Abstract 3077: Preclinical studies of SR-8314, a highly selective ENPP1 inhibitor and an activator of STING pathway
Authors: Weston Alexis , Thode Trason , Munoz Ruben , Daniel Sherin , Soldi Raffaella , Kaadige Mohan , Han Haiyong , Vankayalapti Hariprasad , Sharma Sunil 
Published: August 2019

Abstract
Purpose: STING (stimulator of interferon genes) plays an important role in innate immunity by activating type I interferons in response to cytosolic nucleic acid ligands such as cyclic dinucleotides (CDNs). In recent years, STING has become an attractive therapeutic target for cancer immune therapy and hydrolysis resistant CDNs have been developed as a new class of cancer therapeutics. These CDNs have been shown to possess potent preclinical efficacy but early results from phase I trials have been disappointing. Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1 or NPP1) is natural antagonist of STING pathway. ENPP1 constitutively hydrolyzes 2′3′-cGAMP, a CDN that is the natural ligand for STING. Previously, we reported that SR-8291, a highly selective ENPP1 inhibitor, induces STING activity and demonstrates anti-tumor activity in B16F10 melanoma and CT26 colorectal models. In this study, we extend these findings and report the discovery of SR-8314, an analog of SR-8291 that shows improved physiochemical and developability properties.
Methods: With the application of computational techniques (ICM, Maestro), human NPP1 homology model was built by utilizing the crystal structure of mouse NPP1 (PDB: 4GTW). Series of docking simulations on built in ligands were performed within the substrate binding pocket and that led to the identification of lead NPP1 inhibitor SR-8314. Binding of SR-8314 to ENPP1 was evaluated using thermal shift assay. Activity of recombinant human ENPP1 using ATP as a substrate was measured using Cell Titer Glo (Promega) reagent. IRF-Lucia reporter activity, RT-PCR and Western Blotting were performed to evaluate the effect of SR-8314 in 2′3′-cGAMP primed THP1 dual reporter cells (Invivogen). In vivo efficacy studies of intraperitoneally-dosed SR-8314 and SR-8291 were performed in a syngeneic murine tumor model. Tumor T cell infiltration, tumor and plasma pharmacokinetics were assessed by flow cytometry and mass-spectrometry, respectively.
Results: SR-8314 has higher binding affinity towards ENPP1 and it potently inhibits ENPP1 activity with a Ki value of 0.079µM. A significant increase in gene expression of IFNβ, ISG15 and CXCL10 along with an increase in the secretion of IFNβ was observed in SR-8314 treated THP1 cells. We show anti-tumor activity as well as an increase in CD3+, CD4+ and CD8+ T cells in both SR-8314 and SR-8291 treated tumors. In addition, there was a decrease in tumor associated macrophages in SR-8314 treated tumors.
Conclusions: In summary, we show SR-8314 as a potent inhibitor of ENPP1 that promotes STING activity in vitro. SR-8314 displays promising anti-tumor activity and has ideal candidate properties that warrant further evaluation.
Citation Format: Alexis Weston, Trason Thode, Ruben Munoz, Sherin Daniel, Raffaella Soldi, Mohan Kaadige, Haiyong Han, Hariprasad Vankayalapti, Sunil Sharma. Preclinical studies of SR-8314, a highly selective ENPP1 inhibitor and an activator of STING pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3077.