Abstract 6128: Targeting DHX9 helicase with the novel small-molecule inhibitor ATX968 induces replication stress and innate immunity in small cell lung cancer

Abstract Small cell lung cancer (SCLC) is the most lethal type of lung cancer. The uniqueness of this tumor lies in its initial exquisite response to chemotherapy, due to its high level of genomic instability. However, most patients experience relapse within 6 to 12 months. Although the addition of immune checkpoint blockade (ICB) therapy to standard chemotherapy has been approved by the FDA, only a small fraction of SCLC patients responds to immunotherapy because of an immunologically “cold” tumor microenvironment. Recently, using genetic depletion methods, we have demonstrated that targeting DExD/H-box helicase 9 (DHX9) in SCLC cells triggers tumor-intrinsic innate immunity, enhancing immunotherapy response and suggesting an effective and unexplored strategy to convert immunologically “cold” SCLCs into “hot”. DHX9 is a multi-functional RNA helicase capable of unwinding both RNA and DNA duplexes and more complex nucleic acid structures and has been considered an undruggable nuclear protein. Here, using SCLC models we characterized ATX968, an orally bioavailable DHX9 tool inhibitor developed by Accent Therapeutics. In vitro treatment with ATX968 induced an aberrant accumulation of R-loops (DNA/RNA hybrids), resulting in replication stress and an increase in DNA damage-derived cytoplasmic DNA in SCLC cells. In addition, ATX968 treatment led to an accumulation of cytoplasmic double-stranded RNA (dsRNA), which mainly derived from repetitive element sequences. Both cytoplasmic nucleic acids contributed to the induction of an innate immune response in SCLC cells. Notably, oral treatment with ATX968 monotherapy resulted in robust and durable tumor regression in SCLC mouse xenograft models, which was tolerable over a 28-day treatment period. Together, these findings suggest that DHX9 is a crucial repressor of tumor-intrinsic innate immunity and replication stress, and inhibitors of DHX9 represent a promising new therapeutic strategy for SCLC treatment and potentially other “cold” tumors characterized by replication stress and genomic instability. Citation Format: Takahiko Murayama, Jun Nakayama, Xueying Ma, Siddharth Balachandran, Matthew H. Daniels, Jennifer Castro, Israel Cañadas. Targeting DHX9 helicase with the novel small-molecule inhibitor ATX968 induces replication stress and innate immunity in small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6128.