Abstract Background: HGSOC is the most lethal gynecologic malignancy in the U. S. and is characterized by TP53 mutations and genomic instability. While PARP inhibitors (PARPi) improve outcomes in a subset of BRCA-mutant HGSOC, their activity is limited in BRCA wild-type platinum-resistant tumors, highlighting an urgent need for new therapeutic strategies. We previously identified DHX9, a DNA/RNA helicase, as essential for the survival of platinum- and PARPi-resistant HGSOC cells through DHX9’s function in resolving R-loops and supporting transcription-coupled DNA repair. We hypothesized that pharmacologic inhibition of DHX9 would induce genomic instability and cytotoxicity in drug-resistant HGSOC. Methods: We tested the activity of a DHX9 inhibitor (DHX9i, ATX968) in BRCA wild-type platinum/PARPi-resistant (OVCAR3, OVCAR5, OVCAR8, PEO4) and BRCA-mutant platinum/PARPi-sensitive (UWB1. 289, PEO1) HGSOC cell lines. Long-term proliferation was assessed by colony formation assays. DNA damage and chromosomal instability were quantified using immunofluorescence staining for γH2AX foci and metaphase spread analysis. Apoptosis and cell cycle progression were evaluated by Annexin V staining and flow cytometry. Statistical significance was defined as p 0. 05 (Student’s t-test or ANOVA). Results: DHX9i suppressed colony formation in a subset of HGSOC cell lines (OVCAR3, PEO4, UWB1. 289) with IC50 values ranging from 0. 0004 to 0. 24 μM, independent of BRCA mutation status or platinum/PARPi resistance. To investigate the mechanisms underlying DHX9i response, we focused on OVCAR3 (most sensitive) and OVCAR8 (most resistant). In OVCAR3, DHX9i induced marked DNA damage (percentage of cells with 5 γH2AX foci: 75% vs. 18%, p 0. 01), chromosomal instability (5 breaks in 95% of nuclei, p 0. 001), G2/M arrest (47% vs. 35%, p 0. 05), and apoptosis (34% vs. 3%, p 0. 01) compared to untreated cells, indicating a critical role for DHX9 in maintaining genomic stability. These effects were absent in OVCAR8. Paclitaxel, which disrupts mitosis through microtubule stabilization, has demonstrated activity in platinum-refractory HGSOC. In OVCAR8, co-treatment with paclitaxel and DHX9i significantly reduced proliferation compared to DHX9i alone (79% vs. 12%, p 0. 001). Conclusions: Our data suggest that DHX9i activity appears to occur independently of BRCA mutation status or baseline platinum/PARPi resistance. Cytotoxic effects of DHX9i and paclitaxel in resistant models warrant further investigation as a treatment approach for recurrent HGSOC. Citation Format: Tzu-Ting Huang, Jennifer Castro, Sunaina Nayak, Serena J. Silver, Jung-Min Lee. DHX9 inhibition induces genomic instability and enhances paclitaxel sensitivity in drug-resistant high-grade serous ovarian cancer (HGSOC) cells abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85 (18Suppl): Abstract nr A031.
Huang et al. (Fri,) studied this question.