Abstract PR006: DNA repair inhibition via targeting ATR activates cGAS/STING signaling, promotes anti-tumor immunity, and is a synthetic lethal strategy to augment immunotherapy response in preclinical models and clinical samples of small cell lung cancer

Introduction: Small cell lung cancer (SCLC) is the most aggressive subtype of lung cancer. Clinical trials of immune checkpoint blockade (ICB) combined with chemotherapy delivered only very modest benefits. In this study, we investigated the effect of targeting ataxia telangiectasia and rad3-related (ATR), the primary activator of the replication stress response, in SCLC. Clinical samples from treatment naïve and treated SCLC patients were analyzed by single cell and bulk RNA sequencing to ascertain the intratumoral transcriptional heterogeneity and the surrounding tumor microenvironment (TME). Methods: In this study, we investigated the effect of ATR inhibition either alone or in combination with PD-L1 blockade in multiple immunocompetent mouse models of SCLC. The downstream effects of ATR inhibition were assessed by bulk RNA sequencing, and multicolor flow cytometry. The SCLC clinical samples from treatment naïve and patients treated were analyzed by single cell and bulk RNA sequencing. Results: In multiple immunocompetent SCLC mouse models, ATR inhibition (ATRi) remarkably enhanced the anti-tumor effect of PD-L1 blockade. We next tested the ATR inhibition either alone or in combination with PD-L1 in the second-line regimen for SCLC. We observed that ATR inhibition in combination with PD-L1 blockade significantly reduced tumor volume and prolonged survival of aggressive mice models when compared to PD-L1 alone. Targeting ATR enhanced the expression of PD-L1, activated the cGAS/STING pathway, induced the expression of Type I and II interferon pathways, and caused significant infiltration of cytotoxic and memory/effector T-cells into tumors. Interestingly, ATRi also led to significant induction of MHC class I in SCLC in vitro and in vivo models. Single-cell RNA sequencing data provided a deep annotation of SCLC phenotypes and the surrounding immune TME. Treatment and subtype are associated with substantial phenotypic changes in the SCLC immune microenvironment, with greater T-cell dysfunction in SCLC-N than in SCLC-A. Moreover, a recurrent highly metastatic SCLC subset is associated with exhausted CD8+ T-cells and a pro-fibrotic, immunosuppressive monocyte/macrophage population, suggesting possible tumor-immune coordination to promote metastasis. Specifically, analysis of pre-and post-treatment clinical samples from a proof-of-concept study of a first-in-class ATR inhibitor, VX970 (berzosertib), and topotecan, in patients with relapsed SCLCs, validated the induction of MHC class I and interferon pathway genes, for the first time in this disease. Conclusion: We provide insight into tumor and immune biology in SCLC at single-cell resolution. Further, we show ATRi as a potentially transformative vulnerability of SCLC. Given the increasing importance of immunotherapy for the management of SCLC and that ATR inhibitors are already in clinical trials, combining ATR inhibitors with PD-L1-blockade offers an attractive strategy for the treatment of SCLC and contributes to the rapid translation of this combination into the clinic. Citation Format: Triparna Sen. DNA repair inhibition via targeting ATR activates cGAS/STING signaling, promotes anti-tumor immunity, and is a synthetic lethal strategy to augment immunotherapy response in preclinical models and clinical samples of small cell lung cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr PR006.