Expression of the multidrug resistance‐associated protein in myelodysplastic syndromes

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Abstract
In the myelodysplastic syndromes (MDS), P‐glycoprotein (P‐gp) expression is clinically associated with drug resistance, whereas the clinical significance of multidrug resistance‐associated protein (MRP1) is uncertain. Bone marrow from 56 patients with MDS, including six with refractory anaemia (RA)/RA with ringed sideroblasts (RARS), 23 cases of RA with excess blasts/in transformation (RAEB/T), four patients with chronic myelomonocytic leukaemia (CMML) and 23 cases of MDS having progressed to acute myeloid leukaemia (MDS‐AML), were studied. MRP1 expression was investigated by immunocytochemistry (ICC) and by flow cytometry using MRPm6 monoclonal antibody. The efflux test using calcein‐AM (CAM) ± probenecid to evaluate MRP1 activity was performed in ten of the 56 patients. Twenty‐eight of the 56 cases (50%) expressed MRP1. MRP1 expression was more frequent in MDS‐AML than in MDS (70% vs. 36%). The efflux test using CAM was positive in three out of the ten patients tested. The results were in agreement with expression of MRP1 in six cases, and were discordant in four cases (1 MRP‐/CAM+, 3 MRP+/CAM–). No correlation was observed between MRP1 expression and P‐gp, lung resistance‐associated protein (LRP) or CD34 expression, although there was a trend for more frequent MRP1 expression in P‐gp‐positive cases in MDS‐AML (P = 0·08). Ten of the 26 patients treated with intensive chemotherapy achieved complete remission including six out of 16 MRP1+ and four out of ten MRP1– cases (P = NS). In conclusion, MRP1 expression was correlated with disease stage in MDS in our study. As for P‐gp, discordant expression/function of MRP1 could be found in some cases, suggesting the existence of non‐functional transport proteins in MDS. MRP1 expression did not seem to be a prognostic factor in MDS in our experience.