Abstract 4281: Pharmacological inhibition of thymine DNA glycosylase (TDG) triggers an anti-tumor immunity to suppress p53-deficient cancer

Abstract As the most frequently mutated or silenced tumor suppressor, TP53 mediates cell-cycle arrest, apoptosis, and senescence in cancer development. Although patients with TP53 alterations are unfortunately poor in prognosis and survival, targeting p53 directly is still largely difficult to activate or restore p53 normal function specifically in cancer. Recently, synthetic lethality approach has been successfully applied to treat patients with mutations in tumor suppressor genes (TSGs) like PARPi in BRCA1/2 mutated tumors, potentiating the future direction of next generation p53 therapies. However, it remains a huge challenge to identify the bona fide synthetic lethal partners to TP53 mutations in cancer. In this study, we demonstrated that pharmacological inhibiting thymine DNA glycosylase (TDG), a multifunctional DNA modifying enzyme with essential roles in DNA base excision repair (BER), transcriptional regulation, and DNA active demethylation, can induce synthetic lethality specifically in various p53-deficient solid tumors including lung, breast, and skin cancer, etc. This offers a novel strategy to target mutated TP53 for cancer therapies. We discovered and developed from CytosinLab’s EpigenPLUS platform a series of small-molecule inhibitors that binds specifically and directly to TDG and disrupts its DNA-binding and modifying activities. Among the series, C-271 is an orally bioavailable front-runner with excellent pharmacokinetics (PK) properties and druggabilities and exhibits profound efficacy in suppressing growth of p53-deficient lung cancer cells both in vitro and in vivo, with minimal repression on p53 wide-type cells. In p53-deficient cancer cells, treatment with C-271 reduces transcription of the RNA helicase DHX9, thereby leading to the accumulation of SINE (short interspersed nuclear element) -derived dsRNA and subsequent activation of the anti-tumor immunity pathways. Additionally, TDG inhibition enhances anti-tumor immunity and displays significant synergy with anti-CTLA-4 antibody, the immune checkpoint blockade (ICB). Furthermore, we explored and validated the sensitivity of other p53-deficient cancer types to TDG inhibition, broadening the potential of TDG inhibition to treat a larger patient population. In conclusion, we identified inhibition of TDG as a brand-new synthetic lethality approach and an innovative synthetic immune activation anti-cancer mechanism in TP53-mutant tumors. It underscores the promise of profiling and developing C-271 or other preclinical candidate molecules of TDG into clinic study and extending the breakthrough first-in-class (FIC) therapy to be viable and beneficial for treating p53-deficient cancers. CytosinLab Therapeutics holds full intellectual property (IP) rights to C-271 and its series of TDG-targeting small molecules. Citation Format: Jiannan Guo, Meng Wang, Jiaxin Zhou, Yao Cao, Yiqin Wang, Yilin Liu, Youzhen Wang, Yuan Mi, Yarui Du, Guoliang Xu, Haiping Wu. Pharmacological inhibition of thymine DNA glycosylase (TDG) triggers an anti-tumor immunity to suppress p53-deficient cancer [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 4281.