Abstract 5153: Synergistic efficacy of PARP and DNMT inhibition in ATM and CHEK2 CRISP-engineered in vitro and in vivo models

Introduction: More than a decade of work has demonstrated the benefit of using poly (ADP-ribose) polymerase inhibitors (PARPi) in patients with BRCA1 and BRCA2 mutant tumors, based on their compromised ability to repair DNA damage by homologous recombination (HRD). While the benefits of PARPi monotherapy can be limited, our laboratory is working to identify co-therapeutics to improve therapeutic efficacy. Homologous recombination requires the function of many other proteins besides BRCA1 and BRCA2, and PARP sensitivity of tumors with these mutations has not been well explored. Of particular interest are tumors with mutations in the ATM and CHK2 proteins. Here in preclinical studies, we explore the sensitivity of these mutants to PARP directed therapeutics, and to our current therapeutic combination with a DNA methyltransferase inhibitor (DNMT1; decitabine) that we now have in Phase 1 clinical trials. We hypothesize that tumors with pathogenic ATM and CHK2 mutations may also show PARP inhibitor sensitivity, which might also be potentiated in combination with decitabine. Methods: To test this hypothesis, we generated inducible RNA knockdown variants of the HR proficient triple negative breast cancer cell line MDA-MB-231, targeting ATM and Chek2 transcripts under doxycycline control via dCAS9 technology. Responses to the PARP inhibitor talazoparib, with and without decitabine were assessed with and without doxycycline induction. We evaluated colony formation, cell death, cell cycle fraction alterations, DNA damage and measured PARP1-DNMT1 adduct formation using proximity ligation assays. Results: Preliminary findings show that the combination of talazoparib and decitabine resulted in greater synergistic efficacy at lower doses than talazoparib alone. In the ATM dCas9, and CHEK2 dCas9 cell lines, the combined treatment led to substantial DNA damage, prolonged trapping of PARP1-DNMT1 complexes on DNA, and significant induction of cell death. Accordingly, potent growth inhibition was also seen in xenografts models with clinically relevant ATM and CHEK2 mutations. Conclusion: These results support further investigation into the combined use of PARP and DNMT inhibitors in tumors with these mutations. The synergistic effects observed in the MDA-MB231-derived models suggest that this combination could provide a more effective treatment option for HRD cancers, particularly in overcoming resistance to PARPi monotherapy. Citation Format: Fateme Karimi Dermani, Sibapriya Chaudhuri, Luika Timmerman, Scott Thomas, Pamela Munster. Synergistic efficacy of PARP and DNMT inhibition in ATM and CHEK2 CRISP-engineered in vitro and in vivo models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5153.