Several anticancer drugs such as vinca alkaloids and anthra-cyclines exhibit lower accumulation in the brain than would be expected from their lipophilic properties [42, 43]. Furthermore, tumor cells show a resistance to anticancer drugs such as anthracyclines and other nonrelated compounds . This multidrug resistance is accompanied by overexpression of the transmembrane P-glycoprotein and a decrease of drug concentration in the resistant cells compared with that in drug-sensitive cell lines [45, 46]. P-glycoprotein molecules are located at the luminal side of the endothe-lial cells and their function is to serve as an energy-dependent efflux pump and transport various drugs out of cells, thus decreasing their accumulation in the cytoplasm, and, as a consequence, reducing their efficiency [46-48]. P-glycoprotein has a very broad substrate specificity [49-51].
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