Molecular engineering design of twisted-backbone pure Type-I organic photosensitizers for hypoxic photodynamic therapy

Photodynamic therapy (PDT), an emerging tumor therapeutic strategy has received tremendous attention. Enslaved by the high dependence of oxygen, Type-II photosensitizers (PSs) mediated PDT is restricted by the hypoxic environment of tumors. By transferring electrons to water or other substrates instead of oxygen, Type-I PSs hold the promise of achieving an ideal therapeutic effect under hypoxic conditions. In this study, three twisted-backbone PSs (CBz-TQs-1, CBz-TQs-2 and CBz-TQs-3) are synthesized and studied. Owing to different substituent effects, the ROS generation mechanism transfers from pure Type-II of their prototype PSs (TQs-1, TQs-2 and TQs-3) to mixed Type-I/II of CBz-TQs-1 and CBz-TQs-2 to pure Type-I of CBz-TQs-3. Moreover, CBz-TQs-3 exhibits an ultra-high ROS quantum yield (∼1.0). The in vitro and in vivo PDT effects of water-dissolvable nanoparticles (NPs) of CBz-TQs-3 are investigated. The results show that the phototoxicity of CBz-TQs-3 is not affected by hypoxic environments. In addition, a remarkable tumor ablation can be found after CBz-TQs-3 NPs mediated PDT on Balb/c mice with xenograft tumors. It proves that a twisted backbone strategy is beneficial for designing pure Type-I PSs with high-efficient hypoxic PDT.