Functional multidrug resistance in acute myeloblastic leukaemia: a standardized flow cytometric assay for intracellular daunorubicin accumulation.

We have developed a standardized flow cytometric method for the measurement of in vitro multidrug resistance in acute myeloblastic leukaemia (AML) blasts, using carboxylate microspheres which bind the fluorescent drug daunorubicin. Cells and beads were incubated concurrently with the drug. Fluorescence was expressed as a cell:bead ratio. Bead fluorescence at a fixed cytometer voltage was consistent over at least a 3-month period (CV 5.47%), and repeat assays up to 8 months later correlated well (R = 0.86). Bead to drug binding provides a valuable measure of quality assurance as well as a standard for cellular drug accumulation assays and would therefore be suitable for reproducibly reporting the results of multidrug resistance analysis in a clinical setting.