Abstract 5509: The PI3K inhibitor Paxalisib combines synergistically with the MEK inhibitor Mirdametinib to target atypical teratoid/rhabdoid tumors

Atypical teratoid/rhabdoid tumors (AT/RT) are aggressive pediatric brain tumors and the most common malignant brain tumors of infancy. The four-year event-free survival rate is only 37%. For patients with relapsed AT/RT there are limited treatment options, but precision therapies may help improve survival for patients with this deadly disease. We have previously identified strong activation of the PI3K-AKT-mTOR (mTOR) signaling pathways in AT/RT. Paxalisib is a highly brain-penetrant PI3K inhibitor acting upstream of mTOR. We find that Paxalisib slows tumor growth in orthotopic xenograft models of AT/RT and extends median survival from 40 to 54 days (p=0.001, log-rank test). RNASeq after mTOR pathway inhibition identifies reflexive activation of the RAS-RAF-MEK-ERK (MAPK) pathway as a possible mechanism of therapy resistance (KEGG pathway analysis). The RAS-RAF-MEK-ERK (MAPK) pathway plays a pivotal role in regulating cell growth and survival and activation of the pathway contributes to the aggressive growth of numerous cancers. Mirdametinib is a small, allosteric MEK inhibitor currently used in clinical trials for both pediatric low- and high-grade gliomas (PD0325901). We find that dual inhibition of both the mTOR and MAPK pathways by Paxalisib and Mirdametinib synergize to reduce AT/RT growth and viability (Bliss synergy score 16.77). Paxalisib also combines with Mirdametinib to induce high levels of apoptosis and cell senescence (as determined by western blot: cPARP, pRB, P21, P16). Naïve treatments in mice bearing AT/RT orthotopic tumors suggests the combination therapy decrease mTOR and MAPK pathway activation (as determined by western blot). These data support the use of Paxalisib and Mirdametinib combination therapy as a promising novel combination therapy to treat relapsed AT/RT. Citation Format: Kristen J. Malebranche, Tyler Findlay, Anupa Geethadevi, Charles Eberhart, Eric Raabe, Jeffrey Rubens. The PI3K inhibitor Paxalisib combines synergistically with the MEK inhibitor Mirdametinib to target atypical teratoid/rhabdoid tumors. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5509.