Cathepsin B Activatable Fluorescent Probe for Antitumor Efficiency Feedback: Attempt To Detect Certain Apoptotic Cells

As many treatments kill tumor cells by inducing apoptosis, fluorescent probes that can detect apoptosis are crucial for effective feedback regarding tumor therapy outcomes (in particular, activatable probes for better imaging). Cathepsins are enzymes that are released from lysosomes into the cytoplasm during lysosomal membrane permeabilization-induced apoptosis of many tumor cells, making them potential biomarkers of apoptotic cells. Despite their potential, to the best of our knowledge, no cathepsin-activatable fluorescent probes have been reported for this purpose. To fill this gap, we designed a cathepsin B (CTSB)-activatable fluorescent probe, A-DCO, which can distinguish tumor cells from normal cells in the free state and give feedback of enhanced fluorescence signal under drug stimulation when encapsulated into nanoparticles (NPs), enabling the evaluation of treatment efficacy. As a proof of concept, A-DCO NPs were employed rather than Annexin-V/PI double staining to detect apoptotic 4T1 and MCF-7 tumor cells via flow cytometry. In vivo experiments demonstrated that A-DCO NPs can rapidly detect apoptotic 4T1 tumor cells and are quickly metabolized after imaging, achieving timely, accurate, and safe feedback about tumor treatment effectiveness. The study offers a new tool for detecting the apoptosis of certain tumor cells and provides inspiration and direction for fluorescent probe design in the future.