KillerRed protein based in vivo photodynamic therapy and corresponding tumor metabolic imaging

Photodynamic therapy (PDT) gains wide attention as a useful therapeutic method for cancer. It is mediated by the oxygen and photosensitizer under the specific light irradiation to produce the reactive oxygen species (ROS), which induce cellular toxicity and regulate the redox potential in tumor cells. Nowadays, genetic photosensitizers of low toxicity and easy production are required to be developed. KillerRed, a unique red fluorescent protein exhibiting excellent phototoxic properties, has the potential to act as a photosensitizer in the application of tumor PDT. Meantime, the course of tumor redox metabolism during this treatment was rarely investigated so far. Thus here, we investigated the effects of KillerRed-based PDT on tumor growth in vivo and examined the subsequent tumor metabolic states including the changes of nicotinamide adenine dinucleotide hydrogen (NADH) and flavoprotein (Fp), two important metabolic coenzymes of tumor cells. Results showed the tumor growth had been significantly inhibited by KillerRed-based PDT treatment compared to control groups. A home-made cryo-imaging redox scanner was used to measure intrinsic fluorescence and exogenous KillerRed fluorescence signals in tumors. The Fp signal was elevated by nearly 4.5-fold, while the NADH signal decreased by 66% after light irradiation, indicating that Fp and NADH were oxidized in the course of KillerRed-based PDT. Furthermore, we also observed correlation between the fluorescence distribution of KillerRed and NADH. It suggests that the KillerRed protein based PDT might provide a new approach for tumor therapy accompanied by altering tumor metabolism.