Fluorescent Probes for Nitroreductase Activity in Hypoxic Tumor Imaging

Tumour hypoxia, a hallmark of solid malignancies, arises from imbalances in oxygen supply and consumption and drives resistance to conventional therapies. Nitroreductase (NTR) enzymes, upregulated in hypoxic niches, catalyse the bioreduction of nitroaromatic moieties, offering a selective trigger for activatable fluorescent probes. Recent advances have yielded optical agents that remain silent under normoxia but undergo enzymatic reduction to emit light in the near-infrared region, enabling deep-tissue imaging with high signal-to-background contrast. Many designs couple a nitroaromatic trigger to fluorophores or phosphorescent complexes, sometimes incorporating theranostic functions such as photodynamic activation or drug release. Innovations address challenges of tumour heterogeneity and limited penetration by combining ratiometric or reversible sensing, supramolecular assemblies for signal amplification, and multimodal modalities like photoacoustic or phosphorescence imaging. These tools facilitate real-time monitoring of hypoxia dynamics, guide surgical resection and personalised therapy, and hold promise for global applications in cancer diagnosis and treatment planning.

Research from Nature Portfolio

Researchers have introduced a light-activated ratiometric near-infrared probe integrated within a covalent organic framework. Under illumination, this system quantifies oxygen levels by dual-wavelength emission while consuming local oxygen through photodynamic action. Enhanced hypoxia then triggers prodrug activation, yielding a self-regulating theranostic that both images and amplifies tumour cell killing in vivo. Another team reported a reversible arylazo-conjugated probe for cycling hypoxia imaging. Its fluorescence switches off in normoxia and on in hypoxia, with a stable intermediate that prevents irreversible degradation. This allows repeated monitoring of oxygen fluctuations in cells, zebrafish embryos and mouse models, offering insights into dynamic hypoxia biology. A further study described a dual-activation photosensitiser–fluorophore conjugate linked by an azo group. Under hypoxia, enzymatic cleavage restores both phototoxicity and fluorescence, enabling selective imaging and photodynamic therapy of tumour tissues while minimising off-target effects.

Fluorescent Probes for Nitroreductase Activity in Hypoxic Tumor Imaging publication trend

The graph below shows the total number of articles in fluorescent probes for nitroreductase activity in hypoxic tumor imaging across all publications each year (not limited to Nature Index journals).

Technical terms

Nitroreductase (NTR): An enzyme family that reduces nitroaromatic compounds to amines, upregulated in low-oxygen environments.

Hypoxia: A physiological state of reduced oxygen concentration common in rapidly growing solid tumours.

Fluorescent probe: A molecular construct that emits light upon excitation, used to visualise specific biological processes.

Ratiometric imaging: A technique employing two emission wavelengths to quantify analyte concentration independent of probe distribution.

Near-infrared (NIR) fluorescence: Emission in the 650–900 nm range, offering deeper tissue penetration and reduced background.

Photoacoustic imaging: A modality detecting ultrasonic waves generated by pulsed light absorption, enabling high-resolution deep imaging.

Photodynamic therapy (PDT): Treatment that uses light-activated photosensitisers to generate cytotoxic reactive oxygen species in situ.

References

1. A light-activatable theranostic combination for ratiometric hypoxia imaging and oxygen-deprived drug activity enhancement. Nature Communications (2024).
2. Rational construction of a reversible arylazo-based NIR probe for cycling hypoxia imaging in vivo. Nature Communications (2021).
3. Microenvironment-triggered dual-activation of a photosensitizer- fluorophore conjugate for tumor specific imaging and photodynamic therapy. Scientific Reports (2020).
4. Modularized supramolecular assemblies for hypoxia-activatable fluorescent visualization and image-guided theranostics. Theranostics (2024).
5. Hypoxia-triggered single molecule probe for high-contrast NIR II/PA tumor imaging and robust photothermal therapy. Theranostics (2018).
6. A Novel NIR Fluorescent Probe for Highly Selective Detection of Nitroreductase and Hypoxic-Tumor-Cell Imaging. Molecules (2021).