Intracellular Nitroreductase-Triggered "On" and "Enhanced" Photoacoustic Signals for Sensitive Imaging of Tumor Hypoxia.

Current molecular photoacoustic (PA) probes are designed with either stimulus-turned "on" or assembly-enhanced signals to trace biological analytes/events. PA probes based on CBT-Cys click reaction possess both "turn-on" and "enhanced" PA signals, thus should have higher sensitivity. Nevertheless, such PA probes, particularly those for sensitive imaging of tumor hypoxia, remain scarce. Herein, we rationally designed a PA probe NI-Cys(StBu)-Dap(IR780)-CBT (NI-C-CBT) which, after being internalized by hypoxic tumor cells, was cleaved by nitroreductase under the reduction condition to yield cyclic dimer C-CBT-Dimer to turn the PA signal "ON" and subsequently assembled into nanoparticles C-CBT-NPs with additionally enhanced PA signal ("Enhanced"). NI-C-CBT exhibited 1.7-fold "ON" and 3.2-fold overall "Enhanced" PA signals in vitro. Moreover, it provided 1.9-fold and 2.8-fold overall enhanced PA signals for tumor hypoxia imaging in HeLa cells and HeLa tumor-bearing mice, respectively. We expect this strategy be widely applied to design more "smart" PA probes for sensitive imaging important biological events in vivo in near future. This article is protected by copyright. All rights reserved.