Background Small molecule STING agonists (STINGa) can reverse suppressive tumor microenvironments and promote T-cell mediated tumor killing; however, dose-limiting toxicities due to systemic innate immune activation have limited their clinical application to direct intratumoral injection. To improve the safety and protective capacity of STINGa for treating metastatic cancers through systemic intravenous administration, we developed virus-sized (~ 30 nm diameter) star polymer-STINGa conjugates (‘Star-STINGa’).

Methods These nanoparticles consist of STINGa covalently linked to polymer arms radiating from a central core. The flexibly arrayed arms are designed to reduce systemic toxicity resulting from interactions in the bloodstream by sequestering drug molecules towards the core, while the enzyme-degradable linker provides a mechanism for controlling drug release. We used dimeric aminobenzimidazole (‘diABZI’) as a benchmark model STINGa for comparison with Star-STINGa and evaluated safety and efficacy, as well as the cellular mechanism of action, in mouse tumor models in vivo.

Results Star-STINGa was tuned to reduce systemic toxicity by increasing the density of polymer arms. Nanoparticles with more arms per core had decreased uptake by splenocytes and induced less weight loss in vivo without limiting anti-tumor efficacy. One treatment with optimized low-dose Star-STINGa administered intravenously to mice with established (mean tumor volume = ~90 mm3) MC38 tumors led to complete tumor clearance in up to 70% of treated animals, compared to 0% of animals treated with free diABZI. In the highly aggressive B16-Adpgk-BFP tumor model, three treatments with Star-STINGa cleared established tumors while free diABZI was unable to do so. Using antibody depletion and knockout mouse strains, Star-STINGa treatment was shown to be dependent on CD8 T-cells, cross-presenting cDC1 cells, and Type I and II IFN signaling. To further explore the differences between treatments, tissues were harvested for immune population analysis. Star-STINGa increased the number of cDC1s in the tumor draining lymph node, as well as the activation of T-cells in the tumor and blood. Star-STINGa treatment also decreased the number of immunosuppressive macrophages in these tissues. Free diABZI was unable to induce these changes.

Conclusions Together, these data suggest that synthetic star nanoparticles may be an effective platform for improving the therapeutic index of small molecule immunostimulants (e.g., STINGa) to enable systemic use in cancer treatment. Linking these small molecules to a nanoparticle carrier transforms their ability to induce changes in the tumor microenvironment through IFN dependent alteration of inhibitory macrophages, enhancing DC activation to facilitate T-cell mediated clearance of the tumor.