Abstract B140: Development of EO-4426: A brain-penetrant dual DNA polymerase-α and ribonucleotide reductase inhibitor

Abstract Background: Resistance associated with cytidine deaminase (CDA)-mediated deactivation, a key limitation of existing ribonucleotide reductase (RNR) inhibitors such as gemcitabine and hydroxyurea. EO-4426 (tezacitabine) is an orally bioavailable, brain-penetrant small molecule that inhibits both DNA polymerase alpha (Pol α) and ribonucleotide reductase (RNR)—critical enzymes for DNA replication and repair. This unique dual-targeting mechanism induces replication fork collapse and accumulation of DNA damage, selectively impacting rapidly proliferating tumor cells. Methods: EO-4426 has been evaluated in over 400 patients across multiple Phase 1 and Phase 2 clinical trials, using both intravenous and oral formulations as monotherapy and in combination with standard chemotherapies such as 5-FU and cisplatin. In both preclinical and clinical settings, EO-4426 has demonstrated anti-tumor activity across a broad range of solid tumors and hematologic malignancies, with a favorable tolerability profile. The primary dose-limiting toxicity observed has been reversible neutropenia. Results: In preclinical studies, EO-4426 demonstrated sustained activity in tumor models with high CDA expression, supporting its potential to address this common resistance mechanism. Planned clinical trials will incorporate biomarker-driven patient selection, including microsatellite instability (MSI) and RRM2 overexpression, to identify tumors characterized by replication stress and increased reliance on DNA repair pathways. Conclusions: EO-4426 represents a novel oral, brain-penetrant dual inhibitor of Pol α and RNR with promising preclinical and clinical evidence of activity in aggressive cancers. Its ability to bypass CDA-related resistance and its biomarker-driven development strategy position EO-4426 as potential a first-in-class precision therapy for CNS malignancies. Citation Format: Jeffrey A. Bacha, Richard Daniels, Sarath Kanekal, John M. Langlands, Dennis Brown. Development of EO-4426: A brain-penetrant dual DNA polymerase-α and ribonucleotide reductase inhibitor [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 B140.