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.