Targeting 1q21 Amplification with APG2575 and Lenalidomide to Sensitize BCL-2 Inhibition with the Decrease of MCL-1 Protein in High Risk MM Models
Authors: Xie Yan , FU Chengcheng , Yan Lingzhi , Jin Song , Shang Jingjing , Shi Xiaolan , Pan Jinlan , Yin Yan , Mao Tingting , Deng Jing , Fang Douglas D. , Yang Dajun , Zhai Yifan , Yan Shuang , Zhai Yingying , Yao Ying , Wang Panfeng , Wang Jing , Wu Depei 
Published: November 2020

Background
Multiple myeloma (MM) is a heterogenous plasma cell malignancy. About 30-40% of newly diagnosed and 20-60% of relapsed/refractory MM patients carry 1q21 amplification worldwide, a high-risk indicator for poor prognosis in RRMM. Different BCL-2 family anti-death proteins play important roles in MM survival and drug resistance. High expression of BCL-2 due to t (11;14) renders cell vulnerability to BCL-2 antagonist, however, patients carrying other genetic abnormality including 1q21 amplification have limited response to the newly emerged treatment choice. With MCL-1 upregulation accompanied with 1q21 amplification, we tested whether BCL-2 antagonist combined with IMiDs (immunomodulatory imide drugs), improves cell killing in high-risk MM patient models, including those resistant to bortezomib and lenalidomide. For that aim, we studied APG-2575, a novel and potent BCL-2 inhibitor developed by Ascentage Pharma Group and it is currently in clinical trials for hematologic malignances, including MM.
Methods
1. Cells were treated with APG-2575 single agent or in combination with lenalidomide;
2. Cell line viability was assessed by CellTiter-Glo (CTG) assay;
3. Flow cytometry analysis was used to detect CD138+ cell surface marker in primary cells derived from MM patients;
4. Western blot analysis for BCL-2 family and IMiD signaling proteins.
Results
We first determined the cell sensitivity of APG-2575 as a single agent in a panel of MM cell lines, and as expected, those carrying t (11;14) were very sensitive to APG-2575, with low IC50 values ranging 7-23 nM. The IC50 values for cells carrying other genetic markers were greater than 5-10 μM.
We then evaluated cell-death inducing activity of APG-2575 in MM patient-derived primary cells ex vivo, which were freshly prepared from patients' bone marrow aspirates. Primary cells were treated with APG-2575 or ABT-199 (venetoclax) for 18-24 hours, and the loss of CD138 surface marker was used to quantify cell death. As shown in Figure 1a, APG-2575 induced cell death (CD138+ loss) in a dose-dependent manner, and significant cell death was observed at 0.37-3.3 μM of APG-2575, indicating primary cells more sensitive than MM cell lines. Interestingly, the sensitivity to APG-2575 is similar in primary MM cells with or without 1q21 amplification.
Since moderate cell death inducing activity was observed in MM cells when APG-2575 used as a single agent, we combined APG-2575 with lenalidomide. Cell death was increased in the combination groups compared with single agent, and it was dose-dependent (Figure 1b).Similar enhanced antiproliferative activity was confirmed in RPMI 8226 cell line, which carries 1q21 amplification (Figure 1c).
We furthered to understand the mechanism of action (MoA) of this combination. In Figure 1d, Western blot analysis of RPMI 8226 cell line revealed that IKZF1 and IKZF3 proteins from the NF-KB pathway were downregulated by lenalidomide. And the decrease of MCL-1 protein and the strong induction of pro-death protein BAK were evident in the combination group of APG-2575 and lenalidomide, which helps to illustrate MoA underlying the synergistic effect of APG-2575 and lenalidomide in MM models.
Conclusions
In both cell lines and primary samples derived from MM patients, APG-2575 demonstrates cell death inducing activity as a single agent, and enhanced/synergistic effects when it combines with lenalidomide in RRMM resistant to lenalidomide and bortezomib. The combination decreases IKZF1, IKZF3 and MCL-1 proteins, and upregulates pro-death protein BAK, thus providing a strong rationale to combine BCL-2 inhibitor and lenalidomide to treat high-risk patient populations carrying 1q21 amplification.

Disclosures
FU: Ascentage Pharma (SuZhou) Co., Ltd: Other: research agreement contract . Yin:Ascentage Pharma (SuZhou) Co., Ltd: Current Employment. Mao:Ascentage Pharma (SuZhou) Co., Ltd: Current Employment. Deng:Ascentage Pharma (SuZhou) Co., Ltd: Current Employment. Fang:Ascentage Pharma (SuZhou) Co., Ltd: Current Employment. Yang:Ascentage Pharma (SuZhou) Co., Ltd: Current Employment, Current equity holder in publicly-traded company, Other: Leadership and other ownership interests. Zhai:Ascentage Pharma (SuZhou) Co., Ltd: Current Employment, Current equity holder in publicly-traded company, Other: Leadership and other ownership interests.