Cancer stem cells (CSCs), also called tumor-initiating cells (TICs), have been studied intensively due to their rapid proliferation, migration, and role in the recurrence of cancer. In general, CSC marker-positive cells [CD133, CD44, CD166, aldehyde dehydrogenase (ALDH), and epithelial cell adhesion molecule (EpCAM)] exhibit a 100-fold increased capacity to initiate cancer. Within a heterogeneous tumor mass, only approximately 0.05-3% of cells are suspected to be CSCs and able to proliferate under hypoxia. Interestingly, CSCs, cancer cells, and normal stem cells share many cytochemical properties, such as inhibition of the redox system for reactive oxygen species (ROS) production and high expression of drug resistance transporters. However, compared to normal stem cells, CSCs develop unique metabolic flexibility, which involves switching between oxidative phosphorylation (OXPHOS) and glycolysis as their main source of energy. Due to the similarities between CSCs and other cancer cells and normal stem cells, limited chemotherapeutic and bio-imaging reagents specific for CSCs have been developed. In this short review, we address the current knowledge regarding CSCs with a focus on designing chemotherapeutic and bio-imaging reagents that target CSCs.
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