Abstract 6565: Improving survival of atypical teratoid/rhabdoid tumor orthotopic xenografts through the combination of PI3K inhibitor paxalisib and nucleoside analog gemcitabine
Authors: Findlay Tyler , Malebranche Kristen , Geethadevi Anupa , Eberhart Charles , Rubens Jeffrey , Raabe Eric 
Published: March 2024

Abstract
Atypical teratoid/rhabdoid tumors (AT/RT) are an aggressive infantile brain tumor with a dismal four-year event-free survival of 37%. We have previously identified high activation of mTORC1 and mTORC2 within AT/RT and shown that the highly brain penetrant PI3K inhibitor paxalisib is capable of simultaneously inhibiting mTORC1/2 by acting upstream of both complexes. In two orthotopic xenograft models, paxalisib slowed tumor growth and significantly extended survival (CHLA-06: 40 to 54 days, p=0.001; BT-12: 21 to 35 days, p=0.02). RNAseq analysis following mTOR inhibition in AT/RT cell lines showed an upregulation of genes involved in the activation of the integrated stress response (ISR) (ATF4, CHOP, PPP1R15A t-test p&lt;0.05). Analysis of primary pediatric brain tumors through the Children’s Brain Tumor Network showed increased levels of ATF4 mRNA and eIF2α protein and mRNA in AT/RT compared to other indolent pediatric brain tumors. Increased expression of ISR components suggests this pathway may be a dependency in AT/RT. Activation of this pathway is known to be cytoprotective, however persistent over-activation drives cell death through induction of pro-apoptotic factors. Gemcitabine is a pyrimidine analog functioning to inhibit DNA synthesis and induce DNA damage; notably, recent literature has correlated DNA damage to activation of the ISR with downstream phosphorylation of eIF2α. We hypothesize that due to high baseline activation of ISR for AT/RT, paxalisib combined with gemcitabine will result in an over-activation of the ISR leading to AT/RT cell death. Western blot analysis of combination therapy compared to DMSO control showed induction of phospho-eIF2α, ATF4, and CHOP, denoting an activation of the ISR. Combination therapy in AT/RT cells induces cell death (Western blot analysis cPARP, MUSE Annexin V assay, SynergyFinder BLISS score [viability] CHLA-06: 21.50, CHLA-266: 22.60) and synergizes to decrease cell growth (SynergyFinder BLISS score [inhibition] CHLA-06: 16.80, BT-12: 14.30, CHLA-266: 13.90, BT-37: 11.40, CHLA-05: 10.40). Paxalisib/gemcitabine combination therapy in murine orthotopic xenograft models of AT/RT has slowed tumor growth (bioluminescent imaging) and led to significant extension of survival. Mice bearing BT-37 orthotopic xenografts and treated with vehicle control had a median survival of 56 days, while all of the paxalisib/gemcitabine treated mice were alive at 100 days of treatment (p&lt;0.0001). In CHLA-06 orthotopic tumors combination therapy extended median survival from 22 days for control to 82.5 days (p&lt;0.0001). Based on these findings, the Pacific Pediatric Neuro-Oncology Consortium (PNOC) is planning to include paxalisib/gemcitabine combination therapy in the next international clinical trial for patients with relapsed/recurrent AT/RT.
Citation Format: Tyler Findlay, Kristen Malebranche, Anupa Geethadevi, Charles Eberhart, Jeffrey Rubens, Eric Raabe. Improving survival of atypical teratoid/rhabdoid tumor orthotopic xenografts through the combination of PI3K inhibitor paxalisib and nucleoside analog gemcitabine [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(6_Suppl):Abstract nr 6565.