Clinical significance of P-glycoprotein expression analyzed by immunohistochemical staining in cancer tissues.

P-glycoprotein is a transmembrane protein with increased drug efflux from resistant cells, which is encoded by the MDR1 gene. An overexpression of P-glycoprotein has been reported to correlate with the degree of resistance to anticancer agents, especially to adriamycin. In this study, the expression of P-glycoprotein was analyzed immunohistochemically by using a monoclonal antibody, MRK16 against P-glycoprotein in 18 fresh human tumors. The expression of P-glycoprotein was detected in eight (44 per cent) tumor specimens out of 18 patients. Although six (75.0 per cent) of the 8 P-glycoprotein positive tumors were resistant to adriamycin, the other two tumors showed clinical responses. Furthermore, five (50.0 per cent) of the 10 P-glycoprotein negative tumors exhibited positive clinical responses. These results suggest that P-glycoprotein expression may not be a useful marker to predict intrinsic resistance to adriamycin in fresh human tumors.