For some 25 years, P-glycoprotein (Pgp) has reigned supreme as the most important tool of normal as well as of cancer cells to rid themselves of unwanted substances; in the case of malignant growth, the increased efflux of cytostatics and cytotoxics are the main cause of eventual chemotherapy failure. Trafficking of molecules across the cell membrane is the apparent main function of the ATP-binding cassette (ABC) superfamily of transport proteins that can be traced possibly in all living matter and may be an essential characteristic of survival. While Pgp was one of the first and best understood mechanisms of intracellular efflux, other transporters have joined the family: the MDR-associated proteins (MRP 1 to 7, as of now) and numerous lesser-understood mechanisms, such as the topoisomerases. The most recent newcomer is the mitoxantrone-resistance protein, MXR.
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