Abstract 5991: Novel triple-drug combination strategy for p53 mutant cancers leveraging their DNA damage response liabilities

Abstract Genetic alterations in the tumor suppressor p53 gene (TP53) are found in most solid malignancies, including colorectal (CRC), breast (BC), and pancreatic (PANC) cancers. Mutant p53 contributes to cancer progression and metastasis. Current therapeutic options show low selectivity for p53-mutant cancers and frequently exhibit high toxicity. To address this unmet clinical need, we developed a two-drug therapeutic strategy that selectively targets p53-mutant cancers. This novel strategy combines a thymidine analogue TAS102, acting as an inducer of DNA damage, and PARP inhibitor (PARPi) that works as an amplifier of DNA damage. Our two-drug therapeutic strategy showed greater efficacy in p53 mutant cancer models (including patient-derived xenografts (PDXs)) than either drug alone. The two-drug regimen was well tolerated in animal preclinical PDX and cell-derived xenograft models. This two-drug strategy is now being tested in Phase I clinical trial for patients with advanced CRC (NCT04511039). Clinical data showed no major signs of toxicity and increased progression-free survival compared to TAS102 alone. Mechanistically, the incorporation of thymidine analogues into DNA provokes post-replicative repair, generating single-strand DNA break intermediates. Repair of these intermediates is assisted by PARP, while inhibition of PARP increases more lethal double-strand DNA breaks. Cells with p53 wild-type (p53WT) respond with G1 and G2 -checkpoint arrest and efficiently remove thymidine analogues. In contrast, p53 mutant cells, lacking the G1 checkpoint, accumulate in G2 with lethal DNA breaks. The current work investigated the mechanism of the DNA damage response (DDR) and the G2 checkpoint induction in response to the TAS102-PARPi regimen. Analysis of DDR signaling showed that the G2 checkpoint in response to TAS102-PARPi is mediated by ATR kinase that acts through CHK1 and WEE1 kinases. We next examined a drug combination using the two-drug TAS102-PARPi regimen and the G2 checkpoint kinase inhibitors together. The concurrent triple-drug regimen showed high toxicity to non-tumor cells and cancer cells. In contrast, a sequential drug regimen, the two-drug treatment followed by G2-checkpoint inhibitors demonstrated selective toxicity against p53 mutant cancer cells. This sequential triple-drug regimen was further examined in preclinical CRC model in mice. The triple-drug regimen showed substantially greater anti-tumor activity in p53 mutant CRC xenograft model without adverse side effects (based on mouse body weight and peripheral blood analysis). Based on these preclinical studies, we propose a novel triple-drug therapeutic strategy for p53 mutant cancers that has strong potential to improve cancer treatment and meaningfully impact the health of cancer patients. Citation Format: Mohammed M. Alruwaili, Aidan Mazierski, Erica Norris, Kyeong Beom Jo, Yanqi Guo, Priyanka Rajan, Henry Withers, Thomas Melendy, Christos Fountzilas, Andrei Bakin. Novel triple-drug combination strategy for p53 mutant cancers leveraging their DNA damage response liabilities [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 5991.