Abstract 1911: Targeting PLK1 or WEE1 uncovers a therapeutic vulnerability in BRCA1/2 wild-type, homologous recombination-proficient high-grade serous ovarian cancer.

Abstract Background: High-grade serous ovarian cancer (HGSOC) with BRCA1/2 wild-type (BRCA-WT) and homologous recombination proficiency (HRP) responds poorly to platinum agents and PARP inhibitors and lacks effective targeted therapies. We aimed to determine whether inhibition of PLK1 or WEE1, two G2/M checkpoint kinases, could uncover a therapeutic vulnerability in BRCA-WT/HRP HGSOC. Methods: We evaluated the effects of the PLK1 inhibitor volasertib and the WEE1 inhibitor adavosertib in BRCA-WT/HRP (and BRCA-mutant/HR-deficient (HRD) ovarian cancer models. Cell viability, cell-cycle distribution, apoptosis markers, and DNA damage responses were quantified, including γH2AX, RAD51, and 53BP1 foci. DNA-PKcs inhibition or siRNA was used to assess NHEJ dependence. Antitumor activity was further validated in BRCA-WT cell line-derived xenografts and two PDX models with confirmed BRCA status. Results: BRCA-WT cells demonstrated substantially higher sensitivity to PLK1 or WEE1 inhibition than BRCA-mutant cells. In BRCA-WT models, volasertib induced abnormal spindle assembly, polyploidy, and delayed mitotic progression, whereas adavosertib abrogated the G2/M checkpoint and triggered premature mitotic entry. Both agents increased DNA damage and markedly reduced RAD51 foci, demonstrating HR suppression. Although NHEJ activation was observed, it was insufficient to compensate for impaired HR. In contrast, BRCA-mutant cells showed minimal response and displayed constitutively high DNA-PKcs expression with persistent 53BP1 foci. Pharmacologic or genetic DNA-PKcs inhibition restored sensitivity to PLK1 or WEE1 inhibition in BRCA-mutant cells. In vivo, PLK1 or WEE1 inhibition significantly reduced tumor growth and increased apoptosis in BRCA-WT xenografts and BRCA-WT PDX, whereas BRCA-mutant PDX exhibited limited response. Conclusion: BRCA-WT/HRP HGSOC exhibits distinct susceptibility to PLK1 or WEE1 inhibition through disruption of HR repair and mitotic regulation, revealing a DNA repair-based vulnerability not observed in BRCA-mutant/HRD tumors. These findings provide a strong rationale for biomarker-guided development of PLK1 or WEE1 inhibitors as therapeutic strategies for HRP ovarian cancers, a patient population with significant unmet clinical needs. Citation Format: Akiko Kunita, Qian Xi, Masanori Kawakami, Miho Ogawa, Mirei Ka, Kousuke Watanabe, Tetsuo Ushiku, Hidenori Kage, Kazuhiro Katayama, Katsutoshi Oda. Targeting PLK1 or WEE1 uncovers a therapeutic vulnerability in BRCA1/2 wild-type, homologous recombination-proficient high-grade serous ovarian cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1911.