax and ixazomib alone induced cell death in a dose- and time-dependent manner against lymphoma cell lines. The IC₅₀ is between 0.5 and 8 µM for venetoclax and between 12 and 1250 nM for ixazomib.

The combination of venetoclax (0.03, 0.06, 12.5, 25 nM) with ixazomib (0.03, 0.06, 0.25, 0.5 nM) produced a synergistic effect (CI < 1) after 24 h of treatment in the most lymphoma cells lines leading to inhibition of cell growth and induction of apoptosis between 26 % and 59 % accompanied by increased with cleavage of caspases-3, -9 and PARP. We observed an additive effect (CI = 1) in Jeko1 (MCL) and MEC1 cells (CLL) and antagonist effect (CI > 1) Hut-78 cells (CTCL). Synergistic effect has been seen in two MCL primary patient samples (CI = 0.5 - 0.7). In sensitive lymphoma cells, the combination abrogated colony formation in the methylcellulose medium. When lymphoma cell lines were co-cultured with mesenchymal stromal cells with both drugs we observed a decrease of cell viability and a fraction of apoptotic cells indicating that drug combination may overcome the tumor promoting effects of stromal cells. The apoptosis induced in FL and Granta519 cells (MCL) by drug combination was accompanied by partial downregulation of Bcl-2 and strong upregulation of Bax, Bad, Bim and Noxa proteins. Jeko-1 cells were less sensitive to venetoclax-ixazomib combination-induced apoptosis. Western blot analysis showed a differential expression of Bcl-2, Mcl-1 and Bcl-XL proteins in FL, MCL and HL cell lines. Jeko-1 cells showed a normal expression of Bcl-2 and Mcl-1 proteins and high Bcl-xL protein level. Co-expression of related anti-apoptotic Bcl-2 family proteins could limit activity of treatment. Combined treatment induced G0/G1 cell cycle arrest and increased the sub-G1 population that was linked by the upregulation of p27 and p21. In addition, in RL, WSU-NHL and Granta519, enhanced cell death is associated with AKT inactivation and with a reduction of p-4EBP1, leading to decreased levels of c-MYC.

Conclusion. Venetoclax exhibits strong synergistic activity with ixazomib in lymphoma cells. Studies are still ongoing and signaling pathways that promote the combination of venetoclax with ixazomib are to be analyzed. These data offer a rationale to continue exploring venetoclax-ixazomib combination and suggest that suppression of Bcl-2 family protein driven survival signaling may be one important mechanism for combination synergy.