(A) Upregulation of Pgp expression measured by flow cytometric analysis using MRK 16 binding (solid histogram) compared with an IgG_2a control (open histogram) in Pgp-negative leukemic blasts obtained from a single patient after 16 hours of treatment with 20 ng/mL IDA, 50 ng/mL MX2, and 100 ng/mL EPI. The blasts were isolated from a patient with BT-CML. The inhibitory effect of 1 μmol/L PSC 833 on the upregulation of Pgp is also shown. Pgp levels were expressed as the ratio of the MCF of MRK 16 versus the IgG_2a control as described in Materials and Methods. This ratio (R) is indicated in each case. (B) Flow cytometric analysis of Pgp function based on Rh123 accumulation in the absence (solid histogram) or presence of 2 μmol/L PSC 833 (open histogram) in blasts from the same patient treated as described in (A). Pgp function was expressed as the ratio of MCF in the presence or absence of 2 μmol/L PSC 833 as described in Materials and Methods. This ratio (R) is indicated in each case.