Legumain-Specific Near-Infrared Fluorescence "Turn On" for Tumor-Targeted Imaging.

Legumain is one of the cysteine proteases which can serve as an essential indicator for cancer diagnosis. Near-infrared (NIR) nanoprobes with fluorescence "Turn On" property are advantageous in cancer diagnosis. However, to the best of our knowledge, using a completely organic NIR nanoprobe to image legumain activity either in vitro or in vivo has not been reported. Herein, employing a CBT-Cys click condensation reaction, we used a rationally designed NIR probe Cys(StBu)-Ala-Ala-Asn-Lys(Cy5.5)-CBT (1) to synthesize its nanoprobes 1-NPs with self-quenched fluorescence. Cell and animal experiments indicated that our nanoprobes were able to specifically image legumain activity in living cells and tumors with a NIR fluorescence "Turn On" manner. We envision that the nanoprobes could be applied for the diagnosis of legumain-related diseases in the near future.