A Multi-Threshold Micelle Improves Tumor Accumulation and STING Immunotherapy

Stimulator of interferon genes (STING) is a critical innate immune protein that potentiates anticancer immunity. An adjuvanted micelle, PC7A, directly binds to STING and coordinates robust immune signaling after local tumor delivery but is not optimized for intravenous administration. To improve systemic delivery, we engineered a hybrid, STING-activating micelle (HySTING) composed of an immune-stimulating polymer, PEG-b-PC7A (pK_a = 6.9), and a secondary carrier polymer with a lower pK_a. HySTING variants (HySTING_6.2, HySTING_5.2, and HySTING_4.2─subscript denotes secondary pK_a) show stepwise ultra-pH-sensitivity at each polymer's individual pK_a despite forming a single micellar architecture. HySTING micelles maintain the pharmacological properties of PC7A (i.e., STING-activation and membrane-destabilization) in bone marrow-derived monocytes and cancer cell lines, respectively. In mice, the hybrid formulations drastically improve blood circulation half-life from <1 h (PC7A) to 18 h (HySTING_5.2) after intravenous administration. The improved circulation results in significantly higher tumor accumulation of HySTING_6.2 (17-fold) and HySTING_5.2 (6-fold) as compared to PC7A nanoparticles. Further, we screened a panel of cyclic dinucleotides (CDNs) and discovered thiol substitutions in CDN linkers improved micelle encapsulation and stability. We identified a HySTING^IACS-8803 formulation with improved tumor growth inhibition in a CT26 murine colon carcinoma model and synergy with anti-programmed cell death-1 (αPD-1) antibody. Overall, the multithreshold pH-sensitivity of HySTING improves tumor biodistribution and efficacy, enabling intravenous STING immunotherapy against cancer.