Targeted Singlet Oxygen Delivery: A Bioorthogonal Metabolic Shunt Linking Hypoxia to Fast Singlet Oxygen Release

Singlet oxygen can be generated by thermal cycloreversion of aromatic endoperoxides. However, for any practical potential of chemically generated singlet oxygen within a therapeutic context, the time and place of the release of this cytotoxic species must be tightly regulated. We now show that using a bimodular design with a hypoxia responsive unit and fluoride‐triggered endoperoxide unit, a bioorthogonal metabolic shunt can be established, where an enzymatically generated submicromolar fluoride signal plays a crucial role. Thus, cellular nitroreductase is repurposed in a bioorthogonal enzymatic activity, where it releases fluoride ions upon the reduction of a targeted compound. The fluoride ions released in the initial reaction remove the silyl stopper, yielding a highly accelerated release of singlet oxygen. The result is a remarkable difference in cytotoxicity between hypoxic and normoxic conditions as evidenced by microscopy, viability assays and the use of control compounds. A bimodular endoperoxide with a hypoxia sensitive functionality and fluoride‐reactive stopper for the cycloreversion was synthesized. Under reductive conditions of the hypoxic cancer cells, a bioorthogonal shunt which releases cytotoxic singlet oxygen via the intermediacy of a fluoride signal becomes operative. The result is a very precise spatio‐temporal targeting of singlet oxygen, which opens a new path toward potential therapeutic applications.