Totally Caged Type I Pro‐Photosensitizer for Oxygen‐Independent Synergistic Phototherapy of Hypoxic Tumors

Abstract Activatable type I photosensitizers are an effective way to overcome the insufficiency and imprecision of photodynamic therapy in the treatment of hypoxic tumors, however, the incompletely inhibited photoactivity of pro‐photosensitizer and the limited oxidative phototoxicity of post‐photosensitizer are major limitations. It is still a great challenge to address these issues using a single and facile design. Herein, a series of totally caged type I pro‐photosensitizers (Pro‐I‐PSs) are rationally developed that are only activated in tumor hypoxic environment and combine two oxygen‐independent therapeutic mechanisms under single‐pulse laser irradiation to enhance the phototherapeutic efficacy. Specifically, five benzophenothiazine‐based dyes modified with different nitroaromatic groups, BPN 1−5, are designed and explored as latent hypoxia‐activatable Pro‐I‐PSs. By comparing their optical responses to nitroreductase (NTR), it is identified that the 2‐methoxy‐4‐nitrophenyl decorated dye (BPN 2) is the optimal Pro‐I‐PSs, which can achieve NTR‐activated background‐free fluorescence/photoacoustic dual‐modality tumor imaging. Furthermore, upon activation, BPN 2 can simultaneously produce an oxygen‐independent photoacoustic cavitation effect and a photodynamic type I process at single‐pulse laser irradiation. Detailed studies in vitro and in vivo indicated that BPN 2 can effectively induce cancer cell apoptosis through synergistic effects. This study provides promising potential for overcoming the pitfalls of hypoxic‐tumor photodynamic therapy.