Abstract 3149: Micronuclei (MN) dynamics in preclinical models and in patient reticulocytes as a pharmacodynamic biomarker for the first in class DNA polymerase theta (PolΘ) inhibitor ART6043

Abstract Introduction: ART6043 is a first in class, potent and selective polymerase domain inhibitor of DNA Polθ, a central mediator of the microhomology-mediated end joining (MMEJ) DNA double strand break (DSB) repair pathway. Novel biomarkers are important to demonstrate target engagement of ART6043. Polθ knock out mouse studies highlighted increased levels of MN in mouse reticulocytes (RETs) and normochromatic erythrocytes since MN are a common hallmark of genome instability. Here we describe the utility of an assay designed to monitor MN induction in reticulocytes from both preclinical models and patient samples to evaluate Polθ inhibition. Methods: This analysis builds on the MN-RETs assay, which is traditionally used as a preclinical toxicology assessment (Tometsko et al., 1995). This assay involves fixing whole blood, isolating reticulocytes (specifically in the human assay), and labelling them for nucleic acids to stain any MN. High throughput flow cytometry is then used to quantify the number of reticulocytes with MN. Results: These data demonstrated the utility of MN-RET as a target engagement biomarker for ART6043 in preclinical studies and in the subsequent clinical study (NCT05898399). In preclinical xenograft models ART6043 monotherapy and ART6043 in combination with olaparib induces MN-RET at doses where tumor regressions occurred. The MN-RET data evaluated in the phase 1 trial of ART6043 mirrored the preclinical data. Patients treated with the highest dose of ART6043 evaluated as monotherapy had elevated MN-RETs, while patients treated with ART6043 in combination with olaparib had statistically higher MN-RET levels. This suggests that the combination of two DNA repair pathway inhibitors, targeting separate mechanisms, leads to a synergistic increase in DNA damage. Summary: The increase in MN-RETs observed in patients dosed with ART6043 has demonstrated target engagement of a Polθ inhibitor for the first time in humans. MN-RETs has demonstrated potential utility as a mechanism of action target engagement biomarker for a range of therapeutics targeting different DNA damage repair pathways. References: Tometsko, A. M., Dertinger, S. D., & Torous, D. K. (1995). Analysis of micronucleated cells by flow cytometry. 4. Kinetic analysis of cytogenetic damage in blood. Mutation Research/Environmental Mutagenesis and Related Subjects, 334(1). https://doi.org/10.1016/0165-1161(95)90025-X Citation Format: Colette L. Baxter, Timothy A. Yap, Susanna V. Ulahannan, Nehal Lakhani, Minal Barve, Erika P. Hamilton, Babar Bashir, Helen M. Robinson, Jayesh B. Majithiya, Paula Costales, Sarah V. Holt, Samantha C. Skelton, Marina Roy-Luzarraga, Jeff C. Bemis, Dorothea Torous, Svetlana Avlasevich, Susana Barriga, Bryony Harrop, Gillian A. Langford, Ian Smith, Graeme C. Smith. Micronuclei (MN) dynamics in preclinical models and in patient reticulocytes as a pharmacodynamic biomarker for the first in class DNA polymerase theta (PolΘ) inhibitor ART6043 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3149.