Expression of the multidrug resistance protein (MRP1) in breast cancer

Background: The multidrug resistance protein (MRP1) is expressed in human breast carcinomas but its clinical significance remains unclear: The aim of the present study was to determine the clinical significance of MRP1 in breast cancer patients. Materials and Methods: MRP1 expression of primary carcinomas fr om 100 breast cancer patients was immunohistochemically determined by means of the monoclonal antibodies QCRL-1/QCRL-3. Results: MRP1 was negative in 20 (20%) and positive in 80 (80%) breast carcinomas. MRP1 expression was more frequent in both estrogen receptor-negative carcinomas and progesterone receptor-negative cal carcinomas (p = 0.1 in both cases), but was independent of tumor size and lymph node involvement. Patients with MRP1-negative carcinomas had prolongations of overall survival (p = 0.01 for death due to any cause, p = 0.04 for breast cancer-related death) and disease-free survival (p = 0.07) as compared to those with MRP1-positive carcinomas. Also in subsets of patients (negative lymph nodes; positive lymph nodes; positive estrogen receptor; T1/T2 rumors), overall survival was longer for patients with MRP1-negative carcinomas. In univariate Cox regression analyses, MRP1 positivity was associated with relative risks of 4.9 (95% CI 1.2-20.6; p = 0.03) for death due to any cause, 6.4 (95% CI 0.9-48.0; p = 0.07) for breast cancer-related death and 3.5 (95% CI 0.8-14.9; p = 0.09) for relapse. In multivariate Cox regression analyses, MRP1 positivity had relative risks of 5.1 (95% CI 1.2-21.7; p = 0.03) for death due to any cause, 6.5 (95% CI 0.8-50.1; p = 0.07) for breast cancer-related death and 3.4 (95% CI 0.8-15.1; p = 0.1) for relapse. Conclusions: Our results suggest that MRP1 might be an important factor in breast cancer indicating excellent prognosis for patients with MRP1-negative carcinomas.