Thionated donor-acceptor naphthalimides as efficient heavy-atom-free triplet photosensitizers for type I/II ROS generation in hypoxia-immune photodynamic therapy.

Photodynamic therapy (PDT) is a promising non-invasive cancer treatment but faces challenges from the hypoxic tumor environment and inefficient reactive oxygen species (ROS) generation. To address this, we designed a series of donor-acceptor naphthalimide derivatives via synergistic integration of electron-donating groups and stepwise thionation of the NI core, achieving ROS generation even under hypoxia. The donor-acceptor structure and stepwise thionation synergistically extend absorption to the 500-700 nm red light region and reduce the singlet-triplet energy gap (ΔEST), thereby enhancing triplet state population and boosting ROS generation efficiency. Among them, NI-OCH3-2S exhibits the lowest ΔEST (0.114 eV) and the highest T1 state yield (28 %). Under red light irradiation, NI-OCH₃-2S demonstrates efficient 1O2 (Φ = 44 %) and O2-· generation under hypoxia. After encapsulation by PLGA-PEG and targeted PLGA-PEG-FA, the resulting NI NPs produced ROS induce oxidative stress and lead to mitochondrial dysfunction, activating the caspase-3-mediated apoptosis pathway. In vivo, NI NPs effectively targeted tumors and their generated ROS remodeled the tumor immune microenvironment by repolarizing immunosuppressive M2-TAMs toward the pro-inflammatory M1 phenotype, increasing the M1/M2 ratio by 4.8-fold. ROS-induced tumor cell damage also recruited dendritic cells to tumor sites. These DCs then migrated to secondary lymphoid organs, where they activated splenic helper T cells (CD3+CD4+, 37.8 %) and cytotoxic T lymphocytes (CD3+CD8+, 20.6 %), establishing a systemic anti-tumor immune response. In 4T1-bearing mice, NI NPs achieved 71 % tumor suppression with negligible toxicity, enabling hypoxia-immune synergistic PDT. This study presents a rational design strategy for efficient heavy-atom-free type I/II PSs, addressing ROS inefficiency, hypoxia, and immunosuppression to advance hypoxia-immune synergistic PDT. STATEMENT OF SIGNIFICANCE: Photodynamic therapy (PDT) is hindered by hypoxic tumor microenvironments and insufficient reactive oxygen species generation. Here, we develop a heavy-atom-free donor-acceptor naphthalimide photosensitizer NI-OCH3-2S via synergistic electron-donating group introduction and stepwise thionation. This design extends red-light absorption, narrows the singlet-triplet energy gap, and enables efficient production of both oxygen-dependent 1O2 and oxygen-independent O2-· even under hypoxia. Encapsulated into targeted PLGA-PEG-FA nanoparticles, the photosensitizer NI NPs induces mitochondrial apoptosis and remodels the immunosuppressive tumor microenvironment by repolarizing M2-like macrophages to the M1 phenotype and activating systemic T cell immunity. In 4T1 tumor-bearing mice, NI NPs generated ROS achieve 71 % tumor growth inhibition with low toxicity, providing a generalizable strategy for hypoxia-resistant, immune-modulatory PDT with clinical translation potential.