Ultra-sensitive and fast nitroreductase-responsive fluorescence probe for real-time bioimaging of hypoxia in tumor cells and zebrafish wound inflammation

Hypoxia is intertwined with the occurrence and progression of tumors and inflammation. To date, various fluorescent probes have been constructed for monitoring degrees of hypoxia in tumor cells, however, efficiently evaluating the degrees of hypoxia caused by inflammation in real-time has been rarely exhibited via fluorescence imaging technique. Hence, an ultra-sensitive and fast-response fluorescent probe R19-NTR using rhodamine 19 as the fluorescent reporter and nitro aromatic groups as the recognition unit was performed to estimate the degrees of hypoxia through assessing the changes in the level of nitroreductase. The probe R19-NTR was basically non-fluorescent on account of the intramolecular photo-induced electron transfer process. Upon selectively reacted with nitroreductase, the R19-NTR featured an ultrafast response (< 1 min), and a significant fluorescence enhancement (ca. 114-fold). Encouraged by these outstanding properties, R19-NTR has been utilized for monitoring the degree of hypoxia in tumor cells and zebrafish wound inflammation. The results revealed that tumor cells under normoxic and different degrees of hypoxic conditions can be distinguished successfully, as well as the differentiation between tumor cells and normal cells. Moreover, the in vivo visualization disclosed that the endogenous nitroreductase of 6–10 days post fertilization zebrafish embryos was mainly focused on the liver and intestine. In particularly, the elevation of hypoxia degrees in zebrafish wound inflammation caused by severed tails were successfully visualized for the first time, as well as the variation of hypoxia degrees in gradually dying zebrafish embryos. The present research may provide a promising methodology for the diagnosis of inflammation-related diseases and tumors.