Background: Cancer immunotherapy is a highly effective therapeutic option for cancer patients. However, the overall response rate with checkpoint inhibitors and other related modalities has been modest. Targeting innate immune signaling pathways that induce type I IFN production to reprogram tumor microenvironment and restore antitumor immunity represents a novel immunotherapeutic approach. We have previously reported that SB 11285 is a first-in-class synthetic cyclic dinucleotide STING agonist, which has demonstrated potent antitumor activity, when used alone or combined with other antitumor agents, in several syngeneic mouse and rat tumor models when administered by intratumoral, intravenous or intraperitoneal routes. Presented here are studies that provide additional insights into the mechanism of action of SB 11285 and analogs.

Methods: (a) SB 11285 induces Type I IFN production and other cytokines in human PBMCs and PBMC-derived monocytes. PBMCs and monocytes, isolated from fresh PBMCs using pan monocyte isolation kit (Miltenyi Biotec), were stimulated with SB 11285. Type I IFNs and other cytokines were quantified using regular and multiplex ELISA assays. (b) SB 11285 directly binds STING. To address whether SB 11285 directly binds wild-type human STING, surface plasmon resonance assay was performed with a Biacore T200 device and a biotinylated SB 11285 analog (Biot-SB 11285). (c) Activity against STING polymorphs. Evaluation of SB 11285 and analogs was carried out using both SZ14 reporter cells (HEK293-derived) and HEK293T cells that stably or transiently express STING polymorphic variants. (d) Pharmacodynamic studies. Normal BALB/c mice were injected intravenously with SB 11285 or its analogs at 9 mg/kg. Serum, spleen, and liver samples were collected to quantify RANTES and TNF-α using ELISA. The expression of representative ISGs in spleen samples, including IRF7, IFIT2, and OAS1b were quantified using real-time PCR. (e) Determination of cellular uptake of SB 11285 by PBMCs. The STING agonist activity of Biot-SB 11285 in inducing type I IFN response was confirmed using SZ14 reporter cells and THP1-Dual-WT reporter cells. Human PBMCs were incubated with Biot-SB 11285 and the cellular uptake of compound by immune cells was evaluated using flow cytometry.

Results and Conclusion: Our studies provide significant mechanistic insights into the STING agonistic activity of SB 11285 and analogs in that they: (a) induce Type I IFNs, other cytokines and chemokines in PBMCs and monocytes, monocyte-derived dendritic cells; (b) directly bind STING with nanomolar affinity; (c) activate multiple human STING polymorphic variants; (d) induce cytokines, chemokines, and ISGs in mice following i.v. injection; and (e) are effectively taken up by monocytes and other immune cells. SB 11285 is being advanced to human clinical trials.

Citation Format: Shenghua Zhou, Sreerupa Challa, Vishal Nair, Geeta Meher, Anjaneyulu Sheri, Rayomand Gimi, Seetharamaiyer Padmanabhan, Dillon Cleary, Leena Suppiah, Diane Schmidt, Santosh Khedkar, Radhakrishnan Iyer. Mechanistic insights into the antitumor activity of SB 11285—a novel STING agonist [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B87.