Indolequinone-rhodol conjugate as a fluorescent probe for hypoxic cells: enzymatic activation and fluorescence properties

Hypoxia is an important feature of many diseases such as malignant solid tumors, inflammatory diseases and cardiac ischemia. We herein focused on the development of a novel hypoxia-sensitive fluorescent probe, IQ-R, consisting of an indolequinone unit and a rhodol fluorophore. IQ-R has good solubility in water and longer wavelength for absorption and emission, which are favorable for cellular bioimaging. While the fluorescence of rhodol in the IQ-R conjugate was quenched by the function of intramolecular indolequinone unit, it was restored under hypoxic conditions through the enzymatic one-electron reduction of IQ-R by NADPH:cytochrome P450 reductase to release the nonconjugated free rhodol. When administered to A549 cells, IQ-R was activated and reduced by endogenous reductase preferentially under hypoxic conditions, thereby visualizing hypoxic cancer cells by robust fluorescence.