Abstract B018: Combined AKT and XPO1 Inhibition Thwarts Chimeric Transcription-Factor Driven Pediatric Sarcoma Pathogenesis

Abstract Ewing sarcoma (EwS) is the most common chimeric transcription factor-driven pediatric bone cancer. EwS patients are treated with an intensive, “dose-dense” combination chemotherapy regimen, along with radiation and surgery. One third of EwS patients are refractory to this multimodal treatment strategy, especially those with recurrent or metastatic disease. New targeted therapies for EwS are urgently needed, and we posited targeted combination therapies will be most effective at thwarting sarcoma pathogenesis. Examining reverse phase protein array data from the Cancer Dependency Map (DEPMAP), we found several phosphatidylinositol-3-kinase (PI3K) pathway members with high EwS expression or phosphorylation when compared to all other cell lines in the DEPMAP. This highlights the importance of PI3K pathway activity for EwS. We identified a subset of EwS cell lines deficient in PTEN protein expression, which was associated with increased sensitivity to AKT inhibition. Lower PTEN gene expression in EwS tumors is associated with poorer overall survival. Modulating PTEN levels or enhancing AKT activity altered the response to PI3K pathway inhibition, with AKT inhibitors showing a distinct effect compared to other pathway-targeting agents. Specifically, PTEN loss or increasing AKT phosphorylation heightened sensitivity to AKT blockade, whereas restoring PTEN expression conferred resistance. For both pan-PI3K inhibition and mTORC1/2 inhibition, increasing AKT phosphorylation or losing PTEN created resistance to these compounds while restoring PTEN expression enhanced responsiveness to each respective agent. For EwS cells with high PTEN protein expression, PTEN was both cytoplasmic- and nuclear-localized. We hypothesized treating EwS cells with a nuclear export inhibitor could block PTEN trafficking out of the nucleus, reactivate the PI3K pathway, and synergize with AKT inhibition to thwart EwS pathogenesis. Indeed, we showed treating EwS cells with selinexor, an FDA-approved exportin 1 (XPO1) inhibitor, increased nuclear PTEN protein levels. Additionally, treating EwS cells with selinexor further activated the PI3K pathway. Dual AKT and XPO1 inhibition also synergized to limit EwS cell viability, induced cytotoxicity, and decreased xenografted tumor volume. Furthermore, combined AKT and XPO1 inhibition was also effective against other pediatric sarcomas driven by chimeric transcription factors, including desmoplastic small round cell tumors and PAX3::FOXO1-positive rhabdomyosarcomas. In conclusion, we demonstrated that PI3K pathway signaling is elevated in EwS cells and that low PTEN gene expression correlates with poor survival outcomes. Nuclear export inhibition altered PTEN subcellular localization and increased PI3K activity, contributing to the synergy between AKT and XPO1 inhibition. This combination therapy displayed efficacy across multiple fusion-driven childhood sarcomas, supporting its potential as an effective combination targeted therapeutic strategy for chimeric transcription-factor driven pediatric sarcomas. Citation Format: Cameron Bumbleburg, Suprina Neupane, Henry D Schrecker, Megan VanGarven, Janaia Jackson, Allison Reno, Emma Saunders, Jacob Lenski, Reid Barker, Hannah Gulbronson, Jamie Silverman, Kajol Patel, Elizabeth Maahs, C A Colvert, Abasi-ama Udeme, Moriah Heifetz, Jenna Schwesig, Margaret C Taquey, Abbigayle Szcinski, Abigail Miller, Joshua Monts, B J Kendrick, Evan M Bagley, Casey G Langdon. Combined AKT and XPO1 Inhibition Thwarts Chimeric Transcription-Factor Driven Pediatric Sarcoma Pathogenesis [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Discovery and Innovation in Pediatric Cancer— From Biology to Breakthrough Therapies; 2025 Sep 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl_2):Abstract nr B018.