Abstract SY11-02: Small molecule inhibitors of RNA modifying enzymes as precision cancer therapeutics

Abstract Over 100 modifications to RNA are known to occur in human cells, where they influence many aspects of RNA biology, including protein and nucleic acid interactions. Our survey of human RNA-modifying enzymes with selectively essential phenotypes in specific cancers, suggests that these cancer-essential enzymes fall into four mechanistic categories: group transfer enzymes; base editors; nucleases; and helicases. Our team has discovered nanomolar or picomolar inhibitors of cancer-essential enzymes within each of these four categories. DHX9, a novel cancer target, is a multifunctional DEAH-box RNA helicase which can unwind regions of double-stranded DNA and RNA helices but has a greater propensity for secondary structures such as DNA/RNA hybrids (R-loops), circular RNA and DNA/RNA G-quadruplexes. DHX9 interacts with and regulate a large variety of proteins, including key proteins in DNA damage repair pathways such as BRCA1, ATR, Ku86, and WRN. We previously demonstrated that DHX9 inhibition is selectively efficacious in microsatellite high (MSI-H) /defective mismatch repair (dMMR) tumor models, in vitro and in vivo. Further profiling of DHX9 inhibitors across a broad panel of cancer cell lines reveals that tumor cells with Loss-of-Function (LOF) mutations in the DNA damage repair genes BRCA1 and/or BRCA2 (as defined by somatic mutations including single-nucleotide variants and/or copy number loss), are also selectively responsive to DHX9 inhibitor treatment. Selective dependence on DHX9 was observed in both ovarian and breast cancer cell lines that exhibit BRCA1 and/or BRCA2 LOF. DHX9 inhibition leads to increased RNA/DNA secondary structures such as R-loops and G-quadruplexes, resulting in subsequent DNA damage and increased replication stress. Cell lines that exhibit BRCA1 and/or BRCA2 LOF appear unable to resolve this replication stress and showed S-G2 phase cell cycle arrest prior to onset of apoptosis. An orally bioavailable DHX9 inhibitor was dosed in vivo to assess DHX9 dependency within multiple human xenografts representing triple negative breast cancer and high-grade serous ovarian cancer with BRCA1 and/or BRCA2 LOF. In all models, DHX9 inhibition was well tolerated for a period of up to 28 days, with robust and significant tumor growth inhibition - including tumor regression - observed in multiple BRCA1 and/or BRCA2 LOF models; in contrast, minimal tumor growth inhibition was observed in BRCA1 and BRCA2 wild type models. These results portend DHX9 inhibition as a novel treatment modality for patients with BRCA1 and/or BRCA2 LOF across multiple tumor types, including breast and ovarian cancer. Citation Format: Robert A. Copeland, Jennifer Castro, Matthew H. Daniels, Deepali Gotur, Young-Tae Lee, Shihua Yao, David Brennan, Brian T. Johnston, Monique Laidlaw, Sunaina Pai, Jie Wu, Rishabh Bansal, Anugraha Raman, Shane M. Buker, Julie Liu, E. Allen Sickmier, Kevin Knockenhauer, Chuang Lu, Stephen J. Blakemore, Serena J. Silver, P. Ann Boriack-Sjodin, Kenneth W. Duncan, Jason A. Sager. Small molecule inhibitors of RNA modifying enzymes as precision cancer therapeutics abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (7Suppl): Abstract nr SY11-02.