Abstract 355: BCLxL inhibition enhances copanlisib response in colorectal cancer

Abstract Background: Anti-apoptotic signaling, including through BCLxL, is a potential mechanism of resistance to PI3K inhibition in PIK3CA mutant cancers. We have previously demonstrated enhanced activity of PI3K inhibition in combination with BCLxL inhibition in PIK3CA mutated colorectal cancer (CRC) models. Here we examine the potential impact of BCLxL expression in PIK3CA mutant CRCs and the mechanism of treatment response with the combination of copanlisib and navitoclax. Methods: PIK3CA mutant CRCs were identified within TCGA colorectal adenocarcinoma data set and two cohorts were generated based on BCL2L1 (gene that encodes protein BCLxL) expression. Differentially expressed genes and gene set enrichment analysis were performed. CRC cell lines SW48 and SW48PK (Horizon Discovery, SW48 with a PIK3CAH1047R/+ mutation) were treated with copanlisib (copan) (1-100 nM) and navitoclax (250 nM). Immunoblotting was performed quantified using ImageJ to evaluate PI3K and apoptotic signaling. Flow cytometry using annexin V and propidium iodide (PI) was performed as a measure of apoptosis. Immunohistochemistry (IHC) using Ki67 was performed as a measure of cell proliferation on SW48PK xenografts and quantified as percent positive nuclei at 40x field of view (FOV). Results: Two datasets comprised of PIK3CA mutant CRC patients with either high or low BCL2L1 expression containing 39 and 47 patients, respectively, were used in the TGCA analysis. There was no difference in age, gender, or staging between these two datasets. An increase in DNA repair (false discovery rate q-value= 0. 034) and interferon alpha signaling pathways (q= 0. 022) was seen in PIK3CA mutant and high BCL2L1 expression compared to PIK3CA mutant and low BCL2L1 expression patients. PIK3CA mutant patients with low expression of BCL2L1 had a greater disease-free and overall survival (p= 0. 037 and p= 0. 042, respectively). Combination of copan and navitoclax treatment of the SW48 and SW48PK cell lines resulted in a reduction in activated ribosomal protein S6 (pRPS6) as compared with control or navitoclax alone at both 6 and 24. Cleaved PARP increased in SW48PK cells treated with 100 nM copan plus navitoclax versus control at both 6 and 24 hour timepoints (p= 0. 03, p= 0. 049). SW48 and SW48PK cells that were treated with copan and navitoclax for 24 hours had an increase in annexin v, a marker for apoptosis, compared to cells treated with control or single agents. In SW48PK xenografts, a significant reduction in Ki67 percent positive nuclei was seen in combination compared to control (median percent positive nuclei: 20. 5, control: 33. 2, p0. 001). Conclusions: High BCLxL expression is a sign of poor prognosis for PIK3CA mutant CRC. Inhibiting BCLxL can enhance the sensitivity of PI3K inhibition in PIK3CA mutant cancers and deserves further investigation clinically. Citation Format: Alexa E. Schmitz, Rebecca A. DeStefanis, Alyssa K. DeZeeuw, Autumn M. Olson, Anna L. Lippert, Kennedy J. Maduscha, Katherine A. Johnson, Sean Kraus, Cheri A. Pasch, Dustin A. Deming. BCLxL inhibition enhances copanlisib response in colorectal cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 355.