Multifunctional single-component photosensitizers as metal-free ferroptosis inducers for enhanced photodynamic immunotherapy

Immunotherapy can enhance primary tumor efficacy, restrict distant growth, and combat lung metastasis. Unfortunately, it remains challenging to effectively activate the immune response. Here, tertiary butyl, methoxy, and triphenylamine (TPA) were utilized as electron donors to develop multifunctional photosensitizers (PSs). CNTPA-TPA, featuring TPA as the donor (D) and cyano as the acceptor (A), excelled in reactive oxygen species (ROS) generation due to its smaller singlet-triplet energy gap (ΔES-T) and larger spin-orbit coupling constant (SOC). Additionally, cyano groups reacted with glutamate (Glu) and glutathione (GSH), reducing intracellular GSH levels. This not only enhanced PDT efficacy but also triggered redox dyshomeostasis-mediated ferroptosis. The positive effects of photodynamic therapy (PDT) and ferroptosis promoted immunogenic cell death (ICD) and immune activation. By further combining anti-programmed cell death protein ligand-1 (anti-PD-L1) antibody, the powerful treatments of ferroptosis-assisted photodynamic immunotherapy significantly eradicated the primary tumors, inhibited the growth of distant tumors, and suppressed lung metastasis. In this study, a three-pronged approach was realized by single-component CNTPA-TPA, which simultaneously served as metal-free ferroptosis inducers, type-I photosensitizers, and immunologic adjuvants for near-infrared fluorescence imaging (NIR FLI)-guided multimodal phototheranostics of tumor. (1) CNTPA-TPA shared the smallest singlet-triplet energy gap and the largest spin-orbit coupling constant, which boosted intersystem crossing for efficient type-I photodynamic therapy (PDT); (2) Special reactions between cyano groups with glutamate and glutathione in mild conditions restricted the biosynthesis of intracellular GSH. GSH-depletion efficiently induced glutathione peroxidase 4 inactivation and lipid peroxide, resulting in ferroptosis of tumor cells; (3) The combination treatments of ferroptosis-assisted photodynamic immunotherapy induced by single-component CNTPA-TPA with the participation of anti-PD-L1 antibody resulted in increased T-cell infiltration and profound suppression of both primary and distant tumor growth, as well as lung metastasis.