Water-Soluble Near-Infrared Fluorescent Probes for Specific Detection of Monoamine Oxidase A in Living Biosystems

Monoamine oxidase (MAO) has two isomers (A and B), and the analysis of each one in biological samples is of great potential in studying MAO-related diseases. However, specific detection and imaging of MAO-A in cells and in vivo are still a great challenge at present due to the relative lack of suitable near-infrared (NIR) fluorescent probes. In this work, two new NIR fluorescent probes, Rma-1 and Rma-2, have been developed for the analysis of MAO-A in living biosystems. The probes were designed by conjugating the specific structure of the MAO-A inhibitor to hemicyanine (an NIR fluorophore) directly or through a self-immolative linker. Both probes are water-soluble and showed selective and sensitive NIR (708 nm) responses toward MAO-A rather than MAO-B, of which probe Rma-1 with a self-immolative linker exhibited superior analytical performance with a detection limit of 4.5 ng/mL and 13-fold specificity for MAO-A over MAO-B. Further, probe Rma-1 has been successfully utilized to image the MAO-A activity in cells, zebrafish, and tumor-bearing mice, showing a promising prospect for MAO-A study in carcinoma.