Abstract 6762: A novel therapeutic approach for targeting p53-mutant cancers by leveraging DNA damage response vulnerabilities

Abstract The tumor suppressor gene TP53 is frequently mutated in most solid malignancies, including colorectal and pancreatic cancers, driving tumor progression and metastasis. However, existing treatments often lack selectivity for p53-mutant (p53mut) cancers and are associated with high toxicity. To address this clinical challenge, we developed a two drug therapeutic approach that selectively targets p53-mutant cancers by combining TAS102, with a PARP inhibitor (PARPi). Mechanistically, incorporation of TFT into DNA triggers post-replicative repair, generating single-strand break intermediates. While PARP facilitates their repair, inhibition of PARP converts these intermediates into lethal double-strand breaks. In p53 wild-type (WT) cells, TAS102 and PARPi activate a p53-dependent G1/S checkpoint, enabling DNA repair and preventing excessive damage. In contrast, p53mut cells, lacking this checkpoint, experience uncontrolled DNA damage accumulation, leading to cell death. This combination demonstrated superior anti-tumor efficacy in p53-mutant cell lines and patient-derived xenograft (PDX) models compared to either agent alone, and was well tolerated in preclinical studies. This two-drug strategy is now being tested in Phase I clinical trial (NCT04511039) in advanced CRC patients showed no significant toxicity and improved PFS relative to historical TAS102 monotherapy. To further elucidate its mechanism, we investigated the DNA damage response (DDR) and checkpoint signaling in p53mut cancer cells. Our findings show that TAS102-PARPi induces a p53-independent G2/M checkpoint mediated by ATR kinase, which activates downstream kinases CHK1 and WEE1 to inhibit CDK1, thereby halting entry into mitosis. Our transcriptomic profiling revealed a marked induction of homologous recombination (HR)-associated double-strand break repair genes, including BRCA1, BRCA2 and RAD51, in p53-deficient cells following TAS102-PARPi treatment. This pronounced upregulation likely reflects enhanced DNA repair activity at the G2 checkpoint, orchestrated by ATR kinase. Based on this mechanistic, we tested whether the TAS102-PARPi regimen could be potentiated by targeting G2-checkpoint kinases. We developed a triple-drug therapeutic strategy that combines our two-drug regimen with a G2-checkpoint kinase inhibitor. Subsequent inhibition of checkpoint kinases such as WEE1 or ATR releases the G2-arrested cells into mitosis, resulting in mitotic catastrophe and cell death. Importantly, sequential administration, delaying the G2-kinase inhibitor after TAS102-PARPi, allows p53WT cells time to repair DNA, thereby minimizing toxicity to normal tissues. This sequential triple-drug strategy acts through a synthetic lethality mechanism, producing massive cell death in p53mut cancer models. In PDXs models, this regimen achieved robust tumor suppression without detectable toxicity. Citation Format: Mohammed M. Alruwaili, Yanqi Guo, Justin Zonneville, Thomas Melendy, Robert M. Straubinger, Barbara A. Foster, Priyanka Rajan, Henry G. Withers, Sarah Chatley, Renuka V. Iyer, Christos Fountzilas, Andrei V. Bakin. A novel therapeutic approach for targeting p53-mutant cancers by leveraging DNA damage response vulnerabilities [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 6762.