In cancer cells, the MDR pump promotes drug resistance as it ejects cell-killing chemotherapy molecules, a victory for the cell, but of course not for the patient, who must then rely on some other form of treatment to keep the cancer at bay.
Many important chemotherapy drugs are denied entry into MDR-laden cancer cells, says Stanford University oncologist Brandy Sikic, who for years has studied multidrug resistance in cancer. "The pump is active in a broad spectrum of human cancers, including lymphoma, leukemia, and breast, lung and ovarian cancers," says Sikic. And in most cases, he says, how much MDR is present in tumor cells can predict how well the patient fares treatment. The more MDR pumps cancer cells have, the less effective chemotherapy is likely to be.
Scientists don't yet fully understand why some cancer cells are packed with MDR and some appear to have little or none at all, but they do know that chemotherapy drugs themselves can sometimes prod MDR production within a select group of cancer cells in the body. When that happens, those cells have a distinct growth advantage and quickly take over, rendering the drug useless for its original purpose: attacking the tumor.
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