tokens: PURPOSE : The multidrug resistance protein 2 ( MRP2 ) is a drug efflux transporter that is expressed predominantly at the apical domain of hepatocytes but seems also to be expressed at the apical membrane of brain capillary endothelial cells that form the blood - brain barrier ( BBB ). MRP2 is absent in the transport - deficient ( TR (-)) Wistar rat mutant , so that this rat strain was very helpful in defining substrates of MRP2 by comparing tissue concentrations or functional activities of compounds in MRP2 - deficient rats with those in transport - competent Wistar rats .