Abstract LB181: First disclosure of XRD-0394, a novel dual ATM/DNA-PK inhibitor, that potently radiosensitizes and potentiates PARP and topoisomerase inhibitors

Abstract A majority of cancer patients receive radiation therapy (RT) as part of their treatment regimens, whether using external beam therapy or locally-delivered radioisotopes. While often effective, RT may not be efficacious for some tumors while exerting significant short-term and long-term toxicities. Thus, enhancing therapeutic effectiveness of radiation therapy remains an important goal. ATM (ataxia-telangiectasia, mutated) and DNA-PKcs (deoxyribonucleic acid [DNA]-dependent protein kinase, catalytic subunit) are related protein kinases and both are critical regulators of cellular responses to ionizing radiation. We report here the identification and initial characterization of XRD-0394, a potent, specific dual inhibitor of both ATM and DNA-PKcs, whose chemical structure will be disclosed. A dual ATM/DNA-PK inhibitor has practical and theoretical advantages over compounds which inhibit just one of these kinases: 1) blunting both DNA damage response (DDR) pathways enhances radiation-induced tumor cell kill compared to single target inhibition; and 2) simultaneous inhibition of two DDR pathways should overcome intrinsic or acquired resistance of tumor cells to either target alone. Notably, transient inhibition of both kinases by XRD-0394 has little adverse effects on cells in the absence of irradiation. XRD-0394 is orally bioavailable, with excellent pharmacological profiles, demonstrating significant in vitro and in vivo sensitizing activity in combination with ionizing radiation and in combination with immunotherapy and radiation. In addition, a CRISPR screen identified alterations in the Fanconi A/BRCA pathway as an opportunity for synthetic lethality in tumor cells exposed to XRD-0394, an insight that was validated in multiple tumor cell types. In vitro studies demonstrated synergy of XRD-0394 in combination with PARP inhibitors, particularly in BRCA-mutant cells. XRD-0394 also potentiates the effectiveness of topoisomerase inhibitors in vitro. Single doses of XRD-0394 were well-tolerated in combination with palliative RT in subjects with advanced cancer in a Phase Ia clinical trial (NCT05002140). No DLTs were observed. At a dose of 160 mg, plasma concentrations were well above the preclinical target plasma level of 530 ng/mL (1 µM) for up to 15 h and inhibition of ATM kinase activity was demonstrated by inhibition of radiation-induced pKAP1 in tumor tissue. Importantly, this concentration of drug is sufficient to induce potent radiosensitization in numerous preclinical models. The median tmax (2.3 h) and mean terminal t½ (11.1 h) at the 160 mg dose in subjects support coadministration of XRD-0394 and RT within a clinically feasible time frame. These results provide a rationale for future clinical trials with XRD-0394 in combination with RT, RT and immunotherapy, PARP inhibitors and targeted delivery of topoisomerase inhibitors. Citation Format: Tona M. Gilmer, Chun-Hsiang Lai, Kexiao Guo, Katherine Deland, Kathleen A. Ashcraft, Amy E. Stewart, Yaode Wang, Jianmin Fu, Kris C. Wood, Deborah A. Smith, Jonathan T. Yang, Christopher T. Chen, Michael F. Gensheimer, Paul B. Romesser, Steven H. Lin, David G. Kirsch, Michael B. Kastan. First disclosure of XRD-0394, a novel dual ATM/DNA-PK inhibitor, that potently radiosensitizes and potentiates PARP and topoisomerase inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB181.