Expression of the multidrug-resistance gene product in untreated human breast cancer and its relationship to prognostic markers.

The immunohistochemical expression of the 170-kDa permeability glycoprotein (P-gp) was investigated in 41 primary untreated breast carcinomas, using the monoclonal antibodies C219 and MRK16. DNA ploidy by flow cytometry and estrogen (ER) and progesterone receptor (PgR) contents were also determined. P-gp expression, as revealed by C219 or MRK16, was observed in 6 (14%) of the investigated cancers. P-gp expression had a tendency to occur in non-diploid, high-grade tumors as well as in patients with lymph node negative disease. However, except for lymph node status, these associations were not statistically significant. No positive statistical relationships were observed between other prognostic parameters (age, tumor size, and receptor status) and P-gp expression. Considering the great heterogeneity observed in previous studies and the low expression of P-gp observed hereby, the utility of P-gp immunostaining as a guide for therapy planning in patients with breast cancer remains uncertain.