Abstract 2503: Predicting drug sensitivity to a novel BCL-2 and BCL-xL dual inhibitor in solid tumor PDX trials
Authors: Deng Jing , Huang Xiaojing , Zhai Guoqin , Mao Tingting , Yin Yan , Fang Douglas D. , Yang Dajun , Zhai Yifan 
Published: August 2019

Abstract
Background: BCL-2, an anti-apoptotic protein and oncogene, has been considered one of the top 10 targets for cancer drug development. The inhibition of BCL-2 and other anti-apoptotic proteins will help to restore the apoptotic pathway. Aiming to target much diverse BCL-2 family protein dependency in solid tumors, we have developed a potent BCL-2 and BCL-xL dual inhibitor APG-1252, which is currently in phase 1 trials for solid tumors in USA, China and Australia. Mechanistically, BCL-2 and BCL-xL protect cells from cell death by sequestering pro-death BH3-only proteins like BIM. Thus, BCL-2 and BCL-xL inhibitors are designed to disrupt the protein complexes and release pro-death proteins to trigger downstream apoptotic cascade. In this study, we intended to further understand the mechanism of action of APG-1252 in vivo and factors regulating its sensitivity. To this end, we conducted trials in SCID mice carrying patient-derived xenograft (PDX) tumors, and studied target engagement.
Methods and Experiments: We first selected eleven gastric and esophageal cancer PDX models that bore high levels of BCL-xL (BCL-xLhigh) but normal levels of MCL-1 (MCL-1nor) based on the gene expression profiles and the consideration of that BCL-xL may play a major role in tumorigenesis in solid tumors and MCL-1 a resistant factor for the dual inhibitor. Mice bearing PDX tumors were intravenously treated with 100 mg/kg of APG-1252 twice per week for three weeks. At the end of treatments, tumor samples were collected. An advanced ELISA system (MSD, Meco Scale Discovery) and Western blotting were performed to detect BCL-2 and BCL-xL protein complex and individual proteins.
Results: 1. The MSD assays revealed dominant BCL-xL:BIM complex signals in the solid tumor PDX samples in comparison with hematological malignancy cell lines, in which BCL-2:BIM was the major complex. 2. The dual inhibitor treatment disrupted BCL-xL:BIM complex, thus BCL-xL:BIM complex may serves as a pharmacodynamic (PD) marker for target engagement. 3. BCL-2 family protein profiles showed high BCL-xL and MCL-1 levels in the solid tumor PDXs. The treatment with APG-1252 led to a significant increase in BCL-2 protein levels, while there was no major change on pro-apoptotic BH3-only proteins BIM and PUMA. 4. The more efficacious responders to the treatment exhibited a more substantial decrease in BCL-xL:BIM complex signals and low MCL-1 protein levels, whereas a much less significant decrease in BCL-xL:BIM complex and higher MCL-1 levels were found in the poor or non-responders.
Conclusion: Both BCL-xL protein and BCL-xL:BIM complex are dominant in the solid tumor PDX tissues. The decrease of BCL-xL:BIM complex indicates the on-target activity after the treatment with APG-1252. Thus, the complex may be used to monitor the target engagement and, potentially, a predictive biomarker for APG-1252 efficacy. These assays may be applied to patient samples to guide the clinical trials.
Citation Format: Jing Deng, Xiaojing Huang, Guoqin Zhai, Tingting Mao, Yan Yin, Douglas D. Fang, Dajun Yang, Yifan Zhai. Predicting drug sensitivity to a novel BCL-2 and BCL-xL dual inhibitor in solid tumor PDX trials [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2503.