P-glycoprotein-mediated multidrug resistance and cytotoxic effector cells.

Multidrug resistance (MDR) is one of the major obstacles to successful cancer chemotherapy. MDR is a complex and multifactorial phenomenon. One important and common mechanism used by cancer cells as a defense against cytotoxic drugs is a 170-kD plasma membrane glycoprotein, P-glycoprotein (P-gp). P-gp confers resistance by actively pumping cytotoxic drugs out of cancer cells. Paradoxically, P-gp overexpression on tumor cells is frequently associated with enhanced susceptibility to lymphokine-activated killer cell activity. This enhanced susceptibility is not observed with P-gp- MDR cells, nor is susceptibility to natural killer cells increased. The physiologic, evolutionary and immunologic concepts with regard to the P-gp and the possible intervention of the function of the P-gp in cancer therapy are reviewed.