(A) Flow cytometric analysis of Pgp expression using MRK 16 binding (solid histogram) compared with an IgG_2a control (open histogram) in Pgp-negative blasts from a patient with AML undergoing chemotherapy (in vivo) and after ex vivo experiments. Pgp expression was expressed as the ratio of the MCF of MRK 16 versus the IgG_2a (R) as described in Materials and Methods. The analysis of samples was performed before treatment (0 h), at 4 hours, and at 16 hours of DAU/AraC treatment. In the ex vivo experiments, cells were exposed for 16 hours to medium alone, to 100 ng/mL DAU in the absence or presence of 1 μmol/L PSC 833, or to 10 ng/mL AraC in the absence or presence of 1 μmol/L PSC 833. (B) Flow cytometric analysis of Pgp function based on Rh123 accumulation in blasts from the same patient as shown in (A) at 0, 4, and 16 hours after DAU/AraC combination chemotherapy (in vivo) and in blasts treated with the same drugs ex vivo. Pgp function was determined by flow cytometric analysis using Rh123 accumulation in the absence (solid histogram) or the presence of 2 μmol/L PSC 833 (open histogram). 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.