Abstract 7548: Role of bromodomain extra terminal inhibitor (BETi) in combination with radiotherapy for the treatment of NF1-associated malignant peripheral nerve sheath tumor

Abstract Introduction: Neurofibromatosis Type 1 (NF-1) associated malignant peripheral nerve sheath tumors (MPNST) harbor mutations in the polycomb repressive complex 2 (PRC2) in 70% of tumors. PRC2 loss results in epigenetic changes, which increase histone acetylation and make the DNA vulnerable to binding by bromodomain proteins. This interaction could be targeted to uncouple histone acetylation from transcription using bromodomain extra terminal inhibitors (BETi). As radiation therapy (RT) is a cornerstone of MPNST treatment, we analyzed the effects of BETi with and without RT. Methods: Mice were implanted with patient-derived xenografts (PDX) of PRC2-wildtype model developed in the Torres laboratory, MPNST007. Mice were segregated into vehicle and I-BET 762 treatment groups and treated with either vehicle or drug for one week before the RT was initiated. Half of the mice in each treatment group (n = 10) received radiation (2 Gy per fraction for 5 days for a total of 10 Gy). The study endpoints were (1) tumor growth rates, (2) tumor size and weight at study termination, (3) immunohistochemical (IHC) staining for biomarkers of proliferation (Ki67), apoptosis (cleaved caspase 3, CC3), and DNA damage (γH2AX), and (4) RNA sequencing of PDXs from each experimental arm. Results: Significant tumor regression was observed in PRC2-mutant PDX tumors treated with I-BET 762 based on tumor growth kinetics over 45 days of treatment (p < 0.001). Tumor volume was significantly reduced by I-BET 762 compared to vehicle (p < 0.01). Combining I-BET 762 and RT resulted in significant tumor volume reduction (p <0.001). RNA sequencing and intersection of differentially expressed genes related to “DNA damage repair” and those repressed by I-BET 762 was performed for the MPNST007 PDXs. The expression of 259 genes related to DNA damage repair decreased after I-BET 762 treatment, including XRCC1, ERCC4, RAD50, RAD51D, and LIG4 (data not shown). IHC analysis prompted by the RNA sequencing results of the I-BET 762-treated PDXs revealed higher levels of γH2AX. An increase in necrotic and fibrotic tissue with increased CC3 in the BETi plus radiation group was observed. Conclusions: Genetic mutations in NF-1-associated MPNST tumors result in susceptibility to BETi treatment. In a PDX model of PRC2 wild-type tumors, BETi treatment resulted in tumor regression and sensitized MPNST xenografts to RT. Combining BETi and RT enhanced tumor death and increased expression of genes associated with DNA damage and apoptosis. Citation Format: Alcia A. Gingrich, Sharon Landers, Angela Bhalla, Cristian Rodriguez-Aguayo, Rossana Lazcano, Suresh Satpati, Alines Lebron-Torres, Emily Z. Keung, Christopher P. Scally, Christina L. Roland, Kelly K. Hunt, Heather Lyu, Heather Lillemoe, Ashleigh Guadagnolo, Alexander Lazar, John Slopis, Ian McCutcheon, John A. Livingston, Ahsan Farooqi, Kunal Rai, Keila E. Torres. Role of bromodomain extra terminal inhibitor (BETi) in combination with radiotherapy for the treatment of NF1-associated malignant peripheral nerve sheath tumor [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 7548.