Reduction-Responsive Carbon Dots for Real-Time Ratiometric Monitoring of Anticancer Prodrug Activation in Living Cells

Anticancer prodrugs have been extensively investigated to lower toxic side effects of common chemotherapeutic agents in biomedical fields. To illustrate the activation mechanism of anticancer prodrugs, fluorescent dyes or single-emission intensity alteration-based approaches have been widely used. However, fluorescent dyes often suffer from poor photostability and chemical stability, and single-emission intensity alteration-based methods cannot avoid the influence from uncontrolled microenvironment changes in living samples. To overcome these obstacles, herein, a fluorescence resonance energy transfer (FRET)-based ratiometric approach was successfully developed for real-time monitoring of anticancer prodrug activation. Excitation-wavelength-dependent and full-color-emissive carbon dots (CDs) were used as drug nanocarriers and FRET donor, and a cisplatin(IV) prodrug was selected as the model drug and the linker to load the Dabsyl quencher on the surface of CDs. Owing to the FRET effect, the blue fluorescence of CDs was effectively quenched by the Dabsyl unit. Under reductive conditions in solution or in living cells for the reduction of cisplatin(IV) prodrug to Pt(II) species, the blue fluorescence of CDs increased over time, without apparent intensity change for green or red fluorescence. Thus, the gradually enhanced intensity ratio of blue-to-green or blue-to-red fluorescence could be indicative of the real-time reduction of the cisplatin(IV) prodrug to cytotoxic Pt(II) species. This ratiometric method could exclude the influence from complex biological microenvironments by using green or red fluorescence of CDs as an internal reference, which provides new insights into the activation of the cisplatin(IV) prodrug and offers a great opportunity to design safe and effective anticancer therapeutics.