Molecular mechanism of multidrug resistance in tumor cells.

The ability of tumor cells to develop simultaneous resistance to multiple lipophilic cytotoxic compounds represents a major problem in cancer chemotherapy. This review describes recent molecular biological studies which resulted in the identification and cloning of the gene responsible for multidrug resistance in human tumor cells. This gene, designated mdr1, is overexpressed in all and amplified in many of the multidrug-resistant cell lines analyzed. Gene transfer and expression assays have indicated that the mdr1 gene is both necessary and sufficient for multidrug resistance. The product of the mdr1 gene is P-glycoprotein, a transmembrane protein which shares homology with several bacterial proteins involved in active membrane transport. P-glycoprotein appears to function as an energy-dependent efflux pump responsible for the removal of drugs from multidrug-resistant cells. The functions of the mdr system in normal cells and its potential clinical implications are discussed.