Tracking and Finding Slow‐Proliferating/Quiescent Cancer Stem Cells with Fluorescent Nanodiamonds

Q uiescent cancer stem cells (CSCs) have long been considered to be a source of tumor initiation. However, identifi cation and isolation of these cells have been hampered by the fact that commonly used fl uorescent markers are not suffi ciently stable, both chemically and photophysically, to allow tracking over an extended period of time. Here, it is shown that fl uorescent nanodiamonds (FNDs) are well suited for this application. Genotoxicity tests of FNDs with comet and micronucleus assays for human fi broblasts and breast cancer cells indicate that the nanoparticles neither cause DNA damage nor impair cell growth. Using AS-B145-1R breast cancer cells as the model cell line for CSC, it is found that the FND labeling outperforms 5-ethynyl-2′-deoxyuridine (EdU) and carboxyfl uorescein diacetate succinimidyl ester (CFSE) in regards to its long-term tracking capability (>20 d). Moreover, through a quantifi cation of their stem cell activity by measuring mammosphere-forming effi ciencies (MFEs) and self-renewal rates, the FND-positive cells are identifi ed to have an MFE twice as high as that of the FND-negative cells isolated from the same dissociated mammospheres. Thus, the nanoparticle-based labeling technique provides an effective new tool for tracking and fi nding slow-proliferating/quiescent CSCs in cancer research.