The ABCs of cancer stem cell drug resistance.

Chemotherapy is generally effective as a non-targeted therapy in killing the majority of cells in a tumor; however, a small population of residual cells that are intrinsically resistant to such agents persist after chemotherapy, ultimately resulting in patient relapse. There is evidence that these cells within resistant tumors are cancer stem cells. A common mechanism of multidrug resistance used by residual tumor cells involves the expression of the ATP-binding cassette (ABC) transporters. Understanding the anticancer drug transport properties of these transporters, as well as their physiological functions, in addition to improved efforts to discover and characterize selective inhibitors, will lead to more effective therapeutics for oncology.