ATRT-16. 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%. There are limited treatment options for patients with relapsed AT/RT, 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 which 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 and a current survival study in mice bearing AT/RT orthotopic tumors suggest that combination therapy decreases mTOR and MAPK pathway activation (as determined by western blot). These data support using Paxalisib and Mirdametinib combination therapy as a promising novel therapy to treat relapsed AT/RT.