A General Approach to Design Dual Ratiometric Fluorescent and Photoacoustic Probes for Quantitatively Visualizing Tumor Hypoxia Levels In Vivo

Herein, we describe an energy balance strategy between fluorescence and photoacoustic effects by sulfur substitution to transform existing hemicyanine dyes (Cy) into optimized NIRF/PA dual ratiometric scaffolds. Based on this optimized scaffold, we reported the first dual-ratio response of nitroreductase probe AS-Cy-NO2, which allows quantitative visualization of tumor hypoxia in vivo. AS-Cy-NO2, composed of a new NIRF/PA scaffold thioxanthene-hemicyanine (AS-Cy-1) and a 4-nitrobenzene moiety, showed a 10-fold ratiometric NIRF enhancement (I-773/I-733) and 2.4-fold ratiometric PA enhancement (PA(730)/PA(670)) upon activation by a biomarker (nitroreductase, NTR) associated with tumor hypoxia. Moreover, the dual ratiometric NIRF/PA imaging accurately quantified the hypoxia extent with high sensitivity and high imaging depth in xenograft breast cancer models. More importantly, the 3D maximal intensity projection (MIP) PA images of the probe can precisely differentiate the highly heterogeneous oxygen distribution in solid tumor. Thus, this study provides a promising NIRF/PA scaffold that may be generalized for the dual ratiometric imaging of other disease-relevant biomarkers.

Automated summary

This study describes an energy balance strategy by sulfur substitution to optimize hemicyanine dyes for dual ratiometric fluorescence and photoacoustic imaging. The newly developed nitroreductase probe AS-Cy-NO2 allows quantitative visualization of tumor hypoxia in vivo with high sensitivity and depth. This promising NIRF/PA scaffold may have broader applications in dual ratiometric imaging of other disease-relevant biomarkers.