Photodynamic therapy (PDT) is a promising cancer treatment but has limitations due to its dependence on oxygen and high-power-density photoexcitation. Here, we report polymer-based organic photosensitizers (PSs) through rational PS skeleton design and precise side-chain engineering to generate •O2- and •OH under oxygen-free conditions using ultralow-power 808 nm photoexcitation for tumor-specific photodynamic ablation. The designed organic PS skeletons can generate electron-hole pairs to sensitize H2O into •O2- and •OH under oxygen-free conditions with 808 nm photoexcitation, achieving NIR-photoexcited and oxygen-independent •O2- and •OH production. Further, compared with commonly used alkyl side chains, glycol oligomer as the PS side chain mitigates electron-hole recombination and offers more H2O molecules around the electron-hole pairs generated from the hydrophobic PS skeletons, which can yield 4-fold stronger •O2- and •OH production, thus allowing an ultralow-power photoexcitation to yield high PDT effect. Finally, the feasibility of developing activatable PSs for tumor-specific photodynamic therapy in female mice is further demonstrated under 808 nm irradiation with an ultralow-power of 15 mW cm-2. The study not only provides further insights into the PDT mechanism but also offers a general design guideline to develop an oxygen-independent organic PS using ultralow-power NIR photoexcitation for tumor-specific PDT.

Cancer stem cells, characterized by high tumorigenicity and drug-resistance, are often responsible for tumor progression and metastasis. Aldehyde dehydrogenases, often overexpressed in cancer stem cells enriched tumors, present a potential target for specific anti-cancer stem cells treatment. In this study, we report a self-assembled nano-prodrug composed of aldehyde dehydrogenases activatable photosensitizer and disulfide-linked all-trans retinoic acid for diagnosis and targeted treatment of cancer stem cells enriched tumors. The disulfide-linked all-trans retinoic acid can load with photosensitizer and self-assemble into a stable nano-prodrug, which can be disassembled into all-trans retinoic acid and photosensitizer in cancer stem cells by high level of glutathione. As for the released photosensitizer, overexpressed aldehyde dehydrogenase catalyzes the oxidation of aldehydes to carboxyl under cancer stem cells enriched microenvironment, activating the generation of reactive oxygen species and fluorescence emission. This generation of reactive oxygen species leads to direct killing of cancer stem cells and is accompanied by a noticeable fluorescence enhancement for real-time monitoring of the cancer stem cells enriched microenvironment. Moreover, the released all-trans retinoic acid, as a differentiation agent, reduce the cancer stem cells stemness and improve the cancer stem cells enriched microenvironment, offering a synergistic effect for enhanced anti-cancer stem cells treatment of photosensitizer in inhibition of in vivo tumor growth and metastasis.