Abstract A050: Role of drug induced nuclear CTSL (nCTSL) in DNA damage response in cancer- therapeutic implications

Abstract Introduction: Despite treatment advances, ovarian cancer (OC) remains the deadliest gynecological cancer, with a ∼70% mortality rate. OCs initially respond to platinum chemotherapy but often develop resistance. While PARP inhibitors improve relapse-free survival in responsive cases, they do not extend overall survival. This underscores the need for effective combination therapies beyond BRCA status. The lysosomal protein cathepsin L (CTSL) can traffic to the nucleus, where it can regulate DNA repair and serve as a chromatin modifier. We found that nucleoside analog clofarabine (CLF) induces CTSL nuclear trafficking in OC cells. Here we tested the hypothesis that accumulation nuclear CTSL (nCTSL) is a biomarker of sensitivity to DNA damaging therapy in OCs for improved therapeutic options. Methods: To determine the role of CLF induced nuclear CTSL in DNA damage and its synergy with PARP and CRM1 inhibitors in PARP inhibitor–sensitive and –resistant ovarian cancer (OC) cell lines, ex vivo cultures of ascites-derived patient OC cells (OVA-3D spheroids), primary ovarian tumors, and patient-derived xenograft (PDX) models and validated by colony formation assays. Nuclear CTSL localization was determined in subcellular fractionated lysates by western blots and immunofluorescence cytometry (IFC). Tumor regression was determined in vivo using ex vivo cultures of PDX derived cells. Results: In ovarian cancer (OC) cell lines, CLF monotherapy induced nuclear CTSL (nCTSL) in a subset of models classified as CLF-responsive (CLF-r), sensitizing them to the PARP inhibitors olaparib and rucaparib. In CLF-non-responsive (CLF-nr) models, nCTSL induction and therapeutic synergy were observed only with the CLF+olaparib combination. Synergy was detected in 47% of CLF-r and 24% of CLF-nr patient-derived ascites samples (OVA). Mechanistically, CLF induced nuclear import of CTSL via KPNB1 in CLF-r cells, while this import was triggered by the CLF+olaparib combination in CLF-nr cells. CLF also downregulated the nuclear export protein CRM1 (XPO1) in both groups. Knockdown of CTSL in either CLF-r or CLF-nr cells abrogated sensitivity to the combination. Notably, in 29% of OVA samples classified as CLF-resistant (CLF-Res), CLF failed to suppress CRM1 expression. In these cases, CRM1 inhibition with KPT8602 restored CLF+PARP inhibitor synergy. In vivo, CLF-r and CLF-nr PDX models treated with the combination exhibited increased DNA damage, reduced tumor burden, and extended survival. Conclusion: These findings identify nuclear CTSL (nCTSL) induction as a potential biomarker of response to DNA-damaging therapies. The combination of CLF and olaparib represents a promising strategy to overcome drug resistance in ovarian cancer by promoting CTSL nuclear localization and enhancing DNA damage. Citation Format: Prabhu Thirusangu, Ling Jin, Upasana Ray, Anya Zhao, Julie Staub, Xinyan Wu, Xiaonan Hou, Jamison L. VanBlaricom, Syed Mohamed Musheer Aalam, Sun-Hee Lee, Ann Oberg, Nagarajan Kannan, John Weroha, Jeremy Chien, Scott H. Kaufmann, Jamie N. Bakkum-Gamez, Viji Shridhar. Role of drug induced nuclear CTSL (nCTSL) in DNA damage response in cancer- therapeutic implications [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(18_Suppl):Abstract nr A050.