P-glycoprotein expression in primary breast cancer detected by immunocytochemistry with two monoclonal antibodies.

We have investigated P-glycoprotein (P-gp) expression in samples of primary breast cancer from 29 patients before therapy. We employed immunohistochemical techniques using two monoclonal antibodies (C219 and MRK16) and an indirect alkaline phosphatase method. Heterogeneous expression in epithelial cells was detected with both C219 (21 of 29) and MRK16 (16 of 29). A surprising finding was P-glycoprotein expression in stromal cells with both C219 (26 of 29) and MRK16 (12 of 29). Our results suggest that significant levels of P-glycoprotein expression may be present in breast cancer before exposure to drugs associated with multidrug resistance.