Abstract 4522: Potent induction of a tumor-specific immune response by a cyclic dinucleotide STING agonist
Authors: Gremel Gabriela , Impagnatiello Maria A. , Carotta Sebastian , Schaaf Otmar , Chetta Paolo M. , Oost Thorsten , Zichner Thomas , Hofmann Marco , Blake Sophia , Bretschneider Tom , Fleck Martin , Grube Achim , Nar Herbert , Rast Georg , Schmidt Esther , Klinkhardt Ute , Arndt-Schmitz Kirsten , Laux Thorsten , Zinzalla Vittoria , Sedgwick Jonathon , Kraut Norbert 
Published: September 2020

Abstract
Introduction: Stimulator of interferon genes (STING) is activated by cyclic dinucleotides that are generated by cyclic GMP-AMP synthase in the presence of cytosolic DNA, resulting in a type 1 interferon response and the secretion of pro-inflammatory cytokines. BI-STING mimics the natural ligand of STING and hijacks the STING signaling pathway to trigger a potent anti-tumor immune response. Here we report the pre-clinical characterization of BI-STING in human cells lines in vitro, in human ex vivo systems and in vivo mouse models.
Methods: Activity and specificity of BI-STING were tested in vitro in the human monocytic leukemia cell line THP1. Target engagement was confirmed using ex vivo studies in human whole blood and in human precision cut colorectal cancer slices. For in vivo studies, BI-STING was administered intratumorally (i.tu.) in a range of subcutaneous, syngeneic murine tumor models. The modulation of cytokine secretion over time was recorded and the induction of a tumor-specific immune response demonstrated.
Results: Using THP1 cells, activation of down-stream signaling events by BI-STING was seen specifically in STING wild type, but not STING knock out cells. Ex vivo treatment of human whole blood and human precision cut colorectal cancer slices with BI-STING resulted in the significant induction of cytokine secretion (including IFNβ) and in an array of transcriptional changes associated with the activation of STING signaling. Treatment of tumor-bearing mice with a single dose of BI-STING i.tu. led to a transient increase of cytokine levels in tumor and plasma. In all tested models, i.tu. administration of BI-STING resulted in dose-dependent local tumor control. Importantly, animals whose primary tumor was cured by BI-STING treatment did not develop tumors when re-challenged with the same tumor cell line and an abscopal delay in tumor growth was seen for non-injected (or anenestic) lesions in mice carrying bilateral tumors. Abscopal tumor control was further improved when combined with anti-PD-1. Finally, ELISpot analysis resulted in a significantly increased number of immunospots in splenocytes from BI-STING treated animals as compared to vehicle control mice, confirming the induction of a tumor-specific immune response.
Conclusions: BI-STING is a potent and specific inducer of the STING signaling pathway. In addition to local tumor control, i.tu. treatment with BI-STING results in a systemic anti-tumor immune response in mice, which was enhanced upon anti-PD-1 combination. A clinical trial to evaluate i.tu. administration of BI-STING alone and in combination with anti-PD-1 in patients with advanced solid tumors is ongoing.
Citation Format: Gabriela Gremel, Maria A. Impagnatiello, Sebastian Carotta, Otmar Schaaf, Paolo M. Chetta, Thorsten Oost, Thomas Zichner, Marco Hofmann, Sophia Blake, Tom Bretschneider, Martin Fleck, Achim Grube, Herbert Nar, Georg Rast, Esther Schmidt, Ute Klinkhardt, Kirsten Arndt-Schmitz, Thorsten Laux, Vittoria Zinzalla, Jonathon Sedgwick, Norbert Kraut. Potent induction of a tumor-specific immune response by a cyclic dinucleotide STING agonist [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4522.