Abstract B40: Nucleotide analogs as novel STING agonists for immuno-oncology
Authors: Challa Sreerupa , Zhou Shenghua , Sheri Anjaneyulu , Padmanabhan Seetharamaiyer , Delaney Samantha , Meher Geeta , Cleary Dillon , Nair Vishal , Gimi Rayomand , Khedkar Santosh , Iyer Radhakrishnan 
Published: March 2017

Immunotherapy has recently emerged as a transformative approach for the treatment of cancer; nevertheless, many patients remain unresponsive to treatment. Recent evidence suggests that the activation of Stimulator of Interferon Genes (STING) pathway in tumor cells and/or antigen presenting cells (APCs) within the tumor microenvironment (TME) can induce type I Interferon production leading to apoptosis of tumor cells, as well as, induction of adaptive immune response (through priming of CD8+ T cells to tumor-associated antigens) thereby providing a powerful anti-cancer strategy. Therefore, therapeutic agents that activate STING signaling pathway in tumor cells and APCs in the TME are urgently needed. Herein, we describe the discovery of highly potent and selective first-in-class STING agonists for application in immuno-oncology.

Methods: Using structure-guided drug design, in conjunction with published crystal structures of different cyclic dinucleotides bound to STING, a focused library of nucleotide compounds was prepared using standard phosphoramidite chemistry. The compounds were screened for induction of Interferon regulatory factor (IRF), Interferon-stimulated gene 54 (ISG54), and NF-KB using reporter assays. We used HEK293 cell line stably expressing ISG54 (ISRE)-promoter-driven firefly luciferase reporter gene for initial hit discovery and the actives were further characterized in PBMCs and THP1 cells. The IRF and NF-kB induction was calculated from % fold-change in luminescence compared to DMSO-treated cells and EC50 of the compounds were ascertained using Xlfit. Lead STING agonists were further evaluated for: (a) Binding affinity: Binding assays were conducted by Differential Scanning Fluorimetry (DSF) and Tm was calculated using Thermal Shift software, (b) Induction of pathogen recognition receptors (PRRs), ISGs and Programmed Death Ligands 1 & 2 (PDL1, PDL2) genes: THP1 cells and PBMCs were treated with various concentrations of lead compounds or 2,'3'-cGAMP or DMSO and the gene expression of different PRRs, ISGs, PDL1, and PDL2 was determined by quantitative RT-PCR using ΔΔct method, (c) Apoptosis-inducing activity: PBMCs and THP1 cells were treated with various concentrations of lead compounds, 2',3'-cGAMP, or DMSO control and the apoptotic activity was evaluated using Caspase-Glo® 3/7 Assay (Promega), and (d) In vitro anti-tumor activity: STING-dependent anti-tumor activity of lead compounds in various tumor cell lines was assessed by either high-content imaging or through Cell titer Glo® Cytotoxicity Assay (Promega). Cell survival was calculated based upon % reduction of live cells compared to DMSO control. CC50 of the compounds were generated by curve fit in Xlfit.

Results: Through in vitro assays in conjunction with Structure Activity Relationship (SAR) studies, we have identified several highly potent and selective first-in-class STING agonists. A promising lead nucleotide compound SB 11285 caused STING-dependent induction of: (a) IRF with an EC50 of 2 nM that is 1000-fold more potent than the natural STING agonist 2',3'-cGAMP, (b) NF-kB with an EC50 of 200 nM that is &gt;200-fold more potent than 2',3'-cGAMP, (c) selective apoptosis of human monocyte leukemic cell lines (CC50, 500 nM) as compared to normal PBMCs through induction of IFN, and NF-kB signaling, and (d) expression of various PRRs and ISGs including RIG-I, MDA-5, LGP2, ISG54 and OAS-1, as well as, PDL1 and PDL2. Finally, SB 11285 showed potent in vitro anti-tumor activity in multiple tumor cell lines.

Conclusion: We have discovered highly potent first-in-class STING agonists that show excellent selectivity in induction of IFN, NF-KB, ISGs, and PRRs, and apoptosis of tumor-derived cell lines. The lead STING agonist SB 11285 has potent immune-modulating, as well as, anti-tumor activities and is being advanced for additional preclinical studies for application in immuno-oncology.

Citation Format: Sreerupa Challa, Shenghua Zhou, Anjaneyulu Sheri, Seetharamaiyer Padmanabhan, Samantha Delaney, Geeta Meher, Dillon Cleary, Vishal Nair, Rayomand Gimi, Santosh Khedkar, Radhakrishnan Iyer. Nucleotide analogs as novel STING agonists for immuno-oncology. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2016 Oct 20-23; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2017;5(3 Suppl):Abstract nr B40.