A novel nitro-substituted benzothiadiazole as fluorescent probe for tumor cells under hypoxic condition

Most of solid tumor cells are hypoxic and hard to trace and measure. A new compound, 4,7-bis(4-dodecylthiophen-2-yl)-5,6-dinitrobenzo[c][1,2,5]thiadiazole (BTTD-NO2), was synthesized for labeling the hypoxic cells specially in this paper. BTTD-NO2 showed no cytotoxicity to MG63 cells by MTT method. When MG63 cells were cultured with BTTD-NO2 under hypoxic condition for 24h, strong red fluorescence distribution in cytoplasm was observed. Flow cytometry results showed that 65% of MG63 cells were labeled with strong red fluorescence in hypoxic condition while only 2.4% in oxic condition. Furthermore, Real time RT-PCR proved that BTTD-NO2 could stimulate high gene expression of the nitroreductase in the cells which could improve the conversion rate of BTTD-NO2 to BTTD-NH2 in turn. It proved that the fluorescence of BTTD-NO2 was quenched by its two nitro groups, however, strong red fluorescence could emit in the cytoplasm after the reduction of its nitro groups to amino groups in the tumor cells under hypoxic condition. These results suggested that BTTD-NO2 had the potential as a superior fluorescent probe for tumor detection.