Abstract 382: MTL-STING restores endogenous STING expression for improving efficacy of cancer therapeutics
Authors: Tan Choon Ping , Stathaki Konstantina Skourti , Ryan Brid M , Gomez Valentí , Sinigaglia Laura , Hegre Siv Anita , Habib Robert , Rossi John J , Habib Nagy A 
Published: June 2022

cGAS-cGAMP-STING is essential for sensing foreign DNA from pathogens or self-DNA from dying cancer cells. Activation of this pathway is critical for the innate immune response to cancer and is necessary for the full efficacy of various cancer treatments including checkpoint and PARP inhibitors, radiotherapy, and CAR T-cells. Intense efforts have focused on triggering this pathway with cGAMP analogs, which are small-molecule activators of STING. However, recent reports show that many cancer cells downregulate STING by promoter methylation. Furthermore, T-cells activated by cGAMP undergo apoptosis, which limits the efficacy of T-cells based tumor eradication. Indeed, emerging evidence suggests that the critical cells in which STING activation achieves maximal anti-cancer efficacy is myeloid cells in the tumor microenvironment (TME).

saRNAs are small double-stranded complementary RNA oligos that increase target mRNA expression, they directly modulate traditional "undruggable" targets such as STING. Using single cell RNA sequencing data, we demonstrated that STING expression is decreased in myeloid cells within the TME of cancer patients. Therefore, our hypothesis was that saRNAs targeted against STING would restore its expression to levels that are necessary for pathway activation either alone or in combination with other drugs, including cGAMP analogs.

Using our proprietary algorithm, we designed saRNA sequences that are complementary to positions +2000 to -2000 nucleotides around the transcriptional start site of STING. The lead sequence (named saSTING) was further optimized by our discovery pipeline. Upregulation of STING mRNA was detected up to 168 hours post-transfection of saSTING in multiple cell lines, with an increase in the range of 4 to 11 fold. The upregulation of STING expression was further validated at the protein level. Strand and sequence specificity of saSTING was demonstrated by mutational analysis.

Concerning the mechanism of transcriptional activation via saSTING, we show that saSTING regulates STING expression by increasing both nascent pre-mRNA and mature mRNA. In essence, saSTING causes transcriptional activation across the whole STING locus, which is recapitulated in an upregulation of spliced and polyadenylated (mature) STING mRNA.

Development candidate MTL-STING encapsulates saSTING in NOV340 liposomes for in vivo administration. NOV340 liposomes have previously been clinically validated to efficiently deliver saRNA to myeloid cells, which is the critical cellular subset to benefit from STING activation from an anti-tumor perspective. MTL-STING is pre-clinical development as a combination treatment administered intra-tumorally and is expected to enter Phase 1 in 2023.

Citation Format: Choon Ping Tan, Konstantina Skourti Stathaki, Brid M Ryan, Valentí Gomez, Laura Sinigaglia, Siv Anita Hegre, Robert Habib, John J Rossi, Nagy A Habib. MTL-STING restores endogenous STING expression for improving efficacy of cancer therapeutics [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 382.