Abstract B43: Novel small-molecule human STING agonists generate robust Type I interferon responses in tumors

Background: A significant proportion of human cancers grow unchecked by the immune system. These cold or non-inflamed tumors do not have a T cell infiltrate and it has been shown that activation of innate immune signaling through STING (Stimulator of Interferon Genes) generates a Type I interferon (Type 1 IFN) signal that increases T cell infiltration into these tumors leading to significant regression (Woo et al., Ann Rev Immunol 2015). Activation of STING using small molecules is a novel therapeutic approach gaining significant traction in the translational immuno-oncology research community. Methods: We used a pharmacophore based approach to identify small molecule human STING agonists. Our compounds were assessed (1) in reporter gene assays on HEK293T cells containing stably transfected hSTING polymorphs, (2) by immunoblots to confirm pSTING, pTBK1, pIRF3, (3) by real-time PCR monitoring induction of Type I cytokines in human tumor cell lines, isolated human PBMCs and dendritic cells (hDC). Promising lead compounds were further evaluated for anti-tumor activity in c57/BL6 syngeneic and xenografted SCID mice humanized with hPBMC and dendritic cells from human donors. Results: We have designed and synthesized potent non-macrocyclic, non-nucleoside human STING agonists. Our lead compound CRD-100 described here has good oral bioavailability and drug like properties (Patents filed). Treatment of an isolated cell free system containing purified recombinant STING and TBK1 leads to STING phosphorylation confirming that CRD-100 is a direct STING binder. Evidence of direct binding also comes from low temperature isothermal calorimetry. CRD-100 activates all five common hSTING variants with agonist EC50 comparable to CDNs in the pIRF3 reporter assay. Treatment of hSTING transfected cells, human tumors cell lines or hPBMC with CRD-100 leads to phosphorylation of IRF3, TBK1 and STING. CRD-100 also causes the maturation of hDCs and the release of innate and adaptive inflammatory cytokines such as IFNβ and TNFα from hPBMCs. Once fortnightly doses of CRD-100 delivered intra-tumorally caused tumor regression in syngeneic and xenografted tumors. Conclusions: The ability of hSTING agonists to generate anti-tumor immune responses in cold tumors in murine models makes them a promising therapeutic option either as a single agent or in combination with existing or developing therapies. CRD-100 has conventional small molecule drug like properties making it an attractive drug