Abstract A032: PTEN protein and lipid phosphatase deficiency confers sensitivity to ATR inhibitor-based treatment in high-grade serous ovarian cancer

Abstract ATR inhibition is under evaluation for treatment of high-grade serous ovarian cancer (HGSOC) to reverse acquired resistance to PARP inhibition and to exacerbate chemotherapy-induced replicative stress. Nonetheless, ATR inhibitor combinations with PARP inhibitors have carried overlapping hematologic toxicities, highlighting the need for predictive biomarkers to identify those most likely to benefit. In this study, we have identified PTEN deficiency as a biomarker for ATR inhibition, either as monotherapy or in combination, in HGSOC models. We found that ATR inhibitor-sensitive patient-derived organoids and xenografts were PTEN-deficient and confirmed that PTEN loss confers ATR inhibitor sensitivity using isogenic cell lines. Here, we have shown that loss of both PTEN protein and lipid phosphatase activities is important for sensitization to ATR inhibition. First, PTEN deficiency prevents dephosphorylation of MCM2 during ATR inhibition, preventing the coordinate reduction in helicase and polymerase activities at the replication fork. This uncoupling leads to successive helicase unwinding and excessive single-stranded DNA, inducing greater replication stress and DNA damage than in PTEN-proficient cells. Second, PTEN lipid phosphatase deficiency promotes AKT-mediated phosphorylation and cytoplasmic sequestration of CHK1. This disrupts cell cycle checkpoint control after ATR inhibition, leading to continued DNA synthesis during mitosis and mitotic catastrophe. Critically, these events limit the time during which ATM and DNA-PK can compensate for ATR inhibition, increasing sensitivity in PTEN-deficient cells. Third, the reduced nuclear CHK1 reduces RAD51 loading, destabilizing replication forks in response to ATR inhibition and sensitizing cells to PARP inhibition. In both in vitro and in vivo models, the degree of ATR and PARP inhibitor synergism is greater in PTEN-deficient cells, with enhanced DNA damage and apoptosis, leading to tumor regression and longstanding durability of response beyond the dosing period. Therefore, PTEN deficiency is a biomarker not only for ATR inhibitor as monotherapy, but also for combined ATR and PARP inhibition. Importantly, we have further assessed PTEN as a potential biomarker via biopsy samples from platinum-sensitive HGSOC patients treated with the ATR inhibitor ceralasertib and the PARP inhibitor olaparib. PTEN immunohistochemistry demonstrated a statistically longer progression-free survival among patients with tumors harboring low PTEN levels, compared to patients with high PTEN levels. In summary, we demonstrate PTEN as an important biomarker for prediction of clinical benefit to ATR inhibitor-based treatment. As ATR inhibition continues to move forward as a high-priority strategy in ovarian cancer, our work may ultimately contribute to optimal patient selection where there is high unmet medical need. Citation Format: Jie Hao, Bose Kochupurakkal, Timothy B. Branigan, Ozge Sezin Somuncu, Renyan Liu, Heta Jadhav, Alexandre Andre B.A. da Costa, Yuqing Jiao, David B. Martignetti, Golbahar Sadatrezaei, Sirisha Mukkavalli, Prafulla C. Gokhale, Su-Chun Cheng, Steven J. Skates, Dimitrios Nasioudis, Panagiotis A. Konstantinopoulos, Joyce F. Liu, Stephanie Gaillard, Robert L. Giuntoli II, Lainie Martin, Janos Tanyi, Nawar Latif, Ian Heller, Fiona Simpkins, Kalindi Parmar, Alan D'Andrea, Geoffrey I. Shapiro. PTEN protein and lipid phosphatase deficiency confers sensitivity to ATR inhibitor-based treatment in high-grade serous ovarian cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A032.