Near-infrared fluorescent probe for selective detection of nitroreductase and tumor imaging in breast cancer

Nitroreductase (NTR), an oxygen-sensitive flavoprotein that plays a crucial role in tumor hypoxia, has been widely recognized as a potential biomarker for cancer diagnosis and therapeutic evaluation. Herein, we report the development of FQ-NO₂, a novel near-infrared (NIR) fluorescent probe for the sensitive and selective detection of NTR activity in breast cancer. The probe exhibits an "off-on" response: it remains non-fluorescent in the absence of NTR, but after enzymatic reduction of the nitro group, it emits strong fluorescence at 710 nm upon excitation at 600 nm. With NIR emission, the FQ-NO₂ probe had deep-tissue penetration capability and produced low background signals, and is thus suitable for in vivo imaging. The probe had a low detection limit, high selectivity, and excellent stability under physiological conditions. Cellular studies revealed that FQ-NO₂ was able to distinguish breast cancer cells from normal epithelial cells and manifest changes in NTR activity. In vivo fluorescence imaging further the probe could be rapidly activated after local or systemic administration, and its signals were correlated with NTR activity. Collectively, these results show that FQ-NO₂ is a promising molecular probe for tumor hypoxia-related enzyme detection and breast cancer imaging that has potential applications in cancer diagnosis and therapeutic monitoring.