P-GLYCOPROTEINS AND MULTIDRUG-RESISTANCE

Multidrug resistance represents a major obstacle in the successful the rapy of neoplastic diseases. Studies have demonstrated that this form of drug resistance occurs both in cultured tumor cell lines as well as in human cancers. P-glycoprotein appears to play an important role in such cells by acting as an energy-dependent efflux pump to remove var ious natural product drugs from the cell before they have a chance to exert their cytotoxic effects. Expression of the MDR1 gene product has been associated with a poor prognosis in clinical studies. It has bee n demonstrated in the laboratory that resistance mediated by the P-gly coprotein may be modulated by a wide variety of compounds. These compo unds, which include verapamil and cyclosporin, generally have little o r no effect by themselves on the tumor cells, but when used in conjunc tion with antineoplastic agents, they decrease, and in some instances eliminate, drug resistance. Clinical trials to modulate P-glycoprotein activity are underway at the present time to determine if such strate gies will be feasible. Although the P-glycoprotein is expressed in man y cell lines and occurs in patient tumors, its expression is not a uni versal feature of multidrug resistance, suggesting that other mechanis ms are operating.