Targeting cancer stem cells.

Recent evidence has demonstrated the existence of a small subset of the tumour mass that is wholly responsible for the sustained growth and propagation of the tumour. This cancer stem cell (CSC) compartment is also likely to be responsible both for disease relapse and the resistance to therapy that often accompanies relapse. The evidence for CSCs in various malignancies is presented. The failure of existing therapeutics to eradicate CSCs suggests that they are relatively resistant to present cancer treatments. This resistance may reflect the preservation of normal stem cell protective mechanisms, such as an increased expression of drug efflux pumps or alterations in apoptotic, cell cycle and DNA repair mechanisms. Targeting these mechanisms, and taking advantage of potential differences in the biology of normal stem cells and CSCs, such as differences in surface phenotype, self renewal/quiescence and stem cell-niche interactions are discussed and preliminary preclinical or clinical data are presented. Finally, the authors give their opinion of the direction in which one must travel to successfully target the CSC and improve treatment outcomes in malignant disease.