Abstract Background: Poly (ADP-ribose) polymerase (PARP) 1, a multifunctional protein, is widely recognized for its critical role in the HR-mediated DNA repair, and its inhibition has therefore been introduced into the treatment of various solid tumors. Besides DNA damage response (DDR), PARP1 has also emerged as a key regulator of immune modulation. This is attributed to the accumulation of cytosolic nucleic acids (NAs), such as dsDNA or dsRNA, which are sensed by innate immune pathways like cGAS-STING. However, the precise mechanisms by which PARP1 inhibition induces these cytosolic NAs and activates innate immune response remain to be elucidated. Thus, we investigated the anti-tumor and immune modulatory effects of AZD5305 in gastric cancer cells to explore the underlying mechanism. Methods: To evaluate the in vitro antitumor effect, colony formation assay (CFA) was performed for 14 days with increasing concentrations of AZD5305 (dose range: 0-5nM). The in vivo antitumor activity was assessed using an SNU-601 xenograft model by monitoring tumor growth. Cell cycle distribution was analyzed by flow cytometry and apoptosis was assessed by annexin-V/PI staining. DNA damage was detected by comet assay and the cell immunofluorescence assay (IFA). To investigate immune modulation, the expression of immune-related molecules was quantified by qRT-PCR and western blot, while the formation of dsRNA was detected by cell IFA. RNAseq was performed using the cell lines for differentially expressed gene and gene set enrichment analyses. Results: The RAD51C-deficient SNU-601 cells were highly sensitive to AZD5305 (IC50: 0.25 nM), unlike the moderately sensitive SNU-668 (IC50: 2.05 nM) or resistant KATOIII cells (IC505 nM). Moreover, AZD5305 demonstrated potent antitumor activity in an SNU-601 xenograft model (52.4% TGI, p=0.003). After treatment, SNU-601 cells exhibited G2/M arrest and apoptosis, confirmed by an increased sub-G1 population, positive annexin-V staining, and cleavage of PARP and caspase-7, but not in KATOIII. This cytotoxicity was driven by the accumulation of DNA damage, as indicated by elevated p-RPA, γH2AX, and comet tail formation. Furthermore, AZD5305 increased cytosolic dsRNA and cGAS-STING signaling in SNU-601, triggering the innate immune response. Consistent with this, RNAseq confirmed enrichments of activated IFN-γ response pathways and corresponding increases in the expression of interferon-stimulated genes (ISGs) in SNU-601, but not in KATOIII. Conclusion: AZD5305 exhibits potent cytotoxic effects in HR-deficient gastric cancer cell lines in vitro and in vivo, by inducing DNA damage and apoptotic cell death. Immune modulation by AZD5305 was characterized by innate immune activation with increased interferon signaling, and our findings suggest the formation of cytosolic dsRNA as a key mediator of this effect. Citation Format: Sujin Ham, Hae Min Hwang, Youlim Noh, Jiwon Koh, Chaeyoung Lee, Seohyeon Lim, Minyoung Jeong, Yu-Jin Kim, Minyoung Lee, Changhee Park, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. AZD5305, a PARP1 selective inhibitor, exhibits antitumor effects and stimulates immune response in homologous recombination (HR)-deficient gastric cancer cells 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 513.
Ham et al. (Fri,) studied this question.