Expression of the multidrug transporter, P‐glycoprotein, in chronic myelogenous leukaemia cells in blast crisis

The overexpression of a cell‐surface glycoprotein termed P‐glycoprotein (P‐gp) is frequently associated with multidrug resistance (MDR) in cell lines in vitro. To evaluate the implications of P‐gp expression in clinical drug‐resistance, we examined the expression of P‐gp in fresh leukaemia cells from chronic myelogenous leukaemia (CML) patients in blast crisis. By using immunoblotting with a monoclonal antibody against P‐gp, C219, we showed that leukaemia cells from three CML patients in blast crisis were P‐gp negative at the stage when these patients were in complete remission, and that the cells showed high levels of P‐gp expression at times when the same patients had relapsed and had not responded to chemotherapy. Six out of 11 patients (nine in the refractory state) were P‐gp positive and they rarely responded to chemotherapy. These data suggest that the expression of P‐gp is closely associated with drug‐resistance in CML.