Abstract 1163: Combined AKT and XPO1 inhibition to target chimeric transcription factor-driven pediatric sarcoma pathogenesis

Abstract Ewing sarcoma (EwS) is the most common chimeric transcription factor-driven pediatric bone cancer. One third of EwS patients are refractory to multimodal therapy (combination chemotherapy, surgery, radiation), especially those with recurrent or metastatic disease. New targeted therapies for EwS are urgently needed. We posit 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 elevated expression or phosphorylation of phosphatidylinositol-3-kinase (PI3K) pathway members in EwS cell lines, highlighting the importance of PI3K pathway activity for EwS. We identified a subset PTEN-deficient EwS cell lines 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 PI3K pathway inhibition response, 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 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 (EWSR1::WT1), PAX3::FOXO1-positive rhabdomyosarcomas, and clear cell sarcomas (EWSR1::ATF1). 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 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 Taquey, Abbigayle Szcinski, Abigail Miller, Benjamin Caiello, Joshua Monts, B J. Kendrick, Evan Bagley, Casey G. Langdon. Combined AKT and XPO1 inhibition to target chimeric transcription factor-driven pediatric sarcoma pathogenesis abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1163.