Both Pgp and MRP1 activities using calcein-AM contribute to drug resistance in AML

Thirteen cell lines with different levels of Pgp and MRP1 expression were used to assess the ability of calcein-AM uptake and calcein efflux to measure Pgp and MRP1 functions, respectively. There was a good correlation between MRP1 expression and the modulatory effect of probenecid (a specific modulator of MRP1) on the calcein efflux (r=0.91, p=0.0003) and between Pgp expression and the modulatory effect of CsA on calcein-AM uptake (r=0.96, p<0.0001). On light of the high correlations for both proteins, we tested calcein-AM uptake and efflux in fresh myeloid leukemic cells. In 53 AML patients, there was also a good correlation between MRP1 expression (measured by RT/PCR and by MRPm6 expression by flow cytometry) and the modulatory effect of probenecid on the calcein fluorescence (r=0.92, p<0.0001) and between Pgp expression as measured by UIC2 antibody binding on flow cytometry and the modulatory effect of CsA on calcein-AM uptake (r=0.83, p<0.0001). Pgp activity was higher in CD34+ leukemia than in CD34- leukemia (2.26+/-1.50 vs 1.46+/-1.21 respectively, p=0.003) and MRP1 activity was higher in CD34- leukemia than in CD34+ leukemia (1.77+/-0.40 vs 1.4+/-0.29 respectively, p=0.004). Pgp expression and activity (p=0.004 and p=0.01, respectively), MRP1 activity (p=0.03) but not MRP1 expression were prognostic factors for achievement of CR. The effect of probenecid and CsA together were higher than the effect of either probenecid or CsA alone on calcein-AM uptake. These results suggest that functional testing (with calcein-AM+/-modulator) for the presence of both MRP1 and Pgp activities is of prognostic value and that MRP1 contributes to drug resistance in AML.