Next-generation STING mRNA therapy with reduced toxicity and dual action against cancer by boosting immunity and inducing tumor cell death

Immunotherapy has reshaped the standard of care in oncology, but only a fraction of patients experiences durable benefits. To improve patient benefit from immunotherapy, an effective strategy is to engage innate immune pathways that prime robust anti-tumor immunity. Activating the Simulator of IFN genes (STING) pathway can initiate potent type I interferon signaling and prime cytotoxic T-cell immunity. Despite promising results in rodent syngeneic models, first-generation STING agonists have shown limited clinical efficacy, underscoring the need for improved modalities and delivery strategies. Here, we aim to develop a systemically delivered, mRNA-encoded STING analog that drives durable antitumor immunity. Since STING expression is epigenetically silenced in many cancers reducing the impact of ligand-dependent agonists, we engineered STING mRNA analogs that signal without ligand binding. STING pathway activation was assessed utilizing human HEK-Blue ISG and mouse B16-blue ISG reporter cell lines. We measured IFN-beta secretion by ELISA in human PBMCs and assessed expression of IFN and NF-κB pathway gene expression by qRT-PCR and western blot in cancer and immune cells. Our lead STING mRNA analog produced robust activation of STING pathway with marked increases in IFN-beta secretion and expression of downstream inflammatory genes. We also evaluated direct effects on cell viability in human cancer cell lines and human peripheral blood mononuclear cells (hPBMCs). The STING mRNA therapy selectively suppressed cancer cell proliferation while preserving hPBMC viability, indicating dual mechanisms of action, namely direct tumor growth inhibition and immune activation. To confirm efficacy in vivo , we tested the STING mRNA therapy in syngeneic tumor models after intravenous (IV) or intramuscular (IM) administration. Both systemic administration routes significantly inhibited tumor growth in multiple models. In the LL/2 immune-cold model, we observed reduced pro-tumorigenic immune compartments and increased anti-tumor immune compartments after the STING mRNA administration. In addition, we modified our mRNA therapy platform to prevent excessive accumulation in the liver and reduce toxicity from high STING expression in hepatic tissue. Clinical application of first-generation STING agonists has been limited by poor metabolic stability, inefficient cytosolic delivery, and reduced STING expression in human cancers. Our data show that an mRNA-encoded, ligand-independent STING analog overcomes these barriers, enabling transient systemic activation that translates into sustained anti-tumor immunity. These findings support further development of STING mRNA therapeutics as a next-generation immuno-oncology strategy.