Clinical role of multidrug resistance protein 1 expression in chemotherapy resistance in early-stage breast cancer: The Austrian breast and colorectal cancer study group

Purpose The multidrug resistance protein 1 (MRP1) is expressed in human breast cancer cells and may contribute to the clinical drug resistance of breast cancer patients. Therefore, we determined the impact of MRP1 expression on the clinical outcome of adjuvant therapy in patients with early-stage breast cancer. Patients and Methods Immunostaining for MRP1 was performed on tissue sections from 516 premenopausal, hormone receptor-positive breast cancer patients with stage I and II disease. Statistical analyses were performed to assess the effect of MBP1 expression on survival and to test for interaction between MRP1 expression and treatment. Results MRP1 expression independently predicted shorter relapse-free survival (RFS) and overall survival (OS) in patients treated with cyclophosphamide, methotrexate, and fluorouracil (CMF; RFS: hazard ratio [HR] = 1.48; 95% CI, 1.16 to 1.88; P = .002; OS: HR = 1.82; 95% CI, 1.10 to 3.01; P = .02), but it did not predict shorter RFS and OS in patients who received tamoxifen plus goserelin (RFS: HR = 0.99; 95% CI, 0.74 to 1.31; P = .9; OS: HR = 0.68; 95% CI, 0.40 to 1.15; P = .1). Tests for interaction between MBP1 expression and treatment were statistically significant for both RFS (P = .04) and OS (P = .006). Conclusion Our data suggest that MBP1 expression plays an important role in the clinical resistance to adjuvant CMF chemotherapy but does not seem to affect response to adjuvant endocrine treatment with tannoxifen plus goserelin. Thus, MRP1 may be a useful marker for the selection of patients with early-stage breast cancer for the appropriate adjuvant therapy after prospective confirmatory evaluation. (C) 2005 by American Society of Clinical Oncology.