A purpurin-based metalloimmunostimulant amplifies ROS and modulates STING/TNF-α axis to potentiate melanoma immunotherapy

The low infiltration of pro-inflammatory immune cells and the sustained activation of multiple immunosuppressive signaling pathways in melanoma significantly limit the efficacy of clinical immunotherapy. Therefore, developing an effective immunostimulant with reversing the immunosuppressive tumor microenvironment (TME) is of great significance for improving melanoma immunotherapy. Herein, a degradable metalloimmunostimulant (PurpN/Mn@PEG) is developed for immunotherapy targeting immunosuppressive melanoma. The PurpN/Mn@PEG NPs are fabricated by coordination-driven self-assembly of purpurin and Mn²⁺, followed by polyethylene glycol (PEG) modification. PurpN/Mn@PEG dissociates in acidic pH and high glutathione TME, releasing PurpN and Mn²⁺. The nanoparticle exhibits peroxidase-/oxidase-like activity, generating a reactive oxygen species (ROS) storm that induces immunogenic cell death. PurpN/Mn@PEG amplifies ROS via H₂O₂ production through phenolic oxidation, enhances TNF-α secretion via CCAAT/enhancer-binding protein beta (CEBPB) upregulation, and sensitizes cGAS-STING pathway, synergistically boosting melanoma immunotherapy. In vivo experiments demonstrated that this purpurin-based metalloimmunostimulant exhibits remarkable therapeutic efficacy with an 87.8 % tumor growth inhibition rate in B16-F10 melanoma-bearing mice by activating multiple immune pathways, thereby effectively augmenting melanoma immunotherapy. This study provides an innovative therapeutic strategy that effectively reprograms the immunosuppressive TME to potentiate melanoma immunotherapy.