DDDR-33. EO-4426: A brain-penetrant dual DNA-polymerase α and ribonucleotide reductase inhibitor for high-grade gliomas and other aggressive cancers

BACKGROUND High-grade gliomas (HGG) and other CNS malignancies are marked by replication stress, genomic instability, and resistance to standard-of-care therapies. 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 dual-targeting mechanism induces replication fork collapse and accumulation of DNA damage, selectively impacting rapidly proliferating tumor cells. METHODS EO-4426 has been studied in over 400 patients across multiple Phase 1 and Phase 2 trials as both i.v. and oral formulations, alone and in combination with 5-FU or cisplatin. The drug showed favorable tolerability, with reversible neutropenia as the primary dose-limiting toxicity, and clinical activity across diverse cancers, including CNS tumors. A new Phase 1–2a study is planned to assess oral EO-4426 in patients with advanced solid tumors and hematologic malignancies, including tumors with CNS involvement. RESULTS EO-4426 crosses the blood-brain barrier and achieves therapeutic CNS concentrations in preclinical models. Preclinical data demonstrate potent activity in replication-stressed tumor models and predict efficacy in gliomas. Importantly, EO-4426 overcomes resistance related to cytidine deaminase (CDA)-mediated deactivation, a known mechanism of resistance to FDA-approved RNR inhibitors such as gemcitabine and hydroxyurea. Planned trials will incorporate biomarker-guided patient selection, including microsatellite instability (MSI) and RRM2 overexpression, both associated with replication stress and DNA repair dependency in CNS tumors. 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, including CNS malignancies. Its ability to bypass CDA-related resistance and its biomarker-driven development strategy position EO-4426 as a first-in-class precision therapy for CNS malignancies.