ATRT-12. HYPERACTIVATING THE INTEGRATED STRESS RESPONSE WITH PROTEASOME INHIBITION IN AT/RT

Atypical teratoid/rhabdoid tumor (AT/RT) is an aggressive pediatric brain tumor with a poor prognosis. AT/RT has increased baseline activation of the integrated stress response (ISR), an evolutionarily conserved system that enables cells to tolerate various forms of stress. Elevated expression of ATF4 indicates activation of the ISR. Transient or low-level expression of this transcription factor protects cells from stress, while sustained, high-level activation results in cell death. Ixazomib is an orally bioavailable proteasome inhibitor that causes endoplasmic reticulum stress, which is a major upstream activator of the ISR. Because AT/RT has a high baseline level of ATF4, we hypothesized that this tumor would be susceptible to ISR activators such as ixazomib. After determining the IC50 of ixazomib, AT/RT cell lines were treated with increasing concentrations of the drug. Cleaved caspase-3 immunofluorescence showed a significant increase of apoptosis (CHLA06 p<0.0001 by ANOVA). We are currently testing ixazomib in combination with idarubicin, a brain-penetrant anthracycline, and gemcitabine, a brain-penetrant nucleoside analog. Synergy testing in CHLA06 showed an overall zero-interaction-potency (ZIP) synergy score of 23.464 with inhibitory concentrations of ixazomib and idarubicin in the low nanomolar range (scores of 10 or above indicate synergy). Similarly, ixazomib and gemcitabine synergized to suppress CHLA06 growth and induce apoptosis (67% Annexin V+ cells in combination compared to 14 and 24% Annexin V+ with single agent treatment). In the future, we will determine if ixazomib selectively kills AT/RT while sparing normal cells in iPSC brain organoids. We will also test ixazomib in orthotopic xenografts of AT/RT. These results suggest that ixazomib, especially in combination with idarubicin, gemcitabine or other ISR activators, has the potential to serve as an effective therapy for AT/RT.