Abstract 984: Targeting BCL-xL addiction with APG-1252 (pelcitoclax) to overcome apoptotic blockade in neuroendocrine neoplasm (NEN)
Authors: Guo Yu , Deng Jing , Zhang Lin , Huang Xiaojing , Zhang Kaixiang , Zhou Feng , Li Yuanbao , Luo Fan , Fang Douglas D. , Yang Dajun , Chen Jie , Zhai Yifan 
Published: July 2021

Current targeted therapies such as VEGFR inhibitor sunitinib and mTOR inhibitor everolimus extend progression-free survival (PFS) in patients with Grade 1 (G1) and G2 neuroendocrine tumors (NET), but the objective response rate remains low. The median PFS in patients with advanced neuroendocrine carcinoma (NEC) receiving platinum-based therapy is only 3 to 4 months. Hence, more effective therapy is needed to improve clinical outcomes. This study explored if BCL-2 family antideath proteins play a role in NEN tumorigenesis and whether clinical-stage dual BCL-2/BCL-xL inhibitor APG-1252 might overcome intrinsic apoptotic blockade in NEN. A panel of six NEN cell lines were exposed to APG-1252-M1, which is an active metabolite of APG-1252 typically used for in vitro studies. After 72 hours, the IC50 of APG-1252-M1 ranged from 0.43 to 10 μM. Among the panel, pancreatic NET (pNET) cell lines BON-1 and β-TC3 exhibited the highest sensitivity, with IC50 values of 0.43 and 0.55 µM, respectively, whereas NEC cell line NCI-H460 was insensitive to APG-1252-M1. Interestingly, Western blot analysis revealed higher expression of BCL-xL compared to BCL-2 and MCL-1 proteins in NET cell lines (BON-1, QGP-1, NCI-H727). Higher baseline levels of BCL-xL:BIM and BCL-xL:PUMA complexes in NEN cells indicated APG-1252-M1 sensitivity. Exposure to APG-1252-M1 disrupted these complexes, liberating BIM/PUMA to trigger the apoptotic cascade, as indicated by an increase in BAX:BAK complexes. Conversely, lower BCL-xL complex levels and higher MCL-1 levels correlated with insensitivity of NCI-H460 to APG-1252-M1. Immunohistochemistry staining of BCL-xL, BCL-2, and MCL-1 proteins was performed in formalin-fixed paraffin-embedded tissue sections of 106 NEN patient tumors (including stomach/intestine/pancreatic NEN), of which approximately 70% were NET and 30% NEC. Similar to data in NEN cell lines, BCL-xL expression was higher than BCL-2 expression in NEN patient samples, in which BCL-xL expression was indicated by more than 80% of samples with IHC scoring ≥6 on a scale of 0 to 10, and BCL-2 expression by 30% IHC scoring &lt; 6 or undetectable (60% IHC scoring 0). Unexpectedly, however, MCL-1 expression was also high, with more than 80% of samples IHC scoring ≥6. In summary, the results suggest that BCL-xL plays an important role in NEN. Cellular sensitivity to BCL-2/BCL-xL inhibitor APG-1252-M1 correlates with baseline BCL-xL complex levels. In NEN patient samples, MCL-1 was also highly expressed, implicating its potential negative regulatory effect on sensitivity to APG-1252. Concurrent expression of BCL-xL and MCL-1 proteins suggests that a combination treatment targeting both proteins might be more effective in NEN. Our findings inform development of a BCL-2/BCL-xL inhibitor for NEN therapy.

Citation Format: Yu Guo, Jing Deng, Lin Zhang, Xiaojing Huang, Kaixiang Zhang, Feng Zhou, Yuanbao Li, Fan Luo, Douglas D. Fang, Dajun Yang, Jie Chen, Yifan Zhai. Targeting BCL-xL addiction with APG-1252 (pelcitoclax) to overcome apoptotic blockade in neuroendocrine neoplasm (NEN) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 984.