The ABC transporters MDR1 and MRP2: multiple functions in disposition of xenobiotics and drug resistance.

ATP‐binding cassette (ABC) transporters comprise one of the largest membrane bound protein families. They are involved in transport of numerous compounds. These proteins transport substrates against a concentration gradient with ATP hydrolysis as a driving force across the membrane. Mammalian ABC proteins have important physiological, pharmacological and toxicological functions including the transport of lipids, bile salts, drugs, toxic and environmental agents. The efflux pumps serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs. In general terms, the transporters remove xenobiotics from the cellular environment. For example, in cancer cells, over expression of these molecules may confer to multidrug resistance against cytostatic drugs. In addition, based on diverse structural characteristics and a broad substrate specifity, ABC transport proteins alter the intracellular concentration of a variety of therapeutically used compounds and toxicologically releva...