Abstract 4221: Precision targeting of HDAC3 in ovarian cancer

Abstract Introduction: Addressing chemo-resistant homologous recombination-proficient (HRP) ovarian cancers and CCNE1-amplified tumors, while minimizing treatment-related side effects, remains a significant clinical challenge. Our research demonstrated that HDAC inhibition suppresses tumors in PARP inhibitor (PARPi)-resistant ovarian cancers and enhances PARPi sensitivity in preclinical models. However, pan-HDAC inhibitors (HDACis) have shown high toxicity in clinical trials. To overcome this, we focused on selectively targeting specific HDACs, particularly HDAC3 and HDAC8, which have unique structures and co-repressor complexes. HDAC3 is frequently upregulated in ovarian cancers, making it a promising therapeutic target. Here, we explore HDAC3 genetic depletion/knockout (KO) and precision targeting using HDAC-PROTACs in five cell lines. Methods: We employed siRNA-mediated depletion and CRISPR-Cas9 KO to study the impact of HDAC3 inhibition on CCNE1-amplified OVCAR3 cell proliferation, clonogenicity, and protein expression. Histone modifications were analyzed via mass spectrometry. The anti-proliferative efficacy of HDAC3 (XZ9002), HDAC8 (YL352), and dual HDAC3/8 (YX968) PROTACs was evaluated in OVCAR3, COV318, HRP and HRD mouse fallopian tube cells, and patient-derived primary cell lines. Results: HDAC3 siRNA treatment in OVCAR3 cells led to dose-dependent reductions in HDAC3 protein levels and PCNA expression. HDAC3-KO cells exhibited increased histone acetylation and decreased histone methylation. PROTACs showed differential efficacy across cell lines. In OVCAR3 cells, IC50 values were 1.82 µM for HDAC3-PROTAC, 1.98 µM for HDAC8-PROTAC, and ∼0.67 µM for HDAC3/8 dual PROTAC, indicating superior potency with dual targeting. In COV318 cells, the dual HDAC3/8 PROTAC exhibited an IC50 of 2.5 µM. HRD mouse fallopian tube cells were more sensitive to HDAC8 and HDAC3/8 PROTACs (IC50 ∼6 µM), while HRP cells were less sensitive (IC50: 8 µM for HDAC3/8 PROTAC, 10 µM for HDAC8 PROTAC, >10 µM for HDAC3 PROTAC). Patient-derived primary cell lines demonstrated the highest sensitivity to the dual HDAC3/8 PROTAC (IC50 ∼7 µM), with reduced response to single HDAC PROTACs. Pharmacodynamic analyses via Western blot and qPCR are ongoing. Conclusion: Selective targeting of HDAC3, HDAC8, and HDAC3/8 using PROTACs provides a promising therapeutic strategy to address toxicity limitations of conventional HDACis. These agents hold potential for low-dose applications in ovarian cancers and broader therapeutic use in other malignancies and diseases. Citation Format: Vijayalaxmi G. Gupta, Bisiayo Fashemi, Reni Akande, Preedia Babu, Yukihide Ota, Yufeng Xaio, Guangrong Zhang, Francisca Nathalia Vitorino, Benjamin Garcia, Mary Mullen, Dineo Khabele. Precision targeting of HDAC3 in ovarian 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 4221.