In-Situ Imaging of Azoreductase Activity in the Acute and Chronic Ulcerative Colitis Mice by a Near-Infrared Fluorescent Probe

Azoreductase (AzoR) is an essential reductive enzyme which is closely associated with the intestinal disease such as ulcerative colitis (UC). To date, only a few fluorescent probes for detecting AzoR activity in bacteria or cells have been constructed successfully. It is still challenging to design fluorescent probes for in situ monitoring AzoR in vivo. In this paper, a near-infrared (NIR) fluorescent probe (Cy-Azo) based on hemicyanine is designed and synthesized. The emission of the probe is located at 735 nm in the NIR region, which is favorable for its application in vivo. In addition, Cy-Azo shows high sensitivity to AzoR activity with 17-fold fluorescence enhancement and is particularly selective to AzoR over other enzymes, ions, and amino acids. Meanwhile, a possible response mechanism (the azo group in Cy-Azo is reduced by AzoR and cleaved resulting in the production of Cy-NH2) was proposed and verified by HPLC, MS, and theory calculation. In addition, based on low cell cytotoxicity, Cy-Azo is successfully applied in visualizing the activity of AzoR in two cell lines (HCT116 and HepG2 cells) and three types of bacteria (E. coli, S. aureus, and P. aeruginosa). In particular, due to its NIR emission, the probe can monitor AzoR activity in acute and chronic UC mice models. To our knowledge this is the first fluorescent probe for detecting AzoR activity in vivo, which can provide much important information for the diagnosis and treatment of UC.