An azoreductase-responsive "OFF-ON-OFF" reversible photosensitizer for highly selective antibacterial photodynamic therapy.

Antibacterial photodynamic therapy (aPDT) represents a highly promising approach for combating resistant pathogens without inducing drug resistance. Selective photo-inactivation of bacteria without damage to the mammalian cells is a prerequisite for the clinical application of aPDT. In this work, based on the different azoreductase levels between mammalian cells and Staphylococcus aureus (S. aureus), an azoreductase-responsive "OFF-ON-OFF" photosensitizer, namely Ru-AzoCF3, was designed and synthesized as a proof-of-concept. The aPDT activity of Ru-AzoCF3 is initially suppressed by the presence of the azo group but can be activated by azoreductase, which is overexpressed in S. aureus. After exiting the bacterial cells, the activated photosensitizer reverts to an "OFF" state through a reversible "azo-hydrazine-azo" transformation, thereby precisely limiting the aPDT activity within pathogens and greatly minimize the potential phototoxicity to the mammalian cells. The detailed results demonstrated that Ru-AzoCF3 could selectively image and photo-inactivate S. aureus and methicillin-resistant S. aureus (MRSA) both in vitro and in vivo, surpassing the performance of vancomycin. This study may provide new insights for developing highly selective aPDT agents.