A robust RNAi nanoplatform for precise activation of cGAS-STING pathway and effective immune checkpoint blockade to potentiate cancer immunotherapy.

Activation of cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-interferon gene stimulator (STING) pathway has demonstrated significant potential in cancer treatment due to its crucial role in bridging the innate and adaptive immunity. However, clinical attempts of current cGAS-STING activating approaches remain challenged because of their undesired adverse effects and low therapeutic efficacy. We herein developed a new and robust immunostimulatory RNA interfering (RNAi) nanoplatform to potentiate breast cancer (BCa) immunotherapy through precise activation of cGAS-STING pathway and effective immune checkpoint blockade. This nanoplatform comprises the electrostatic complexes of small interfering RNA (siRNA) targeting oncogene coactivator-associated arginine methyltransferase 1 (Carm1) and metformin prodrug. Using orthotopic and metastatic BCa tumors, we demonstrated this nanoplatform could suppress the proliferation of BCa cells via siRNA-mediated Carm1 silencing and down-regulate programmed death-ligand 1 (PD-L1) expression via metformin-mediated ubiquitin-proteasome degradation. More importantly, due to the important role of oncogene Carm1 in repairing damaged double stand DNA (dsDNA), Carm1 silencing could specifically enhance the accumulation of damaged dsDNA and cytosolic release of dsDNA fragments to precisely activate the cGAS-STING pathway in BCa cells, which could thus promote their expression and secretion of interferon-β (IFN-β) to induce a significant inhibition of BCa tumor growth via leveraging both the innate and adaptive immunity.