P-glycoprotein is a membrane efflux transporter protein discovered by Juliano and Ling in 1976 in the plasma membrane of Chinese hamster ovary cells selected for resistance to colchicine.23 These cells also displayed cross-resistance to a wide range of structurally and functionally unrelated drugs, a phenomenon nowadays known as multidrug resistance (MDR). The consistent observation of this membrane protein in several MDR cell lines selected with different drugs23-26 and the positive correlation found between the level of P-glycoprotein expression and drug resistance in a variety of MDR cell lines27,28 strongly suggested that P-glycoprotein may play a key role in mediating MDR. This was subsequently confirmed by studies,29,30 in which transfection of P-glycoprotein cDNA was shown to confer the MDR phenotype upon otherwise drug-sensitive cells. The mechanism by which P-glycoprotein mediates MDR is believed to be that P-glycoprotein functions as an ATP-dependent efflux pump, actively extruding a wide range of cytotoxic agents, such as anthracyclines, vinca alkaloids, epipodophyllotoxins, and taxol, from inside the cell to the extracellular space, resulting in inadequate intracellular accumulation of these agents for efficient cell killing.1,31-34 It is well established that P-glycopro-tein overexpression is one of the major mechanisms responsible for the development of MDR.2,35 The clinical relevance of this MDR mechanism was substantiated by the findings that P-glycoprotein was often detected in numerous resistant human tumors and that the expression of this protein represents a poor

prognosis factor.36-44

The genes encoding P-glycoprotein have been cloned and belong to a small family of closely related genes designated mdr. The family consists of two members (MDR1 and MDR3) in humans and three members (mdrla, mdrlb, and mdr2) in rodents.45-48 Despite the high homology between different members of the family, only human MDR1 and its mouse homolog mdrla and mdrlb protein can confer MDR and drug transport capabilities, while human MDR3 and its mouse homolog, mdr2 protein, apparently cannot.29,30,47,49-54 The latter was shown to be more concentrated in the liver canalicular membranes and functions as a phos-phatidylcholine translocase or flippase.55-58 Human P-glycoprotein has 1280 amino acids, and the polypeptide component of the protein has a molecular weight of 120 to 140 kDa.45 The apparent molecular weight of P-glycoprotein, however, could vary between 130 and 190 kDa, depending on the level of glycosylation. The molecular structure of the protein was predicted to consist of two homologous halves, each consisting of six transmembrane domains, and a hydrophilic nucleotide binding domain with Walker A, Walker B and ABC signature sequences, characteristic of ABC proteins (Figure 18.1). The nucleotide binding sites are located intracellu-larly and exhibit ATPase activity, which hydrolyzes ATP and provides the energy for the pumping function of the protein.59,60

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