Abstract A118: Targeting Replication Stress Promotes Anti-tumour Immune Responses that are Suppressed by Tumour-associated Myeloid cells

Abstract Replication stress is a common feature of solid cancers, and drugs targeting replication stress such as Checkpoint kinase 1 inhibitors (CHK1i) have demonstrated significant preclinical activity especially in combination with replication stress promoting chemotherapies. However, this has not translated into an effective clinical treatment, primarily due to high normal tissue toxicity. We have previously demonstrated that a combination of CHK1i with a subclinical dose of hydroxyurea selectively targets a range of tumour types, importantly with little normal tissue toxicity to even chemo-sensitive tissues. The CHK1i combination also promotes a pro-inflammatory response and immunogenic cell death. In vivo, this drug combination induces tumour regression which is dependent on an adaptive immune response. Here we report the immune responses triggered by this combination in mouse models and demonstrate that this response is suppressed by tumour-associated myeloid cells. Syngeneic mouse melanoma and ovarian cancer models were treated in vivo with the combination of CHK1i SRA737 and low dose hydroxyurea. The tumour immune microenvironment and peripheral immune cell responses were assessed by single cell expression analysis and immune cell marker multiparameter flow cytometry. In the panel of cancer models tested in syngeneic mice, the CHK1i combination, controlled tumour growth. Immune responses were elicited by the CHK1i combination in all tumours but found different types of immune cell responses in the different models and cancers investigated. The combination enhanced immune responses independent of the initial tumour immune microenvironment, including in tumours that were immunologically “cold”. The common features of the immune responses in all the models are increased cytolytic activity and reduced immune suppression in the tumour microenvironment. The responses were dependent on CD8+ T cells. Myeloid cells in the tumour microenvironment were immunosuppressive, expressing high levels of PD-L1 in response to treatment. This could be reduced by depletion with CSF-1R antibody or reducing tumour CSF-1 expression. This demonstrates the CHK1i combination is highly selective with minimal normal tissue toxicity, does not adversely affect immune responses, and can trigger an effective anti-tumour immune response in range of tumour settings, including current treatment resistant tumours. Reducing tumour associated myeloid number or activity was associated with enhanced anti-tumour immune responses. This work suggests that the myeloid component of tumours may significantly alter treatment responses by suppressing anti-tumour immune activity and that targeting this population may enhance the durability of the anti-tumour immune response even in immunologically cold tumours. Citation Format: Zhen Zeng, Ritu Bhatt, Anastasia Gandini, Jazmina Gonzalez Cruz, Martina Proctor, Katharine Irvine, Riccardo Dolcetti, James Wells, Brian Gabrielli. Targeting Replication Stress Promotes Anti-tumour Immune Responses that are Suppressed by Tumour-associated Myeloid cells [abstract]. In: Proceedings of the AACR IO Conference: Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2025 Feb 23-26; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2025;13(2 Suppl):Abstract nr A118.