P-Glycoprotein — A Marker of Cancer-Cell Behavior

Resistance to a broad array of cytotoxic drugs — multidrug resistance — is thought to be a major reason chemotherapy fails to cure most cancers. Multidrug resistance has been studied intensively since the human MDR1 gene was identified almost 10 years ago.1 Increased levels of the MDR1 product, called P-glycoprotein, are often associated in vitro with reduced intracellular concentrations of several anticancer drugs derived from plants, such as anthracyclines (e.g., doxorubicin), epipodophyllotoxins (e.g., etoposide), vinca alkaloids (e.g., vincristine), dactinomycin, and paclitaxel. When cells are grown in increasing concentrations of one of these cytotoxic drugs, populations of cells that overexpress the . . .