Nitro-Pyrazinotriazapentalene scaffolds– nitroreductase quantification and in vitro fluorescence imaging of hypoxia

Abstract

Nitroreductases (NTRs), a family of flavin-containing enzymes, can be overexpressed in regions of tumor hypoxia, i.e., areas deprived of oxygen. The detection of NTR may be applied for monitoring the hypoxia level in tumors. To quantify NTR, novel sensors were designed based on the conjugation of pyridazino-1,3a,6a-triazapentalene to a para-nitrophenyl, directly or with an alkyne linker, resulting in two probes denoted as 1-NO2 and 2-NO2. Both probes had a weak fluorescence (form off), while their reduction by NTR led to the over 15-fold enhancement of fluorescence intensity (form on) regardless of the oxygen concentration in their environment. The detection limit was as low as ca. 20−30 ng/mL of NTR. Interestingly, the presence of human serum albumin significantly enhanced the observed fluorescence turned on by NTR in particular for the 2-NO2 probe. The in vitro response to both probes was evaluated on the highly metastatic human melanoma A2058 cell line, where NTR levels increased under hypoxic conditions. Their low toxicity, high photostability, and efficient uptake combined with a strong correlation between the enhancement of fluorescence and hypoxia in cells indicate the high potential of the 1-NO2 and 2-NO2 probes for the assessment of the hypoxic environment in biomedical research. The designed compounds allowed for a fast determination of the difference in NTR content of cells without the need for special sample preparation such as cell lysis. The changes can be monitored using a plate reader, flow cytometer, and fluorescence microscope, which that makes these probes a remarkably universal tool.