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 24 h, 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.