Abstract B06: Targeting PI3K/mTOR in mouse rhabdomyosarcoma models driven by FGFR4 activation
Authors: McKinnon Timothy , Venier Rosemarie , Yohe Marielle , Gryder Berkley E. , Dickson Brendan , Schleicher Krista , Davani Dariush , Wei Winnie , Guidos Cynthia , Gupta Abha , Khan Javed , Gladdy Rebecca 
Published: August 2018

Abstract
Rhabdomyosarcoma (RMS) is the most common pediatric soft-tissue sarcoma and while survival rates have increased over the past few decades, outcomes for intermediate and high-risk patients remain dismal. Genomic analyses show that 93% of RMS have RTK/RAS/PI3K alterations and that fibroblast growth factor receptor 4 (FGFR4) is frequently mutated or overexpressed. The point mutation V550E constitutively activates FGFR4, stimulating downstream signaling pathways. In this translational study, we demonstrate that FGFR4V550E is a potent oncogene in mouse models of RMS and that secondary tumor models can be generated following transplantation of FGFR4V550E overexpressing tumor cells into immunocompetent host mice. These models uncovered mechanisms of transformation in FGFR4V550E-driven RMS and helped identify novel compounds that inhibit RMS growth. Specifically, AKT and mTOR signaling pathways were activated in myoblasts overexpressing FGFR4V550E and similarly, FGFR4V550E overexpressing tumors and tumor-derived cells exhibited increased AKT and mTOR phosphorylation by immunoblot. Murine tumor cells overexpressing FGFR4V550E were tested in an in vitro dose-response drug screen along with human RMS cell lines. Compounds were grouped by target class, and potency was determined using average percent area under the dose response curve (AUC). Using this technique, FGFR4V550E overexpressing tumor cells were highly sensitive to PI3K/mTOR inhibitors. In particular, GSK2126458 (omipalisib) was a potent inhibitor of FGFR4V550E tumor-derived cell and human RMS cell viability. FGFR4V550E overexpressing myoblasts and tumor cells had low nanomolar GSK2126458 EC50 values. Mass cytometry using mouse and human RMS cell lines validated GSK2126458 specificity at single cell resolution, decreasing the abundance of phosphorylated Akt as well as decreasing phosphorylation of the downstream mTOR effectors 4ebp1, Eif4e, and S6. In a preclinical study, GSK2126458 inhibited tumor growth in vivo. A statistically significant increase in disease-specific survival was observed in mice treated with GSK2126458 compared to mice treated with vehicle alone (p&lt;0.001) or standard of care, vincristine (p&lt;0.05). In summary, RMS driver mutations were validated in vivo and our model system was effectively used as a preclinical tumor model to identify and test therapeutic agents. Importantly, these results suggest a role for PI3K/mTOR inhibition in precision therapy regimens for RMS with FGFR4 mutations. This study provides further evidence for RMS clinical studies involving mTOR inhibitors (e.g., temsirolimus).
Citation Format: Timothy McKinnon, Rosemarie Venier, Marielle Yohe, Berkley E. Gryder, Brendan Dickson, Krista Schleicher, Dariush Davani, Winnie Wei, Cynthia Guidos, Abha Gupta, Javed Khan, Rebecca Gladdy. Targeting PI3K/mTOR in mouse rhabdomyosarcoma models driven by FGFR4 activation [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr B06.