Thiol “Click” Chromene Ring Opening and Subsequent Cascade Nucleophilic Cyclization NIR Fluorescence Imaging Reveal High Levels of Thiol in Drug-Resistant Cells

As the structural unit of natural products, chromene derivatives show a wide range of biological activity and pharmacological activity due to their unique photophysical and chemical properties. Ten years ago, our research group discovered the "thiol-chromene" click reaction, which achieved the selective detection of thiols through the change of the optical spectrum. Afterward, we attempted to develop various chromene-based fluorescent probes for imaging including near-infrared (NIR) probe, ratiometric probe, and multifunctional probe. However, how to integrate the fluorophore and reaction sites into the chromene-based skeleton remains challenging. In this work, we connected the chromene motif with the NIR fluorophore methylene blue utilizing a carbamate spacer to provide a new fluorescent probe (CM-NIR), which is triggered by thiols to open the pyran ring followed by attacking the carbamate by phenolate to releases the methylene blue. This novel cascade mechanism avoids the formation of para-quinone methides, which proved to be toxic to normal cells. CM-NIR also showed the specific imaging of thiols in living cells and mice. More importantly, the thiols level in drug-resistant cancer cells was found to be significantly higher than that in the corresponding cancer cell, which indicated that the thiols level may have an important role in cancer cells developing drug resistance.