Abstract 7590: Novel therapeutic approach for targeting p53 mutant colorectal cancers by affecting post-replicative DNA repair

Abstract Colorectal Cancer (CRC) is the third most common cause of cancer mortality in the US. A major problem in CRC management is relapse and progression to metastatic disease leading to poor overall survival. The tumor suppressor p53 is mutated in most CRCs and often drives metastasis and therapy resistance. Current therapeutic options targeting p53 mutant cancers show poor efficacy and frequently exhibit high toxicity. To address this clinical problem, we developed a novel therapeutic strategy for selective targeting of p53 mutant cancers. The strategy combines a thymidine analogue (e. g. , trifluorthymidine/TFT, a component of TAS102) and poly (ADP-ribose) polymerase inhibitor (PARPi). We observed that TAS102 does not block DNA replication, but rather prompts post-replicative base-excision DNA repair (BER), and PARPi increases double-strand DNA breaks (DSBs) in p53 mutant cancer cells. Normal p53 wild type (p53wt) cells are arrested in the G1 phase and repair DNA. Thus, the TAS102-PARPi combination selectively targets p53-mutant cancer cells, resulting in the accumulation of DSBs and cell death. This novel strategy was examined in p53wt and p53-mutant tumor cell-derived xenograft (CDX) and patient-derived xenograft (PDX) models. The drug combination was significantly more effective in inhibiting p53 mutant tumor growth and in prolonging survival compared to monotherapies. This strategy is currently being tested in a first-in-human dose-escalation Phase I study (NCT04511039) in patients with advanced CRC. Further investigation revealed that the TAS102-PARPi combination induced DNA damage response (DDR) and the G2 checkpoint, leading to G2 arrest in p53 mutant cancer cells. We observed that TAS102-PARPi induced phosphorylation of CDC2 (also known as CDK1) but did not increase phosphorylation of histone H3 (a marker of mitosis), therefore indicating activation of the G2 checkpoint. Furthermore, the roles of ATM and ATR kinases in the DDR and G2 responses were clarified using highly specific kinase inhibitors. The analysis of the DDR/G2 signaling response in isogenic p53wt and mutant CRC cell lines showed that inhibition of WEE1 kinase can selectively abrogate the G2 checkpoint in p53-mutant cells, resulting in marked DNA damage and cell death. Importantly, the blockade of WEE1 strongly synergized with the TAS102-PARPi combination, further enhancing its potency and efficacy against p53-mutant cancer cells. In summary, our research provides a foundation for a novel therapeutic strategy for p53 mutant cancers that may provide a paradigm shift in the treatment of these deadly cancers, increasing efficacy while minimizing toxic adverse effects. Citation Format: Mohammed M. Alruwaili, Natsumi Naranjo, Priyanka Rajan, Kyeong Beom Jo, Dae-Kyum Kim, Thomas Melendy, Robert Straubinger, Christos Fountzilas, Andrei Bakin. Novel therapeutic approach for targeting p53 mutant colorectal cancers by affecting post-replicative DNA repair abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 7590.