In the field of cancer immunotherapy, activating STING (Stimulator of Interferon Genes) is a key strategy to stimulate anti-cancer immune responses. STING, as an important innate immune protein, enhances anti-cancer immunity through type I interferon signaling. However, traditional STING agonists face numerous challenges during intravenous administration, such as short blood circulation half-life and insufficient tumor accumulation. The previously developed PC7A micelle can directly bind to STING and activate immune signaling, but its effectiveness during intravenous administration has not reached an ideal state. Optimizing the systemic administration method has become a key scientific issue to enhance the efficacy of STING immunotherapy.
Based on this, the research team designed a multi-threshold pH-sensitive HySTING micelle. This micelle is composed of the immune-stimulating polymer PEG-b-PC7A (pKa=6.9) and a carrier polymer with a lower pKa, forming a single micelle structure that exhibits stepwise super pH sensitivity at their respective pKa values. This design is akin to a “series electronic transistor” logic, allowing the micelle to gradually dissociate in different pH environments, retaining the pharmacological properties of PC7A to activate STING and disrupt cell membranes, while extending blood circulation time through the carrier polymer and optimizing tumor-targeted distribution.
Solution characterization data fully validate the unique properties of HySTING. pH titration experiments show that different HySTING variants (such as HySTING5.2, HySTING4.2) have two equivalence points corresponding to the ionization events of each polymer; dynamic light scattering analysis indicates that the micelle size and count rate change stepwise with pH; transmission electron microscopy observed that at pH 7.4, HySTING5.2 is in micelle form, at pH 6.4, a mixed state of micelle and polymer appears, and at pH 4.0, it presents a monomer state like PC7A; heteroFRET experiments further confirm that the distance between polymers within the micelle is less than 10nm, and exhibits stepwise fluorescence response under different pH, validating its pH-sensitive sequential dissociation model.
The biological evaluation results at the cellular and animal levels are encouraging. In cellular experiments, HySTING effectively induces hemolysis of red blood cells, inhibits the viability of CT26 cancer cells, triggers ATP and HMGB1 release, and induces bone marrow-derived dendritic cells to secrete IFN-β at the pH 6.9 threshold, completely retaining the pH-gated pharmacological activity of PC7A. Animal experiments show that after intravenous injection, the blood circulation half-life of HySTING variants is significantly extended from < 1 hour for PC7A to 16-24 hours, with HySTING5.2 reaching 18 hours, and tumor accumulation increased by 6-17 times compared to PC7A. In the CT26 mouse colon cancer model, the selected HySTING-IACS-8803 formulation significantly inhibits tumor growth and shows a synergistic effect when combined with anti-PD-1 antibodies. Mechanistically, HySTING achieves precise responses to the tumor microenvironment through multi-threshold pH sensitivity, combined with CDN screening optimization (such as thiol substitution of CDN linker to enhance micelle encapsulation rate and stability), ultimately enhancing the efficacy of STING immunotherapy.
This study cleverly addresses the key challenge of intravenous administration of STING agonists through the design of multi-threshold pH-sensitive micelles. HySTING not only extends blood circulation time and increases tumor accumulation but also provides a new approach for systemic STING immunotherapy through optimization combinations with CDN and synergistic effects with immune checkpoint inhibitors. Its stepwise pH response mechanism serves as a model for nanodrug design and is expected to promote the clinical translation of STING agonists in cancer immunotherapy, bringing new treatment hope to more cancer patients.
Original link:
https://doi.org/10.1021/jacs.4c17082
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