Abstract A027: CHK1/2 inhibition with prexasertib reveals therapeutic vulnerabilities in advanced prostate cancer

Abstract Introduction: Advanced prostate cancer remains challenging to treat, particularly after the development of resistance to androgen receptor-targeted therapies. Current options have significant limitations as Poly ADP-ribose polymerase (PARP) inhibitors benefit only tumors with homologous recombination deficiency, and radiation therapy is often unsuitable for metastatic disease. Genomic alterations common in advanced prostate cancer (e.g., TP53 loss, MYC amplification, BRCA2 mutations) increase replication stress, creating therapeutic vulnerability. Checkpoint kinase 1 (CHK1), a key regulator of replication stress response, is an emerging target. Prexasertib, a CHK1/2 inhibitor under clinical investigation in other cancers, has not been systematically studied in prostate cancer. Here, we evaluated prexasertib efficacy and explored molecular mechanisms of sensitivity. Methods: Prexasertib was tested as a monotherapy across 27 metastatic prostate cancer patient-derived xenograft (PDX) models with diverse histopathology and genomic backgrounds. Response was quantified by comparing tumor growth slopes between treated and vehicle groups, and classified as responder, intermediate responder, or non-responder. Pre-treatment PDX RNA-seq was used to identify transcriptional signatures associated with treatment response. To investigate mechanisms, we performed CRISPR/Cas9 knockout screens in PC-3, 22Rv1, and LNCaP cells using a custom sgRNA library targeting ∼1800 DNA damage response genes under prexasertib treatment at IC20. Results: Prexasertib inhibited tumor growth in 26% (7/27) PDX models. RNA-seq analysis revealed enriched E2F signaling in sensitive tumors. CRISPR/Cas9 knockout screens identified key genes whose loss enhanced sensitivity, including Wee1-like protein kinase 1 (WEE1), a checkpoint kinase linked to CHK1 inhibitor resistance. Integration of transcriptomic and functional genomic data highlighted overlapping candidate genes currently under investigation to refine therapeutic strategies. Conclusion: Prexasertib demonstrated efficacy in a subset of metastatic prostate cancer PDX models. Combined transcriptomic and functional genomic analyses identified candidate mediators of sensitivity, providing a foundation for advancing CHK1/2 inhibitor-based therapies in prostate cancer. Citation Format: Jiachen Ji, Aliaa Alamoudi, Nadia Boufaied, Connor J. Sessions, Holly M. Nguyen, Ilsa M. Coleman, Maria C. Fernandez, Eva Corey, David P. Labbé. CHK1/2 inhibition with prexasertib reveals therapeutic vulnerabilities in advanced prostate cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Innovations in Prostate Cancer Research and Treatment; 2026 Jan 20-22; Philadelphia PA. Philadelphia (PA): AACR; Cancer Res 2026;86(2_Suppl):Abstract nr A027.