Activatable Type I Photosensitizer with Quenched Photosensitization Pre and Post Photodynamic Therapy

The phototoxicity of photosensitizers (PSs) pre and post photodynamic therapy (PDT), and the hypoxic tumor microenvironment are two major problems limiting the application of PDT. While activatable PSs can successfully address the PS phototoxicity pre PDT, and type I PS can generate reactive oxygen species (ROS) effectively in hypoxic environment, very limited approaches are available for addressing the phototoxicity post PDT. There is virtually no solution available to address all these issues using a single design. Herein, we propose a proof‐of‐concept on‐demand switchable photosensitizer with quenched photosensitization pre and post PDT, which could be activated only in tumor hypoxic environment. Particularly, a hypoxia‐normoxia cycling responsive type I PS TPFN‐AzoCF3 was designed to demonstrate the concept, which was further formulated into TPFN‐AzoCF3 nanoparticles (NPs) using DSPE‐PEG‐2000 as the encapsulation matrix. The NPs could be activated only in hypoxic tumors to generate type I ROS during PDT treatment, but remain non‐toxic in normal tissues, pre or after PDT, thus minimizing side effects and improving the therapeutic effect. With promising results in in vitro and in vivo tumor treatment, this presented strategy will pave the way for the design of more on‐demand switchable photosensitizers with minimized side effects in the future. Photodynamic therapy (PDT) is considered as one of the most momentous strategies to ablate cancer cells. However, the phototoxicity of photosensitizers pre and post PDT, and the tumor hypoxia environment, are the two major problems that limit the application of PDT. In this paper, we report an activatable type I photosensitizer TPFN‐AzoCF3, which could respond to cycling normoxia‐hypoxia and undergo redox cycles in a fully reversible manner.