Synthesis and Luminescence Properties of Near-Infrared N-Heterocyclic Luciferin Analogues for In Vivo Optical Imaging

As a means of achieving highly sensitive bioluminescence imaging of deep tissues utilizing the firefly luciferin-luciferase (L-L) reaction, we previously reported a luciferin analogue, AkaLumine, which exhibits high cell-permeability and emits near-infrared (NIR) light with high tissue-penetration by the L-L reaction. However, while AkaLumine enables us to observe targets in deep tissues, its poor solubility in aqueous media limits its utility for in vivo imaging. Herein, to address this issue, we have synthesized three AkaLumine derivatives with N-heterocyclic aromatic rings as new red luciferin analogues that have substantially higher solubility than that of AkaLumine in phosphate buffered saline solution. One of the derivatives (herein termed seMpai) exhibits an emission maximum at 675 nm upon L-L reaction with Photinus pyralis luciferase and presents an activity in mouse-tissue imaging similar to that of AkaLumine. It is hoped that seMpai will extend the application of high-sensitivity NIR bioluminescence imaging in a wide range of biomedical research fields.